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

[LeftyStrat]

At this point I'd start from easiest to hardest.

- With the amp off, see if there is any resistance between eyelets on the board. Perhaps moisture in the board material?
- Pull and test the coupling cap after v1b, or just replace it.
- Rip out the tone stack and test every part individually.

I'm just clutching at straws here.

[Bob-I]

Another stab in the dark. But I would consider completely taking the tone stack out of the circuit, removing the connections to both v1a and the volume before v1b. Then run a new coupling cap from v1a to the volume control before v1b. Then I would listen with an audio probe at each stage, after the stages coupling cap.

This way you'll know if it is the tone stack, or something else weird going on downstream.

Last think you want to do is rebuild a tone stack only to discover it was something else, like a bad coupling cap somewhere else.

Good idea.

I disconnected all the leads from the tone caps, then one by one put a jumper from the output of each tone cap to the input of the volume control. I doesn't seem to ever sound right. I'd expect that the mid control, .022 would be a fairly neutral sound, but it's not.

That was a big help. It's very possible I've been barking up the wrong tree, maybe it's V1b.

[briane]

If I dime the bass it's not terrible,

that prolly says it all - not why but where. Is the bass pot ok?

[Structo]

I've said this before but I bought a few NOS Allen Bradley pots to use in my 100w clone.
I failed to do something I do with most components in that I never measured the total resistance or the sweep on them.

My amp didn't seem to work right.
Very little head room, but it actually sounded pretty good.

Once I talked to someone about how it was acting they suggested that I measure the pots.

Both AB pots in my amp were defective.

Put some new Alpha's in it and it was right.

So I guess the moral to this story was never assume anything.

[Bob-I]

The bass pot measures and sweeps correctly, so do both of the replacements

Somewhere I'm losing all bottom end. I shouldn't need to dime the bass to get a balanced tone. I used to run the bass at about 12-12:30.

[Structo]

Weird.

On my amp I can't run the bass past 10:00 and that is with a 2x12 EVM 12L cab.

Although it does kind of sit near the corner of the room so that gives more bass.

That is a real puzzler Bob.

[bluesfendermanblues]

Weird.

Although it does kind of sit near the corner of the room so that gives more bass.

That is a real puzzler Bob.

Oh yeah

Speaker on the floor away from walls +3db bass boost
Speaker on the floor and close to back wall + 6db bass boost
Speaker on the floor and in corner + 9 db bass boost


Which is why most active subwoofers have a switch for placement; 'Corner', 'Wall', 'Free'

I like my open back 1x12" speaker on a chair or similar, so I can turn the bass knob above 10 o'clock.

[Structo]

Thanks Claus, those numbers are helpful.

[briane]

I used to run the bass at about 12-12:30.

yep - on most dumble style builds that would be too much bass for many - definetly not working right.

I think the one comment I always get - is dumbles have great bass tone (maybe too much)- and the frequency spectrum is divided - so individual strings really come through the mix.

[LeftyStrat]

Bob, sorry to hear you're giving up on this one. I did have one last idea you may still want to try.

If there was something wrong with the area around the preamp out jack, it would behave as you've seen. Another preamp into this power amp would bypass it. This preamp into another power amp would have the problem.

Have you considered replacing the preamp out jack? Or checking the area around the preamp out?

I know it is a long shot, but it's the only area that perhaps you haven't checked.

[Bob-I]

Bob, sorry to hear you're giving up on this one. I did have one last idea you may still want to try.

If there was something wrong with the area around the preamp out jack, it would behave as you've seen. Another preamp into this power amp would bypass it. This preamp into another power amp would have the problem.

Have you considered replacing the preamp out jack? Or checking the area around the preamp out?

I know it is a long shot, but it's the only area that perhaps you haven't checked.

Thx, but I've actually removed the loop, relays, replaced the input jack and literally every component on V1 including wire. I'm totally stumped.

[talbany]

Bob
Here is something you could try..

Lift the wire on the top of the treble (wiper going to the vol control) and run that straight into the entrance to the PI and see how the tone controls respond..If it's still thin you know for sure it's in V1a and tone stack somewhere..Let us know what you find..This has me thinkin now..

Tony

[LeftyStrat]

I'd really love to help you solve this issue, because I think we could all learn something from it.

It might be related to this thread:

https://tubeamparchive.com/viewtopic.php?t=19533

Just to make sure I have all the facts, would you mind a few more questions?

- Did it happen gradually over time?

- Have you tried running a signal directly into V1b, with the volume control disconnected from the V1b grid, any without any connections to the input jack and previous circuitry? Take a new jack, solder a coax cable to the hot and ground, solder the hot to the input grid of V1b with nothing else connected, and connect the shield to the chassis. Plug a guitar in. Is it still trebly?

- http://www.youtube.com/watch?v=uiCRZLr9oRw

[Bob-I]

I appreciate your persistence. I'm really just spending too much time and have other things to do, I will keep at this.

Yes, it seemed to happen over time, but I can't be sure because when I gig I use in ears and only listen to the FOH mix, not the amp.

I originally put this on the bench because the reverb crapped out, just a bad tube. I'm also thinking there may be something more to that, possibly something damaged in the PS from that tube. I did check the filter caps, but just now realized there's a separate rail for the reverb I haven't looked at.

I disconnected the volume pot and used a jack on a clip lead to connect to v1b grid resistor. I'll try your suggestion.

[LeftyStrat]

Awesome news Bob. Now we all know yet another thing to check.

So was the cap leaking?