5 Tube Amp idea for the new Chassis Doug Sell - Gibson GA-80 lite

[kagliostro]

Yesterday I was looking to the Gibson Service Book that Doug has on the database

1959 Gibson GA-80T Vari-Tone Amp DEMO

http://www.vintageguitar.com/16351/gibson-ga-80t-vari-tone/

And comes to me an idea for a compact 2 x 6L6GB Amp that I think will be a choice for the new 5 tube chassis

I really think this will be a fine "small" amp

may be someone don't want to use the GZ34 rectifier, so a vibrato circuit can be added to the project

or prefer to use 6V6 tubes instead of the big 6L6 (a bit of arrangment will be necessary)

All is based on the Gibson GA-80 Amp



in a Lite way



There are two things to be determined

The % of the intake on the 2M Tone pot
(this pot can be obtained using a 24 position rotary switch, and connecting an intermediate "pin", not very difficult to do)

And the value of the coupling capacitor to be put between V2 and V1b
(on the original there is in the middle a 100k Mix resistor that is no more necessary as the amp has only one channel)

I hope you like this idea, what do you think ?

Franco

p.s.: I changed the schematic of the 1M resistor connection on the input, on the GA-80 it seemed connected in an odd way to me

[tubenit]

I like this idea a lot!  Why the caps in series going into V1b grid?  And I might consider a lower value grid resistor into V1-2.

With respect, Tubenit

[kagliostro]

Ciao Tubenit

I might consider a lower value grid resistor into V1-2

SORRY, to which resistor do you refer ? Do you mean V1a and V2 plate resistors ?

---

The .002 capacitors is on the original GA80 schematic, I really don't know the reason, but there is a .01 capacitor, followed by a 100k resistor (whose purpose to me is of Mix resistor) and then a .002 capacitor

if you look to the original schematic on the junction of the 100k Mix resistor with the .002 capacitor, there is connected one other 100k Mix resistor that is followed by a .0047+1M+.0047+1M filter (I think is to eliminate the vibrato oscillator frequency), so no DC can arrive to the grid of V1b and I really don't understand the purpose of the .002 capacitor, to me the coupling function is absolved by the .01 capacitor that is connected to the plate of V2



Franco

[PRR]

> I really don't understand the purpose of the .002 capacitor

Not needed in theory.

But the cathodyne grid is a VERY high impedance. Roughly Mu*470K, or 10 Meg, and floating at maybe 80 Volts.

Even a "slight" leakage here will pull the cathodyne grid down, mis-bias the cathodyne, which will distort before the power tubes give all the Power you paid for.

I do think it would almost-always work without that cap. Maybe excess caution?

[kagliostro]

Many Thanks PRR

Very good explanation

Franco