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Had a 12AQ5 P-P amp with mosfet PI (PI with help from you guys) that I put in a small chassis and was planning on putting it in a narrow panel Tweed Champ cabinet. Had it with volume and tone control controls along with a switch that I used to change the NFB. Eventually I tried adding a bass control that could be switched in that was to replace the NFB switch.
Found it did not do too much, mind you both the tone and bass control should have entered the top of the volume control, I had the tone (treble) still hooked up as the treble bypass Tweed style. So it got me thinking, convoluted path to the following schematic. The first stage using the LND150 or 2N5485 with the first stage of a BSIAB. I have not decided which way to go, going to have to breadboard both as the two drawings I pulled together from posts on the web.
Have some numbers that are a little useful (to me at least). Looked up some Fender schematics and came up with the gain of a 57 Bassman RI sections using the Amp's service manual voltages. Input is voltage gain of 50, second stage 41, the tone stack -0.6. The PI has a gain of >1 and the 12AQ5's need 15V for full output. First stage should have more than enough gain with the second stage biased at 1.25V. The second stage gain of 41x being hit with a 1V signal should put out 40V. Cut that in half through the tone stack and you have 20V to the 12AQ5's.
So depending on tone stack positions the second stage should start to clip around roughly when the outputs do. Have the amp running on 240V right now (Tweed tone controls separating two triodes) but am thinking of going with a voltage doubler for roughly 300V with the transformer being rewired. Have a 70V transformer for the output, if I am lucky I would guess 7W out of it. Just want a small practice amp out of this one and using it to try out something different.
Found it did not do too much, mind you both the tone and bass control should have entered the top of the volume control, I had the tone (treble) still hooked up as the treble bypass Tweed style. So it got me thinking, convoluted path to the following schematic. The first stage using the LND150 or 2N5485 with the first stage of a BSIAB. I have not decided which way to go, going to have to breadboard both as the two drawings I pulled together from posts on the web.
Have some numbers that are a little useful (to me at least). Looked up some Fender schematics and came up with the gain of a 57 Bassman RI sections using the Amp's service manual voltages. Input is voltage gain of 50, second stage 41, the tone stack -0.6. The PI has a gain of >1 and the 12AQ5's need 15V for full output. First stage should have more than enough gain with the second stage biased at 1.25V. The second stage gain of 41x being hit with a 1V signal should put out 40V. Cut that in half through the tone stack and you have 20V to the 12AQ5's.
So depending on tone stack positions the second stage should start to clip around roughly when the outputs do. Have the amp running on 240V right now (Tweed tone controls separating two triodes) but am thinking of going with a voltage doubler for roughly 300V with the transformer being rewired. Have a 70V transformer for the output, if I am lucky I would guess 7W out of it. Just want a small practice amp out of this one and using it to try out something different.
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