[TV MAN]

I'm into a mid 1990's tube guitar amp that failed a 6CA7 output by developing a carbon path between the plate pin and heater pin. Tube tested short, and found the burn marks on the tube socket and later tube when I went to replace. So, tube sockets on the way and a pair of Mullard outputs in hand. Now, my question - this uses "modern" electrolytics in the power supply. They are more than 20 years old. No signs of overheating. Should those be replaced as a preventive or left as long as things look and work ok? I dont want a repeat.

[morzh]

If it has worked recently and you have seen no degradation (hum, clipping) - you could leave them be.
You could possibly unsolder few wires and check the capacitance to make sure it is OK.

[TV MAN]

Kinda thinking the same. Its not something that is going to get a lot of use.  Here's a couple pics of things.
   
   

[mikethedruid]

Condensers are not that expensive. Were it I, I would replace the filter condensers as a preventative measure, if they will fit, with ones of a higher voltage rating.

[Eliot Ness]

I can't see what brand those are, but if they are from a quality manufacturer I would consider leaving them, otherwise I'd replace them with a known quality brand from a vendor like Mouser. A.E.S. carries Solen 630V Metalized Polypropylene that I often like to use in place of electrolytic caps but they don't seem to carry radial leads or values above 47 uf.

https://www.tubesandmore.com/products/ca...nd%3DSolen

[Ron Ramirez]

+1 what John (Eliot) said.

If they are Nichicon or Panasonic, or possibly United Chemicon and there is no hum, then you might chance leaving them. Any other brand, then replace - with Nichicon, Panasonic or United Chemicon (in that order with Nichicon being the preferred brand). Relatively cheap insurance.

[TV MAN]

Samwha caps. Sounds Japan to me.

[Ron Ramirez]

Samwho?

Never heard of that brand. Replace...Nichicon is your friend. Or Panasonic.

[morzh]

I am not sure Chinese caps were popular (or even existed) in the beginning of 90s on American market.

[TV MAN]

I finally had a chance to update. I replaced all the power HV supply caps with the good Nichicon brand from Mouser. I didnt replace any of the lower voltage caps, but I may need to. I put new ceramic tube sockets in, which was fun since they were soldered between the PC board and bottom of the tube socket instead of opposite sides. I installed a new pair of matched Mullard output tubes and bias looks about right (-33V if I recall). Im not sure just how well they are matched tho. There is a slight difference in cathode current when I monitor, but is small. They both draw slightly either side of 50ma at idle. I cleaned all the controls and fixed the speaker terminal connector after it broke off when disconnecting. I have a couple minor issues, which one I believe is the 12AX7A preamp tubes. I have some static, hissing or whatever when I first power up, but diminishes once it warms up for a few minutes. I plugged a new short plate JJ tube (only had 1) in and it helped quite a bit, but still had some noise. It is very quiet with the 1st preamp tube pulled and the JJ tube in the 2nd tube/splitter. It is noiser with the original tube, so I have a new set of Tung Sol tubes coming. Ill see if this silences it. 

The next is a low level 60/120 cycle hum that is dependent on the tone controls. Its almost not noticeable with the tone controls zero'd. So, I will probably go after the e caps in that section and on the DC line of the filaments. 

   
   

[morzh]

My guitar teacher owns several Trainwreck amps.
He knew Ken Fisher well and those were gifted to him.
Nowadays those are worth a fortune.

[TV MAN]

To me, this is one of the best sounding amps I've heard! I really like the fact you can set it for an acoustic or use a bass guitar. Most amps I've seen are one or the other. I put the new pre tubes in and it silenced the noise, so down to tracking down a hum in the preamp section. It's not bad, and would pass like it is, but if I can diminish it, that would be great. Got a couple low voltage e caps on the supply line for a couple mosfets on the cathode of the 12AX7a's. May be the culprit. I'm posting a PDF schematic. 

Ampeg-SJ-12R-Amp-Schematic.pdf Size: 48.07 KB  Downloads: 175

[TV MAN]

I pronounce it fixed. I replaced the rest of the e caps, put back together and found some of the cracking static came back until it warmed up. I pulled both boards and resoldered a bunch of suspicious looking connections. I put back together kinda loose to be able to flex things and pull again and had no sound at all! Huh ... so after some messing around I found flexing the board made the sound (and static) come and go and could see the output tubes flash inside (not sparking, you know -the look inside when the tubes are handling current). Well, I discovered the outputs were grounded through the screws and stand-offs that hold the octal tube sockets to the metal chassis! So, I went back and cleaned and tightened those screws very tight - quiet as a church mouse! So, if anyone has one of these amps with static/cracking noises and cleaning controls and replacing tubes doesnt completely quiet it, clean those studs or solder a ground to those to chassis ground. I still have a very light, low level hum, but I think it is the nature of the beast for the most part. It's not really HiFi, so it may not be completely quiet like a Fisher or Dyna.

[Ron Ramirez]

JJ might be OK for preamp tubes, but when it comes to audio output tubes - avoid JJ.