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I wasn't sure if this should be here or in "Theory..." But here goes.
I have one of my custom builds on the bench and it's a Vox/Fender switcher. The Vox configuration uses a straight up classic Top Boost preamp right down to the voltage readings. But I'm seeing something I'm not certain about...
The first stage couples to the second via the stock 500pf/500k/100pf bright cap. No grid stop resistor. At higher pot settings I'm getting asymmetrical cutoff clipping. I didn't think it should be as asymmetrical as I was seeing. I checked DC on the second stage grid and found that it's loading about -1V analogous to clipping. Such that there is no possibility of producing symmetrical clipping even when adjusting the cathode bias resistor value. So this stage clips cold due to grid loading. Fine. But unexpected with a 500pf coupling cap so I'm not sure that this is normal. I know this preamp is notorious for it's unique sound, and the amp does sound like I would expect. I was just wondering if this is "normal" or if I should be looking for a problem?
EDIT: You know what? Never mind. After careful review there's nothing "incorrect" about the circuit as I have it wired. Apparently it just does this. I could put a big grid stopper in there, but that wouldn't be accurate to the original design. Not sure why I've never noticed this action before in Marshall type circuits I've built. Maybe I just never looked for it. So... I hope this observation can help someone else. And thanks for looking.
I have one of my custom builds on the bench and it's a Vox/Fender switcher. The Vox configuration uses a straight up classic Top Boost preamp right down to the voltage readings. But I'm seeing something I'm not certain about...
The first stage couples to the second via the stock 500pf/500k/100pf bright cap. No grid stop resistor. At higher pot settings I'm getting asymmetrical cutoff clipping. I didn't think it should be as asymmetrical as I was seeing. I checked DC on the second stage grid and found that it's loading about -1V analogous to clipping. Such that there is no possibility of producing symmetrical clipping even when adjusting the cathode bias resistor value. So this stage clips cold due to grid loading. Fine. But unexpected with a 500pf coupling cap so I'm not sure that this is normal. I know this preamp is notorious for it's unique sound, and the amp does sound like I would expect. I was just wondering if this is "normal" or if I should be looking for a problem?
EDIT: You know what? Never mind. After careful review there's nothing "incorrect" about the circuit as I have it wired. Apparently it just does this. I could put a big grid stopper in there, but that wouldn't be accurate to the original design. Not sure why I've never noticed this action before in Marshall type circuits I've built. Maybe I just never looked for it. So... I hope this observation can help someone else. And thanks for looking.
Also, this amp has a mid control, so, no good way to wire that up incorrectly also.
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