OT: Tube Amp Debug

[aron]

I was debugging my troublesome amp tonight and I thought I would post a little on how I solved it.

First off, this amp sounded great EXCEPT that if you whacked it, it made horrible sounds (i.e. microphonic or?).  I kind of gave up on the 2nd channel and focused on the first channel of my Bassman amp.

This amp has had almost every possible mod applicable to it.

Snubbers, FREDS, Rectifier simulator, cathode bias etc....

Anyway, I loved the sound of the first channel so I decided to get rid of all the baggage of the 2nd channel. After studying signal flow, I started removing all excess circuitry.

In the process of doing this, the amp became horrible!

Hitting low bass notes made ugly distortion even when the amp was set clean. I couldn't believe it! I swear I didn't touch a thing on that channel's side.

First thing, I changed tubes... nope.

OK, what to do.... geofex.com of course!

http://geofex.com/ampdbug/distort.htm

Ahhhh, ok, tubes or failing coupling cap. THAT MUST BE IT! I pull all the tubes and start measuring the grids.... hmmmm some tiny voltage but nothing big.... hmmmmm.....

OK, I start measuring ground and WHAT? On the 2nd stage, the cathode GROUND is around 2.6V!? Now how could that be? I test adjacent grounds and they are fine. How can one cathode be so much higher than the rest?

I try to trace the ground wire but it's impossible. I star-grounded this thing and bundles the ground wires together.... I start buzzing the star ground and grounds and what? It seems some are not "connected". Turns out I was using a couple of ground "tabs" and they were connected by a wire, but that wire was simply touching one of the ground tabs; it wasn't soldered - or maybe it was and broke or?

I apply solder and fire it up. FIXED!!!!!!

Maybe that explains some of the other problems I had.... oh well... I ripped out the 2nd channel completely.

Don't worry, it was only a D*mble circuit
:shock:

[Doug_H]

This amp has had almost every possible mod applicable to it.

Snubbers, FREDS, Rectifier simulator, cathode bias etc....

Do you have snubbers and FREDS in at the same time? They basically do the same thing. :lol:  :lol:

This sounds like a real "Ampage" amp: all the latest tricks bundled in one! :lol:  :lol:

Don't worry, it was only a D*mble circuit
:shock:

Haha!!! :lol:

Well, with all that switching stuff and etc you told me you had in there, it probably does sound much better now.

Congratulations Aron, I'm glad you got it working! So many times it ends up being something dirt simple doesn't it?

Doug

[Paul Marossy]

I just star grounded my '65 Gregory Mark X. I swear that after I did this, the amp had way more UMPH!  :shock:  Of course, the hum level went way, way down.  

Some of these sorts of things seem inexplicable! :wink:

[aron]

It may have been dirt simple, but you couldn't see this problem. Only the multimeter told the story and only if you believed it!

I was ready to pull out my scope but I still managed to solve it without it. Now to put back the 3 stage circuit - now that's going to have DISTORTION!  :twisted:

[Paul Marossy]

Rock on dude! Or is that fusion on?!  :wink:

[aron]

Hehehehe. Thanks.

I might try it next week. I'm a little hesitant because I'm just worried about any noise issues at the club. This amp DID have buzzing issures before but not now. We will see.