Fender brown preamps

[Paul P]

The preamp circuits in my amp are from the brown Fender Bandmaster,
6G7-A. The two channels are identical except for the output of the
vibrato channel preamp which looks like :



Now that I have both preamps working in my amp I can say that the
vibrato channel is not anywhere near as loud as the normal channel.
I suppose the following vibrato circuit, which I haven't installed yet,
needs a lower input than what would come out of the normal preamp.

Am I right in calculating that the voltage divider provides only 10% of
the normal signal to the coupling capacitor ?

What is the purpose of C1, the .003uf capacitor ?

Paul P

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[Bruce / Mission Amps]

C1 is a high frequency snubber... it rolls a lot of the high freq treble off by sending the highs right across the plate load resistance to the B+ filter cap, which is a very low impedance (AC) to ground.
Yes, the two resistors are a simple AC voltage divider used like that.
The split resistor anode load is not really uncommon and it is a way to reduce a gain stages total AC voltage output while maintaining the actual plate load resistance.
This works quite well and I have used the method many times with excellent results when tweaking a gain stage.

[Paul P]

This capacitor + voltage divider also looks to convert sine waves into
triangle waves, the more so the higher the frequency. Here are the
signals before and after this bit of circuit :




Since the output of the vibrato channel is 2.5 times less than the normal
channel I'm going to try 30K and 82K resistors in this divider and see what
happens.

Paul P

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