Please Help troubleshoot Vibroverb

[DanC]

Hi all,

I'm new here. I'm a guitarist/pedal steel guitarist trying to service my own tube amps with not much technical know-how.
I copied a 6G16 Vibtroverb circuit using the same values shown in the original Fender layout. With the specified 33K resistor in the bias supply, the reading on the 6L6 power tubes is only about 19 mA. I've been trying lower value resistors in place of the 33K to bring that reading up because I don't much like the way the amp sounds. Last value I tried is about 24K, which brings the mA reading up to 23 BUT I get an immediate shrill howling out of the speaker when I turn on the amp - this is with the intensity knob at >0. Any ideas what I might be doing wrong? I tried wiring in a bias adjustment looking at the Weber 6G16 kit layout(https://taweber.powweb.com/store/6g16_layout.jpg), trying both a 10K pot and 50K pot with the same results: Hoooowwwllll.
Any advice would be greatly appreciated!

Dan
PS I first posted this in the Music Electronics forum - Sorry, admin, if I've already faux-pas'd

[pdf64]

Try swapping over the output transformer wires to the plates.

[DanC]

Try swapping over the output transformer wires to the plates.

Thanks for the reply. I did see that advice given in some other post. I will try that. Didn't before because I figured, since it was functioning fine though sub-par before, that wouldn't be an issue. And it has always been wired per the schematic with a Mercury OT. Could you explain the idea behind swapping the wires?

Dan

[Bob M.]

Bias pot...

Copy an existing known-to-work-well bias circuit with, at least, an adjustable bias amount pot. Then you'll be able to get your power tube's mAs in the ballpark, depending what plate voltage you have. You know, this 6G16 circuit is quite brown sounding and not really the greatest thing for pedal steel. If you've got the typical two-channel setup, try a BF/SF preamp on one side with it's attendant scooped mids and bight thing going on and the steel might like that side alot better.

Bob M.

[DanC]

Thanks, Bob. The only example I could find of a bias-type tremelo circuit with an adjustable bias is the Weber kit (I gave the link in my first post). I think that's a 25K pot in the Weber kit, which I sent for in the meantime.
BTW, I'm going to use this amp for guitar, not pedal steel. I agree that pedal steel doesn't get along well with Brown Fenders - tried it once through a 1962 Pro - yuck.

Dan

[Alex R]

Could you explain the idea behind swapping the wires?

Dan

If they are the wrong way around the negative feedback becomes positive feedback instead. Not something the schematic can make clear as it depends which way around the signal wave is fed into the power amp.

[DanC]

Thanks, Alex. The OT wires are color-coded and the Fender layout indicates that the blue wire is connected to pin 3 of V8, while the brown wire is connected to pin 3 of V7. Could it nevertheless be the case that the wires should be reversed?
Dan

[pdf64]

Yes, if it howls then the phase has got reversed somewhere - the OT manufacturer maybe.
If you like where the OT wires are placed, you could swap over the the input grid wires instead, between the tubes, that will accomplish the same thing.

[DanC]

Looks like you guys were right. Thank you!!

Dan

[Alex R]

- glad you got it sorted Dan.