After:
It was for a jazzer who wanted a clean valve pre-amp, and is based on the Fender AB763 circuit.
It was designed to run on an easy to source 12VDC supply; a 12AX7/ECC83 can be wired for 12VDC operation (12.6 VDC to be really accurate), and I used a charge pump kit from
Tube Town for the HT, which we adjusted to 140 VDC. The kit is well-designed and gets my recommendation.
There where two issues I thought needed addressing; firstly the voltage output from the pre-amp could potentially be way too much for a normal amp input (or indeed a line input), so I attenuated the signal from the second triode using a split anode resistor, and then added a master volume.
The second issue is that output impedance would potentially be quite high, so I used a high voltage MOSFET as a source follower buffer to give a reasonably low output impedance (probably around 150-200 ohm) so the unit can drive a typical 10k line input impedance.
Comments
Nice one J!
Did the ht generator use the UC3843A chip? If so I hope you used a higher rated resistor in the feedback than "we" did at first. It went high to O/C and HT soared to 4-500V!
Yes there are some high signal voltages present in the pedals, 100V+ pk-pk. They used a conventional attenuator to bring them down to drive standard op amps but since there are +&-15V rails there is still a lot of headroom!
"They" had not thought of a CLEAN valve pre amp AFAIK. To late mow mate!
Hot cakes I reckon.
Dave.
I don't think yours uses it, but I hadn't seen the -ve voltage add-on to that circuit before - interesting stuff from Rick. I'd also vouch for the basic voltage converter circuit being pretty robust and reliable - and it's a lot easier to live with than the MAX1771 designs!
How possible would it be to add a parallel speaker-emulated output to that? Maybe an fx loop as well?