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Equalization in 1950s Era Amplifiers

ew, feedback loop tone controls. Mind linking to the schematic you're using? I'm finding several different ones here.

still, off-value parts in the tone control circuits will screw things up. Pots tend to be bad about drifting and that skews the response of the controls considerably.

Output transformers as a rule haven't gotten better, but thats not to say all old ones are perfect either. I will tell you that if you do a transformer swap in a feedback amp its pretty much guaranteed to perform differently. Whether its good or bad depends on the specifics but its going to need the feedback and stability evaluated and very likely re-tuned. The supersonic characteristics of every transformer is different and its easy to have something go off the rails with improper tuning. Tone controls in the loop make this all vastly more annoying. A power amp designed for flat response is a lot easier, stick a bunch of caps and resistors in there which screw up the phase response and its very much a gamble.
 
gadget73 said:
Mind linking to the schematic you're using? I'm finding several different ones here.
Click to expand...
I believe this one, from a 10 year old thread here, may be the correct one. The other one I have, with EL84 outputs, also has a number of resistor values that differ from this one.
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ok so yeah again ew on feedback loop tone controls. I was figuring this was a more typical integrated with a line stage that involved tone controls upstream of the power amp.

some HF roll-off would be explained by that 1M volume control. It would go away at full up but at partial travel you get some Miller effect causing roll-off. Could probably experiment with small pf value caps from the top of the pot to the wiper or maybe even the loudness tap to compensate that. It would never be perfect but it might be tunable to be less-bad. If the tone pots have drifted significantly that will also skew this.
 
gadget73 said:
some HF roll-off would be explained by that 1M volume control. It would go away at full up but at partial travel you get some Miller effect causing roll-off. Could probably experiment with small pf value caps from the top of the pot to the wiper or maybe even the loudness tap to compensate that. It would never be perfect but it might be tunable to be less-bad. If the tone pots have drifted significantly that will also skew this.
Click to expand...
Thanks for pointing that out. I have entered the main amplifier portion of the schematic into MicroCap and simulated it. Sure enough the top end of the frequency response does change depending on the volume control setting. This is not exactly surprising due to the unconventional use of the "Contour" control in the circuit. I'm not certain what other effects it may produce from the changes in input impedance it presents to the output of the preamplifier, but certainly there will be some rolloff in frequency response when the loading starts to significantly affect the roughly 50k output impedance of the preamplifier.

I will try a small capacitor as you suggest to see if the HF response dependency on the volume setting can be mitigated.
 
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