Geloso 272A & Marshall Silver Jubilee pre - a diabolic mixture?

[IvanMan]

Hello everybody.
I recently bought one old Geloso 272A amplifier (originally a PA, attached the schematic).
Internally I've to replace completely all the components (they are in a very poor state), the boards and the cabling. At the end I save the transformers, choke, tubes and probably the pots.
So I'd like to transform it in a guitar amp, with the pre part and the phase inverter of a Marshall SJ Silver Jubilee (attached the schematic).
So a few questions to you gurus to start the project:
1) what have I to do to adapt the output of the SJ phase inverter to the input of the original Geloso power stage, based on 807 tubes, eliminating all of existing Geloso pre and phase inverter up to the 12BH7 included?
2) what about the Geloso power stage, to be transformed from linear to distorted stage?
3) what about fixed biasing of 2)?
4) what about the OT primary impedance in the new configuration?

Any comment (also worst) will be well accepted, also about the whole project itself May be also suggestion of already realized project with similar tubes (I know the 807 are quite similar to 6L6).
Thanks in advance to all contributors.

[PRR]

The Geloso's PT and OT were designed to work together with the somewhat extreme way the 807s are run: moderate Vg2, high Vp, and very significant grid current. You want the Geloso's 12AT7 12BH7 driver, or a complete re-design of the power section.

The Geloso power stage WILL get "dirty": play louder!

Because the Marshall has a whole different approach to gain-staging than an affordable PA amp, the input sections will need re-thinking. You could just build the entire Marshall preamp and then pad-down for the Geloso's power stage input. I don't think all this will fit on the chassis.

Too late now, but I think you could have just fixed what was actually sick and played guitar through it.

[IvanMan]

The Geloso's PT and OT were designed to work together with the somewhat extreme way the 807s are run: moderate Vg2, high Vp, and very significant grid current. You want the Geloso's 12AT7 12BH7 driver, or a complete re-design of the power section.

The Geloso power stage WILL get "dirty": play louder!

Because the Marshall has a whole different approach to gain-staging than an affordable PA amp, the input sections will need re-thinking. You could just build the entire Marshall preamp and then pad-down for the Geloso's power stage input. I don't think all this will fit on the chassis.

Too late now, but I think you could have just fixed what was actually sick and played guitar through it.

In any case I have to rebuild anything, saving only /if I'm lucky) tubes, transformers, choke, pots and the chassis. The other part are completely unuseful and rebuild the Goloso in the original manner is not what I need as a kind of amp.
Have you a schematic example that can help me in the right direction (to interface the Marshall preamp with the Geloso's power stage). The chassis is quite large and will not be a problem.

[DummyLoad]

The Geloso's PT and OT were designed to work together with the somewhat extreme way the 807s are run: moderate Vg2, high Vp, and very significant grid current. You want the Geloso's 12AT7 12BH7 driver, or a complete re-design of the power section.

The Geloso power stage WILL get "dirty": play louder!

Because the Marshall has a whole different approach to gain-staging than an affordable PA amp, the input sections will need re-thinking. You could just build the entire Marshall preamp and then pad-down for the Geloso's power stage input. I don't think all this will fit on the chassis.

Too late now, but I think you could have just fixed what was actually sick and played guitar through it.

output power rating also suggests they run the 807 pair in ICAS AB2.  sorry - reread your post - you already implied that. doh! 

1.25ohm - 500ohm out Z select + a 70V out. you may need the manual to strap secondaries proper.

click me for full specs..

30-15KHz - +/- 3dB. not bad - bass anp? leave it as is with minor mods? james stack is nice - i'm thinking ampeg-ish bass/guitar amp?


--pete

[DummyLoad]

assuming you have 120V mains: strap power transformer primary for 100V - line to 120V tap and neutral to -20V tap - this will bump up 267V secondary to give about 440V B+ unloaded. if you have 220V mains there are other combinations of connections for the primary you can make to get B+ up to about 450V using one of the secondaries.

run a pair of KT88 in fixed bias with 1/2 load indicated (e.g., 4 ohm spk. load on 8 ohm tap etc.) on OT secondary - this will give about 3.5K reflected Z with the stock OT.

replace the filament transformer with a 5A 6.3VCT hammond or similar.

use the 206V PT secondary for the bias power supply.

should make about 60W.

if it were mine, i'd run with the 807s with the PS / output as designed and not gig with it. 

[DummyLoad]

university on holiday + i got bored... see attached.

simpler conversion and idf it were mine, probably the route id take only IF i had to gut it entirely. left out the filament transformer circuit so just copy that. i'd also ditch the standby switch. DC filaments is a + for V1-3 so keep -36V supply.
 
run with NOS 12BH7.


--pete

[IvanMan]

university on holiday + i got bored... see attached.

simpler conversion and idf it were mine, probably the route id take only IF i had to gut it entirely. left out the filament transformer circuit so just copy that. i'd also ditch the standby switch. DC filaments is a + for V1-3 so keep -36V supply.
 
run with NOS 12BH7.


--pete

Hello DummyLoad.
I'd like to conserve the 803 too. What do you think about the attached schematic.
I can realize the Marshall SJ pre but substitute the 12AX7 PI with a 12AT7.

In addition I realize that the OT inside is not the original one but a replacement with different characteristics.
In attachment the calculation for the reflected impedance (I hope there are right).
The reflected impedence seems low for the 807, with the exception of the 16 Ohm charge on a couple of wire (7000 Ohm).
May be the right value for the 807 in class AB2?

[IvanMan]

university on holiday + i got bored... see attached.

simpler conversion and idf it were mine, probably the route id take only IF i had to gut it entirely. left out the filament transformer circuit so just copy that. i'd also ditch the standby switch. DC filaments is a + for V1-3 so keep -36V supply.
 
run with NOS 12BH7.


--pete

Hi. Which software can I use to open the .sch files?

[shooter]

http://el34world.com/Forum/index.php?topic=590.0

try there

[sluckey]

Hi. Which software can I use to open the .sch files?

ExpressSCH or JSchem