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Paia Tubehead questions

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Some very naive questions here....hopefully someone will help me out.

I'm going to use the tubehead schematic as the basis for a (unity gain) tube exciter. I'll be making a number of modifications to the circuit, but because of my lack of familiarity with tubes (this will be my first project) I'm afraid to change a few things.

1) What is the deal with the esoteric voltage tripler using the 4049 CMOS IC? Could a standard voltage tripler be used, or is the square-wave output desirable?

What about a large filter cap on this supply?

2) It is possible to use regulated DC for the heater, correct? (pins 4,5 in the schematic?) If so, what voltage (6-7v?), and is a current limiting resistor required?

3) Due to the large resistor values, would it be advisable to, say, have a gain of two on the input buffer and a gain of 1/2 at the output buffer, to attenuate the tube stage noise?

Thanks very much for any replies.

(The full schematic can be found
here. This design is C1995 PAIA)


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I built a PAIA tube mic preamp a few years ago, very similar to the tubehead.
The voltage tripler needs to be isolated from the +/- 15 supply. The 4049 oscillator is a very cheap way to do this, but it's noisy. I don't think bigger filter caps would help.
Better would be to design a proper power supply. This will require more transformers than PAIA's simple wall wart.
I personally didn't like the sound of the "starved" tube. I tried different tubes including 12at7 and 12au7 but they all sounded...starved. I prefer the conventional tube voltages and biasing arrangements. With a 12ax7, its pretty hard to go wrong with the classic "Leo Fender" gainstage as a starting point. See any of the older Fender guitar amp schematics for an example. Common mods include lowering the values of coupling caps and cathode bypass caps to prevent flubbiness under overdriven conditions.

Thanks for the input. I was planning on doing a basic regulated supply for the opamps, so I'll probably use a more standard voltage tripler, or hunt for a suitable xform.

Can you comment on the proper heater supply?

I'll take a look at the Fender design; I'm sure to experiment with conventionally-driven tubes at some point. Part of the allure of the tubehead design was I already had everything but the tubes.

Each 12a_7 filament draws 150ma at 12.6v or 300ma at 6.3v.

My Dynaco PAS-3 preamp uses a simple passive C-R-C filter which works good enough, especially when you double the stock capacitor values. You could use a 7812 or LM317. I've used a basic discreet regulator consisting of a 2n3055, a zener, couple of resistors and a couple of caps, not the best but good enough. Some designers prefer to go all out with a fancy regulator.

If proper care is taken with lead dress, AC filaments can be surprisingly quiet. Especially if you are dealing with line level signals. All my tube power amps are AC filaments. All my tube guitar amps are AC filaments, and a guitar signal is nominally about 100mv, well below line level.

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