Thread: Fisher 30-A amp
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Big Bad Bob Big Bad Bob is offline
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Default Fisher 30-A amp

On 2020-03-01 00:56, G.Re wrote:
Hi folks,

What is the reason the Fisher 30-A uses (very) different valued grid
resistors for the power tubes?
- R9 = 2K2 for V2
- R4 = 33K for V3
https://www.flickr.com/photos/derekv...n/photostream/

A later (or earlier?) 30-A version has no grid resistors,Â* also no
balance circuit for the phase inverter.


screen grid?

probably did a 'cost reduction' version, and different resistor values
(in your case) as a hard-set "balance" of sort.

Also see if the output transformer is actually driving the screens in
'ultra-linear' mode or not. Makes all the difference in the world for
screen circuit if it's ultra-linear... which may have NO screen
resistors - lots of things dependent on one another in design, screen
current limit, screen volt limit, whether or not it's ultra-linear and
where the taps are, supply voltage, bias, yotta yotta.


--
(aka 'Bombastic Bob' in case you wondered)

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