Thread: Fisher 30-A amp
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G.Re G.Re is offline
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Default Fisher 30-A amp



"Eike Lantzsch, ZP6CGE" wrote in message
...

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

* Thanks Eike, that makes sense.
I had a closer look at both the early & late version schematics.
The earlier version is dated 1958-xx, it shows no EL84 control grid
resistors nor balance pot.
The later version is dated 1959-10, it shows (different) control grid
resistors plus balance pot.
Both mentioned dates are the documention printer's (H.W. Sams) date.
Thats no proof of Fisher's 30-A versions issue order but it's likely
documentation issue and 30-A issue are in the same order.
So, probably the change was to gain amplifier performance.
Best regards, Gio.