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Geloso 272A & Marshall Silver Jubilee pre
Geloso 272A & Marshall Silver Jubilee pre
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Old 21st November 2018, 12:55 PM   #1
SimoS is offline SimoS
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Default Geloso 272A & Marshall Silver Jubilee pre

Hello everybody.
I recently bought one old Geloso 272A amplifier (originally a PA, attached the schematic).
g272a.jpg
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 trasformers, 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).
Marshall_25aniv_silverjubilee_50_100w_2555.pdf
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.

Last edited by SimoS; 21st November 2018 at 01:27 PM.
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Old 3rd February 2019, 03:12 AM   #2
OrbitalLex is offline OrbitalLex  United States
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Pins 3 & 8 of the 12bh7 is the signal heading into the power amp stage. I would leave everything there as is. Tie your presence controls for sj the same as the geloso.

I would keep every thing on the sj from c30/c31 forward geloso and see how you feel about tone before going any further.
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Old 3rd February 2019, 07:09 PM   #3
SimoS is offline SimoS
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Hello OrbitalLex.
Some weeks are passed from my post and I moved a little differently.
After c30/c31 of sj (after the PI) I go directly into the 807 (I find a schematic, attached, of a different model of Geloso that follow more or less this approach).
G-261.jpg
It was also a matter of available space. The SJ is quite complex and requires space that was not available. I moved the tone stack near the pots and cabled on-air.
I near to complete all the cabling, only the output jacks and impedance selection missing, and the commands for the lead channel and double master volume (I included this mod).
20190203_204028.jpg
Next week, before cabling the outputs, I'd switch on the amp without the tubes and measure all the voltages to verify how the 807 should work.
I've also in mind to use differently the 807 to reduce the output power (now the declared value is 100W).
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Old 3rd February 2019, 11:59 PM   #4
OrbitalLex is offline OrbitalLex  United States
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Good progress!
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Old 16th February 2019, 12:02 PM   #5
SimoS is offline SimoS
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First request for help.
Completed mounting and cabling and it works! This time a strange hybrid: the pre until the PI is a Marshall Silver Jubelee 2550, the final stage excluding the Bias is the original Geloso G272. To be my second amp I consider it almost a success.
I wonder to hear your opinion on a couple of things:
1) One of the 807 has a red plating at medium-low volume after a few minutes of sound, while raising the volume even at max everything returns to normal. The redness follows the valve so I guess it may be a problem of his own. Tips? I installed separate Bias trimmers for the two valves so I could also make different adjustments. Currently the Bias Voltage is at -30Vdc as per specification (I think).
2) The volume is really high while at the lower level there is no distortion at end stage. Do you have any suggestions on how to use the 807 in a different configuration (keeping the trafo) to have lower power and a more pronounced distortion. Thanks in advance.
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Old 16th February 2019, 04:51 PM   #6
OrbitalLex is offline OrbitalLex  United States
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1. If the RP follows the tube, toss the tube.

2. Experiment. Play with negative feedback adjustments. Change to true PPIMV master volume. Rewire sockets for 6L6(electrically identical to 807), KT66 or EL34. Convert power supply to a tube rectifier or reduce the stiffness with sag mods. Study power amp stages like the JTM45 for example and compare to your circuit.

I would be inclined to think most improvement in this department would be found in power supply mods. There is a lot of dynamic tone being lost in a p/s as stiff as yours. I would definitely be interested in doing some work on that if it were mine. My best advice is to experiment.

Guys will come and here and tell you exactly what to do and nothing else will work, when in reality there is no shortage of things to try that may achieve what you are looking for.
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Old 18th February 2019, 12:05 AM   #7
thoglette is online now thoglette  Australia
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Quote:
Originally Posted by OrbitalLex View Post
Rewire sockets for 6L6(electrically identical to 807), KT66 or EL34.
Can't do that Dave. (The 807 has a 5 pin socket, not octal)

807s are common and cheap. But check the bias and current (it's possibly, no, likely, that the valves are a long way from matched)

Then do some homework (e.g. read what Rob Robinette has to say about how amps work).
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Old 18th February 2019, 12:11 AM   #8
OrbitalLex is offline OrbitalLex  United States
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True...I misspoke. You want flexibility, nix the 807 5 pin sockets. They are $1 each.
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