Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tobyk on September 22, 2020, 08:36:10 am

Title: Gibson clone
Post by: tobyk on September 22, 2020, 08:36:10 am

Hello, I’m having some problems with a Gibson clone I’ve built.
It’s a GA19/GA20T hybrid.
It sound great clean but when being overdriven for a short while, the sound suddenly first gets anemic and is then zapped altogether.
Is it a matter of tube starvation and the plate resistors being too large? Thanks!

Title: Re: Gibson clone
Post by: tubenit on September 22, 2020, 01:00:18 pm

Thanks for sharing the schematic!

Can you post voltages for the plates and cathodes of the tubes, please?

Can you post a hi-rez picture of the chassis interior layout and wiring?

Have you tried "known to be good" tubes?

Is the problem more connected to the 5879 channel or 12AX7 channel?

It almost sound like an oscillation issue with a tube?

with respect, Tubenit

Title: Re: Gibson clone
Post by: tobyk on September 22, 2020, 01:50:23 pm

I’m almost certain this is a circuit design issue, not a layout or tube issue. Tubes are all new or NOS.
The amp also makes a loud click when switching off, which I think is part if the same problem. I remedied that with a cap over the on/off switch, though.
I think it might be the high plate resistors (470K and 510K) combined draws too much current. It happens in the high gain 12AX7 input, not sure about the others. The 5879 channel has lower gain thanks to the low impedance inputs. Will check voltages when I get the chance, thanks.

Title: Re: Gibson clone
Post by: sluckey on September 22, 2020, 01:56:36 pm

Measure voltages for all filter caps and all tube pins while the amp is behaving. Then measure voltages for all filter caps and all tube pins while the amp is misbehaving. Any clues?

Title: Re: Gibson clone
Post by: PRR on September 22, 2020, 02:58:59 pm

> overdriven for a short while, the sound suddenly first gets anemic and is then zapped altogether.

Sounds like a bad/missing grid-leak resistor or joint.

Title: Re: Gibson clone
Post by: sluckey on September 22, 2020, 03:42:49 pm

Why do you have a 470K connected to the grid of the PI? The original does not.

Title: Re: Gibson clone
Post by: tobyk on September 22, 2020, 04:10:16 pm

>Why do you have a 470K connected to the grid of the PI? The original does not.

Good catch, I added that grid stopper to remedy any overdriven cathodyne nastyness.

Title: Re: Gibson clone
Post by: tobyk on September 22, 2020, 04:13:15 pm

Quote from: PRR on September 22, 2020, 02:58:59 pm

> overdriven for a short while, the sound suddenly first gets anemic and is then zapped altogether.

Sounds like a bad/missing grid-leak resistor or joint.

Thanks, will check..

Title: Re: Gibson clone
Post by: tobyk on September 22, 2020, 04:16:09 pm

Nobody thinks it has to do with the large plate resistors?

Title: Re: Gibson clone
Post by: shooter on September 22, 2020, 04:34:36 pm

gator clip a second one in parallel, 1/2 the R twice the fun

Title: Re: Gibson clone
Post by: PRR on September 22, 2020, 10:51:30 pm

> Nobody thinks it has to do with the large plate resistors?

Nope.

They aint what Fender gravitated to but there is LOTS of audio with resistances like that.