I have a 1974 Peavey Classic 100 Series Amp. It is 50 watts RMS, 100 watts peak with 2x12 speakers.

I have had it since new.

It is a solid state preamp and a 2 tube 6L6GC final output amplifier.

With a headphone amp and headphones all the guitars sound great.

With the Peavey amp, the notes and chords sound off pitch.

Neither the Bass, Mid, or treble controls make much of a difference.

The amp over drives into distortion very easily. Only very low volumes sound clean.

Everything is OEM and 50 years old.

I think there is a good probability the electrolytic capacitors are probably bad and I am going to replace them.

Is there anything else I should look at either before or after replacing the caps

Looking at the schematic from 1974 it has numerous electrolytic capacitors.

The all have a format but not sure what it means.

The non electrolytic makes sense, a calacitor symbol and a value. Ie 100pf

The electrolytic have the capacitor symbol with a +- on opposite sides showing polarity. Makes sense but the values are written as
100/25
25/25
2/25

Etc

Is the first number the value of the capacitor with the unit being assumed to be uf or pf or something else and the second number the max voltage rating?

I have no seen a calacitor value without a unit listed ie uf or pf or mf etc

Is uf. Micro farad assumed if no unit is listed?



Attached File



ETA:

Nevermind about uf or pf, the small print on the page says all capacitors are in MFD unless noted.


So I am guessing the number after the slash is the voltage rating based on the voltages on the schematic

Yeah, that is correct.

Check out the second post here as well - tech says the 2uf/35v are always bad, and do the others as well.
https://forums.peavey.com/viewtopic.php?t=53356
I'd definitely start with that and bias it. Test the power tubes if still having issues.

If everything is OEM in the amp, I would try changing the pre and power amp tubes first.
And be extremely careful when working on tube amps, they can kill you even if it's been turned off for a while.
You have to discharge the capacitors, only then is it safe to work on.

If the tubes are old, I wouldn't change them without getting them tested first. Particularly the preamp tubes see very little wear - they don't make them like they used to.

The electrolytic caps always go bad over time however. They need to be replaced at ~10-15 years.


https://www.amprepairparts.com/peaveycapkits.htm

That is a weird condition. Going into distortion is one thing, but phase or tone shifting is another.

Have you recorded a few samples and asked if someone else can hear it?

First thing I always check with tube equipment is house power. Might be nothing. Does the house have a good neutral? I'd take a meter to the outlets and the breaker box and look there. Especially if that outlet has a lot of other things on it.

Next thing I would do is make a known good tone source, and inject that into the amp, and see how it is colored.

If it starts becoming obvious that there is an issue, I would start then cleaning up connections. Pull the tubes, clean the pins. Clean the power cord connections. Measure resistance in the ground wire. Common ground points. Jacks. Then I would clean the pots. Then I would look for cold solder joints, and maybe even remelt a few.

On audio gear, the absolute last thing I do is replace components. Once you do that, you will change the sound of it forever, and you won't be able to go back in many cases. Vintage caps and tubes are highly, highly sought after, and had I known, I would have bought boxes of both super cheap as a retirement plan...

Good points. I think he had a previous post about going through it and cleaning everything already. Changing the coupling capacitors that conduct the signal should be avoided at all costs. The filter caps or electrolytic caps he is talking about however have minimal if any affect on the sound and are routine maintenance.

Here's the power filter cap section from a completely dead Bassman I revived for a friend. I do the rest of electrolytics at the same time as a matter of course (e.g. green and black filters caps on the board are new, blue signal caps and everything else stays).






I did okay on tubes - a little late on the uptake but nothing like today's prices.

I am aware, I have worked on ham radio tube amplifiers.

ok thanks, yeah I can definitely replace the electrolytic caps and I should be able to check and adjust the bias for the tubes without killing myself.

I have already cleaned the control pots because they were scratchy and the jack connections.

I am in the process of borrowing an oscilloscope to compare input and out put of the amp but there is no way any of those electrolytic are good. I will save them but as long as I can find the parts they are coming out

unfortunately there are lots of electrolytic caps in the preamp section directly in the signal path. They are those 2/35 caps and they do need to be replaced after 50 years. I however do not know WHY they are electrolytic. That makes no sense to me other than being cheap and smaller . All they do is block DC to the next amplifier stage and in this case, to the next transistor. I am very temped to put film caps in those signal path places instead.

Letting DC pass would saturate the next transistor and cause distortion.

The electro caps going bad in the bass, mid and treble filters would screw up the filter frequencies coloring the sound.

All the other electrolytic caps in the preamp section and all the caps in the power amp tube section are there to prevent DC voltage sag or filter  to ground

Oh, I hear what you're saying. Those are simply providing the power and filtering ripple, etc etc etc. I also don't work on people's guitar or high end stereo amps anymore because they swear they can hear the difference. I can't see it on a scope. But they can hear it. (shrugs) System won't work without working caps, and caps don't like heat and overvolting, which is what they see in those applications. oh well

This link may have gotten buried - it looks like it has the main ones listed as a set part way down the page. There is also a parts kits on ebay that has the cap can as well, but it's 2x the price.

https://www.amprepairparts.com/peaveycapkits.htm

https://www.ebay.com/itm/196001220869?itmmeta=01HX4PYR3BZY6TEV802QHVRPZ6&hash=item2da2954905:g:V-YAAOSw-E9lGRhx&itmprp=enc%3AAQAJAAAA8Fx4MuougXlipqaA1BhuYYFvpCrEaxdb8uQVHT%2B7Pd3oaxypICmxphg%2BHa9t2XUeeTCRMd59jyuOwVPawu2qxP0shWLQve6GK3c09Tnzo%2FPRr%2BgvUplWlho6qedlg2C1lOutbqhnmhc%2BRVR1pQUK8aCFkEgI1MzztuQ%2Bx0GIFOM%2BSyfW0pHj%2BuJTx4Hhc92MrL8hL75JpUjxCdPGuvN9ytF6H%2FadcOSNzi%2B%2FCXGF80MmasOoAjZVpOyIEjHYQgfrC6Q5e1MukfJQmcsp8fx7Q1nPIya6o6S8hvieJnJrPjyQzRhyLMD38Q2NeTSXflFKKg%3D%3D%7Ctkp%3ABFBM7IH7lulj

Huh - no advise there but that's a little strange for sure.

Thanks!  The ebay link looks like it has the right caps, the first link doesn't and it says mid 1990s, so maybe the 1974 amp is different values.

I am going to post the schematics that I have. But I also have to open it up again and check I have the right schematics before I order the parts.

It appears using electrolytic caps in the pre-amp signal path was common in the 1970 I think mostly because those caps are smaller and cheaper and it was before the capacitor revolution making cheaper small film and ceramic caps, but I am just guessing.

I honestly do not see any need to use electrolytic caps in the signal path, all they do is block DC so the transistors only get an audio signal with no DC offset.

Caps that do DC supply voltage stabilization on the board is a different story and electrolytic caps make sense there because they need to be much bigger values and are not in the signal path.

I am really tempted to put ceramic or film caps in the signal path and see how it sounds. I can always replace them if I don't like it, but I do need to replace those electrolytic caps in the signal path with something.

FIXED

Thanks for the links.

 I installed the ebay capacitor kit. It was the only one that had the correct parts

That kit included 7 x  2.2 uF Tantalum caps which are very low leakage ( low noise ) and fast caps ( clarity )  for the pre amp signal path caps. I dont  know if it sounds different than the original electrolytics when new and not 50 years old.

 Amp sounds great,  filters work again and very loud with no distortion. It no longer sounds "off"

Awesome, glad it all worked out. I found it to be quite a satisfying thing to DIY and ended up getting into my first amp build a little later