Metropoulos Forum

Questions
1. Lowering the value of V1A cathode capacitor does what exactly?
2. What is the lowest voltage rating you can go on 10uf bias capacitors?
3. Lowering the value on the tone stack treble capacitor does what?
4. Lowering the value of the bright cap does what?

Thanks gents I appreciate your knowledge!

Guns4hire03 wrote: ↑

Wed May 09, 2018 5:51 pm

Questions
1. Lowering the value of V1A cathode capacitor does what exactly?
2. What is the lowest voltage rating you can go on 10uf bias capacitors?
3. Lowering the value on the tone stack treble capacitor does what?
4. Lowering the value of the bright cap does what?

Thanks gents I appreciate your knowledge!

1). The V1a cathode resistor sets the bias point of that triode, for a 12AX7/ECC83 the 820R is a pretty hot bias and 10k is cold. Putting a capacitor fine tunes the effect of that resistor by removing certain frequencies of negative feedback and will increase the gain of the stage, a smaller value (e.g. 0.68uF used on split cathode Marshalls gives an upper mid boost, whereas a big capacitor e.g. 250uF passes most frequencies with very little selection of focus of the gain.
To look at other values take a peek at the on-line Duncan tone stack calculator.

2). I usually use 200v bias caps as they are a 'vintage' size an span turrets neatly but 150v is ok.

3). The tone stack treble capacitor of 250pF gives a mids focus and 500pF more of an upper focus, but again in conjunction with the slope resistor.

4). Bright cap values of 100pF gives a slight upper frequency boost at low pot settings but is out of the equation later, a really big cap 5,000pF boosts most frequencies so the gain comes in with a bang. I find 500pF the spikiest value, YMMV.

http://www.duncanamps.com/tsc/

So lowering the V1A cathode capacitor value focuses the the gain? Thanks again for your response.