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

[rockybottom16]

duplicate message, computer's acting up...

[rockybottom16]

It would be nice to isolate the problem to the preamp or power amp. But I'm confused as to how to do this exactly. Obviously I'm not an engineer...

[g1]

You can connect a guitar patch cord from the vol. wiper (& ground) to another amp. Preferably to that amps 'power amp in' or 'effects return' jack if it has a loop. Otherwise into the regular input and you can still use the volume control on your 18 watt to avoid overloading the input (of 2nd amp).
Just use the wiper and ground lugs from the volume control to the tip and sleeve of your patch cable.

[lowell]

Have you checked the bias? I havent read any post about the bias of the el84s. If they're cold it could be crossover distortion. Be sure and check bias on both secondaries of the OT to be sure theyre at least close to identical. Seems everything is identical except the OT. May need slightly different cathode resistor with this OT...or one of the windings are bad.

[rockybottom16]

One step forward... I took g1's advice and ran the preamp into another amp. It was clean as a whistle. So the problem is in rest of the amp somewhere. This leads me back to suspecting the output transformer. Since I have a new one coming this week, I may just wait and rule that out before I tear into the power amp section. lowell: I haven't checked the power tube bias, but I have tried other tube sets with no change in the noise. And all voltages are where they should be.

[Chuck H]

I took g1's advice and ran the preamp into another amp. It was clean as a whistle. So the problem is in rest of the amp somewhere.

Probably.?. One question though. Did you disconnect the power amp of the troubled amp when you connected the preamp to a different amp? I ask because with no signal passing through the later stages of the troubled amp there would be no proximal interaction with earlier stages. So it could still be parasitic oscillation. Which wouldn't technically be in the pre amp or power amp, but a potential interaction between them.

So maybe a second similar test... Can you run the preamp of a different amp into the power amp of the troubled amp.?.

[rockybottom16]

Chuck - No, I did not disconnect the power amp of the troubled amp. I just used alligator clips on the vol. pot, to a guitar cord which I plugged into another amp. I ran a long speaker cord from the troubled amp and faced the speaker away from me so I wouldn't hear it. The good amp was right on the bench next to me.

[lowell]

Assuming your cathode resistor is within tolerance (150ohms) then your 11v cathode voltage looks on point regarding current. 11/150=.074. Each tube should be conducting half that at 37ma. Subtract screen current so about 35ma at 335v is 11.7W. Looks good. If your OT had a compromised winding one tube could be pulling more current and the other tube less. Could cause a cold bias crossover buzz. But you'd most likely hear some hum as well so if there isn't an audible hum from the power amp it is probably ok.

[Chuck H]

That's great. I'd roll with troubleshooting the power amp then.

I think lowell was suspecting a possible cold bias resulting in crossover distortion. I can see the 150R bias resistor, seemingly wired correctly, in the photo's. I would think that's hot enough to prevent crossover distortion on clean tones UNLESS you have a leaky coupling cap. The possibility of a new film cap being leaky are really small though. I might look to the PI circuit next. I'll look at the pics again. DC voltages for the PI tube might help too.

Too bad you don't have a scope, man. If it's not a parasitic oscillation, and it seems it may not be, it would be easy to find with a scope.

[rockybottom16]

Chuck - Here's the DC voltages for the PI tube:

1 - 207
2 - 33
3 - 68
6 - 207
7 - 33
8 - 68

[Chuck H]

Well those look fine.

[Chuck H]

I'm running out of sane ideas (we may get to the insane ones ), but look in the yellow circle. The OT secondary and the power tube grid leads are in close proximity. Positive phase interaction might be causing a problem here. There is a lot of current in that secondary so it may be coupling signal to the grid leads. You can try re routing the grid leads as shown (also in yellow). If it helps, and you want the neater appearance of the original routing you could try some shielded lead.

[rockybottom16]

Chuck - I just tried that and there's no change. I'm beginning to think I have a faulty component somewhere, O.T. or otherwise.

[lowell]

Please do double check that your first 33uf filter has solid solder joints, as well as the OT primaries and power tube cathodes/plates. You did say lead dress was good but just making sure cause this symptom is common with iffy solder joints.

[Chuck H]

We need a "dislike" button

Without having it on my bench I'm about out of ideas (the sane ones anyway). I don't imagine there's anything about the OT that should cause such a problem.

There is one other possibility. You may be grid loading due to the time constant of the PI coupling cap discharge. Your master volume arrangement is one I haven't seen before, though I thought it was a good one. It parallels the pot load with the stock grid load. That SHOULD keep the time constant lower than it would be with the 470k load resistors alone for the power tube grids. I haven't entirely wrapped my head around what it might do at the coupling cap end, but I didn't see a problem. Also, the coupling caps you're using are of a relatively low value and that also reduces the time constant. Even so, if there is some grid loading that can cool the bias and cause crossover distortion. This would be easy to see on a scope (which we don't have). The things to do about grid loading don't always solve the problem, but include:

Increase grid stop value
Decrease coupling cap value
decrease grid load value

Your amp doesn't seem to be in gross error of any of these parameters. You might still have it though. IMHE some amps just do it even when you take these parameters into account. When that doesn't work there are other circuits developed to mitigate grid loading. I've worked with them plenty and we may get there. But first we should stay with looking for a problem because, as you mentioned, you've built this amp before without this new issue.

Power the amp down and let it rest for a bit and clip the OT primary CT to ground. This is sometimes necessary to prevent residual voltage in the filter caps from skewing the meter reading. Now measure resistance from either end of the OT primary to ground. If they're grossly dissimilar then maybe the OT could be causing the problem.?. I know it's the only difference you report from previous builds, but is that strictly true? Were the other builds done with the vol, vol, tone panel arrangement on the same chassis with the same component location? Did the other builds include the master volume circuit? If the OT tests with reasonable DCR balance (and there's a good bit of leeway) then ANY other difference between this build and the others should probably be suspected first.

Have you tried disconnecting the master entirely yet?