Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: skeezbo on February 17, 2013, 09:17:14 pm
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Daydreaming about amp design... Would it make any difference to a Marshall style cathode follower tone stack if the triode had a plate resistor of equal value to the cathode resistor? I would like to be able to take an inverted signal off the plate of the trode currently set up to drive the tone stack for a KOC inspired side chain loop. This would essentially turn the cathode follower triode into a split load inverter. Would taking a signal off the plate cathodyne style affect the original signal on the cathode follower tone stack? Would I need to add a coupling cap before the split load?
Skeez
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Would it make any difference to a Marshall style cathode follower tone stack if the triode had a plate resistor of equal value to the cathode resistor?
no
Would taking a signal off the plate cathodyne style affect the original signal on the cathode follower tone stack?
no
Would I need to add a coupling cap before the split load?
What do you mean? The only cap you will need to add is a coupling cap from the plate of the tube. If your plate resistor is the same size as your cathode resistor the plate signal will have no gain. If you need a little gain you can use a larger plate resistor.
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Steve
Thanks so much for the quick response. I thought it would work, but needed some expert advice. My question about the coupling cap was based on the fact that the cathodyne inverter schematic I was looking at had one between the two stages, but the cathode follower looked to be direct coupled.
I know it won't have gain, but it may seem to have more gain compared to the cathode follower side with the tone stack loss. I am thinking of a two stage reverb loop with tone and dwell that will get re-inverted when it re-enters Matchless Chieftain style at the long tail pair phase inverter.
Thanks again,
Skeezbop
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Think of the typical 12AX7 plate load, 100k.
In a cathode follower, that same load is slid through the tube to the cathode side, 100k load. But this causes degeneration, 100% feedback and gain less than 1. Load line is still the same, but you have to do extra thinking to see what the gridlines really represent.
In a split load, the 100k load didn't slide all the way to the bottom of the cathode; half below, half up top. That should be a 50k resistor on either side, but 56k is a good compromise in a standard part value.
So 100k on top and bottom would be the loadline for a 200k plate load. Consider this, and whether it will impact your situation (it may not).
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Matching 56k plate and cathode resistors in a split load will create a similar load line for the cathode signal when compared to a 100k cathode resistor in a cathode follower setup.
Got it, thanks again!
Skeez