Another Geloso G-227A To Bassman

Tube or solid-state, this section goes to eleven!
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sarakisof
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Post by sarakisof »

Hello everyone, first post here.
Calling @modman, found a Geloso G 227A and after fully repaired, want now to mod its one input only like in Bassman. I am planning to add a cathode res in first pre tube (pin 3) 1K and 25uF in parallel to gnd and replace the grid 3.3M with 1M resistor. Do i need to also remove 470K and 22nF and add the usual 68K or it is not necessairy?

https://www.rsp-italy.it/Electronics/Au ... lifier.pdf

https://robrobinette.com/5F6A_Modifications.htm

Big ups modman for your offer on diy world so far.

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ppluis0
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Post by ppluis0 »

sarakisof wrote: 09 Jan 2021, 18:22 I am planning to add a cathode res in first pre tube (pin 3) 1K and 25uF in parallel to gnd and replace the grid 3.3M with 1M resistor. Do i need to also remove 470K and 22nF and add the usual 68K or it is not necessairy?
Hi sarakisoff and welcome to this forum.

The changes you describe are correct.
The resistor in series with the input must be of 39K due old Fender normally has two input jacks (to have two input sensitivities) but when high input is in use the remaining jack short both 68K in parallel.

Another change is to replace the plate resistor of 68K to 100K at the section of the first 12AX7 you intend to modify to work as instrument input.

BTW nice diagram of this unit as the two 12AX7 in the preamp section have their heaters feed with DC to reduce hum. 8)

Cheers,
Jose

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sarakisof
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Post by sarakisof »

Thanks Jose now i see. But i have to remove 470K and 22nF anyway, right? Just a High input signal to grid with the 1M resistor to gnd.
Yeap the 100K plate res mod looks promising too have also heard about that.
BTW nice diagram of this unit as the two 12AX7 in the preamp section have their heaters feed with DC to reduce hum. 8)

Yes it's a great beast with lot of soul in it. Was designed for PA purposes like most Geloso's back then, but with some little mods it can do really well.
Also checked and got measurements for cathode bias, those two old EL34s still run ok. I will leave it as is, got 22.4W and 21.2W dissipation, about 90% and 86%.

:shred:

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Post by ppluis0 »

Hi sarakisof,

Yes, you must remove 470K and 22nF, replace grid resistor from 3.3M to 1M, place a 39K in series between input jack and the first grid, and add the cathode resistor and condenser you mentioned.

This article explain about the first stage and volume potentiometers issue in Fender amps and also apply to your amp:
https://torresengineering.stores.yahoo. ... uitar.html

Regarding EL34's you can use an independent cathode and condenser at each of the output tubes as per the following diagram:
Image
This way you can trim a little each of these resistors to have the same idle current in both tubes.

Cheers,
Jose

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Post by sarakisof »

Great explanation thank you ppluis0 !

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Post by ppluis0 »

sarakisof wrote: 11 Jan 2021, 23:37 Great explanation thank you ppluis0 !
You're welcome !
Good luck with the upgrade of this amplifier. :thumbsup

Cheers,
Jose

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