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FDP Forum / Amp Mods, Repairs, and Projects / Some 63 Super questions.

Roly
Contributing Member
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Whitehorse Canada

I don't get out much
Aug 19th, 2019 09:46 PM   Edit   Profile  

What is the function of the cap that's across the 100k/82k resistors in the phase inverter circuit?
My wee Peak Atlas passive component tester says it's 117.8 pF in circuit.
Should I put 1k5 resistors in line to pin 5 of the power tubes?
Would there be any benefit in replacing V6 with a 12AT7 and if so, should I replace resistors in the negative feedback circuit with stock blackface values.
Thanks folks.
cheers

pdf64

UK

Aug 20th, 2019 10:25 AM   Edit   Profile  

That cap helps to keep the circuit stable.
Kinda instead of the 1k5 power tube control grid stoppers fitted to later amps.
It rolls off and shifts the phase of the high end response.
The capacitance is so low that to get an accurate reading, you'd need to lift a leg.
Whatever, as long as the amp is working ok, I wouldn't mess with it.
A 12AT7 may sound a bit different in V6 so try it if you fancy but that may result in 1k5 grid stoppers needing to be fitted; the 12AT7 plate impedance is a lot lower, so that cap won't act until much higher frequency, perhaps too high to maintain stability.
Don't mess with the feedback resistor values; BF values would probably all but stop the presence control from working.

Roly
Contributing Member
**********
**********
*****

Whitehorse Canada

I don't get out much
Aug 20th, 2019 12:01 PM   Edit   Profile  

Hi Peter

Thanks very much for your help.
cheers

slider313

NC

Aug 23rd, 2019 06:51 AM   Edit   Profile  

I replaced V6 with a 5751 in my brown Super with good results.

FDP Forum / Amp Mods, Repairs, and Projects / Some 63 Super questions.




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