Sufficient B+ for Dumble kind of preamp?

[Mr. Lime]

Hi guys,

having a lot of amps standing around and taking my place to live, I would like to build a stompbox preamp with two 12AX7.
There's an available voltage pump kit that supplies a maximum of 15mA @ 220V DC out of 4 VA @ 12V DC input.

I don't expect this preamp to be a close reproduction of the Dumble sound but would 220V get me in the ballpark?

Thanks a lot!

[pompeiisneaks]

I found this looking through the #124 section: https://ampgarage.com/forum/viewtopic.php?f=5&t=5719

B+ is 440VDC on the 6L6 plates, which seems low. The remaining plate voltages are: V1A and V1B approximately 190VDC each; V2A and V2B 10 volts higher; on the order of 280VDC for the non-feedback side of the phase inverter plate, and about 10 volts higher for the feedback side.

Which if I do the math means;

V1: 190VDC, V2: 200VDC/210VDC

Then another note later:

22) Were any voltages measured when the amp was examined?
Yes, as follows:
V1 190v
V2 200v
6L6 Plates 440v
PI plates 280v and 290v

So yes, that voltage 'should' put you in the ballpark, but you'll need to play with the a dropper resistor before the V2 tube and then another for V1...

But...

That's definitely only one component in the variable list...

~Phil

[martin manning]

Which if I do the math means;
V1: 190VDC, V2: 200VDC/210VDC

Yes, preamp tube plate voltages are 190-200V, but that means the supply voltage would have to be about 300V at 5mA to replicate the ODS operating points.

[pompeiisneaks]

Which if I do the math means;
V1: 190VDC, V2: 200VDC/210VDC

Yes, preamp tube plate voltages are 190-200V, but that means the supply voltage would have to be about 300V at 5mA to replicate the ODS operating points.

Ahh you're right, I didn't think of that exactly, so that's direclty ON the pin...

So yeah, that definitely won't work unless you use a FWB Rectifier, which should make the output voltage be over 300V? (FWB is 1.4x the output AC whereas FW is .9) that would work I think?

~Phil

[martin manning]

The OP is wanting to use a SMPS of some kind to get the HT from a 12VDC input. Looks like the one he is considering tops out at 220V.

[pompeiisneaks]

I'll hush and stop derailing with my misunderstandings then.

~Phil

[Mr. Lime]

Right, the PSU delivers a maximum of 220V - is it worth trying?

[martin manning]

Right, the PSU delivers a maximum of 220V - is it worth trying?

It will work, but your plate voltages will be low, around 140V instead of around 200V, so you will lose headroom. I'm sure it's possible to build a 300V, 5mA SMPS.

[Mr. Lime]

That's the voltage pump I would use:
https://tube-town.net/info/datenblaette ... t_v1_6.pdf