• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12BH7A driver stage help


2007-02-13 7:10 am
My quest is a bit odd. I'm trying to make a hybrid amp from Quad 405 clone with a bit weird solution. My friend gave it a go and built it with Russian 6N6P (6Н6П) in SRPP and it works fine. For some reason, I am rather interested in using a pair of RCA 12BH7A for the job. Not sure how familiar people are here with the basics of the Quad 405, and it's not really important for this. In a nutshell at the input (original Quad 405) there's an OpA and the signal goes through it, at the same time the same OpA does the error correction. I have a nice pair of electrostatics with an odd and quite demanding impedance characteristics, and found that this amp (within a reasonable budget and power consumption) is the only one I'm capable of building and matching with the ESLs. However I prefer a bit different sound. Guess I'm way too much used to my small 6Watt SE with EL34 downstairs ;)

Anyway, there is a simple way to avoid the OpA on the way of the music by introducing a completely separate line stage which should have an amplification factor of 15 (that much has the OpA stage as it is) and the output of that stage is to be connected to the input of the next stage in the power amp, at the same time, the original input point is to be shorted to the ground. That much introduction should be enough, I guess.

I'm not sure what sort of construction would the collective wisdom suggest. The MU (amplification factor) of the 12BH7A is 16.5 which is really close to the desired 15 I need. The tube is robust, and has more than enough power to drive the rest of the amp. I'm limited somewhat with the rest of the stuff I have, namely, the transformer which is an 80VA toroid, has enough current to do the heating ([email protected] per tube) but the HT secondary is @ 260VAC, and I think that I can't get enough DC Volts to go for a SRPP (I think I'd need about 420VDC to do a properly supplied SRPP with these tubes)

So, I need some help, suggestions on what to do. I'll do it anyway, I'd just prefer to do it the right way right from the start.

If anyone cares, I tried to match my ESLs with some SS power amps also some tube amps and PASS F5 clone, but in all attempts, it lead to clipping before my ears started bleeding. :D

I have a solution with ECC83 (not SRPP), but somehow, it just doesn't seem to be what I want.