Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: birt on May 30, 2010, 04:31:17 pm
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i want to use a part of the fender studio bass schematic in another amp.
the schematic: http://www.albertkreuzer.com/pics/el/studiobass/schem/studiobass.gif
i want to use the preamp without the 5-band EX section. so V1B goes directly to V3. problem is that i have to use a 12AU7 triode for V3 and i think it wil lack gain but i'm not sure.
V2 is not a gainstage i think, it looks like it acts as a phase splitter and doesn't add gain. then there's a 47K series resistor and a 2K7 shunt to ground after the EQ. it looks like there is quite some signal loss there. if i omit the EQ and those resistors the signal will be significantly higher right? but is it high enough to get the same preamp output if V3 is a 12AU7 instead?
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V3 is part of the EQ.
Also there is massive attenuation in front of V3: 47K:2.7K.
V1B would be able to drive the Output Level pot directly if it had a cathode bypass.
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thanks PRR, that saves me some chassis space for the V4B and Sunn preamps i'm gonna add :p
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and while drawing my schematic further i have the same question for the V4B preamp
http://www.schematicheaven.com/ampegamps/v4bpreamp.pdf
http://www.schematicheaven.com/ampegamps/v4bpowramp.pdf
there is a triode (V3b) in the poweramp section. this is the higher gain halve of the 12DW7 so the gain is like a 12AX7.
again i can see there is quite some loss of gain at the first halve of that tube (V3a). it doesn't add gain as it is a cathode follower, but there is also some series and shunt resistance.
if i can adjust the gain loss there, could i use a 12AU7 triode instead of the high gain 12DW7 triode? i don't really understand the cathode setup with all those resistors at V3a. are they part of the tone shaping like the part with the mid control in the previous stage? that would mean i have to leave it as is to keep the tonal character of the preamp right?
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I've said you can mix-and-match preamps with power amps.
However you have found the three extremes.
The 1971 Ampegs take a lot of gain, drop down through tone networks, and have a high gain power amp. About 0.2V into the power amp.
That Fender has gain, tone, gain into a fairly low-gain power section. About 1.4V into the power amp.
I don't have your Sunn plan to hand. Early Sunn had high-gain power stages.
This is a major re-design project.
You will be re-discovering many design decisions made by three different designers.
And I'm not sure they came to VERY different results. Aside from the obviously different tone stacks. I applaud skipping Fender's not-graphic 5-band EQ, but the Ampeg has the same thing as a mid-only control, and impemented very complexly.