12B4 Preamp

[Salas]

Here it is. The grid stopper resistors are essential. They must be carbon composition and soldered directly to BOTH the indicated pins with minimum leg distance. 3k3 proved OK but some difficult 12B4 may need 4k7 if it still oscillates. Oscillation tendencies proved to be the problem of those valves seeming not matchable. In this preamp previously unworkable 12B4s settled for 1V anode difference between channels for the CCS set exactly at 15mA. You will trim the CCS anode load until you see 18V cathode to ground across the 1k2 cathode resistor. That is 15mA.

[Salas]

For CCS we used this one. Elaborate because we still have not got the DN depletion Mosfets that are easy for minimum parts count. Sounds splendid non the less. There may be faster smaller Mosfets too, but those we had on hand. No hiss, great sound so far, even with those. Lower Mosfet needs small heatsink.

[milen007]

nice

thanks for the schematic.
questions:
1. what kind of trimmer is that? whats the 145 10% mean? 500ohm trimmer? parallel to 220R resistor?
2. any specific led used? is 3v led ok?
3. can your simple high voltage regulator using the same mosfet be use to supply the 330v to the ccs?

TIA

erwin

[Salas]

1. It is a 220R 0.25W trimmer. The 145 10% means that with 1.7V Leds the actual trimmed value will end up around 145R.

2. Green or red 1.7V 3 pieces as in HV shunt.

3. Of course it can. And it will sound amongst best solutions. Use 60mA constant current running in it, if you gonna use a common Salas HV shunt for both channels.

Regards.

[tmblack]

I'd be very interested to know if anyone has compared the 12B4 preamps to the Euridice preamp or Choky's WOT preamp?

That has a output transformer for a EC8010 tube.
I would want to try it with a 6SN7 tube and fixed voltage biasing.

Tom

[milen007]

Originally posted by salas

2. Green or red 1.7V 3 pieces as in HV shunt.

Regards.

hi Salas

i got a bit problem finding the 1.7v led. will the 2.2v or 3v led do any good?

how bout this one?
Kingbright L-53HD
http://singapore.rs-online.com/web/s...5988&x=27&y=14

erwin

[Salas]

The 2V Kingbright ones will not give problems. You will just adjust your trimmer until you can measure 18V from cathode to ground across the 1k2 cathode resistor. When you get that, you got 15mA for sure. In the case of the HV shunt's CCS, just measure the voltage drop across R1 with given LEDS & Mosfet, and then divide by its R value. If the outcome is far more or far less than your target, exchange for the proper resistor value. Don't use stronger than 2V LEDS, unless you understand the circuits well and you can recalculate them, so not to start way off, even dangerously so with standard values I have given.

[milen007]

Originally posted by salas
In the case of the HV shunt's CCS, just measure the voltage drop across R1 with given LEDS & Mosfet, and then divide by its R value. If the outcome is far more or far less than your target, exchange for the proper resistor value. Don't use stronger than 2V LEDS, unless you understand the circuits well and you can recalculate them, so not to start way off, even dangerously so with standard values I have given.

hi Salas

thanks alot for the confirmation. you are the man

will just play safe and get the 2v kingbright cant find those locally.

Sorry guys OTT

for the HV shunt's, whats my target voltage drop across R1?

anyway thanks again

Erwin

[Salas]

Originally posted by milen007
3. can your simple high voltage regulator using the same mosfet be use to supply the 330v to the ccs?

TIA

erwin

In the original HV shunt thread I forewarned that for handling over 300V Vout, the MPSA 93 goes over max and someone must employ a 400V PNP like MPSA 94 or 2N6520 (350V) etc. I hope that you noted that, and also that you will need 380V DC input to the reg for good quality 330V output.

[Salas]

Originally posted by milen007


Sorry guys OTT

for the HV shunt's, whats my target voltage drop across R1?

anyway thanks again

Erwin

Your voltage drop is governed by the target CCS current you look for. Say you have a 47R for R1 and you look for 60mA (double the 2X15mA needed for 2 12B4 channels, so to perform nicely). It will be OK to measure 2.82-3V for instance.