Valve amps are dangerous - Page 1
Forum section: electronics / repair

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 15, 2021, 07:15:20 pm
content: No matter how careful you are with a valve amp, you can still get caught out.Happened to me today.Here is a Marshall DSL 201, unplugged from the mains. HT voltages on the valve bases were measured insignificant, I checked that.Yet the main HT cap doesn't have a bleed resistor, and the full HT voltage remains on the standby switch.Normally I check directly on the PSU caps themselves, but this has PCB mounted radial types so you can't access them directly.Ouch, that hurt when I grabbed the amp to turn it over and my thumb hit the standby switch with my other hand on the chassis.

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2021, 07:47:23 pm
content: Youch! There's a lot of noobs in these old companies making design mistakes. They're assuming the O/P tubes are always in and will discharge things I guess. The factory test fixture for the board... big liability if Manufacturing pulls a board off the test fixture and gets zapped. I've seen that happen.On home-brew gear I always put in a LED, resistor, diode indicator just to remind anyone to engage brain before going in. But I have bleeder resistors always.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: October 15, 2021, 07:50:55 pm
content: I design my own valve amps and always have LED in series with a resistor across power supply capacitor.If its red dont touch.I remember my very first valve project.It wasnt working so turned it off and grabbed the pcb and got a shock.Tutor laughed and said that I needed to discharge the cap.So next time I discharged the cap and grabbed the pcb and got a shock again.Forgot to turn it off !Amazingly that was 42 years ago and I am still alive.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 15, 2021, 08:01:27 pm
content: It really should be illegal NOT to have bleed resistors IMHO, I have seen the same on other modern/reissue amps.This amp is a 1999 manufacture date.My thumb has a burn hole, my heart survived though.Newbies' mileage may vary.

author: KevinA
author_rank: Regular Contributor
timestamp: October 15, 2021, 09:27:54 pm
content: Every tech should have an A0 sized copy of this pinned to the wall above their bench, and look at it every ten minutes.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 15, 2021, 09:36:46 pm
content: That actually does kind of look like a RF power triode setup.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 16, 2021, 03:50:59 am
content: It really should be illegal for a tech to service equipment without a schematic and adequate checking of the circuitry and assessment of hazards.Audiorepair, I don't get the impression you have acknowledged a lack of safety awareness and seem to want to give the impression that you were as careful as you could be, which is a bit concerning as you are noting that the main filter capacitor(s) could not be directly probed and did not have bleed resistors.

author: bob91343
author_rank: Super Contributor
timestamp: October 16, 2021, 04:00:51 am
content: I have never like Marshall amplifiers, for reasons I won't bring up here. But they are not built by those who go the extra mile.

author: KE5FX
author_rank: Super Contributor
timestamp: October 16, 2021, 05:02:59 am
content: No kidding. That thing looks like what the lowest-cost Chinese assembly people took apart to learn from.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 16, 2021, 05:27:08 am
content: No kidding, that looks like a typical UK or US designed amplifier sent off-shore for manufacture by a well-renowned company that imports the equipment back in to sell, warrant and service under their well-known and respected name.

author: TMM
author_rank: Frequent Contributor
timestamp: October 16, 2021, 06:03:21 am
content: My question here would be why is the power switch between the DC smoothing caps and the tubes? If you switch the AC mains you don't get this problem as the tubes themselves will bleed it down to at the very most a few tens of volts, probably down to only a few volts if you give it enough time.

author: med6753
author_rank: Super Contributor
timestamp: October 16, 2021, 07:15:31 am
content: Tube audio gear is dangerous only if you are careless. I do a lot of work on tube Tek gear and I typically wear rubber gloves and observe the "other hand behind your back rule". If you don't it can kill you dead.And do me a favor. Your test gear is also part of your safety protocol and should be kept in top condition. That Fluke looks like it was dragged through the mud. Why don't you treat it to a good cleaning?

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 16, 2021, 08:23:38 am
content: Quote from: med6753 on October 16, 2021, 07:15:31 amThe coating is somewhere between ESD safe and HV risky

author: wraper
author_rank: Unknown
timestamp: October 16, 2021, 08:40:32 am
content: Quote from: floobydust on October 15, 2021, 07:47:23 pmIt's not a design mistake since there is no need to discharge capacitors and constantly waste power. And it's not any different from many SMPS where capacitors remain charged for a long time too.

author: Gyro
author_rank: Super Contributor
timestamp: October 16, 2021, 09:19:03 am
content: Personally, I think standby switches are pretty dangerous. They allow the HT caps to charge to full unloaded rail voltage (are they rated 450V or 500V?), way above their normal operating voltage, for no good reason. I'd lay odds that that standby switch isn't rated for 460VDC.

author: PKTKS
author_rank: Super Contributor
timestamp: October 16, 2021, 09:34:16 am
content: Use gloves to service tubes.. !!!!Got zapped countless times before doing so.By that time parts were a lot more expensive..saving one or two here and there was a must do..Today is no longer like that..But 400V and above is really something to not care less aboutPaul

author: CaptDon
author_rank: Super Contributor
timestamp: October 16, 2021, 02:04:09 pm
content: I agree with Bob, Marshalls were always designed with as few parts and qualityin mind. I have seen over 550vdc in standby on 450vdc rated capacitors. I havehad more than one fried power transformer where the line fuse didn't blow untilthe transformer was completely nuked. Guitar players seem to have a loyalty tothe brand but personally I think Marshalls suck. A far over-rated piece of yesteryeargear. My go-to amp for the last 25 years is a TelRay Syncordia (the same folks whomake Fender amps) 'SuperNova' twin 12 knockoff of a Fender twin 12 solid state.Mine was equipped with the Morley Oil Can 'rotating sound' as well as tremelo andreverb. I rebuilt the output section with two 100wrms solid state bricks and addeda mono output jack and stereo return jack for external effects. The jacks are normalledso the amp will play without external effects plugged in. Almost no one has ever seena SuperNova amp and few know what a Morley Oil Can is, but all agree this ampis a killer combo amp! I loaded it with Peavey Scorpions and this thing can play cleanacoustic or killer electric overtones. Granted it doesn't have the warm even harmonicdistortion of a tube amp however.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: October 16, 2021, 02:15:59 pm
content: now i am curious, what is the electrical std called, that apply to this, such that it be made safe? IEC 1234? UL 5678?

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2021, 02:19:35 pm
content: Re: standby switches on tube gear.Unfortunately, most tube gear uses a multiple-secondary power transformer so switching the primary kills both the HV and the heater voltages.If one has a separate heater transformer, then switching the primary winding of the HV transformer allows the tubes to discharge the filter capacitors (somewhat, at least) in standby mode.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: October 16, 2021, 02:23:38 pm
content: I agree with the opinion about Marshall; far too overpriced for no real adequate quality; though Fender did some design errors too, like my Pro Juniors which require quite some mods to reach an acceptable noise level (buzz, hum, hiss).never forget the iron rule of the valve sphere: always one hand in the pocket

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 16, 2021, 02:47:14 pm
content: When I got an HT shock for the first time many years ago now, I adopted the one hand in back pocket procedure. Nobody wants that to happen twice.And until now it hasn't.The issue here is that after testing the pre-amp section with the output valves out, I pulled out the mains plug as I always do, before flipping the amp over to put the output valves back in again.It was then that my thumb caught the spade terminal of the Standby switch.I will now adopt the additional policy of "always check the Standby Switch" before flipping amps.I suggest others do the same.There are issues with the amps design that could have easily been made to prevent this, I feel.The spade terminals on the Standby switch are not insulated, and the live wire is uppermost where it can easily be touched.Reversing the wires and using insulated Spades would make this event pretty unlikely.I don't think the extra cost in doing this would have broken the bank, and you could still have saved a couple of cents by not having a bleed resistor.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 16, 2021, 03:25:12 pm
content: Quote from: TMM on October 16, 2021, 06:03:21 amExactly!Thousands of tube radios & amplifiers were constructed over many years without a separate HT switch.Such things were normally reserved for transmitters.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 16, 2021, 03:29:23 pm
content: Quote from: PKTKS on October 16, 2021, 09:34:16 amTechs repaired tube stuff for decades without needing gloves, or other nonsense.What they did have, was a functional brain, something that seems scarce these days.

author: med6753
author_rank: Super Contributor
timestamp: October 16, 2021, 05:28:03 pm
content: Quote from: vk6zgo on October 16, 2021, 03:29:23 pmAgreed. But my hands aren't as steady as they used to be so as an extra measure of safety I wear the rubber gloves.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 16, 2021, 05:53:45 pm
content: Quote from: vk6zgo on October 16, 2021, 03:25:12 pmI believe the Standby switch was invented so that you could mute a noisy amp when you weren't playing it, and quickly unmute it when you wanted to.It otherwise serves little purpose.Edit:In fact, I have read on several occasions it is a popular myth that the Standby Switch is there to prevent "cathode stripping" on warmup, but the reality is that only very high voltage transmitter tubes suffer from this.Is this a thing?

author: SeanB
author_rank: Super Contributor
timestamp: October 16, 2021, 06:09:25 pm
content: Just remember one thing about high voltage, and high value bleeder resistors, is that they can often quietly go open circuit, so never trust them to discharge the power rail capacitors. I keep a few 10W 68k ceramic resistors around to discharge those capacitors, you see the small spark, but it will not damage the capacitor. Just check the resistor value every so often and see if it is drifting up, and replace with another one from the box.Standby was normally to allow you to set up and get the amplifier ready, with hot heaters, so you could turn it on and be ready to go in under a second, without having any noise out of the speakers. Plus it also kept thermal shock down in the larger tubes, as they often also would reduce filament voltage in standby, so the filaments were warm, but not hot to fully emit, and thus did not suffer any failures from turn on surges. Warm to hot is a lot less stress, and a lower surge on a fragile filament.

author: ogden
author_rank: Super Contributor
timestamp: October 16, 2021, 06:27:57 pm
content: Quote from: wraper on October 16, 2021, 08:40:32 amBe careful with blanket statementsSafety standards requires X-capacitor bleeder resistors. For instance IEC 62368-1 even requires *two* redundant bleeders.

author: rsjsouza
author_rank: Super Contributor
timestamp: October 16, 2021, 06:34:32 pm
content: Quote from: SeanB on October 16, 2021, 06:09:25 pmI adopt the same philosophy as well, although I am a bit more impatient and use a 2k2/10W - the U1273A ZLow 2k resistor is too low power for hundreds of volts, otherwise I would use it instead

author: Gyro
author_rank: Super Contributor
timestamp: October 16, 2021, 06:39:25 pm
content: Quote from: Audiorepair on October 16, 2021, 05:53:45 pmThere are plenty of ways of muting a valve amp without resorting to switching the DC HT - a muting switch on the input would take most noise out, as would shorting the output (something you can do on a valve amp of course, it's open circuit that they hate). I think it was a brainless design decision, probably done on the fly when a musician flagged it on an early model.QuoteCathode stripping is caused by pulling so many electrons out of the cathode that it loses its space charge and is exposed to ion bombardment. It's certainly a thing inhighvoltage tubes where full HT is applied while the heater is still warming up. It's debatable (and frequently debated!) whether this is a factor down as low as 450V - at a few kV it certainly is, but this is a 'danger zone' between where oxide coated cathodes are still useable and where they transition to thorated or pure tungsten cathodes (bigindustrial or transmitting station tubes, X-ray tubes, Kenotrons etc. working at 10s of kV.At the same time, suddenly hitting a hot amplifier with (over)full HT via the standby switch might hit the tube cathodes with a momentary over-current situation depending on the cathode / grid biassing arrangements, charging of coupling capacitors etc. That might count as stripping,causedby the standby switch (and poor design).Whatisa thing is Cathode poisoning, where the tubes sit for extended periods with heaters running and no bias on them. This creates an insulating cathode interface layer under/over(?) the oxide coating that reduces its emission. That's the one I would be more concerned about with amplifiers accidentally sitting in standby for extended periods. [Edit: this used to be arealproblem in so called 'quick start' valve TVs until they learned and started reducing the heater voltages too ('slightly slower start'?)]

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 16, 2021, 06:52:25 pm
content: Interesting stuff.It is possible to mute a typical push/pull valve amp by disabling the Screen grid voltage to the output tubes.I am perplexed, after once seeing it done in an amp I can't remember now, why it was never widely adopted instead of the Standby Switch, which as you describe, is a pretty brutal approach.

author: floobydust
author_rank: Super Contributor
timestamp: October 16, 2021, 08:32:46 pm
content: Quote from: ogden on October 16, 2021, 06:27:57 pmAnyone inside a power supply is a "skilled person", not the "user/operator". I've seen bleeders required only to prevent a user electric shock occuring at the power cord, as in an SMPS primary capacitor backfeeding through the bridge-rectifier.The Marshall at B+ 500VDC is too much for a single electrolytic, you'd use two in series with balance resistors which are the bleeders as well. Did they do this?Cathode-stripping is more of a happening with solid-state rectifiers, a 5U4 takes long enough to warm up and moderates it. I've seen stripping happen, you can see some pretty strange flashes or glows from tubes if you watch them on power up. Just watch a 5U4 someday. The Marshall can have 500VDC up long before anyone else is warmed up so I would leave Standby on until it's warmed up.Cathode poisoning I don't think was ever as problem (outside of nixies). Old TV/radio station transmitters had the spare tubes in sockets with filaments lit, to shorten down time.I did have many white LED's fail on HVDC like 220k+LED on 350VDC. They would go dim, or just fail. Drove me nuts and adding a reverse-diode seemed to fix it. Strange but the dropping resistor's parasitics must be a problem. Anyone else run into this?

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2021, 09:01:00 pm
content: Actually, 5U4s (with directly-heated cathode, i.e. filament) warm up very quickly. The 5AR4 and other tubes with indirectly-heated cathodes were preferred in audio equipment to delay application ofVbbwhile the rest of the cathodes warmed up. Did the Marshall unit in question actually have a single high-voltage electrolytic after the rectifier? I believe the poster referred to measuring 550 V across a 450 V rated single part. That is a good example of non-conservative design practice.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 16, 2021, 09:18:46 pm
content: Quote from: TimFox on October 16, 2021, 09:01:00 pmTim Fox, the OP claimed no such thing. You just made that up.

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2021, 09:54:08 pm
content: Floobydust raised the 5U4 question.Capt Don raised the 550 V on a 450 V capacitor question.My reference to "the poster" was sloppy--I did not mean "the original poster".

author: trobbins
author_rank: Frequent Contributor
timestamp: October 16, 2021, 10:04:04 pm
content: Quote from: floobydust on October 16, 2021, 08:32:46 pm500Vdc is not too much for a single electrolytic - it is at the spec limit for a 450Vdc rated cap during a power on event. Without the amp's schematic and some actual measurements it is just conjecture about what voltage the first filter capacitor could be sitting at long term with the standby on. The DSL201 schematic on Ampwares does not show the HT1 power supply circuitry and only shows 450V rated parts after the standby switch. There are 500Vdc rated e-caps made for the guitar amp market.There is also some doubt about whether the DSL201 did not have a bleed resistor, as the DSL401 schematic on Ampwares shows a bleed resistor is in circuit.

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2021, 10:07:36 pm
content: In Reply #15 above, Capt Don stated that he had measured over 550 V on the 450 V capacitor during standby.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 16, 2021, 10:16:43 pm
content: Quote from: TimFox on October 16, 2021, 09:54:08 pmOk, I take that back.Sorry.

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2021, 10:18:04 pm
content: Another issue with older vacuum-tube equipment (in the US) is that the transformers were originally specified for a 110 V primary voltage, but line voltages have increased since then and the present values are 120 V nominal, 125 V or so (depending on utility) maximum, for consumer service. Thus, the rectifier outputs may be 13% higher than in the original design. That doesn't affect the personal safety question (against shocks), but does affect capacitor voltage rating questions.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 16, 2021, 10:22:41 pm
content: The full schematic is on drtube and in a thread over at ssguitarhttps://www.ssguitar.com/index.php?topic=3357.15There is a bleed resistor across the first filter cap before the standby switch.

author: floobydust
author_rank: Super Contributor
timestamp: October 16, 2021, 11:26:59 pm
content: DSL201 full schematicpg 1&2.I see a single filter cap C32 100uF 450V and R133 470k 1/2W as a bleeder resistor... which is a couple minutes to discharge the cap to something safe. 1/2W is low cost but takes forever if there are no O/P tubes helping out. 500V electrolytics are special construction and extra cost. With high line the 450V part is getting pushed in standby but this is rock and roll.Look at a 5U4 on power up, there's always an odd blue flash when emission starts that I assume is not good, it doesn't matter as it is the sacrificial lamb as far as cathode stripping. Directly-heated filaments I have seen advice to wait until they are heated up before pushing current through them.

author: Teledog
author_rank: Regular Contributor
timestamp: October 16, 2021, 11:28:00 pm
content: Have an old Fender Bassman with an AC polarity switch.After one too many shocks with bare feet touching ground, replaced the original non-polarized plug with a 3 prong grounded plug (N/C to the gnd pin)Put a brass cover over the polarity switch -to keep the chassis in original shape, of course- without someone inadvertently flipping the polarity.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 16, 2021, 11:56:58 pm
content: Quote from: floobydust on October 16, 2021, 11:26:59 pmCheck out the bias pot. VR12What is it doing?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 17, 2021, 12:32:42 am
content: Perhaps mod it to switch off the HV at the transformer allowing the cap to discharge in standby?

author: floobydust
author_rank: Super Contributor
timestamp: October 17, 2021, 01:50:10 am
content: Quote from: Audiorepair on October 16, 2021, 11:56:58 pmThe schematic is drawn a bit wonky, what a mess. Pot VR12 is dual 200k and it's drawn near the bias section but is MASTER VOLUME.SCREEN_A,B,C,D seems to be an AC shield, a capacitor to ground and the leg perhaps to the pot's housing, another to the front trim etc. It has metal shield to SCREEN_D, no taps.Bias is set by PR1 as quiescent current across the common 10R cathode resistor R33 675mV at (Test CON5 pin 1 or 3 to pin 2 GND) giving 33.75mA (by my math) each, including screen current. Seems a little hot.The Marshall manual seems to have a math typo for the bias specs for the DSL201? This is what the sheet says:DSL410 (Two pair EL84, single 10R cathode resistor R33, so divided by four)1.3V=32.5mA/EL841.4V=35mA/EL84DSL201 (One pair EL84, single 10R cathode resistor R33, so divided by two... but not)0.65V=16.25mA/EL840.7V=17.5mA/EL84I think that's wrong, out by a factor of two? Like this:DSL2010.325V=16.25mA/EL840.35V =17.5mA/EL84

author: vk6zgo
author_rank: Super Contributor
timestamp: October 17, 2021, 03:59:06 am
content: Quote from: floobydust on October 16, 2021, 08:32:46 pmBack in the day, in Oz, 600v rated electrolytics were quite common--apparently not so much, now!QuoteI have worked at a lot of TV/Radio sites, & never seen that actually done.From memory, one Tx had extra sockets, but they never had anything in them.Thinking about it, any time saving is pretty much illusory, as the whole transmitter has to be powered down, the spare tubes shifted to a used socket, then the full heat up time has to happen as per the built in timer, before the application of HT.Add to that necessary cooldown time before you can safely handle the tubes (they getbloodyhot!), & you save very little time.A tube change in a TV transmitter usually requires a check of the swept response of the Tx after such a change, stretching the time necessary out even further.Most places have a standby transmitter, or if running parallel transmitters, can go to one Tx only, so there is not that much urgency.Quote

author: trobbins
author_rank: Frequent Contributor
timestamp: October 17, 2021, 05:08:23 am
content: Quote from: vk6zgo on October 17, 2021, 03:59:06 amThat was likely to be '600V surge' or 600V VP (with 500V VW) on chassis mounted can caps from Ducon.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 17, 2021, 09:27:33 am
content: Quote from: trobbins on October 17, 2021, 05:08:23 amAnd UCC!They made "axial" ones in that voltage rating, too!The 500v "0ver run" of the HT line referred to in this thread would have been comfortably handled by such devices.For higher VW, large rectangular oil-filled caps were often chosen.These, in most applications would operate flawlessly for decades.

author: m3vuv
author_rank: Super Contributor
timestamp: October 17, 2021, 11:23:53 am
content: reminds me of working on a homebrew 1kw rf amp a few years ago,is had a 200uf cap bank with 2.5kv on it, the rf cage had an interlock microswitch that bled off the hv thru a 50 ohm welwyn 10 watt wirewound resistor,i forgot to defeat the switch once the resistor exploded in less than a second when i turned the amp on,had to change my underwear shortly afterwards,lesson learned!!

author: m3vuv
author_rank: Super Contributor
timestamp: October 17, 2021, 11:32:59 am
content: one hand in the pocket or one foot in the grave!!!!

author: CaptDon
author_rank: Super Contributor
timestamp: October 17, 2021, 02:12:22 pm
content: I had an old Earth guitar amplifier years ago. As an indication of how solidly builtit was, the amplifier head survived a bar fire (Not the cabinet however). I builta new cabinet for it and cleaned up the chassis and it played beautifully. I didfinally sell it. Here is the interesting thing, it had two pairs of 6L6's in push pulland when the input was absent or very very low they would bias the outputsto near cut off!!! Screen voltage was derived from plate voltage and the screenswere also run in a sort of ultra-linear fashion. I think it had one of the best tube-ampsounds I ever heard and with 4 X 6L6 it got really loud if you needed it to.

author: TimFox
author_rank: Super Contributor
timestamp: October 17, 2021, 03:37:02 pm
content: The biasing arrangement you described:Was this approximately class-B biasing of the control grids (near cut-off), or was there an active circuit that changed the bias towards cut-off as a function of signal level?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 17, 2021, 04:14:50 pm
content: Quote from: floobydust on October 17, 2021, 01:50:10 amYes, at first I thought it was some kind bias balance arrangement, and looked all over the PCB for some kind of trimmers to adjust that aren't there.Took me a while to discover VR12 is the Master Volume pot, never seen this arrangement before, using a stereo pot.

author: floobydust
author_rank: Super Contributor
timestamp: October 17, 2021, 05:07:03 pm
content: I thought it was an AC balance pot lol.Master Volume is as far down the chain as you can get, for best signal/noise ratio. At zero volume, you can only hear noise from the EL84's and not all the pre-amp chain, effects etc. It might be for getting more crunch where you can overdrive everything upstream yet still play at lower levels.Let us know how the bias setup goes, I say ~0.33V at the test point giving 16.5mA per tube is ballpark. It's heavily into Class A so they are going to get hot maybe 8W each? Marshall biasing to ~12W the max. rating and they would likely glow dull red. They really do need a cooling fan pulling that off on a PCB nowadays, the sockets and board just cook.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 17, 2021, 05:11:15 pm
content: The OPT is faulty on this one, have ordered another.Bit of Googling shows OPT failure on these is alarmingly common.

author: floobydust
author_rank: Super Contributor
timestamp: October 17, 2021, 05:56:52 pm
content: How did it fail? The amp has no loop feedback so an open loudspeaker jack would make a Tesla coil. The secondary has a pseudo-ground CH2 which seems silly, 70mA ground loop before D5, D6 turn on. The schematic tags the OPT a "safety critical component".

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 17, 2021, 06:27:42 pm
content: I'm not sure how it failed, DC resistance measurements on each half of the primary are similar, so no clues there.I took a photo of the output into a dummy load, then, because as you say there is no feedback, I just swapped the primary windings around, and the output waveform became inverted.So I guess that proves the OPT faulty.Oh, and some of you may notice my Rigol scope is not covered in mud, like my Fluke.The Fluke is used all day, 5 days a week, 50 weeks per year, and is about 6 or 7 years old.I get grubby amps and PA speakers and the like in every day, so yes, it does become grubby too.If I thought it would improve its performance by constantly cleaning it, I might consider doing that. As it is, it has a job to do and it does it very well.Clearly the Rigol is newer, and has some catching up to do.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 17, 2021, 08:30:23 pm
content: Ah, actually, I'm not sure that proves the OPT is ok.....I scoped the phase splitter pins 1/6 and they were fine big sine waves. Need to go back and check tomorrow.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 17, 2021, 11:20:52 pm
content: OPT's can also be checked by applying a low mains frequency AC voltage to the secondary (perhaps start low but aim to increase to near equivalent full power output voltage) and confirm the unloaded/unconnected primary half-windings each measure the same (getting to quite high) ACV - that aims to confirm no leakage/breakdown within the primary windings, although the other important test to make is an insulation resistance test from primary to core at 1kVdc (if you are doing electrical testing then an insulation resistance meter is an important tool to have).

author: vk6zgo
author_rank: Super Contributor
timestamp: October 17, 2021, 11:45:49 pm
content: Quote from: floobydust on October 17, 2021, 05:56:52 pmThe "Q" of some output transformers is quite high.Many years ago, I built a Mullard "3-3" mono amplifier.(3 valve, 3 watts).It made quite a lot of noise for three watts, used with an efficient 12" speaker.One day, the speaker connection fell off, & I was treated to the sight & sound of zapping across the valve socket.Going purely by eye & ear, it was similar to what you get by holding a sparkplug lead a short distance from the car engine .It didn't damage the old Ferguson output transformer------they were made of sterner stuff in those days!

author: vk6zgo
author_rank: Super Contributor
timestamp: October 18, 2021, 01:02:47 am
content: Quote from: floobydust on October 16, 2021, 11:26:59 pmThat schematic is the most complicated heap of crap I've seen for a long while, & all for a miserable couple of EL84s!The old Mullard designers would "turn over in their graves!"Even if you ignore the various "audio effects" circuits, the remaining audio amp stuff is way more complex than necessary.Marshall may have been a legitimate manufacturer at one time, but it seems they have succumbed to the "make something that looks like black magic" school of thought..The suckers will be thrilled, & go "Oooo! Shiny!"Marshall customer:-

author: CaptDon
author_rank: Super Contributor
timestamp: October 18, 2021, 02:29:34 am
content: I think they had an active 'slide-bias' arrangement where when you played it hard the finalsshifted toward AB1 or AB2, but when there was a no signal condition it drifted toward 'B' atwhat must have been as close to cutoff as you could get. In the active mode with no inputsignal you could actually place your hand on the 6L6's without getting burned, that was oneof the first things I noticed while working on the amp. It had bleeders and the standby switchlifted the center tap of the H.V. winding through a double pole 'double break' single throwswitch. You would still do the 60HZ or would it be 120HZ shuffle if you got a hand on theStand-By switch (Terminals were not insulated). This amp seemed to be perhaps a limitedproduction or one of a few Beta releases of something they were toying with. Most of theEarth amps were exact knock-offs of Fender or Peavey, This thing was different in many waysand did not resemble any picture of Earth amps I can find. Oh, and the bias thing, when itslid to near cutoff or even real cutoff it was quiet as a mouse, no hum no buzz (unless youhad one of the shitty guitar coil cords so popular back then!!) Another indication of 'slide-bias'was the blue halo glow in the 6L6's going fully away after about 10 seconds of no input signal.Wish I still owned it!!! I have a video of me playing a Gibson Grabber Bass through it at anoutdoor concert.

author: rsjsouza
author_rank: Super Contributor
timestamp: October 18, 2021, 03:11:16 am
content: Quote from: vk6zgo on October 18, 2021, 01:02:47 amOof... Indeed. This is a brutal schematics.Someone was tasked to draw the engineers' original designs and didn't realize that paper sizes larger than A4 existed. That or the schematics distributed to the public are mangled on purpose.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 19, 2021, 07:03:51 pm
content: Well it seems I diagnosed the OPT faulty when it wasn't.When I swapped the primary over and the asymmetrical secondary waveform also swapped over, for some reason I blamed the OPT.In reality, the OPT is shown to just pass through the primary waveform, it is THAT that is asymmetrical.Funny how simple logic can get so confused.Anyway, turns out the stereo Master Volume pot was bad. One side way lower than the other.I had ordered one along with the OPT (which fortunately was very cheap from Marshall).So now, with the new Master Volume Pot and the original OPT, the amp is happy again.It doesn't actually have a bleed resistor, R133 doesn't go to ground, but to some point marked Ch2?I dunno, this schematic is indeed very crap and confusing.The main PSU cap and thus Standby Switch still had 260v on it after being off for 2 days.Bias wise, as I think Floobydust suggested, the Marshall readings are WAY too high, what were they thinking?I adjusted to onset of crossover distortion before clipping, and it was 500mV at the test point.I haven't measured actual currents/power as yet.

author: floobydust
author_rank: Super Contributor
timestamp: October 19, 2021, 07:18:43 pm
content: I thought it wasn't the OPT but didn't know the work and readings you'd taken. It's no fun scoping the plates, I avoid that measurement it's dangerous.You have to trigger the scope from the signal generator to keep absolute phase correct when looking at one side or the other.Lop-sided output is usually a weak tube or high value screen resistor. The potentiometers out of china are very low quality, I have replaced many. If you can get Alps or Panasonic 9mm or 12mm that fit they are better.500mV is 25mA/EL84 but don't know what plate voltage that is, for heat.I say it's a squawky Marshall design having no loop feedback makes the damping factor poor and loudspeaker/cabinet/room resonances will cause peaky freq. response. So they have to run bias very high to help tame that. Even the simplest Fender/Vox use loop feedback and lower distortion output stage.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 19, 2021, 07:49:56 pm
content: I think I got fooled into blaming the OPT because it came in with one dead output valve and its burnt out screen grid resistor.New valves and resistor resulted in asymmetric output, yet the phase splitter outputs were good.(I would not be scoping the power tube Anodes either. I scoped phase splitter and OPT outputs only)Interesting what you say about lack of feedback and bias.Should I be biassing this much hotter than normal then?

author: floobydust
author_rank: Super Contributor
timestamp: October 19, 2021, 08:10:59 pm
content: The Marshall DSL201 bias spec reads "0.65-0.7V" which IMHO is near Chernobyl level for the EL84's. You get a fatter sound of course but a constant need for new tubes, cooked sockets and repairs. You can work out the voltage/current at rest and see how close to max plate dissipation ratings it is. Your 0.5V seems better if B+ didn't come up too much with the lighter load.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 19, 2021, 08:41:26 pm
content: Ok, I'll measure tomorrow, and maybe set it halfway between sensible and Chernobyl or something.I do tend to set the bias so the clean is just totally clean, and the tubes have longest life.Thanks.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 19, 2021, 10:32:49 pm
content: I'd recommend checking the main cap bleed path - R133 goes to a gnd lift circuit which should have a 10R resistor to gnd - if that path is not working as a bleed then you have a fault that should be repaired wrt safety. Imho this is an issue with people repairing equipment that don't have the skill to appreciate schematics and safety and getting zapped.

author: floobydust
author_rank: Super Contributor
timestamp: October 20, 2021, 04:50:34 am
content: It's 470k and around 2 minutes to discharge (if the O/P tubes don't drain B+) but it should eventually discharge the filter cap. At a 1/2W rating, not really suitable for near 500V either.I have no idea why Marshall put that 1mA into the ground lift CH2 (10R) it seems wrong. The schematic doesn't show PE ground etc. What a dog's breakfast.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 20, 2021, 06:06:16 am
content: CH2 on page 1 appears to be associated with a 'metal washer' safety critical marking, and CH could mean a chassis connection, as CH1 and CH3 are also identified for circuit sections that make sense for local chassis grounding. Not being the service agent for Marshall with access to training and other documentation, and just relying on a schematic, is a risk to any unauthorised repair work. Companies do just prepare schematics for in-house use, so being a critical outside observer is a bit rich. It's lucky that schematics have made it in to the public domain, as that is not the case for some manufacturers (eg. TEK and Picotest are two instruments I have that I would love to get schematics of) and a schematic may only become available via the effort of a diyer doing some reverse-engineering.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 20, 2021, 07:29:49 am
content: Quote from: trobbins on October 20, 2021, 06:06:16 amIt's not rocket science---it's just a valve type audio amplifier, the like of which were manufactured in their 100s of thousands back in the day.(mostly vastly superior ones, too!)Comparing that to Tektronics equipment (all the older analog stuff not only provided schematics, but detailed repair manuals) is like comparing a duck poo to a diamond!

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 29, 2021, 08:10:14 pm
content: Amp returned to customer. Customer happy.After the myriad of other stuff to sort, turned out 470k bleed was open circuit, replaced with 1 Watt.Heatshrinked all the Standby Switch connectors, so you would really have to try hard to get zapped now.I didn't feel the need to modify the bias further.Clean is clean.

author: coppercone2
author_rank: Super Contributor
timestamp: October 31, 2021, 03:51:07 pm
content: They need to identify hazards with stickers, its the best way, because eventually you will run into some hidden electrolytic behind a wiring harness or some shit like that.You need to dig through it real slow like an archeologist and use nonconductive hooks to look past movable obstacles. Lifting a panel to have a board mated with something else without being able to see the other side is downright frustrating though. Ideally you would use a boroscope but no one is gonna do that. You can always bath it so long its not potted.The worst I have seen I think is in a hughes amp, if the bleed resistor was not there, it would be very dangerous, since there was a cable harness infront of a hidden screw stud capacitor, if I recall correctly. So if you reach near the cables to maybe yank on one, attach a sensor (magnetic field), or reseat, you could brush your fingers on a hidden electrolytic that is fairly substantial. However, they did have a socket crimp leg power bleed resistor over the cap that drained it in a hurry.. I would have however appreciated a screw on shield or clip on shield between the cap and the cable harness, to increase the safety factor. Ever notice that with tek scopes too, the early 90's one, they have cable harnesses just running about possibly making direct contact with the chassis too? I managed to nick one when I was closing a box. God forbid they put some clamps for that kind of stuff.Yeah this is how I felt with that capacitor, attwo minutes, complete with the overkill breaker switch.I just say this, hey dumbass, theraptorscapacitors belong in a pen.But, I know why it happens. Its not the electrical designers fault because you can easily lose your lungs and your job screaming at desperate mechanical integration engineers that make life so unpleasant.

author: floobydust
author_rank: Super Contributor
timestamp: October 31, 2021, 04:57:50 pm
content: This is about putting profit over quality. DSL-201 did retail at $600 "Made in England" and here we have a R133 470k 1/2W sitting at ~450VDC... pretty much the max. ratings.I think Jim Marshall would've preferred hiring designers that can do Ohm's Law and not cater to the executive trying to save $0.10 towards his new Jag lol.Modern Vox amps no better, the PCB arcs and burns up because the designers bungled the HV spacingsFor SMPS primary caps, I did make an LED+resistor in a old ballpoint pen for checking HV and slow discharging. Something to make it easy to add this as a habit in your workflow.It needs an update to using a PTC or larger 5W bleeder resistor, or maybe an SCR would be best.

author: duckduck
author_rank: Frequent Contributor
timestamp: October 31, 2021, 06:26:54 pm
content: Quote from: Audiorepair on October 16, 2021, 05:53:45 pmI think this guy covers all of the pros and cons of standby switches on vacuum tube guitar amplifiers. The explanation that resonates most with me is "Fender was a cheap-ass and Marshall was a copy-cat".https://debontamps.com/standby-switches/