What is a good value for B+ ripple on  480VDC - Page 1
Forum section: electronics / projects

author: Planobilly
author_rank: Regular Contributor
timestamp: December 01, 2016, 02:07:32 am
content: Hi,I have a new 100 watt tube amp design/build I just started up which has a large amount of hum. I am investigating the power supply first and then the rail. Next to no ripple past the B+.The loaded B+ voltage is 480VDC and the ripple is 5.19 VAC RMS (PK-PK 16.2) This only supplies the plates on the four output tubes through the output transformer. All the other HT supplies are well filtered and show more or less no ripple.That is less than 2% and I don't think it could cause much of an issue but just need a second opinion.Thanks,Billy

author: barry14
author_rank: Regular Contributor
timestamp: December 01, 2016, 11:28:56 pm
content: I believe that is too much ripple. I don't think the ripple level can be related to the DC level. It is more of a function of the amplifier design and how well it is able to reject noise on the DC lines. For an audio amplifier, that ripple should probably be no more than 100-200 millivolts peak-to-peak. Have you checked the filtering capacitors for capacitance and dissipation factor?

author: Andy Watson
author_rank: Super Contributor
timestamp: December 02, 2016, 12:56:06 am
content: If you know, or have some estimate of the current you can make an approximation by assuming that the capacitors discharge linearly between each mains cycle (actually every half-cycle - assuming fullwave rectification). \$V_{ripple} \approx \frac{I \times T}{C}\$ This should give you a ball-park figure for the maximum ripple voltage.Assuming this is the usual paired-tube arrangement operating push-pull on an output transformer, I would expect the stage to be reasonably well balanced and not susceptible to ripple on the B+.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 02, 2016, 01:09:47 am
content: 1- Are your filaments powered with AC or DC? (especially the input tubes)2- Got a big inductor lying around? Add it temporarily to the B+ line to smooth it more. See if that makes any difference.As a side note what output tubes are you using that are happy with such a high B+? 480V seems unusually high to me, but it's been a long time since I played with tubes.Good luck!

author: calexanian
author_rank: Super Contributor
timestamp: December 02, 2016, 03:27:15 am
content: Too many variables to make a suggestion. Based on the voltage I am assuming this is a push pull circuit. can you post a schematic? for some kinds of designs that is completely acceptable. For others that is terrible. It just depends on the design.

author: Zero999
author_rank: Super Contributor
timestamp: December 02, 2016, 09:02:40 am
content: Quote from: barry14 on December 01, 2016, 11:28:56 pmIf it's a well designed audio amplifier, then that amount of ripple shouldn't be a problem.QuoteI doubt 100mV to 200mV of ripple on a 480VDC supply was possible in the valve era? It certainly would have been costly to build a power supply that clean.

author: Planobilly
author_rank: Regular Contributor
timestamp: December 03, 2016, 06:46:06 am
content: Thanks guys for all the feedback. The hum turned out to be from something I screwed up in the bias supply.The amp runs AB push pull and yes 480 VDC plate voltage is high. We push these tube past the normal limit all the time in high gain amps. Keeps the repair guys working...lol The heaters are AC on the 4 output tubes and DC on the 7 preamp tubes.Also looking at several other similar amps, it appears that < 2% ripple is pretty normal for just the B+.I am pretty new to all this and this is a fairly complex amp to build on a turret board at least for me. Here is a photo. I am still experimenting and have several issues to solve.Thanks,Billy

author: calexanian
author_rank: Super Contributor
timestamp: December 03, 2016, 06:54:53 am
content: Without a schematic I really cant help you. too many variables.

author: tautech
author_rank: Super Contributor
timestamp: December 03, 2016, 07:10:55 am
content: Quote from: Planobilly on December 03, 2016, 06:46:06 amI would have thought ~2% ripple a little high but considering the current draw of beasts like those ^^^ maybe not.A 100mA 320V valve project PSU I built back in the 70's had a 2V ripple with zero draw other than a 22K bleed resistor. That's well under 1% ripple.I well remember my Science teacher being horrified that as a 14 yr old I was fiddling with stuff like that.How did he know I was....well the only oscilloscope I'd ever seen in the flesh was in his lab and I needed to use it to check this PSU for ripple. Well I didn't blow it up (pure luck) and got my PSU's ripple measurement.The remnants of that PSU are still on a shelf in my lab some 40 yrs later, wonder if it still goes...............

author: cvanc
author_rank: Frequent Contributor
timestamp: December 03, 2016, 02:40:20 pm
content: (smacks forehead) Hum on the bias voltage! Of course. I shoulda thought of that.Glad you got it sorted out!

author: Planobilly
author_rank: Regular Contributor
timestamp: December 03, 2016, 06:34:27 pm
content: The next issue.....Here is how the DC heater supply is connected as built.I have 6.3 VDC at the preamp heaters with .480 VAC at the heaters.Scope screenshots....DC coupled on one side of the bridge to chassis groundDC coupled on both sided of the bridge.I am not very skilled with a scope so I am not sure what I a seeing plus I did not expect .480 VACPerhaps someone can comment.Thanks,Billy

author: Planobilly
author_rank: Regular Contributor
timestamp: December 03, 2016, 06:40:01 pm
content: Basic schematic to the amp.Billy

author: tautech
author_rank: Super Contributor
timestamp: December 03, 2016, 06:56:32 pm
content: Quote from: Planobilly on December 03, 2016, 06:34:27 pmAs you want to see the AC component riding on the DC voltage use AC input coupling, DC coupled trigger (always) and 10:1 (min) probes (preferably 100:1...think safety) and Vertical settings will be in the 10's of mA/div.Be sure the channel's 0V level is visible on screenshots, maybe shift it lower to show larger/all the waveform.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 03, 2016, 07:58:04 pm
content: The output tube B+ supply is before the fuse but the screen grid supply is after the fuse. What happens in your output stage when the fuse blows? The screen grids will go to zero volts but the plates will still be on...I don't see the bias supply on the drawings, but the same worry applies if it will turn off when the fuse blows.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 03, 2016, 08:14:00 pm
content: Quote from: Planobilly on December 03, 2016, 06:34:27 pmIsn't that a 6.3VA.C.supply for the output tubes - that's being rectified and smoothed to deliver D.C. to the preamp tubes ? Even allowing a generous 2V drop across the diode bridge you'll still be over-driving the preamp heaters.

author: Planobilly
author_rank: Regular Contributor
timestamp: December 03, 2016, 09:02:10 pm
content: I have decided to power the preamp AC for the moment and go on to resolve some other issues.Thanks for all the feedback so far.Billy

author: calexanian
author_rank: Super Contributor
timestamp: December 03, 2016, 09:45:46 pm
content: There appears be a few issues in that schematic. First the secondary of the power transformer is drawn with a tap to ground. Somebody else mentioned this. Depending on how that transformer is made will determine what you need to do with that tap. What is the total AC voltage across that secondary? That will help with that. You may find that that tap needs to go between the two main filter caps such as in later fender amplifiers. As far as the filament supply. DC on filaments are never worth it. The unbalanced DC hum finds it way into the signal more than a balanced AC filament supply does. Just ensure proper lead dress and balance the filament supply with a couple of 100 ohm resistors referenced to ground. This appears to be an approximation of a fender style circuit. The standby switch needs to be after the main filter caps and the B+ supply after that with no capacitor there. It will cause nothing but an annoying noise, pop, or whistle when switched the way you have it there. Apart from that you should measure the ripple with the amp biased properly. I see nothing here that stands out as a potential problem. Under load fender amps were to have a less than 20 volt ripple. That was considered acceptable, under load. The push pull nature of the circuit common mode rejects most of it. The screen supply should be less, and by the time you reach the lower supplies it should be very little if any measurable. I suggest doing all tests at this point with only the power tubes and phase inverter tubes installed. and feed test signal directly to the phase inverter input. Lift the primary side of its input cap as to not introduce power supply noise directly into the phase inverter input. If you still have hum in your output at that point it makes testing that much easier to have eliminated the whole preamp.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 03, 2016, 11:19:43 pm
content: Quote from: cvanc on December 03, 2016, 07:58:04 pmNot a major problem---the other way round is what kills tubes!

author: calexanian
author_rank: Super Contributor
timestamp: December 04, 2016, 12:09:17 am
content: I personally have never really liked the idea of fuses on the B+ supply. To dangerous to change. Make your power transformer right and rate the mains fuse properly.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 04, 2016, 12:27:53 am
content: Quote from: calexanian on December 03, 2016, 09:45:46 pmYou're right; I hadn't spotted that. Also notice another ground, below main filter cap C38. There's no way it can actually be built this way, with both of these grounds - the secondary would be shorted to ground thru the bridge rectifier on both half-cycles.

author: John Heath
author_rank: Frequent Contributor
timestamp: December 04, 2016, 04:14:45 am
content: Quote from: Planobilly on December 03, 2016, 06:46:06 amThat looks like a nice amp. Well built on par with a fender or marshal. And a big thumbs up for taking it on. Watch out for the 480 volt power as it will definitely get your attention if you touch it. Also cool radio shack meter. It will log data if you have a computer with a com port. This is good to monitor AC current going to the amp for 24 hours to check for thermal runaway issues in the output.

author: Circlotron
author_rank: Super Contributor
timestamp: December 04, 2016, 04:45:55 am
content: A bit of ripple on the anode supply to the output tubes is not a big deal because being beam tetrodes (same with pentodes) the anode appears more or less as a constant current sink. A change in anode voltage produces very little change in anode current. Look at the characteristic curves and you see they go up vertically and then fairly horizontal. horizontal means large change in anode voltage but little change in anode current, so causes little hum to be sent to the output tranny.A triode is much different in this regard. Also, as has been mentioned, a balanced push-pull circuit rejects hum from the DC supply.

author: Planobilly
author_rank: Regular Contributor
timestamp: December 04, 2016, 05:35:34 am
content: Thanks so much guys. There is a lot in you post for me to read and think about.I think I will go back to AC heaters for everything. The way things are hooked up now all the voltages on the tubes are very good. It is currently biased at 40ma. Both channels are working more or less normally as far as I can tell. I still have some hum but I have not redone the lead dress to the preamp tubes. I have not tested the reverb yet or have the effects loop hooked up. Also the NFB is not hooked up yet.I am not sure if the volume control on channel two is working correctly...I was never sure just how the channel two controls would interact.I am building a little PCBs for the channel switching atm.This is a really complex amp for my level of experience. I think this amp could prove to be a difficult build for most anyone. For sure I am in the deep end of the pool..lolThe power transformer is a Mercury Magnetics APS FTW135-P. HT is centered taped and the AC voltage is 186 VAC to CT for 372 VAC total and rated at 750 ma. 55VAC bias tap. 6.3VAC filament tap rated at 7 amps. The output transformer is a Mercury Magnetics Tone Clone O100JM-VZ. Pretty pricy stuff...I hope it is worth the money.Cheers,Billy

author: calexanian
author_rank: Super Contributor
timestamp: December 04, 2016, 07:51:35 am
content: Based on that you may want to eliminate that fuse and connect that tap to the mid point of your main filter caps.http://www.thevintagesound.com/ffg/schem/twin_reverb_sf_135_schem.jpgHave a look at how it is implemented here.

author: JeremyG
author_rank: Newbie
timestamp: December 04, 2016, 11:27:58 am
content: Hi Planobilly and everyone,Nice construction of this amp, everything is clean.In the schematic, there is no ground connection on the output tube cathodes. Is it present on the actual construction?For safety reason, I suggest you to place 2 serie resistors (100k or 220k) accros the capacitors C37 and C38 (one resistor for one capacitor, like on the schematic posted bycalexanian) because if the fuse blow, you still have the lethal votlage accros these capacitors.Also, the middle point of R72 and R73 should be connected on the middle point of C39 and C40 for sharing the voltage accros the capacitors.As pointed out on previous posts, the push-pull topology is pretty immune to ripple if the transformer is well designed and the outputs tubes well matched.Ps: sorry for my bad english

author: Planobilly
author_rank: Regular Contributor
timestamp: December 04, 2016, 07:47:53 pm
content: Quote from: calexanian on December 04, 2016, 07:51:35 amI have seen several schematics with this sort of HT rail design. The one thing that is never shown is the AC values of the power transformer. Based on the power transformer I have which has a 377 VAC ( measured RMS unloaded voltage end to end) HT secondary which has a center tap,do you think the schematic you linked will work?Secondly, The way I have the current HT circuit built is providing the acceptable voltages and acceptable amounts of ripple (I think).I can draw a as built schematic if needed but in general the HT center tap is going to ground through a fuse atm. The B+ voltage measures 478 V RMS. Having said all that my gut level feeling is the power supply as built needs to be redesigned and rebuilt. Obviously the stand by switch is not connected in the correct place and there is the "pop" most of the time on shut down.I think for us to have a meaningful conversation, I need to bite the bullet and draw a as built set of drawings.Removing the DC components for the preamp heaters also gives me the room to place the HT caps on a new board that fits in that space.Here is a link to the EVH 5150 that I took channel two from and the power supply that I sort of copied.http://support.evhgear.com/schematics/EVH_5150III_AmpHead_Schematics.pdfCheers,BillyBTW...If you look at the EVH schematic, do you think we need to also install the zener diodes to clamp the voltage?

author: calexanian
author_rank: Super Contributor
timestamp: December 05, 2016, 03:38:30 am
content: Connecting the center tap will improve power supply filtering and regulation. The caps will be happier too.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 08, 2016, 01:07:52 am
content: The grounded centre-tap shown in your schematic in reply 11 is a major 'no-no".On every half cycle,one diode will be directly across half the secondary,effectively shorting it out.This is not good for the transformer or the diode.I think the reason that your supply works at all is that the fuse probably blew on the first half cycle.Returning the centre tap to the junction of your two series connected filter capacitors as suggested bycalexanianis a good idea,as it allows you to use the full capacitance of each cap,as well as reducing the ESR as compared to the standard series connection.It is really a "work-around" to allow the use of high capacitance,but lower voltage rated capacitors.This kind of connection was rarely seen during the heyday of valve/tube amplifiers,as 600v rated electrolytics were common in the values usually used.The high overall values of capacitance common today & which are necessary with RC filtering were not readily availableor requiredusing LC filtering,which was all but universal.

author: calexanian
author_rank: Super Contributor
timestamp: December 08, 2016, 06:17:00 am
content: The bridge rectifier circuit in fenders was a Ed Jhans thing. You could get better quality lower voltage capacitors in the 70's. 600 and even 500 volt caps were already on their way out and the lower voltage caps were getting better and more compact and with far lower ESR. Additionally your power rectifiers could have a lover PIV because they will never see more than rectified and filtered DC max potential across then rather than double that in the traditional full wave rectifier circuit. Also you are taking advantage of the entire secondary and you could make the gauge of the secondary winding with a larger gauge wire thereby reducing losses and heat. Bridge rectifier for these circuits is the most efficient way to go.

author: wagnerlip
author_rank: Newbie
timestamp: December 05, 2018, 07:09:56 pm
content: It is just me and my 50+ yrs knowledge of vacuum tubes is just vanishing away, or those output 6L6's have the cathodes floating?

author: Circlotron
author_rank: Super Contributor
timestamp: December 05, 2018, 08:44:06 pm
content: Quote from: wagnerlip on December 05, 2018, 07:09:56 pmYeah, that is a bit naughty.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 05, 2018, 09:49:38 pm
content: Dunno why I didn't pick up on something in the OP in my earlier answers "way back when", but havinga higher hum level on the higher voltage HT for the Output tubes was a standard design in the old days.It saved having to fit more high voltage, high capacitance, high cost electrolytics.The hum is "common mode", so is present at both push pull anodes, with the result that it is cancelled in the output transformer.The stages up to the phase splitter need much better HT filtering, because they are not balanced.With the " Type 3 " amplifiers we used in my old job, you always knew if one of the EL34s had croaked, because you could hear audible hum.