Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 3choplex on July 05, 2013, 12:33:41 am

Title: Gibson Scout Tremolo Distorted
Post by: 3choplex on July 05, 2013, 12:33:41 am

I recently inherited a non-working gibson scout. I fixed a number of issues, but can't seem to get the trem figured out. It has a cathode bias trem--the problem is that it distorts on the trem peaks. I've replaced the .01 and .022 caps and the 330k and 150k (the one going to the speed, not the plate) resistors in the trem circuit, and tried different tubes. Any thoughts? Is it pulling into cutoff? 

The schem is:
http://www.gibson.com/Files/schematics/GA-17RVT%20Amp%20Scout.pdf (http://www.gibson.com/Files/schematics/GA-17RVT%20Amp%20Scout.pdf)

Title: Re: Gibson Scout Tremolo Distorted
Post by: tubeswell on July 05, 2013, 05:14:00 am

What are the LFO (plate and cathode) idle voltages (including the LFO supply HT node voltage)?

Title: Re: Gibson Scout Tremolo Distorted
Post by: 3choplex on July 05, 2013, 10:25:09 pm

Plate: 96v
Cathode: 0v (tied to ground)
Node: 176v before the 150k resistor, 96v on the LFO side

The schem says I should expect 118v on the plate.

Title: Re: Gibson Scout Tremolo Distorted
Post by: sluckey on July 06, 2013, 06:43:54 am

I think it may be taking the output tube grids too positive. Try this. Connect the anode of a 1N4007 diode to the junction of the 220K grid return resistors (R23 and R24). Connect the diode cathode to ground. I'd use gator clip leads. Any help?

You can also increase the size of that 1M (R34?) that's connected to those 220K resistors. This will also decrease the strength of the trem.

Title: Re: Gibson Scout Tremolo Distorted
Post by: 3choplex on July 06, 2013, 10:12:48 am

Thanks! I'll try those.

Title: Re: Gibson Scout Tremolo Distorted
Post by: PRR on July 06, 2013, 02:30:33 pm

No Depth knob on that trem??

Any chance of adding one? If not on front or back, inside trimmer?

Otherwise, I'd add a few hundred K to R34(?), as Sluckey suggests, and see what changes.

> Node: 176v before the 150k resistor

Schem show +265V.

Follow the chain up to the +305V, see what else is droopy.

Title: Re: Gibson Scout Tremolo Distorted
Post by: 3choplex on July 07, 2013, 12:03:37 am

I bumped the resistor up to 4m, lower trek, but still distorted. The diodes fixed it, it lowered the effect as well but that's fine. Now to deal with the thumping..

Title: Re: Gibson Scout Tremolo Distorted
Post by: 3choplex on July 07, 2013, 12:05:01 am

Can you tell me what exactly the diode is doing? Just dumping voltage to ground?

Title: Re: Gibson Scout Tremolo Distorted
Post by: sluckey on July 07, 2013, 08:18:41 am

The tremolo signal is just a low frequency sine wave. The diode shunts the positive half cycle of the tremolo signal to ground. Now only the negative half of the tremolo signal can affect the grid bias.

Title: Re: Gibson Scout Tremolo Distorted
Post by: 3choplex on July 07, 2013, 11:39:55 am

I should have known that. (Slaps forehead) thanks!