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Dual-Drive designs
The Marshall 1987x and a host of other amps (including the Trainer YBA1, Viva Canada!) have dual inputs, each with its own volume. The parallel inputs come together by way of equal-value resistors (470k in the Marshalls; 100k in the YBA1), often with one of the resistors bypassed with a cap. Many people mod the YBA by swapping out the 100k resistors with 470k resistors in order to 'plexi-fy' the Bass Master. I have several questions about these resistors:

1. Why are they there? If they act as grid stoppers, why not have just one?
2. In what way does their value matter?
3. If bypassed with a cap, as on the brighter input of the 1987x, does this form a bright boost or cut? Initially, I assumed that it was a boost, but now I'm thinking that it interacts with the tube's miller capacitance to form a cut. 

Thanks in advance for the edification... Smile

Messages In This Thread
Dual-Drive designs - by jmcd - 10-31-2018, 01:23 AM
RE: Dual-Drive designs - by K O'Connor - 10-31-2018, 11:42 AM
RE: Dual-Drive designs - by jmcd - 10-31-2018, 12:22 PM

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