Problem with new build (dark sounding)

[HBamps]

I just built the second of two identical amps much like a deluxe reverb but without Trem or verb and with cathode bias. The first one came out great so I built another for a friend. this on is like it's got a blanket over it, just sounds muffled. Behaves like the other one as far as gain and all the tone controls seem to work, it just is lifeless and I can't figure out why.

Any educated guesses? I am stumped.

Thanks

[01bcat]

This may seem obvious - Did you try running it through a different speaker (your old one)? I would swap out tubes as well, I assume you are doing a side by side?

[HBamps]

yes, I am listening side by side. I have swapped tubes gone over all the component values, looked at solder joints, even swapped OT's. I know it's gotta be something simple am over looking. One difference is that dark one has carbon comp resistors for the PI plates (going out on a limb here) but could it be hat they are mismatched to a point that there is some cancelation in the PI?

[01bcat]

I mentioned the speaker because new ones can be stiff until they are broken in. I am now assuming this difference isn't that subtle.
I would go over all the resistors and caps, and check the inputs are wired right - do you have a bright and normal inputs? Are you using different cables on the guitar?
It also wouldn't hurt to switch the "known" different resistors.

[Gtr_tech]

Double check your work. Sounds like you have a cap going somewhere it shouldn't....to gnd.

[txstrat]

Can you feed the preamp signal of the "new" amp to the power stage of the "old" one and vice versa?
That way you could find out where the loss of treble happens, in the preamp or the power amp of the "new" build.
At least you got a smaller area to troubleshoot.

[HBamps]

Can you feed the preamp signal of the "new" amp to the power stage of the "old" one and vice versa?
That way you could find out where the loss of treble happens, in the preamp or the power amp of the "new" build.
At least you got a smaller area to troubleshoot.

That's a great idea but not sure how to implement it.

FWIW, this amp previously had a different preamp in it. I never liked how it sounded so I ripped that pre out and put this one in knowing how much I liked it in another amp. Well, after the swap, it still has what i didn't like about it initially. That leads me to believe it's in the power section (the part of the amp unchanged). LTPI into cathode biased 6v6's.

[HBamps]

I mentioned the speaker because new ones can be stiff until they are broken in. I am now assuming this difference isn't that subtle.
I would go over all the resistors and caps, and check the inputs are wired right - do you have a bright and normal inputs? Are you using different cables on the guitar?
It also wouldn't hurt to switch the "known" different resistors.

I swapped out these this morning, changed to carbon film (100k) like the others. I also swapped the power tube cathode resistor from a 300 ohm to 270 ohm (like the other). I find it hard to believe that 25 ohms will make any tonal change.

Couldn't test yet cuz wife and baby were sleeping.....

Thanks for the tips. This forum rules!

[txstrat]

That's a great idea but not sure how to implement it.

Use leads with alligator clips from the old amps preamp to the new amps PI. If you don't wanna tinker around in a "live circuit" put the clips in while the amps are off and switch 'em on after that.

[Enzo]

Look at the chassis side by side. Visually compare each and every part going down the part board. Is ther ANY difference between the two? And verify the part values for each and every part. use teh good amp as the model. It is real easy to install a 100 ohm resistor instead of a 100k resistor. Or a 0.01 cap instead of a 0.001 cap, especially if they are labelled 104 rather than .01. And check each wire as to where it goes. All it takes is a wire coming from the wrong eyelet.


And don't skip ANY parts, don;t let yourself say, "Well I KNOW that group is OK, I already checked it." Start from scratch and check every part.

[Albert Kreuzer]

...especially if they are labelled 104 rather than .01.

Yes, a 104 surely would sound different than a .01

SCNR

[Enzo]

yes, they would. I should have used the same value. I meant that it can be easier to confuse values when they are written in code rather than value.

[HBamps]

I have been looking at this thing all night, comparing it to the other amp. all the components, voltages etc. I did find one thing that may be suspect. This amp has a 12AT7 LTP. It has 100k plate resistors, 820 bias, 22k tail, and a 1k5 for the neg feedback.

For plate voltages I got (V2a is signal side);

"good" sounding amp
V2a = 160V V2b = 154V

'bad" sounding amp
V2a = 147V V2b = 151

It got me thinking, maybe it's a PI balance thing....? It just seems a bot odd that the signal side has higher on one tube and lower on the other. I put a 82k on the signal side but can't check till tomorrow.

[HBamps]

I tried everything I can think of, so much to the point that the board is looking like a rats nest. I am resolved to gut it and re-build. Now I need to decide how much to tear out. I was thinking
- Removing the board, all leads, all volume and tone controls.
- Leaving the PT wiring, OT, and filament wiring in place.
- Build entirely new board and rewire.

Now I am thinking, the only tube I did not sub was the 5Y3 rect. maybe I'll try that first.

Thanks for all the help.

[01bcat]

That can make a difference!