Peavey Interstage/Driver transformer information wanted. - Page 1
Forum section: electronics / beginners

author: Supernaut
author_rank: Contributor
timestamp: June 25, 2025, 04:28:04 pm
content: Looking for advice on the operation of these old transformers that can be found in vintage solid state Peavey amplifiers. They are mounted to the PCB directly and share a relationship with the output transistors.I currently have three Peavey amplifiers with these transformers installed, two of the amplifiers are both 1976 Peavey Pacer's and the other is a 1973 Peavey Century. In all three amplifiers, the transformer seems to be fed via a TIP31C transistor, they also have a few 5W resistors that are in the circuit too, 1000ohm and 0.68ohm respectively. (two of each value) There are also a few smaller resistors (1/2w) that also share a relationship with the transformer.I came across a strange issue in both of my Pacers that someone on the Peavey forums done some digging on, apparently these transformers can cause slight oscillations and other strange behaviour, the user on the forum modelled the output section in LTSPICE and claims they were able to trace the oscillation to two 15ohm 1/2w resistors, they bumped the value up to 18ohm and apparently this addressed the problem, I did try this on one of my Peavey's and there was a slight change but Im not convinced the problem is completely dealt with. The user no longer posts on the Peavey forum so I cant ask further questions.So Im wondering if someone on here could maybe give me some advice with these, I was curious to know if swapping out any resistor that has a relationship with the transformer to 1% tolerance to try tighten up the output section, Im wondering if I might be experiencing issues with phasing due to the OEM resistors being 10% tolerance (and also 50 years old) so the output transistors aren't getting the same feed as each other?The issues are as follows:*Notes sometimes wont sustain and die off on there own, sometimes it will sustain slightly and then abruptly stop, as if someone turned the amplifier off as the note was sustaining.*Mushy sound - Sometimes the sound isn't very focused and sounds a bit flabby, again, im wondering if this is due to an imbalance in the output section due to resistor tolerance and possibly component fatigue? Both of my Peavey Pacers behave like this, someone has changed the filter caps on one of them and the other is completely original and in untouched condition, both exhibit the same behaviour. My Peavey Century is in need of repair so I cant test that one but the driver transformer setup is similar. (schematic and picture included below)*Doesn't seem to like heavy bass notes sometimes - If I turn the distortion up and chug on the E string on the guitar (the thickest one) sometimes its not as full as I would expect, its like its holding back or muffled somehow.As it stands I currently have 17 guitar amps that I have repaired myself (14 solid state 3 valve), Ive checked the usual suspects with regards to connections, contact cleaner, took some measurements on the output transistors etc and everything seems functional on that end, the only difference with these Peavey amps is the driver transformer that they have installed, none of my other amps use these transformers and ive never experienced this problem before.Apologies for the long winded post, all I have available is a multi meter and all of my others amps get on well and don't exhibit strange behaviour like these ones do, the only thing that separates them is the driver transformer's.Ive included pictures, schematics and a screenshot of the individual who claimed to model the output section in LTSPICE and found an oscillation in the circuit. Im just a hobbyist and I repair old guitar amps for my own use, usually bought as spares or repair and usually they just have trivial faults that are easy to diagnose but this is beyond my skill set and I don't have a scope to poke around with either.The Peavey Pacer schematic has 2 variants of the Pacer circuit, mines is the B series at the bottom of the PDF although I think the output sections are the same anyway. Ive included screenshots of my Peavey Century and a schematic too, that amp was in poor condition when I got it so its currently untested and it pieces but it has a similar setup so I thought I would include it anyway.Any help appreciated!

author: MathWizard
author_rank: Super Contributor
timestamp: June 26, 2025, 01:57:20 pm
content: Do you have a schematic? IDK if this is the right modelhttps://www.audioservicemanuals.com/p/peavey/peavey-pacer/1335347-peavey-pacer-a-b-series-schematicsI'd highly recommend getting an oscilloscope, even an old CRT 20MHz scope, or some of the little cheap "toy" ones, would help see what's going on. And an automatic tone generator, besides a guitar, would really help too. Even just a 555-timer circuit that could make a few frequencies, should do for most cases.IDK how good/bad the common resistors and caps were in the 1970's, I'd want to check some of them out of circuit, I wouldn't trust any electrolytic caps. I'm not sure what transistor age like either. Solder joints wear out too, maybe some are cracked.

author: Supernaut
author_rank: Contributor
timestamp: June 26, 2025, 08:00:09 pm
content: Schematics for both amps are in my post above, the one you have posted is for the earlier version of the Peavey Pacer although Im sure output sections on both are the same.I usually do re-cap old units like this by default but because these amps seem to have a peculiar issue with the driver transformer I was hoping I could get to the bottom of that first. In my above post there's a screenshot of a user on the Peavey forums who has one of these amps that is already re-capped and has some transistors changed and he still experienced this issue, so it makes me think that a cap job wont remedy this problem. He claimed he was also able to model the output section in LTSPICE and observed an oscillation in the output stage too.I would like to get a scope but I don't even know which ones are worth buying, im from the automotive trade and we always used PicoScope, Ive never used a scope outside of that though.I suppose if anything, im more curious to know if my reasoning holds any weight with regards to resistor tolerance at the output stage, would it make sense to tighten those up to 1% tolerance as opposed to the 10% tolerance they currently are? I only came to this conclusion after thinking about balanced phase invertors in valve amps, I wondered if trying to balance and tighten up the output from the driver transformer would make any difference?Im due to buy a truckload of axial caps from Mouser in the coming weeks so il have new caps available soon.

author: floobydust
author_rank: Super Contributor
timestamp: June 26, 2025, 09:21:53 pm
content: I wouldn't fault the design- until you know the old electrolytic capacitors are out.In most equipment re-caps, modern caps can be on the lower tolerance end so I'll upsize their value on power rails, i.e. 50uF to 68uF on the driver. In 1974, capacitors were at their rated value and bit expensive. But it can affect the overload sound if this is how the amp is run.The Pacer A has the driver transformer with loop feedback around it - so there is a LF pole which can oscillate but the 2uF part should cover that. The amp must have a load (loudspeaker) connected.

author: Seekonk
author_rank: Super Contributor
timestamp: June 26, 2025, 11:16:48 pm
content: Holy crossover distortion Batman. That output is about as crude as you can get.That 15 ohm resistor sets the bias on the transistor. Increasing the resistance increases the idle current and will lower crossover distortion. That also makes the amp run hotter which then changes the bias.If those are carbon comp resistors, the higher value resistor may have changed too. Even on cheap resistors today the tolerance is closer to 1% due to better manufacturing methods.I'd like to see some resistance loading on the transformer.You can't tell much without a scope. I get a number of cheap digital scopes sent to me free. They can be very misleading with what being displayed having very little to do with the actual signal. For the same money ($50) a USB scope on your laptop is miles better and you can easily save images.Guitar amps tend to have little overall feedback and tend to add more feedback to each stage for more natural clipping and faster recovery. Too bad you are not close. This would be an interesting project.

author: floobydust
author_rank: Super Contributor
timestamp: June 27, 2025, 01:44:49 am
content: Late 50's to early 70's it was common to use driver transformers in audio power amplifiers, to the output stage.Pacer-A is circa 1974 but I see the later Century-A same circuit but added clamp diode+470uF 6V across the 15R resistors, to give some thermal feedback.I don't think it will have cross-over distortion as much as it roasts things when the amp heats up, for the Pacer-A.I find Peavey gear has a coloured sound to it that some musicians really like.

author: MathWizard
author_rank: Super Contributor
timestamp: June 27, 2025, 04:49:51 am
content: I made a model of my cheap little practice amp in LTSpice, the clean audio used 1 op-amp, and a power amp IC. I'll have to try and make a model one of these old amps, and see how they compare. I wish they included the inductances and turns ratio's, in the schematic. But I'm sure there's lots of old amps with that info available.There's lots of threads and video's about choosing a scope. The old CRT ones are big and heavy, but there's lots of them on places like ebay, for $100- $200. Then there's all the modern ones with nice big screens, and all kinds of options, they might start around $300. Then there's USB types like the Picoscope.There's also the most basic ones, that can cost $20-$100 and maybe more, that are more handheld types, usually lacking safety features, and nice buttons, etc. But they can be fine for some stuff too.I forget you're in the UK, but I'm sure there's tons of used scopes for sale there too.

author: Supernaut
author_rank: Contributor
timestamp: June 27, 2025, 09:44:29 pm
content: Quote from: Seekonk on June 26, 2025, 11:16:48 pmThanks for your response, im glad i have a better understanding of those 15ohm resistors now, this only really grabbed my attention because of the post i found on the Peavey forum from a few years ago. I don't have the skills to model anything in LTSPICE so when a user said they found an oscillation in the output stage i was quite interested to know more although the user doesn't post on the forum anymore so i couldn't find out more information.Most vintage solid state Peavey amps all have a fairly crude design i think, im sure it was an old US radio company that did the early designs for Peavey but coming in to the very late 70's and early 80's they had a guy called Jack Sondermeyer who used to be at RCA, apparently he was a big deal with regards to solid state amplification and the designs improved quite a bit with him on-board.Quote from: floobydust on June 27, 2025, 01:44:49 amThat's interesting to know, these really old Peavey's are the only ones ive found with a driver transformer before, i wasn't super interested at first but now i have 3 old Peaveys with this output setup i felt like i should probably learn more about the operation, i would never have thought much about the transformer until i seen someone comment on a potential oscillation from it, that's mostly what grabbed my attention.The old Peaveys do have a very particular sound to them, ive made my way through quite a few solid state amps now and the old Peavey amps sound unique, they have a very fuzzy sound by default, if you push it with an overdrive pedal it sounds even more aggressive. Its like poor-mans Black Sabbath in a box.Quote from: MathWizard on June 27, 2025, 04:49:51 amOne of the main reasons ive not bought a scope is due to most cheaper ones being modern Chinese stuff with some random brand name, im not a huge fan of cheap tools and inaccurate measuring equipment so ive held off until i get a better handle on the situation. I could also use a reliable transistor tester and a ESR meter, i can tell if caps look bad visually and i can test the capacitance with my meter but i would like to do ESR too. I also use my multi meter to test transistors but sometimes trying to stuff those 3 legs in the socket of the meter doesn't work out to well and i end up second guessing the readings which i dont like. Il probably start saving up a few hundred on the side to invest in more testing tools as i find myself doing this work fairly often now with the things i buy (which are mostly vintage)The Peavey schematic does give the resistance for the windings im sure, so if i measured the wire thickness someone who's much smarter than i am could maybe figure out the winding (maybe?) I would personally like to know because the transformers are unique, they are soldered directly to the circuit board with no mechanical fasteners and then the wires themselves go straight down through holes on the PCB and are soldered in place, if the transformer decided to grenade itself one day i dont know if it could ever be fixed?If you would like to model or clone any of the amps i have il gladly take hi-res photos and backlight the PCB if i can so its easier to see traces and components together. My circuit design understanding is rock bottom so i like backlighting PCB's when possible as i like the visual aid it provides.Thank you all very much for the input with regards to this, il probably re-cap the whole thing first and see where that takes me, if the issues i described persist then i think a close look at the driver transformer and its components may be needed

author: MathWizard
author_rank: Super Contributor
timestamp: June 28, 2025, 07:40:05 am
content: In LTSpice I made a copy of the Peavey Pacer A, and so far with the BJT's and inductor values I guessed/semi-calculated, it's pretty noisy and oscillates a lot. I'm sure the real thing wouldn't be this bad.If money is no issue, and you like to collect tools that you only use once and a while, then there's lots of entry level scopes to look at, like from Siglent or Rigol, they are the most popular, more affordable brands, for your typical hobby use, but they are also used in real labs and companies too.

author: MathWizard
author_rank: Super Contributor
timestamp: June 28, 2025, 03:11:33 pm
content: Using the correct BJTs in LTSpice, and your coil resistances, and 60Hz Vsoucre, it really wants to oscillate, at 60Hz, and other stuff too. Most of the voltages are about what your pdf says. I've done some stuff on transformers and mutual inductance before, but I really don't know what size inductors to use. I'm used to using the 2N3055 lately, but I can't get anywhere near the current out of them as required for 50 or 100W. I've tried changing the coil phases too.With a 1V sinewave input, it gets ugly pretty quickly, but some stage is not amplifying very well, and the signal never gets very big. The most power I've gotten on the 8R output so far, was under 1W.Ok 1 cap was marked as 100, should be a 100pF, now it has tons of gain, and looks like decent enough frequency response. Now I have to go over impedance matching for transformers, do choose some good inductor values.

author: Supernaut
author_rank: Contributor
timestamp: June 28, 2025, 03:29:18 pm
content: Interesting that you found an oscillation when you simulate the circuit, the output sections of A and B series Pacers are basically the same i think, the real difference is that the B series has clipping diodes and an overdrive knob, both amps also have a reverb tank on them too.A Peavey Pacer A series has 6 control knobs and a Pacer B series has 7 control knobs, the B series has two diodes used for clipping at the pre amp section, the extra knob on the B series is the overdrive knob.I was looking around last night and i found a Pacer B series for sale and the seller had photographed the internals, immediately i noticed the output stage has been messed with and also seen some black smoke/fire.To my eyes it looks like the two 15ohm resistors have been changed to diodes, although i have seen very tight tolerance resistors packaged in a similar way i think so its hard to say for sure, the photo isn't super clear but you can see something has went wrong, the PCB is charred black and the paper that covers the transformer winding is clearly burned. Both 0.68ohm 5W resistors have been changed and the two 1k 5W OEM resistors don't look like they are having a great time either.Not much use for my problem although i thought it was interesting so ive saved the photo for future reference. I would also add that these amps are known for having a fuzzy sounding quality to them so distortion in the circuit might not be much of a red flag but i would wonder why it oscillates?At the rear of the amplifier it states that it should have 45w output and it does use two 2N3055, Peavey use in-house part numbers for transistors but they have a little explainer on the schematic to tell you which numbers correspond with each transistor, it used to throw me off when i first got in to old Peavey stuff.Here's a couple threads from the Peavey forum discussing this issue, may be some food for thought.https://forums.peavey.com/viewtopic.php?t=53789https://forums.peavey.com/viewtopic.php?t=46553Thanks again for your help

author: MathWizard
author_rank: Super Contributor
timestamp: June 28, 2025, 04:46:53 pm
content: I should have worked on it more before posting about the bad results, the schematic had a cap marked 100, and it seems it should be 100pF, not uF. So works a lot better now.

author: floobydust
author_rank: Super Contributor
timestamp: June 28, 2025, 07:04:29 pm
content: Oh that's a big difference in cap value.Note that older Pacer-A bias current could go to thermal runaway if the amp was run hot, and it might crater.Usually a thermistor or the bias diodes are on the heatsink but I think that's the what the Century-A got.These amps are not hi-fi because that sounds so boring and they are intended to have character. Play a guitar through your home stereo and that sounds so dead and bland.In the repair shop I will inject music from a CD player into an amp, to find obvious distortions and speaker rattles, lack of mid-bass etc. They should not sound really really bad.

author: CaptDon
author_rank: Super Contributor
timestamp: July 01, 2025, 01:24:14 pm
content: Years ago my friend ran ROMAC Audio in York Pa. I was his go-to guy for the DECA stuff and local musicians and D.J.'s even brought their in-warranty DECA stuff to me because I could usually give them next day turn around repairs. Back then everyone was simply sending the amps back to Peavey because they didn't understand the circuitry. Anyway, I bought a bunch of those interstage transformers from him to build my own amplifiers from scratch. The transformers have good headroom and also good low frequency response. They have one very weird issue, they have a self resonance in the audio midrange area which by way of resonance 'Q' gives an increased midrange gain. Because most amplifiers would have a horrible 'open loop gain' frequency response they add the negative feedback to produce a more flat closed loop gain. Those thermal bias control diodes and the capacitors tend to rectify some of the A.C. signal component in the bias circuit when the amplifier is pushed really hard and that shifts the bias point causing some shift in the guitar 'sustain'. Also the 50u@50vdc used to stabilize the 32 volt driver side voltage rail could be increased to 200uf to help maintain the rail when the driver transistor is pushed into its non-linear areas. I should think some sort of a loading resistor across the primary side of the transformer would help to stabilize the closed loop gain a bit. I would maybe start with a 2.2K 2 watt carbon resistor directly across the primary side. I have had excellent luck using that transformer in my home brew designs but being aware that is does have some kind of self resonance in the mid range and compensating in the feedback design.

author: Supernaut
author_rank: Contributor
timestamp: July 01, 2025, 11:17:49 pm
content: Quote from: MathWizard on June 28, 2025, 04:46:53 pmThat's fine, i don't even know how to use LTspice anyway so im glad someone at least tried to model the circuit, i never expected anyone to be able to accurately diagnose anything for me anyway, im just glad some more experienced eyes than mines can throw some opinions in with regards to the circuit design because its not my strong point. I can do basic overhaul and diagnostics but circuit design is beyond me for the most part.My next port of call is to re-cap it, so it might give me a better base to work from compared with the almost half a century old caps that are currently in place just now.Quote from: floobydust on June 28, 2025, 07:04:29 pmThe Peavey amps of this era usually sound great, i have a few solid state guitar amps and the Peavey's have the best build quality and character. When you run a fuzz distortion pedal in front of them they sound even more crazy. With regards to the 14 solid state amps i have just now 6 are Peavey and im always looking for others.I usually test vintage speakers with music, its too laborious installing them in a cabinet and hooking up my amp and guitar just to test if the speaker works so i play a tune by UK electronic group The Chemical Brothers called "Under The Influence", it has a great bass sweep at the start of the song and quickly finds rubbing voice coils or other issuesQuote from: CaptDon on July 01, 2025, 01:24:14 pmThank you very much for the additional information, its a set up that i only have in 3 of my vintage Peavey amps. Later versions moved over to driver transistors among other things and all of my other solid states are transistor/op-amp based, ive never seen an interstage transformer on any other amps yet so its unique to me anyway.Il probably dig deeper once i have it re-capped, many past users on the Peavey forums claim a re-cap makes a big difference (which i don't doubt), but im usually guilty of using the shotgun approach with caps that are 40+ years old regardless of how they perform or look, this time i wanted to try understand the problem better before hitting it for caps.I also have a few Ohmite 0.68ohm 1% 5W resistors laying around so il probably just order a few 1K 1% 5W resistors too. I might change the existing 1/2w 15ohm 5% to 1% items too and see if that does anything. Il try do the job in stages and check each time to see if i have resolved the problem.Ideally i need a scope but funds don't allow for that just now, this issue with the Peavey's is the only time ive felt like i needed a scope to get a better idea of what's happening, most other issues i run in to are usually single component failure which can be easily removed and tested to confirm operation.I should be buying a bag of axial caps from Mouser soon so hopefully i can move forward with this in the coming weeks.Thanks again for the input everyone, its been very useful so far

author: floobydust
author_rank: Super Contributor
timestamp: July 02, 2025, 03:37:56 am
content: No harm in shotgun replacing all the electrolytics, I do this because it ends up less work or hassle.I would not change component vales based on an Internet post that might have originally been a capacitor problem.That will change the sound character and surely the amp originally worked...

author: Supernaut
author_rank: Contributor
timestamp: July 02, 2025, 02:42:32 pm
content: Quote from: floobydust on July 02, 2025, 03:37:56 amIronically in the guitar world side of electronics people generally dislike the shotgun approach with old electrolytics, ive had people talk me down because i have done this in the past, even though i know 50 year old caps aren't much use to anyone and many measure above or below the usual 20% threshold, or show signs of leakage and bulging. People often avoid servicing vintage amps in case it changes the sound, in some instances the concerns are warranted but in many instances the caps need replaced.Just to be clear, it is not my goal to change the sound of these amplifiers. I can clearly hear an issue with the output section, the most obvious one being that notes sometimes will not sustain and they will cut off prematurely, you can think of this like a noise gate that reaches a certain threshold, it just kills the sound completely as the note is sustaining. Beyond that i hear inconsistencies in the notes that almost sounds like an minor intermittent phase or cancellation issue, no other amplifiers i have perform like this, my later Peavey amps that use driver transistors don't exhibit this issue, its only the Peaveys that use driver/interstage transformers.Im personally not talented enough to model anything to do with this circuit so when i found a user on the Peavey forum highlight the issue i was curious, im not one to randomly change components based off a post i found on the internet which is why i posted in the beginners section on here to seek some guidance on how these transformers perform and how any inconsistencies in the circuit design might introduce potential problems. So far, the only change i attempted was to move the 1/2w 15ohm resistors up to 18ohm as the user on the Peavey forum suggested this was related to the issue after modelling the output section on the computer, i wasn't convinced by the change and i had no intentions of doing anything else until i was able to seek advice from people more knowledgeable than myself. If i do change anything related to capacitance values then i can A/B the amplifier as i have two of them, i can keep one with OEM specifications and modify one to see how it changes the circuits behaviour.Quote from: CaptDon on July 01, 2025, 01:24:14 pmI remembered last night while looking at photos of the amplifier that there is a 100ohm 1W carbon comp resistor directly underneath the winding that you cant really see, ive photographed it today and ive found it on the schematic too. Is it possible that this might be causing the issue if the value has drifted significantly over time? I have had problematic carbon comps in the past that drifted drastically (50% drift in some instances). Is it possible for this transformer to go into some kind of off condition if not loaded correctly? That would possibly explain why the note's wont sustain properly...?