Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on April 03, 2011, 11:23:16 am

Title: Geloso G. 213-A - How to mod for guitar use ?
Post by: kagliostro on April 03, 2011, 11:23:16 am

Yesterday I got another Geloso amp

that is an old G. 213-A model

mounting:

1 x 5V4-G (missing because replaced by the previous owner with 2 diodes)

2 x 12SL7-GT

2 x 6V6-G

If the V1 was a 12ax7 I've lot of  schematicas to be copied as to modify the input in order to have a guitar input

but octal tubes in this position are new for me

1) how to arrange the V1a - V1b to have a standard guitar amp input ?

2) which ToneStack is better to put between V1b and V2a ?
    or is better to maintain a one knob control (as is or with which modifies) ?

3) any mod to the PI section ?

4) I'm confused about Filament Supply and Bias Supply

Thanks

Kagliostro

p.s.: that in the photo is one I've find in the web, not mine (and it is modified)

Title: Re: Geloso G. 213-A - How to mod for guitar use ?
Post by: sluckey on April 03, 2011, 11:39:31 am

Here's what I did to a similar octal amp. V1 is now a higher gain 6SL7. Page 4 is the modified 'for guitar' amp. May give you some ideas...

http://home.comcast.net/~seluckey/amps/rocky/rocky.pdf

Title: Re: Geloso G. 213-A - How to mod for guitar use ?
Post by: kagliostro on April 03, 2011, 12:00:13 pm

Thanks Sluckey

your Rock-Ola Mod looks very interesting and very well documented

a good source of ideas

with your solution if I don't want to make new holes on the front panel I must find double 250k and 1M pots (not ganged) and put a resistor in parallel as to use one of the 1M pots as raw control

I must verify the voltages of the Geloso, I don't know it

Kagliostro