New 18 watt lite build, faint buzz/distortion on low notes- HELP!

[rockybottom16]

Chuck - When I took the master volume completely out of the circuit and ran shortest feasible grid wires, there was no change in the noise. I measured the OT as you suggested: 97.3/97.4 ohms. No leeway needed there. I did notice in my snooping around that the MV pot halves are about 10% out of balance. Of course, that's a moot point for now.

All the other builds using this schematic had the same control layout (same chassis), but only one of them had a MV. On the ones without the MV I would just plug the second vol. hole with nifty little black plastic hole covers. Same component location. A couple of them used diode rectification.

[rockybottom16]

Chuck - When I took the master volume completely out of the circuit and ran shortest feasible grid wires, there was no change in the noise. I measured the OT as you suggested: 97.3/97.4 ohms. No leeway needed there. I did notice in my snooping around that the MV pot halves are about 10% out of balance. Of course, that's a moot point for now.

All the other builds using this schematic had the same control layout (same chassis), but only one of them had a MV. On the ones without the MV I would just plug the second vol. hole with nifty little black plastic hole covers. Same component location. A couple of them used diode rectification.

[Chuck H]

Hrmf.!.

Ok. Insane attempt number one then...

Try reversing the OT primary leads. I know the design doesn't use NFB, but other threads here have demonstrated that it can still make a difference. It doesn't look TOO crowded to it without undoing other stuff.

[rockybottom16]

Just for grins I tried piggybacking a couple 470K resistors onto the grid load resistors, making them 235K each. No change in the noise. I tried this because I looked at the schematic for the Weber Smokin' Joe II, which has a slightly different preamp, but the same basic power section with the same MV layout. Their schematic showed 270K load resistors so I thought that might be something. Apparently, that isn't the problem. This is gettin' old.

[rockybottom16]

Tried reversing the OT primary leads. No change.

[lowell]

Hey what about the secondary connections. Just considering things that havent been discussed. It seems voltages look good and you guys are on to insane hypotheses... so figure you may double check the 2 secondaries of the OT and speaker jack etc. And have you tried another speaker cable? Not sure a cable would do this but hey ...we're onto insanity.

[Chuck H]

You could try adding the cursory 47pf cap between the PI plates (a la Fender/Marshall). If it's an oscillation it may help.

Please record voltages at idle for the power tube plate, screen and cathode. Then measure for DC at each grid when you pluck the E string. If either seems to have a high jump in DC (a little one is normal) you may have a leaky PI coupling cap.

[rockybottom16]

Guess what guys.... the new transformer from Weber came today. I threw it in there and problem solved! Thanks to all of you who offered insight and suggestions, and especially Chuck, Lowell & g1 who, like always, really hung in there with me. But in this case, my gut suspicion was right. Whoda thunk. Cheers and Happy Holidays.

[Chuck H]

I didn't doubt that the other OT could solve the problem. You seemed pretty straight up and it was the only significant difference between the amps that worked and the one that didn't. I suppose you could have saved yourself a lot of trouble. OTOH it would have been cool to isolate and conquer the offending mechanism. No such luck this time.

Like I said before, there's nothing about an OT of reasonable quality that should cause the problem. So, to my logic there must be something about the new OT that mitigates a problem. I love a good corker, but I'll take a working amp first every time

[g1]

That was definitely an odd one. Never heard of an OT doing that before.
It seems when we 'double down' most intently against it being an OT issue, that's when it turns around and bites us in the ass.

Edit: considering what Chuck said about the chance of the new OT mitigating another issue, it would be interesting to see if the bad OT does the same in another amp.