Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Imntgrumpy on April 02, 2017, 02:23:19 pm
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OK,
It's been awhile since I've been on the board. Life goes on. It's time to re-cap my Princeton Clone and I've just noticed a difference between the aa1164 and B1270 Schematics that has me wondering, "What is the purpose of the 2000pf Cap located in parallel at pin2 V3B on the B1270 Schematic?"
It appears to be the only difference between the aa164 and b1270 designs.
Jim
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Cut some highs? Can't think of what else it could be there for...
Around that time Fender was adding small caps here and there. The internet lore is that they were having problems with oscillations due to poor lead dress after Leo sold the company to CBS and the caps (in general, not necessarily this particular one) were added to reduce the oscillations. I don't know if that's true or not; it makes sense, which makes it exactly the kind of wild guess that people believe to be truth.
But I think all of that is moot in this case as I can't imagine what oscillation that 2000pF cap would quell.
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"What is the purpose of the 2000pf Cap located in parallel at pin2 V3B on the B1270 Schematic?" ...
Make the reverb a little less-bright.
Pin 2 V3B is the grid of the reverb recovery stage. The 2000pF cap to ground shunts some of the highs coming back from the reverb pan.
I suppose there's an outside chance it is also there to reduce hiss in the amp. Most of the hiss in any blackface Fender with reverb is in the Reverb circuit. Turn the Reverb pot off, and the noise goes away. Cutting highs at the recovery stage grid might be a play to reduce the noise added by the reverb circuit.
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"What is the purpose of the 2000pf Cap located in parallel at pin2 V3B on the B1270 Schematic?"
The reverb tank output coil and the 2000pf cap. have a resonant frequency. At this resonant frequency, the output coil puts out a peak signal. The signal drops off on both sides of this peak like a parametric equalizer. In the case, the resonant frequency is around 6KHz, so it is like having the equalizer set to 6KHz with the gain set to maximum and the Q value set high. This gives the reverb a brighter sound. Noticeably brighter.
Whaddaya think?
Stability?
Duller?
Hiss Reduction?
Brighter?
None of the Above?