Tobyk wrote: ↑Sun Sep 11, 2022 8:41 pm
… I think on a cathodyne, altering the PI cathode tail resistor would be the easiest way to imbalance it, no?
Btw, you didn’t answer why you don’t like paraphase inverters
Yes, dissimilar values for anode and cathode load resistors seems the most obvious means to alter the balance of cathodyne outputs.
Regarding paraphase, if it shares an unbypassed cathode resistor with the preceding stage, the overdriven response can be unpleasant, due to a positive feedback loop. Whilst the loop’s degree of feedback can’t sustain fee running oscillation, it can add a momentary blip of oscillation that parasitises the overdriven signal.
Separating the cathodes eliminates the issue.
Due to the shared cathode, I recall that Merlin advises it’s possible for an LTP to exhibit the above issue.
IMHO part of the LTP sound IS the way it works with feedback: when overdriving, the sound will continuously flip flops between feedback controlled and uncontrolled scenarios in the same waveform, and this is part of that specific sound.
LTP are self balancing, they hold up well and maintain balance, as the tube ages and the dynamic characteristics drift.
Paraphase invertors are actually harder to maintain a good balance, the balance changes with gain.
The harder you hit it, the more the balance drifts.
Remember the basic requirement for a push/pull inverter, 2 equal but opposite phase signals. The tubes should be equal in characteristics and plate voltage.
Its easy to use a meter and grab the difference between sides of the inverter DC at plates and AC out.
I might start there, explore the different variations of paraphase invertors for what happens to the tone while trying to
set and maintain DC and AC balance for different Vin and Vout conditions.
There are many schematic examples for controlling the DC and AC balances, have fun with it.
Tobyk wrote: ↑Sun Sep 11, 2022 8:32 pm
And by ’pure 2x frequency’ you mean ’pure one octave above’ added? I’ve actually heard something at least close to a pure octave above, from the grid leak biased mic channel on a 50’s GA-20 Gibson. Do you have any idea what could cause that in an old analog amp like that?
Yes, only second harmonic, or twice the input frequency. At least on pure sine waves as an input.
My best guess without actual measurements is that it's a bias condition. In some triode setups, you can bias the tube to near cutoff, and it will heavily compress - but not clip - the positive-going side of the plate waveform. This causes a prominent even-harmonic distortion, with the second harmonic (2x frequency) being the most prominent, and the other intermodulation products being very much lower. It can sound like a clean-ish octave up.
Here is a paraphase I use, it is old school but I like it. The NFB can be switched in or out. With the INV cathode bypassed it has a nice aggressive sound. With the INV cathode unbypassed and no NFB it sounds warm. With NFB switched it cleans the amp up. The 100n cap on the NFB is optional as presence. The 100 pf cap and the 22nf/100nf mismatch are just enough for stability with NFB. Old Maggie stereos look similar.
You can balance it or unbalance according to taste. You can even ground the non-INV triode input for a pseudo SE mode. The unbypassed power tube bias resistor comes into play there.
Even if the circuit is adjusted to be balanced at low levels, the non-INV triode does a little cold clipper type action when it is driven hard, so it is back to imbalanced, but in a way that I like.
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max_lwedge wrote: ↑Thu Sep 29, 2022 10:02 pm
Here is a paraphase I use, it is old school but I like it. The NFB can be switched in or out. With the INV cathode bypassed it has a nice aggressive sound. With the INV cathode unbypassed and no NFB it sounds warm. With NFB switched it cleans the amp up. The 100n cap on the NFB is optional as presence. The 100 pf cap and the 22nf/100nf mismatch are just enough for stability with NFB. Old Maggie stereos look similar.
You can balance it or unbalance according to taste. You can even ground the non-INV triode input for a pseudo SE mode. The unbypassed power tube bias resistor comes into play there.
Even if the circuit is adjusted to be balanced at low levels, the non-INV triode does a little cold clipper type action when it is driven hard, so it is back to imbalanced, but in a way that I like.
Sorry for the slow response – thanks for the nice schematic. Would you care to explain in what way imbalancing the PI with the balance knob changes the tone at overdrive – more buzzy, more harmonics? etc..
Also, what is the value of that balance pot, it says B50K and then above it says 39K?
