Tube amp troubleshooting

Buddha Pickups · Sun Dec 10, 2023 10:53 am

Question for tube amp gurus, which I know very little regarding transformers and reading schematics.
I pulled this amp out of storage and it worked for about 5 minutes then stopped. I assumed it was tube but I eventually noticed the speaker jack was loose and when I messed with it the sound intermittently went in and out then stopped.
I checked the fuse next to the power plug and they look fine. Just wondering if the speaker jack cutting in and out might have ended up blowing my output transformer.
Seems easy enough to replace but the only walk through I found uses a classic tone transformer I can't find.
40-18037
The amp is a Kustom v15 combo
@andrewsrea
@mozz

mozz · Sun Dec 10, 2023 12:26 pm

Classic tone is out of business. As to that amp, the output stage is not a common type. But that being said, you should be able to test the output transformer with a meter. Primary might be a few hundred ohms, half of that to the center tap. Output side is gonna be very low, 1 ohm or less from common to all taps. You may have blown a tube or output transformer, once there is no speaker load, the voltages go very high and often things will arc. Check that output selector switch too. See if you get any signal out of the direct out too.

Buddha Pickups · Sun Dec 10, 2023 12:35 pm

mozz wrote: ↑Sun Dec 10, 2023 12:26 pm Classic tone is out of business. As to that amp, the output stage is not a common type. But that being said, you should be able to test the output transformer with a meter. Primary might be a few hundred ohms, half of that to the center tap. Output side is gonna be very low, 1 ohm or less from common to all taps. You may have blown a tube or output transformer, once there is no speaker load, the voltages go very high and often things will arc. Check that output selector switch too. See if you get any signal out of the direct out too.

I tried new tubes and that didn't do it so I assume it must be the OT. Any places to shop for a replacement?
I'm not looking to upgrade per say, just want to get it working again as I was trying to sell it.
Looks like the stock one was 10k but looks like 8k should work.
https://www.unbrokenstring.com/kustom-d ... and-tubes/

mozz · Sun Dec 10, 2023 12:41 pm

Just read the webpage, maybe it's a common problem?
I would still try to measure the primary and secondary before ordering a new part. You may have to disconnect it from the circuit, which i think it has spade terminals. Any replacement that has 4/8/16 ohm outputs is gonna cost more than a generic 8 ohm part. 10k or 8k would be fine for the primary. Like i said, 8k impedance is gonna measure less than 1000 ohms, they vary from 300 and up.

https://www.ebay.com/itm/166430290121?h ... SwZFNjjk9x

Buddha Pickups · Sun Dec 10, 2023 12:45 pm

mozz wrote: ↑Sun Dec 10, 2023 12:41 pm Just read the webpage, maybe it's a common problem?
I would still try to measure the primary and secondary before ordering a new part. You may have to disconnect it from the circuit, which i think it has spade terminals. Any replacement that has 4/8/16 ohm outputs is gonna cost more than a generic 8 ohm part. 10k or 8k would be fine for the primary. Like i said, 8k impedance is gonna measure less than 1000 ohms, they vary from 300 and up.

I'm assuming a generic 8ohm would work and would just disconnect or block the ability to do 4 or 16ohm?
I only had it listed for 125 and I may have a taker as is for 75 so might just do that.
But yes of I do keep it I'll try and take some readings

andrewsrea · Mon Dec 11, 2023 11:34 am

Just saw this. Was the output jack mounting nut loose and you tightened it? If so, did you check for a broken wire or solder joint for that jack?

I would check to see if the jack is making the correct contact first.

Not to say it cannot be the transformer, or something else. In my experience with amps, wear items or items that can receive impact are typically the culprit.

Also, it is very rare for the primary of an output transformer to fail without a lot of other failures which go with it. Arcing as Mozz mentioned, will be heard in the speaker as thumping and crackling while playing aggressively. It may eventually blow a fuse or create a sagging sound (it may look asymmetrical on a scope). A primary or secondary failing open (no connection) will cause the output tubes into a condition called 'run-away', where their plates will get a glowing red dot or oval on the gray plate and eventually blow a fuse. A short in the secondary will result in the tubes getting really hot and possibly blowing a fuse.

If you do find it is the OT, contact matt <matt@musicalpowersupplies.com>. Tell him you need >15w push-pull for EL84 tubes. He owns the company that makes the transformer in Mozz's eBay link. Matt knows all the tube plate load sweet spots for harmonic balance and provides an affordable product.

mozz · Mon Dec 11, 2023 12:53 pm

From google searches, it appears this is a fairly common problem with these amps. Pulling the spade terminals and measuring will confirm if it's bad. To sell it as is $75 and repaired for $125 equals about the same, not counting any of your labor.

Buddha Pickups · Mon Dec 11, 2023 7:13 pm

mozz wrote: ↑Mon Dec 11, 2023 12:53 pm From google searches, it appears this is a fairly common problem with these amps. Pulling the spade terminals and measuring will confirm if it's bad. To sell it as is $75 and repaired for $125 equals about the same, not counting any of your labor.

Yep. I just sold it as is for $75.
I figured if I put the time and money into fixing it I'd probably end up keeping it

Buddha Pickups · Mon Dec 11, 2023 7:15 pm

andrewsrea wrote: ↑Mon Dec 11, 2023 11:34 am Just saw this. Was the output jack mounting nut loose and you tightened it? If so, did you check for a broken wire or solder joint for that jack?

I would check to see if the jack is making the correct contact first.

Not to say it cannot be the transformer, or something else. In my experience with amps, wear items or items that can receive impact are typically the culprit.

Also, it is very rare for the primary of an output transformer to fail without a lot of other failures which go with it. Arcing as Mozz mentioned, will be heard in the speaker as thumping and crackling while playing aggressively. It may eventually blow a fuse or create a sagging sound (it may look asymmetrical on a scope). A primary or secondary failing open (no connection) will cause the output tubes into a condition called 'run-away', where their plates will get a glowing red dot or oval on the gray plate and eventually blow a fuse. A short in the secondary will result in the tubes getting really hot and possibly blowing a fuse.

If you do find it is the OT, contact matt <matt@musicalpowersupplies.com>. Tell him you need >15w push-pull for EL84 tubes. He owns the company that makes the transformer in Mozz's eBay link. Matt knows all the tube plate load sweet spots for harmonic balance and provides an affordable product.

That's a good contact to know for the future as I have a few Bugeras I just got out of storage too. Wouldn't be surprised if one of them needs an OT