Gibson GA-8

[Leevi]

I'm wondering the missing grid stopper from the first power tube. The second tube has 470R. Is there some idea or is it just a fault in the schematic?


Furthermore I thought to replace the 500R voltage drop resistor with a choke.



/Leevi

[shooter]

can't recall if there was an error, The couple I worked on matched up like the schematic though


as to the 500R vs choke;
If you ask most players that have used that amp, it has mojo, NOBODY ever wanted me to do anything but the minimum required to fix it, even then they were afraid i'd mess up the mojo   

[Latole]

Schematic is all wrong

[shooter]

wrong version, that's the PP, NOT parallel single-ended

[Latole]

wrong version, that's the PP, NOT parallel single-ended

There are many GA-8 

[Latole]

Gibson and Epiphone owner manuals and schematics:

https://jt30.com/gibson/index.html

https://jt30.com/gibson/pg_0106.jpg

[Leevi]

There are many GA-8

Yes there are. This model is 1959 GA-8 Gibsonette. The first GA-8 models are from late 40's. There are also models from early 60's


/Leevi

[Leevi]

If you ask most players that have used that amp, it has mojo, NOBODY ever wanted me to do anything but the minimum required to fix it, even then they were afraid i'd mess up the mojo

I tested it with a choke and the ground hum level is lower with it. Furthermore the choke drops the voltage less. 5Y3 is weak to keep the needed voltage and the tubes operate with low dissipation.

And I don't like hot resistors.

/Leevi

[PRR]

> missing grid stopper from the first power tube

You sometimes have to isolate the two grids from each other. That does not always mean resistors everywhere.

I would not mess. It has the feel of try-and-see, that someone came to this plan after some testing (for minimal cost).

Why would you remove the choke? 10 Watts is not enough but 11 Watts is fully satisfying? Or your arms need the work-out? If you truly wanted More Power and Less Hum, you would do push-pull. The OT might even be cheaper, but the PI driver is a buck more.

[Leevi]

I ended up to replace the 500R with a choke, otherwise I kept the circuit as it was planned.The 5Y3 rectifier can be replaced either with GZ34 or SS-rectifier plug and no bias change is needed.The SS rectifier increases the B+ by 55V.
/Leevi