Transformer Failure on Classic 30??

[tsmilliner]

Hi Folks,
Hoping someone can give me an opinion. I was playing this afternoon, and after about and hour and a half of failrly steady playing my amp suddenly started to lose volume and then went down to where it barely puts out a sound at all.

I powered it off for a about 20-30 minutes and then powered back, and now it sounds almost normal, but maybe a little off on the output (could be my imagination on last part).

When I turned it back on the same thing happens after about 4 or 5 minutes (once amp heats up).

When I turned it back on, I checked and all tubes seem to be glowing normally.

The problem is the same on both channels.

I am pretty sure it has to be the transformer, but looking for any advice.

This seems to be one of the more tech savvy forums, so hoping for solid advice.

If it is the transformer anyone have any advice on upgrades for a replacement??

Thnks Tom

[Don Moose]

It sounds more like something thermal and less like a transformer - those are actually pretty hard to kill if you avoid the One Bad Thing of powering on and trying to play with no load connected.

Are there modes that work better than others (drive/clean/boost/reverb ...)?
Try plugging a known-good patch cord from the FX send to the FX return.
Do you have another amp with an FX loop?

Hope this helps!

[tsmilliner]

Hi, I tried all modes and the patch from send to return. Nothing, now when I turn it on it powers up but no sound. Tubes are glowing, all look about the same. I will try swapping them around.
Will get back in a while.

Thnks Tom

[tsmilliner]

I swapped all tubes around and nothing, no sound at all. I guess I need to look for an amp tech tomorrow.

Any other advice appreciated.

[tubeswell]

If it powers up, the PT must be okay. Check both fuses on the secondaries

[Jerry]

If you plug your guitar into the fx return jack, do you get sound?

[Chuck H]

I'm with Don. It's something thermal. I would be surprised if it was the OT.

Looking at the schematic I can see that it's an all tube signal path without the loop in service. So that shoots down my silicootie failure theory. But just because a tube is glowing does not mean it isn't failing. The filament circuit and the actual amplifier parts of a tube are two different circuits. Swapping tubes around won't help because you would still have the defective tube in the amp. You would need to swap with known good tubes to be sure that none of the tubes are the problem.

That you have no sound now is a problem. Usually when tubes are failing you still get some sound out of the rest. The schem doesn't show any internal fuses, but I've heard they are there from guy's who have repaired these amps. If so I would check them and get some new power tubes.

It would help to know your amp repair chops. Have you done this before? Do you own a DMM? A soldering iron?

Chuck

[tubeswell]

The schem doesn't show any internal fuses, but I've heard they are there from guy's who have repaired these amps.
Chuck

F2 and F3 are after the PT secondaries. While the heaters might be going, the HT fuse (F2) might be blown maybe

[tsmilliner]

Thanks for the insight. I will check the fuses. I assume they are located inside the chasis when I pull out it out of the cab should be able to see them. Would that be a safe assumption??

I did check all of the tubes using spare tubes I had from an old swap that were good when I pulled them out. I used 2 spares checking each position twice to ensure that the spare wasn't bad. It is always possible that both spares are bad as well as one of the installed tubes, but the odds are against in moy opnion.

I will check the fuses tonight when I get home.

Thanks Tom

[6267]

I have seen the slow power fade specifically when one of the output tubes was gassy and went into bias runaway. Make sure that the plates don't have even a dull red glow. Sometimes this looks like glare or reflection on the glass. The best way to check is with the lights out. This will be more of a dull red compared to the red/orange of the heaters and will appear on the big grey/black thing (anode).

Here's your schematic:
http://www.schematicheaven.com/newam..._classic30.pdf

I would suggest testing/having tested the transistors, too...they like to do funky stuff based on temperature and there appear (after a 33 second glance) to be a few in the signal path (Qs, Js and Us on the schematic). These are the most likely culprit if there is no redplating, working B+ and proper bias on the power tubes. Don't rule out mechanical problems though, make sure all the solders look good and that all cables, speakers & jacks are good.

The good news is it's probably a 22cent transistor and not a 65 dollar transformer

[tsmilliner]

Hi, I just checked the fuses and took a look inside and there appears to be no issues with broken solder joints, etc.., but there was a wire that looks like it doesn't belong. It is soldered to a resistor just next to where the fuse is and kind of twisted together and soldered. I assume this is a previous repair. (I bought the amp used with no history known) , so who knows what it could be.
Will look at taking it to an amp tech to get an estimate. If it is a simple repair maybe I can get away with 70-80 bucks rather than buying a new amp. Usually I am a little braver, but the board is some kind of square looking folded up thing, I am sure I would just make things worse. I have done mods on a few pieces of (older) gear, but this one is too intricate to mess with.

Thanks again for all your help. If anyone knows a good amp guy in tampa bay area please chime in.

Once again thanks and god bless.

Regards, Tom

[tsmilliner]

forgot to mention fuses tested good.

[tubeswell]

Usually I am a little braver, but the board is some kind of square looking folded up thing, I am sure I would just make things worse. I have done mods on a few pieces of (older) gear, but this one is too intricate to mess with.

I know what you mean - I've had my C30 completely dismantled lots of times, which (I'm not ashamed to say) is "apart more times than a hookers knees" (I believe that is the correct US expression - which was relayed to me by Enzo of all people) - including 13 times in one weekend (trying to find where I'd goofed) . Anyway - anything can go amiss with those inter-board jumpers, and it probably has. If you give each little jumper a tug with a pair of pliers, you'll probably find at least one of them is broken. Anyway good luck

[Enzo]

I said something like that? Wow, I must have no class. Nope, none at all.

[tsmilliner]

Thanks I will check it again one more time before I give up the ghost and take to the shop. Also neglected to mention there is no glow at all on the PT side plates as a previous person asked. They are flat cold, I haven't checked with a meter, but visually no glow and no warmth at all.

As you know the shop repair costs can quickly eat the entire value of these small amps. Unfortunatley I am quite attached to it, like an old pet so to speak, so will keep trying to make it work before I put her down for good even if I have to pay.

Does anyone know if they make a modded or upgrade board or kit for these amps that might fix my troubles and make it better at same time??

Thnks Tom