Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: topbrent on October 16, 2012, 03:25:36 pm
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Question.
What differences would there be in tone/feel/operation in feeding the preamp and cathode follower from a shared B+ Node versus using a separate cap in the B+ rail specifically for Preamp and for the Cathode Follower tonestack?
(Lets ignore the difference in capacitance from the JTM45 to the later Marshall circuits for this discussion. Lets pretend both amps are using the same value caps in the b+ rail.)
For example, the 5F6A bassman & Marshall JTM45 used a shared B+ node to feed both the preamp and cathode follower tonestack whereas all of the later Marshall iterations of the circuit, ie 1986,1959/1987, 2203 had a separate cap feeding those circuits.
Discuss.
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Schematics are much more nice to determine what is going on than layout drawings.
The reason for the change is probably to make the amp more stable, decouple the stages. Doubt there is much tonal change.
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The difference was probably to trim the B+ voltage down for the earlier preamp stages.
Look *very* closely at both layouts you posted. The JTM45 has a single cap feed the cathode follower and the preamp stage immediately before it, but also the two earlier preamp stages. The later Plexi circuit has the preamp/cathode follower stage fed by a cap, with a dropping resistor and another cap feeding the two input preamp stages.
"Feel" change is probably minor. The real change, if any, is likely due to higher B+ voltage for the output stage and phase inverter, with the corresponding need to drop the voltage more as it works its way back over to the first preamp stages.