Heyo, this is a consideration for a future build but, who know, maybe implement in a current one.
Besides placing a bucking transformer in the mains, has anyone ever tried implementing one in only the HV coil? I realize that we would be talking a higher secondary voltage but wondering if any reason why that wouldn't be feasible or effective. I haven't had any luck finding threads on the effects of the lower heater voltage on tone and response but surely remain curious. Any one ever get around to playing with that?
Thanks,
Phil d.,
some other things about bucking transformering
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some other things about bucking transformering
I’m only one person (most of the time)
Re: some other things about bucking transformering
My bucking transformer has a 120V primary and a 12VCT secondary. I would not connect it to a HV secondary.
Re: some other things about bucking transformering
I'll have to go look up the source, book somewhere in the boxes, I think, but here are a few tidbits.
Heater voltage is all about getting the filaments hot enough to boil off >enough< electrons for the circuit's operation. If the circuit has enough cathode emisssion for the peaks of the circuit's draw, little or no effect results in the "tone". [Note: extra points if you can clearly and succinctly define "tone"
]
Tube heater/cathode life goes down approximately with the fourth power of applied voltage. If heater voltage rises by 10%, heater/cathode life goes down by 1/ (1.1 * 1.1* 1.1* 1.1) = 1/1.46521 = 0.683. 32% life drop for raising heater to 10% above nominal. Why didn't they spec heater voltage tighter than +/- 10%? Because heaters in the "Golden Age" were universally run from transformer windings, and even with perfect transformers, the AC power voltage could wander up and down 10%. There was no point in specing them tighter.
The opposite is true, too. Drop heater voltage 10%, the heater and cathode lifetime gets longer - a lot. This works great for preamp tubes, as they hardly ever need as much current as their oxide cathodes can dish out. Works poorly for power tubes, as they have peak currents so high that they can, in normal operation, deplete the electron cloud over the cathode and let the cathode get bombarded by stray-gas ions, slowly poisoning the cathodes. In power tubes, the high currents can cause lower life if the heaters are too cold. Still, you ought to try to get nominal, as that's where the inner structures are designed to work, including the oxides on the cathode.
Bucking just the high voltage secondary sections up probably works fine, given that you leave the heaters at nominal and adjust bias and ventilation for the extra high voltage that a bucked-up B+ gives. There can be some tricky bits here, as bucking up just the high voltage secondary sections needs two windings, one attached to each half-secondary. Most transformer winding practice can stand 300-500V between secondaries, and that's awfully close to where they'll be flying when boosting the high voltage. As to sound? It's all in the changes you'll have to make to get the output tubes to live under the new bias and power dissipation conditions, I think. Tubes have a limited view back through the power supply to the details of what is powering them. The rectifiers and caps are specifically intended to hide everything earlier.
If you raise your high voltage, the bias points, currents, plate voltages, etc. all rise on the preamp tubes as well, so you may have to re-bias those too.
Heater voltage is all about getting the filaments hot enough to boil off >enough< electrons for the circuit's operation. If the circuit has enough cathode emisssion for the peaks of the circuit's draw, little or no effect results in the "tone". [Note: extra points if you can clearly and succinctly define "tone"
] Tube heater/cathode life goes down approximately with the fourth power of applied voltage. If heater voltage rises by 10%, heater/cathode life goes down by 1/ (1.1 * 1.1* 1.1* 1.1) = 1/1.46521 = 0.683. 32% life drop for raising heater to 10% above nominal. Why didn't they spec heater voltage tighter than +/- 10%? Because heaters in the "Golden Age" were universally run from transformer windings, and even with perfect transformers, the AC power voltage could wander up and down 10%. There was no point in specing them tighter.
The opposite is true, too. Drop heater voltage 10%, the heater and cathode lifetime gets longer - a lot. This works great for preamp tubes, as they hardly ever need as much current as their oxide cathodes can dish out. Works poorly for power tubes, as they have peak currents so high that they can, in normal operation, deplete the electron cloud over the cathode and let the cathode get bombarded by stray-gas ions, slowly poisoning the cathodes. In power tubes, the high currents can cause lower life if the heaters are too cold. Still, you ought to try to get nominal, as that's where the inner structures are designed to work, including the oxides on the cathode.
Bucking just the high voltage secondary sections up probably works fine, given that you leave the heaters at nominal and adjust bias and ventilation for the extra high voltage that a bucked-up B+ gives. There can be some tricky bits here, as bucking up just the high voltage secondary sections needs two windings, one attached to each half-secondary. Most transformer winding practice can stand 300-500V between secondaries, and that's awfully close to where they'll be flying when boosting the high voltage. As to sound? It's all in the changes you'll have to make to get the output tubes to live under the new bias and power dissipation conditions, I think. Tubes have a limited view back through the power supply to the details of what is powering them. The rectifiers and caps are specifically intended to hide everything earlier.
If you raise your high voltage, the bias points, currents, plate voltages, etc. all rise on the preamp tubes as well, so you may have to re-bias those too.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Mark Twain