Thread: Fisher 30-A amp
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Eike Lantzsch, ZP6CGE Eike Lantzsch, ZP6CGE is offline
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Default Fisher 30-A amp

Eike Lantzsch, ZP6CGE wrote:
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, also no balance
circuit for the phase inverter.

When the EL84 start pulling grid current the difference of
impedance of the cathode side and the anode side of the phase
splitter comes into play. To balance the grid current the anode
side grid resistor is of lower value than the cathode side
resistor. That is careful design. The engineers probably found
out that, for consumer grade equipment, the distortion due to this
effect are negligable compared to the the distortion due to
overload and canceled the resistors to save some cents.
Kind regards, Eike


not good etiquette to answer to ones own posting ...
anyway
in the 70ies, when still an adolecent, I built an amplifier of
very similar design with ECC83 and EL84. I also put the two
unequal grid resistors in. Only difference with my amp was that
each EL84 has its own cathode resistor and cap.
This amplifier still does service here after almost 50 years but
after 2x restoration of course.
Kind regards, Eike