Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tristanc on December 09, 2024, 11:30:07 am
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I'd like to increase the attenuation between two gain stages where a bridged-T mid-cut filter is being used.
Should I replace the proceeding 1Meg resistor to ground with a pot (A in the attached) wired as a resistive divider, or put a resistive divider after the filter (B) or do something different?
Simulating A gives some high end roll-off as the attenuation is increased, presumably due to loading?
Thanks for any tips.
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Here's the 1st version I tested with the 1Meg turned in to a pot divider.
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Too little information.
What amp is this from?
Is it a real Gibson or is it a clone, or just something you built? (I said Gibson because they used bridged T filters.)
Many guys take those T filters out of Gibson's, lot of insertion loss with those. But you want more attenuation.
Taking out C13/16 will get less gain.
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Here's the 1st version I tested with the 1Meg turned in to a pot divider.
You could tack in that 1M pot, a L taper pot might be easier to use for this. Tack in the pot, only use the wiper and 1 end of the pot, play with the setting until you find what you like, that is if it works for you, then carefully measure the resistance. Then put in the closest R value to the pot measurement. Done.
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One I’ve built based on Merlin’s High Gain preamp. What (I think) I’m finding is the 3rd stage grid is getting hit too hard even at low settings of the gain control.
I’ve the same preamp with the filter replaced with a ~800k/200k divider that sounds excellent. So my idea was to attenuate slightly _and_ keep the filter.
Schematic here with the 1st attempt here:
https://github.com/tristancollins/Guitar-JamNight/blob/main/Guitar-JamNight.pdf
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Disconnecting the cathode bypass cap (C13) will remove gain and improve the tonal response. YMMV
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Disconnecting the cathode bypass cap (C13)
:laugh:
what I was thinking earlier, I figured I'd them the OP do the complicated 1st :icon_biggrin:
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OK, so I'm hearing there are simpler ways to reduce gain hitting the next stage. But how is this different to a TMB and volume control like in Fender preamps? This is where my knowledge is lacking: how to do this without the divider and filter interacting with themselves and with the surrounding stages.
As I said, I have another preamp with the same circuit (similar voltages) and the mid filter is replaced by a divider that drops quite a bit of signal compared to a typical 470k/470k. I'd like to have (more or less) that situation with the mid control on full, with the cut then doing its thing as the control is turned down.
Maybe I'm barking up the wrong tree.
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If the gain pot is before the tone stack (as you have drawn), then the tone stack will see a wildly changing source impedance depending on the rotation of the gain pot. This will radically mess with the tone control response.
But if the gain pot is after the tone stack, then the tone stack sees no change in source or load impedance regardless of the gain pot rotation, so the tone response is not affected.
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If the gain pot is before the tone stack (as you have drawn), then the tone stack will see a wildly changing source impedance depending on the rotation of the gain pot. This will radically mess with the tone control response.
But if the gain pot is after the tone stack, then the tone stack sees no change in source or load impedance regardless of the gain pot rotation, so the tone response is not affected.
Thanks Merlin - makes total sense. I'll try that.
Incidentally, did you ever experiment with the tone stack and move away from the Bandmaster? Anecdotally, the Bone Ray stuck on the end was a winner with other guitarists. I moved to the Bandmaster (and then the mid cut) and don't get quite the same glowing response.
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Incidentally, did you ever experiment with the tone stack and move away from the Bandmaster?
No, that circuit got recycled. More than once, ahem...
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I swore I wouldn't spend time tweaking. Alas, here I am again.... Just play the damn thing! :BangHead:
Attached is a bode plot at the grid of the following stage - with zero and 50% attenuation and the mid control at 1 & 0. Shows what I was after - roughly same response but a bit quieter.
I've quickly made the change (a trim pot between mid cut output and 100k grid stopper) and am using the amp in 40mins time in a pub jam night. Fingers crossed it still works!