Help, why is my amp all highs?... Problem resolved

[Bob-I]

New post 11-16-2012

Thx Leftystrat for getting me back to this.

I disconnected the reverb B+ rail, I'll be damned if the amp didn't get back to it's original big sound. Reconnected the B+ rail, bottom dropped out. I've been looking in the wrong area completely.

Now the strangest part, I put the reverb tube back in and the reverb control acts like a gain for the entire amp.

I gotta dig out the reverb schem and see what's going on there.

Edit...

What a nightmare.

Turned out to be a bad filter cap on the reverb supply rail.

I have a B+ rail parallel to the preamp. Both come off the screen node, the preamp is 4 nodes of 22uF each, the reverb was 2 nodes of 22uF and 10uF driving an ECL86. The 22 supplies both the driver and return, the 10 supplies the screen on the driver.

I don't know what's wrong with the 22, but once I replaced it all the bottom came back and reverb depth increased dramatically, same intensity at 9:00 as I was getting at 2:00. I replaced the 10 with another 22 for good measure.

UNCLE...

I give up. This amp has served me so well for the past 6-7 years but right now, in sailors terms, fuckin fucker's fucked.

I have no idea what else to try. I've replaced everything in the clean pre.

I'm taking it off the bench for now.

Original post...

I have an amp I built about 6-7 years ago from a Peavey 2x12 hybrid, SS preamp 2x6L6 power amp. I've gigged with it steady for 5 + years and it's always been a big rich tone. I used a fairly standard high plate design, tweaked a tad to get what I wanted, HRM and one tube reverb with an ECL86.

Over the past few gigs I've been increasingly unhappy with the tone so I put it on the bench. I listened, no bottom at all and it sounded like it was through a 3" radio speaker.

1) connect to another speaker, same
2) Connect another amp to it's speakers, they're fine
3) Connect another preamp to the power amp in, sounds great
4) Connect the preamp out to another power amp, tarrible.
5) Swapped V1 with a known good tube, same

Then I started with the real stuff.
1) Swapped the cathode bypass caps on V1, new Rk at the same time, no change
2) Swapped the tone caps in spite of them testing good on a cap meter, no change
3) Swapped the V1B coupling cap
4) Disconnected the reverb circuit
5) Disconnected the PAB
6) Ran a jumper directly from the output cap of V1B to input cap of the PI
7) Checked all the tone pots
8 ) Disconnected the Jazz/Rock switch

I'm about to rip the preamp out and start over, but I thought I'd float this here first. I've never heard anything like this before.

The only conclusion I've made so far is that this is in V1A or V1B since I've bypassed everything else and testing the power amp with another preamp.

Thoughts?

[bluesfendermanblues]

a) Try resoldering the V1 soldering lugs.

b) If it doesnt work retighten the socket connectors which 'grab' the the tube pins

c) put in a new V1 chassis socket

[crbowman]

Do you notice any volume loss?
Almost sounds like phase cancellation.
Check to see if you have a short from V1A cathode to V1B grid or something of the sort.
Had this happen on a Vox AC30 once. Took me forever to sort it out.

[stelligan]

a) Try resoldering the V1 soldering lugs.

b) If it doesnt work retighten the socket connectors which 'grab' the the tube pins

c) put in a new V1 chassis socket

My money is with bfmb on checking or swapping out the V1 tube socket.

[talbany]

Bob

What are voltages plates/Cathodes V1 also check input jack..

Tony

[Bob-I]

Thx everyone so far.

I swapped V1 tube socket, no change.

Voltages on V1 check out. 180/1.2, 210/1.8

I Disconnected the loop jacks, relays, ran a jumper from V1b coupler to Pi input, still sucks.

[stelligan]

bummer.... and I lost the bet. If you need a loaner for gigs, let me know. I have a pile o' amps.

[mcinku]

What about a slope resistor...is it still within spec?

[talbany]

Thx everyone so far.

I swapped V1 tube socket, no change.

Voltages on V1 check out. 180/1.2, 210/1.8

I Disconnected the loop jacks, relays, ran a jumper from V1b coupler to Pi input, still sucks.

Bob
Yeah this is a strange one..Sounds to me it's in the tonestack somewhere.. At this point I would pull out the scope and scope the stack..The treble pot acts as a sort of mixer circuit blending both bass and treble frequencys and exiting out of the top of the pot.. I would want to see if I was getting good frequency response from both treble cap and bass pot.. If you don't have a scope you could disconnect the top of the treble pot and run a wire from there into the input of a volume pot on another amp..Since you pretty much already ruled out V1b..If it still does it then you know it's in the stack somewhere and could rebuild it first (including slope resistor and tone caps).. Also while your at it jump out the vol control after the stack just to make sure that has not lifted from the trace..If I think of anything else I'll get back..

Good Luck!!

Tony

[Bob-I]

Grrr...

I replaced the volume pot, been meaning to do that anyway. It was one of the little push pull and I didn't have much confidence in it.

I think I'm gonna rip out the tone stack completely and rebuild it from scratch. It all seems to be in that area.

[Bob-I]

bummer.... and I lost the bet. If you need a loaner for gigs, let me know. I have a pile o' amps.

Thx my friend, but I have a pile too. It's more about wanting to get to the bottom of this than need.

[talbany]

Grrr...

I replaced the volume pot, been meaning to do that anyway. It was one of the little push pull and I didn't have much confidence in it.

I think I'm gonna rip out the tone stack completely and rebuild it from scratch. It all seems to be in that area.

Good Luck..BTW does that amp have a PAB..How does it respond with the PAB on..Just wondering..

Tony

[Bob-I]

Good Luck..BTW does that amp have a PAB..How does it respond with the PAB on..Just wondering..

Tony

I disconnected the PAB. Good thought, I'll reconnect and see.

[Guitarman18]

I had something similar a few years back, turned out to be an Alpha input volume pot wiper doing some very strange things. Just a thought....

Good luck Bob.

edit- I obviously type too slow, I see you have tried that already!

Cheers,

Paul.

[Bob-I]

Damn, it's still bright. Gotta mean something