General Tube amp distortion question - Page 1
Forum section: electronics / projects

author: Planobilly
author_rank: Regular Contributor
timestamp: January 10, 2016, 04:01:31 pm
content: Hi guys,http://www.tangible-technology.com/schematics/fender/HR-Deville/Hot%20Rod%20DeVille.pdfI am working a 2002 Fender Hot Rod De Ville 212. When the amp arrived, I fixed a couple of things to get it to play...tube sockets issues ect.I ran out of 12AX7 pre amp tubes about the time it arrived....only had one on hand. I have more on order which will arrive in a few days.The signal was distorted at the plate of V1A and not distorted at the grid. I replaced the tube and that fixed the issue on V1. The other two 12AX7's test very low so I assume changing them both will most likely fix the issue with the amp.All the voltages are more or less normal across all the tubes in the amp. I poked around in the amp a bit checking values and found nothing.In general what other common failures will cause distortion in a tube amp?Also, what troubleshooting process do you use when tracking down this type of fault?Thanks,BillyBTW, I changed the circuit a bit...lol

author: Audioguru
author_rank: Super Contributor
timestamp: January 10, 2016, 04:33:12 pm
content: Vacuum tubes normally cause distortion. They quickly wear out. That is one of many reasons why they are extinct.Some bands replace all vacuum tubes before each gig.

author: calexanian
author_rank: Super Contributor
timestamp: January 10, 2016, 05:29:13 pm
content: How much signal voltage are you feeding it? Reduce the input voltage until you get a clean sinewave and then compare the input to output ac voltage of your signal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2016, 09:44:29 pm
content: In general, tube emission decreases over their lifetime. The circuit can be designed to maintain bias current and therefore voltages, to a point, and signal gain to a point as well, but usually the gain suffers first, and the bias later. The result is more distortion at lower levels (less undistorted power range) and quieter operation / lower gain.Depending on where in the signal chain it happens, it could simply be acting quiet, or the power range and distortion are affected severely.Usually, a preamp tube only needs to produce 10V to overdrive the next stage, but is capable of 100V or more, so you'd never experience limiting there, only gain. However, a driver or output tube has to swing a much larger signal, and may suffer from both.Tim

author: Planobilly
author_rank: Regular Contributor
timestamp: January 10, 2016, 11:02:07 pm
content: To answer Charles question about input voltage I do one of several things depending on what is going on with the amp.Inject a 1000hz 100mv to start, or turn on a CD player and listen, or plug in a guitar and play it. When these simple test fail to produce results I listen to the signal with a simple probe to hear what is going on. After that it is time to turn on the scope and dig deeper.If I get a amp with no information from the owner,I first check the fuse, then the speakers, then make a visual inspection before turning it on while bringing the AC voltage up on a variac with a light bulb in the circuit. At that point I generally plug it in to my speaker system or a dummy load. I have a good tube tester, A TV-10 Navy device which is in good shape. I generally work from the speaker back through the amp to the input.It is funny how many issues can be solved just by looking and smelling what is going on. Many times the amp has been banged around and the pots are broken. Lots of strange things happen on stage with guitar players...lolMy question was more about what other things other than tubes failing cause distortion when normal voltages are seen on the tubes and a good look around the amp does not show up anything. I guess what I am looking for is a list of things that cause distortion in order of failure rate. I am new to the repair of amps for other people and only have experience with fixing my own amps over the years.Also, thanks Tim for the feed back.Billy

author: Shock
author_rank: Super Contributor
timestamp: January 11, 2016, 12:25:11 am
content: Bad vs good distortion? The output transformer, plate resistors, bias, previous stage, impedance problems, poor connection?The more you approach troubleshooting from an electronics perspective, e.g. small circuits making up a larger circuit the easier it is. Most basic circuits can be troubleshooted by looking at input vs output, the supply and grounding and the quality and availability of the supply causing either noise or problems from unstable power. Then after that you have a dozen or so components in a circuit to test if in spec and behave correctly when under load.

author: AF6LJ
author_rank: Unknown
timestamp: January 11, 2016, 01:04:24 am
content: Quote from: Audioguru on January 10, 2016, 04:33:12 pmIn properly designed circuits vacuum tubes Don't wear out so fast.I have vacuum tube gear here that has tubes from fifty years ago, and the gear still functions fine.

author: Audioguru
author_rank: Super Contributor
timestamp: January 11, 2016, 02:29:12 am
content: In 1963 I took my Heathkit vacuum tubes amplifier to a MacIntosh clinic who measured 25% distortion at its rated output. The tubes were 3 months old. I replaced the tubes and the distortion was better at 0.5%. Then 3 months later the distortion was 30%. I sold that amplifier and bought an HH Scott solid state receiver that never had any audible distortion and I still use it with no distortion today.

author: AF6LJ
author_rank: Unknown
timestamp: January 11, 2016, 03:51:49 am
content: Quote from: Audioguru on January 11, 2016, 02:29:12 amPerhaps something was wrong with the design...I'm not fond of tube audio amps microphonics become an issue as the tubes age and the internals loosen up. Not a big issue with small signal tubes and at RF.

author: Cubdriver
author_rank: Unknown
timestamp: January 11, 2016, 04:18:50 am
content: Quote from: Audioguru on January 11, 2016, 02:29:12 amSounds like there was something seriously wrong with either the wiring itself or the biasing of the tubes. I rather like tubes for audio, and am currently running an updated version of the Dynaco ST-70 that I built from a kit about two years ago. It has several hundred hours on it at this point, and still sounds fine on the original set of tubes I put in it. There's also a three year old output transformerless tube amp in the living room. It doesn't see as much use as the Dynaco clone, but is also happily running its original complement of glowing bottles.In properly designed circuits, tubes are quite robust, and as long as they're not run at the ragged edge of their limits they'll happily chug along for a good long time in my experience. Abuse them and they'll die, but the same is true for semiconductors, perhaps even more quickly.-Pat

author: vk6zgo
author_rank: Super Contributor
timestamp: January 11, 2016, 05:03:12 am
content: Quote from: Audioguru on January 11, 2016, 02:29:12 amBack in 1963,tubes were made to a consistently high standard,so the probability of successive tubes failing after three months is remote.It looks more like you had an intermittent---probably a dry joint on one of the tube sockets.The action of changing the tubes cleared the problem for a while,then it returned.Tube/valve Amplifiers were used in the thousands over decades,with a high degree of reliability.A typical device is the "Type 3" Amplifier,which was extensively used in Oz at Broadcast Studios & Transmittter sites:-http://www.oneillassociates.com.au/~poneill/pdf/TT_T3.pdfOne interesting thing was that HT to the output stage wasn't filtered as much as for the low level stages.This was not normally a problem,as the 100Hz ripple was cancelled out by the Push-Pull design,but if one of the output tubes completely died,you were confronted with high level hum roaring out of the speaker.It made for easy fault finding,though!

author: vk6zgo
author_rank: Super Contributor
timestamp: January 11, 2016, 05:32:45 am
content: Actually,the Type 3 is a nice,simple schematic,with which to demonstrate the possible causes of distortion in tube amplifiers.V2,the phase splitter,is an obvious suspect------if one section has died,the Push-Pull drive will be asymmetrical.R9&R10are other suspects,as at 100k each,they are right in the frame to go high in value.If one goes high,& the other doesn't,this will again cause distortion.ItC6becomes short circuit,it will also unbalance the phase splitter,hence causing distortion.A favourite "Trade Test" was to give the Candidate a Type 3 with a fault placed in it by the Examiner,an Audio Oscillator,a resistive load to replace the speaker,& an Oscilloscope,& say :-"Find the cause of the distortion".The standard fault was a Drawing Pin stuck throughC6!

author: tautech
author_rank: Super Contributor
timestamp: January 11, 2016, 05:45:18 am
content: Why has nobody mentioned the various electrolytics in the schematic? Not those just in the PSU either.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2016, 07:30:43 am
content: Quote from: AF6LJ on January 11, 2016, 01:04:24 amYeah, but I'm betting it hasn't been in daily use either.Storage doesn't count against the clock (unless they were badly made in the first place -- vacuum leak), only "engine hours" so to speak.Quote from: vk6zgo on January 11, 2016, 05:03:12 amMore likely, bias was wrong, so the power dissipation or cathode current was waaayyy out of spec (as cubdriver noted). Operation outside of spec causes rapid deterioration.I think I'd tend to agree, regarding quality in general. But commercial tubes still weren't made to the purity, methods, tolerances and standards that premium quality industrial tubes were, and those had about the same difference again, in comparison to the absolute gold standard (literally!), tubes made for undersea cables. (High precision construction, high purity materials, lots of gold for the grids, ceramic supports, the works. Not to mention a burn-in (several thousand hours) that would challenge most commercial tubes (designed to an MTBF in the 5-10khr range), and a reliability record challenging even semiconductor circuitry! All that obviously came at a price (and not just because of the gold), which is why these beasties were so particularly special-purpose.)So even if you have good, average commercial tubes,expectfailure after some thousands of hours, give or take how hard they're operating (in terms of envelope temperature and cathode current capacity). Yes, you will occasionally get the rare specimen that outlasts even the capacitors around it (especially if operated infrequently), but remember not to put too much importance on that, because that's a sample size of one, an anecdote. And the plural of anecdote is not data!Also, regarding reliability: survivor's fallacy / confirmation bias. All the 99.999% of crappy old TVs died long ago, soof courseonly the random Methuselah antiques survive today (or appear as memorable cases).Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: January 11, 2016, 09:34:11 am
content: Tens of thousands of vacuum tubes were used in Communications & Broadcasting,over decades.I'm not talking about Transmitting tubes here,---plain old receiving type tubes.They definitely did fail after many thousands of hours,but it wasn't consistently at around the same number.The Type 3 Amplifiers I referred to operated for around 7300 hours a year in the places I worked.Out of 10 or so in one Transmitter Hall,you may have had to replace the output tubes in 2 or 3 in any given year.The Speech input equipment at the same site operated for the same time duration,but with very much lower MTBF.The Transmitters themselves,used many such tubes in the Exciters.Comms & Broadcast equipment is normally designed to operate correctly over the full life of the active devices,whereas Domestic equipment is often marginal------that's why we pay the big bucks for the former.Many tube TV Sets were retired after the Picture tube lost emission,rather than due to failure of any of the other tubes.More often,however,still functional TV Sets were discarded following the purchase of a new Solid State TV.Much the same has been the fate of analog Solid State TVs,following the changeover to Digital TV.

author: dom0
author_rank: Super Contributor
timestamp: January 11, 2016, 09:57:43 am
content: Well, there where tube computers and it was seen there how reliable tubes where. Not very. Of course, these things had many many more tubes than any audio amplifier, but that's actually an advantage when you want to look at average failure rates.

author: coppice
author_rank: Super Contributor
timestamp: January 11, 2016, 10:17:29 am
content: Quote from: dom0 on January 11, 2016, 09:57:43 amThe failure rates of switching tubes had very little to do with the failure rates of audio/radio/TV tubes. Switching tubes were specialised devices, made very small and with the heaters underrun to maximise their lifetime. They still failed pretty frequently, though. Vacuum tube computers seldom ran for more than a few hours between repairs.Until the mid 70s B&O was a maker of premium engineered consumer products, at higher than average, but not crazy, prices. They only later became a supplier of fashion items, at silly prices. During their serious engineering years they made extremely reliable TVs by under-running tubes, and using extra stages to make up for the resulting reduced gain. During that period they matched any solid state TV for reliability, and their long term stability was excellent.

author: Shock
author_rank: Super Contributor
timestamp: January 11, 2016, 11:09:25 am
content: Quote from: tautech on January 11, 2016, 05:45:18 amBecause then you have to mention two more components, potentiometers and resistors and then every component that can fail in a tube amp has been talked about.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 11, 2016, 01:46:06 pm
content: Quote from: dom0 on January 11, 2016, 09:57:43 amBut not many more tubes than a TV or Radio Studio,or a Communications Network.Broadcasting & Communications organisations had ample opportunity to determine tube reliability,without having to rely on statistics from other applications.Tube computers used 12AT7s or similar,in modular bi-stable stages,which made the replacement of such stages simple,whether necessary or not..Also,by & large,computers had a larger budget than the major users of tubes,so they could be profligate in tube replacement.It is a fantasy to imply that all equipment using tubes could only operate for extremely short periods without failure.Nobody is denying the great increase in reliability offered by well designed Solid State equipment,but tubes were the devices which carried forth the progress of Electronics over many years.

author: AF6LJ
author_rank: Unknown
timestamp: January 11, 2016, 02:26:25 pm
content: Quote from: T3sl4co1l on January 11, 2016, 07:30:43 amYou are spot on when it comes to industrial tubes.Here is one for you that I found somewhat remarkable.When I worked in two way radio (career ended in 1994) We had a Motorola Research Line base station we maintained for the Navy. When it was taken out of service in 1992 it was forty years old. Yah transmitter, receiver strips, tube rectifiers in the power supply an 829B in the final... We are talking real neolithic stuff here.The base station used all 7V loctal tubes running on 6.3vac with the exception of the rectifier tubes every one was original, even the final. The transmitter had other work done so did the DC remote control and power supply strips.When they rolled it off to the landfill it would still meet all specifications.It was a six foot high tribute to careful and well thought design.It was the only piece of gear we had on contract that was older than me.

author: Cubdriver
author_rank: Unknown
timestamp: January 11, 2016, 03:54:59 pm
content: Sue, actually most likely mainly a tribute to conservative, well executed design. The loktal/loctal based tubes, despite their 7 and 14 prefixes, actually had 6.3 and 12.6V filaments, respectively. When the new base was designed, the different numbers were used to differentiate the new tube design from the more conventional octal base. The filament voltages were referred to as 'nominally' 7 and 14V to let the nomenclature fit, but it was obviously a VERY nominal spec - if you look in the data books you'll find that they're actually spec'd at 6.3 and 12.6V.I find it fascinating to dig into the history of these things.-Pat

author: Audioguru
author_rank: Super Contributor
timestamp: January 11, 2016, 04:29:55 pm
content: When I was a kid, vacuum tubes failed so often that they were tested and sold in convenience stores all over the place.

author: retrolefty
author_rank: Super Contributor
timestamp: January 11, 2016, 05:17:02 pm
content: Quote from: Audioguru on January 11, 2016, 04:29:55 pmI recall those days. Typically some poor fool would not know what to do but pull all his tubes from his TV, run to the local supermarket machine and find 3 weak, purchase them at elevated prices, rush back home, insert all tubes and find nothing changed in the set symptoms. It was a money maker for some.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 12, 2016, 01:34:03 am
content: Quote from: retrolefty on January 11, 2016, 05:17:02 pmThis was apparently a North American thing--in Oz,you generally had to go to an Electronic Parts store to buy "valves".There were always a few people who would go into the City & buy a full set of tubes,come back,forget which sockets they belonged in,& probably cook a couple by plugging them into the rectifier socket,but this never became common practice in Australia,The old "Mantel Set"would often churn on for years,with little more in service than a light dusting of the outside..Solid State devices took off in the early days,not because of perceived higher reliability,but because of very much smaller size,lower power drain & less stress on components running at lower voltages,all of which allowed the manufacture of very small portable equipment with greatly extended battery life.Those of us who were around in the late 1950s first saw Solid State personified in the ubiquitous pocket Transistor Radios of that era.Reliability of early devices wasn't marvellous,despite some "rubbery" figures generated by the Sales Dept of various Semiconductor manufacturers.The bare device may have been highly reliable,but the early packaging left a lot to be desired,with corrosion of leads,both external & internal being not uncommon.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 12, 2016, 03:35:08 am
content: Quote from: vk6zgo on January 12, 2016, 01:34:03 amThe bare device was a bit questionable, too -- low purity, surface contaminants (and other surface physics; which held back FET introduction by many decades*), not using glass passivation, and poor immunity to external influences (transients, mechanical shock).*FETs were known theoretically since the '20s, I think. The BJT is harder to analyze (taking Shockley, et al. until 1942, was it?), but much easier to make, since it can be made in bulk material, away from surface troubles, and tolerates lower purity better, especially in germanium.Tim

author: N2IXK
author_rank: Frequent Contributor
timestamp: January 12, 2016, 03:40:25 am
content: Quote from: AF6LJ on January 11, 2016, 02:26:25 pmThat was actually what those tubes were rated for. For some strange reason (does anyone know?), the RMA decided to ignore the "1st digit in type number = filament voltage" rule for the Loctal types. All the 7-series Loctals had standard 6.3V heaters, and the 14-series loctals were really 12.6V tubes. All the 7 and 14 seem to signify is the fact that they were Loctal base.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 12, 2016, 03:42:37 am
content: Tubes were able to withstand the 100000G acceleration, or "jerk", or setback, and 20000RPM rotation from being fired out a cannon in WWII.https://en.wikipedia.org/wiki/Proximity_fuze#/media/File:MK53_fuze.jpgTubes were reliable enough to power submarine cable repeaters.http://lampes-et-tubes.info/sp/sp056.php?l=eTubes were reliable enough for deep space probes.http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880010181.pdfTubes scale up indefinitely... What is a particle accelerator but a really big vacuum donut with electrons wizzing inside?What is an electron microscope but a really long tube with a door to shove housefly babies into?Sure, I guess transistors are practical or whatever, but tubes make life interesting!edit: fixed link

author: coppice
author_rank: Super Contributor
timestamp: January 12, 2016, 03:51:44 am
content: Quote from: T3sl4co1l on January 12, 2016, 03:35:08 amThe first FET type device was patented in 1926 by Julius Lilienfeld. In the subsequent decade he patented a variety of junction FET and MOSFET like devices, but nobody could make one work. The first point contact bipolar transistor was made in 1947. The first epitaxial transistor, more suitable for high volume use, was made in 1951. The first junction FET was made in the mid 50s. The first MOSFET was made in 1959. It still took a few more years for the MOSFET to become viable for volume manufacture, so it was the end of the 60s before MOSFETs started to ramp.

author: coppice
author_rank: Super Contributor
timestamp: January 12, 2016, 03:53:41 am
content: Quote from: Alex Eisenhut on January 12, 2016, 03:42:37 amTWTs are still the PA of choice for comms satellites.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 12, 2016, 04:02:18 am
content: Quote from: coppice on January 12, 2016, 03:51:44 amI have to jump in and link this:http://www-smirc.stanford.edu/papers/chapter1.pdfThe history of how we got "here" is fascinating: how people were able to come up with this stuff from so little? If I project myself to 1902 for example, what could I have invented? Not much.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 12, 2016, 04:04:06 am
content: Quote from: coppice on January 12, 2016, 03:53:41 amWho are we to argue with space dudes? I understand satellites are mostly chunks of ferrites, isolators and circulators too.

author: Cubdriver
author_rank: Unknown
timestamp: January 12, 2016, 05:11:35 am
content: Quote from: N2IXK on January 12, 2016, 03:40:25 amMy understanding is that the 7 and 14 are simply to identify them as Loktal based tubes. Apparently it's considered a 'nominal' operating voltage. I'm not certain how Sylvania was able to get that designation.-Pat

author: vk6zgo
author_rank: Super Contributor
timestamp: January 12, 2016, 09:26:29 am
content: Quote from: tautech on January 11, 2016, 05:45:18 amValid point------Again using the "Type 3" as an exampleC10/C!1are oil-filled caps,but if they were electros,obviously a failure would cause increased HT ripple.A faultyC7orC8could unbalanceV3/V4causing distortion in the output stage.C2&C3withR7&R13, can be looked at as extra HT filtering as I previously did,or as decoupling.If either of the caps have gone low capacitance with age,or the resistors high in value,there may be:Increased ripple on the low level stage HT.Unwanted coupling betweenV1&V2,Frequency response errors,asR7&R13are no longer effectually bypassed at low frequencies.