New life for an old Geloso amplifier

[IvanMan]

Hello guys.
I recently found this old (around 1960-65) Geloso 100w push-pull amplifier and had the insane idea to transform it to a guitar amplifier.
Basically I'd like to mantain the power part and rebuild the pre using a Marshall schematic (I'm a Marshall fan and love the sound of the Silver Jubilee series).
What do you think about? Suggestion, directions, but also slating are welcome

[Kagliostro]

Give a look here

https://el34world.com/Forum/index.php?t ... #msg163099 <-- read here





Ciao

Franco

[trobbins]

Racing on vintageamps.com/plexiboard/ did a lot of Geloso transformations - maybe the wayback machine is a way to catch that info.

How technical are you - there is a lot that could be done with that type of restoration, especially related to the powering and biasing, and working through all the design aspects, such as output transformer windings.

[IvanMan]

Racing on vintageamps.com/plexiboard/ did a lot of Geloso transformations - maybe the wayback machine is a way to catch that info.

How technical are you - there is a lot that could be done with that type of restoration, especially related to the powering and biasing, and working through all the design aspects, such as output transformer windings.

Hello trobbins, and sorry for my very late answer.
I dedicated this months to the renovation of a JCM800 clone, with a lot of issues. but at the end it's roaring again!

Now I go back to this Geloso project and looking better inside the amp I realized that 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 the idea is to realize a pre based on the Marshall Silver Jubilee (I love Marshall sound, in attachment the schematic), and mantain the power stage based on the 807.

Marshall_25aniv_silverjubilee_50_100w_2555.pdf

In you opinion:
1) what have I to do to adapt the output of the Silver Jubilee phase inverter to the input of the original Geloso power stage, based on 807 tubes? Do you think is better the substitute the 12AX7 Phase Inverter of Silver Jubille with a 12AT7 and follow the Geloso schematic above?
2) what about the Geloso power stage, to be transformed from linear to distorted stage?
3) what about fixed biasing of 2)?

I also 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).

OT.pdf

The reflected impedence seems low for the 807, with the exception of 7000 Ohm for one windings. May be the right value for the 807 in class AB2?

Thanks in advance for your help.

[IvanMan]

Anyone can help?
In particular I'm in difficulty to confirm if the output signal of the 12AX7 (or the 12AT7) Phase Inverter, considered in the Marshall Silver Jubilee implementation, can feed correctly the pair of 807 (in the Geloso implementation)?
Or what kind of modification are needed.
Thanks.

[trobbins]

Given that you are not really using a Geloso circuit any more (ie. only just using the valves and magnetics as parts), if you can't identify one complete amplifier circuit to clone then you may face many design issues trying to graft half of one design to half of another. Grafting can be a great learning curve if you have the time and inclination to appreciate the issues at play, and have the bench facilities to confirm and test that everything is ok.

I would recommend you try and identify the closest possible single amp schematic(s) that can fit in to your Geloso chassis (ie. using existing valves bases, and panel controls, and have transformers with approximately the same heater and HT ratings), and then modify a schematic to identify all the parts that you may want to alter, or have at hand to use, and try and identify what each altered part would do to related voltage and current levels.

If there are no identifying marks on the OT, then you are flying a bit blind. The turns ratios are one aspect, but the core and lamination size, and wire sizes, and interleaving, and dc resistances, and primary inductance, are ways to generalise whether the OT can manage the application you are after - just because the replacement part was in the amp doesn't mean it is a suitable replacement part. Of course you can just suck-it-and-see. For hi-fi use, the 807 datasheets indicate the P-P impedance range for B+ range that could get you certain power levels. If the aim is a guitar amp, then you can go markedly away from the target PP impedance and enjoy the difference. For trying to reverse engineer an OT, I usually choose a likely PP impedance, and determine what the secondary winding impedances match to, and see if any of those values make sense in a vintage application sense. So for a 10k PP, your secondaries are nominally 175, 90, 45 and 23 ohm. PA amp OT's are often designed to achieve certain speaker line voltage levels, but that assessment also requires an educated guess at the PP plate voltage swing. For example a 100Vrms speaker line output in to 175 ohm is about 60W output rating, and would need a (7.51/2) x 140Vpk = 500Vpk half-primary swing, which could be achievable with 600V B+.

But more importantly, the OT inter-winding secondary taps can give you 4, 7, and 15 ohm. The linked article has a spreadsheet link in it for the calculator I use:
https://www.dalmura.com.au/static/Renov ... 20amps.pdf

Ciao, Tim