NEW CAPS INTO CARY 2A3 POWER AMP

[bimasta]

Amp is a Cary 2A3 push-pull. Two of the electrolytics, soldered to the tube sockets, have failed — they read open-circuit, and that channel is dead. I found 2 new 'lytics of the correct value in my drawer. I say "new" meaning unused: I've no idea how long I've had them, may be as much as 20 years.

Can I just solder them in and switch on the amp, i.e. full 120V?

Or should I use a Variac gradually to "form" them?

Anyone have experience with this?

 

[triode17]

I would hook them to a high voltage power supply, with a 10k, 2w in series and slowly bring them up to their rated voltage. All the time while monitoring the charging current. In lieu of that, definitely bring the amp. up slowly with a Variac. I would start at 60vac if it has a tube rectifier, if SS, start at 10vac. Monitor the temperature of the caps. with a carefully placed finger or a temp. gauge. It shouldn't get very warm while charging. If it does, STOP, it's going to burst.=bad cap.

 

[Selmerdave]

Soldered to which tube sockets? The 2A3s? In that case they're probably cathode bypass, in an amp like that I think it would be prudent to buy new caps and have no questions.

 

[Bob]

probably coupling caps, if there's no sound, the cathode bypass if opened reduces the gain from what I remember,
so you could try variac'ing the unit up to "re-form" the new caps in situ. if it's the coupling cap, and you choose a high voltage
replacement, ...

 

[Selmerdave]

Coupling caps wouldn't be electrolytic...but then any cap that is *good* should read an open circuit (infinite resistance). Cathode bypass caps in circuit should read the value of the cathode resistor though, not an open circuit. The issue may not be the caps.

 

[bimasta]

Great advice guys, thanks. Sadly it came too late. When I got no response for a few days, I thought none would be coming. I contacted a professional tech directly and asked his advice. He wrote "No harm in bringing it up gently on the variac IMO. The caps won't really care, unless there's other problems." So I proceeded.

I should have waited, but I was impatient.

When I first switched on, after using the Variac, the dead channel played MUSIC! Hallelujah! Fixed it!

But after 2 seconds it began to sputter, then died. Then I smelled burning, and a wisp of smoke rose from a vent. Unplugged it fast.

I only worked on the 2 lytics I knew were bad, didn't touch anything else. I didn't remove them from the amp, they're secured in place and this was just a test. I snipped one lead on each to remove them from circuit, and made sure the now-loose wires touched nothing. I put the "new" caps in place, carefully observing polarity. Used the Variac 5–6 hours to re-form them.

I traced the smoke to a big resistor, far away from the area I worked on. I should have unplugged the amp at the first whiff of burning but I let a few seconds pass so I could see where it was coming from — thankfully it wasn't one of the transformers..

Maybe the "new" caps, purchased long ago, were too old to re-form, despite the Variac. Selmerdave was so right when he said "in an amp like that [a Cary] I think it would be prudent to buy new caps and have no questions." And so was Triode: "I would start at 60vac if it has a tube rectifier, if SS, start at 10vac. Monitor the temperature of the caps. with a carefully placed finger or a temp. gauge." It does have tube rectifiers, and I didn't know it made a difference. Nor did I check the caps' temps.

I don't know what went wrong or (I hope not) what I did wrong. It seemed such a simple task, and I was told so — but maybe something else was damaged in the original tube accident, which I didn't know about.

I closed the amp up and put it away because looking at it bummed me out. I didn't take pics. But I'll reopen it and post photos — someone will help ID the smoking resistor: it's an odd one.

All blame is on me; that doesn't make me feel good but I don't want anyone thinking I'm pointing fingers. I'll post photos, and ask advice on next steps, if any — it might be "take it to a pro". And this time I'll wait for a body of opinion before doing a damn thing on my own.

 

[Selmerdave]

Sorry to hear that. Truthfully much more information is needed about the caps in question as well as the resistor, primarily where they are in the circuit and what they are connected to as well as the values. With the voltages going on in tube amps nothing is really "simple" in that if things are wrong damage can happen in an instant. With an amp that is showing problems it's best not to proceed without a handle on what is going on and where the problem lies. Good luck with it.

 

[primosounds]

I closed the amp up and put it away because looking at it bummed me out. I didn't take pics. But I'll reopen it and post photos — someone will help ID the smoking resistor: it's an odd one.

All blame is on me; that doesn't make me feel good but I don't want anyone thinking I'm pointing fingers. I'll post photos, and ask advice on next steps, if any — it might be "take it to a pro". And this time I'll wait for a body of opinion before doing a damn thing on my own.

I know you are bummed out about the result of your repair, but really we all have had that happen. Sometimes you don't realize what you did was incorrectly hooked up or you missed another issue. Anyway, my response to such an issue is to get back to it while the impressions and familliarity of the circuit is still fresh in your mind. You don't want to let it sit too long. It is probably good to start comparing voltages from the good channel with the bad channel while using a SSR and at a low input ACV. That should find your problem without jeopardizing any other components. good luck