Fender Style Reverb Questions

[Alexo]

An interesting point is that the Fender brown Vibroverb had spectacular reverb that went on forever, and this was due to different circuitry - a fully cathode bypassed 12AX7 driver, although the output blending resistors are both 470k.

That amp has an interesting circuit, sort of a "what if" Fender... as in "What if the 12AT7 or LED had never been invented?" "What if tapped pots weren't impossible to find?"

The Matchless Chieftain also uses an AX7 as a driver tube. The 12AX7 does not have the (relatively) high wattage output of a high current tube like a 12AT7 in this application, but just like striking a guitar string, the lighter you hit a reverb tank, the more sustain you get, and the less "boing." The downside is you need more makeup gain, and with it comes noise.

[orrong65]

Yes, you are right Bob-I.

Fiddling with the channel coupling resistor is probably not the best thing to do, except that some models like the Fender 30 have a pull-on boost that drops this resistor value to increase the gain. The channel is coupled with the 3.3m/10pf pair, and the boost effectively drops the 3.3m resistor to 2.2m.

ON the AB763 I would more likely reduce the value of the 470k resistor on the reverb pot wiper to increase the reverb return signal. I reduced this to 220k on one amp to get the reverb intensity up.