5F6-A - Solid State Rectification?

[j-stylez]

Ok that was a very nice idea. The bogens do exactly what I want!
So I'm adding the coupling cap. Whats the purpose of the 56k resistor to ground after the coupling cap? Is that an alternative to using a series resistor of something like 200k?

[MWJB]

Yes, really. You're now looking to drop rather less voltage from the AC source (from 350 odd down to 200), so I wouldn't hold on too much to the idea of that 200K dropper. I'd probably copy the CHB 50 Bogen circuit and sub the grounded 68K after the bias diode with a 100K trim pot, wired as a variable resistor. Find the value that allows you to bias to say 45-50mA per tube MAX, then insert this value between the trim pot & ground. Or use 68K from the Bogen schem in series with a 50K trim pot (wired as above).

Set your negative voltage at 6L6 pin 5 BEFORE installing the power tubes, maybe -50vdc at 400vdc plate voltage, -60 at 450vdc plate voltage...this should make sure that you are not under biasing the tubes at initial installation. Or, if bringing up on a variac, with tubes installed - start at 1/3 wall AC (you will then see 1/3 plate voltage at pin 3 & 1/3 neg dc voltage at pin 5) and monitor as you bring up the volts.

[j-stylez]

Actually, I'm not dropping down from 300 something, but from 200 something.. because the bias circuit draws off of my tap running on 200 (190 or something like that with a load...)
So right now I put a 56k resistor with a 25k trimpot to adjust the bias sweep, and a 50k bias pot with 33k to adjust bias. I'll see how this works out once i have everything hooked up, before installing any tubes.
I'll finetune it, maybe sub the 58k or the 33k, I will see what I have to change.
Hope its going to work out. Thanks for all the help again.

[j-stylez]

Allright... I got it all wired up.

Problem now: I'm getting about 540V (no tubes installed) after my voltage doubler rectifier. Thats too much.

Any ideas what I could do to lower that a bit to get closer to 430?

[MWJB]

"Problem now: I'm getting about 540V (no tubes installed) after my voltage doubler rectifier. Thats too much." How do you know if you have no tubes installed? Try and get -64vdc to -70vdc at pin 5 of the 6L6s, install your tubes and bias to 30mA per tube. Then tell us what voltage you have at the plates.

I have seen amps that run 490vdc @ 30mA on the plates rise to 540v at lower currents, even with all tubes installed.

"Any ideas what I could do to lower that a bit to get closer to 430?" Why would you want 430vdc if you are building 5F6A style amp?

[j-stylez]

I thought my goal should be 430 with tubes because the original schematic states 432 Volts at the first filtercaps.
I didnt know this much more was ok, but it is nice to know this.

Anyway, I'm still trying to get the bias circuit working.

It doesnt work though, I'm getting 0 V from it. I looked over the Bogen and Marshall schematics again, but cant seem to find the problem. I thought that my voltage wouldnt be ideal yet, at least I was expecting a voltage I could work with.

Before I troubleshoot more, can someone spot an error here?

And does the cap before the diode have to be one with polarity? I'm using a film cap.

Its wired like this:

[MWJB]

Do you have 200VAC before the .47 cap (non polarised is fine)? Do you have VAC at the bias diode and vdc after it...at what point between the B+ secondary and your 220K grid resistors does the bias voltage disappear? Are you sure that you are simply not getting an overload reading, your voltage divider does not seem to drop enough voltage to work with your power tubes (it would seem that you are still hitting the grids of the 6L6s WITH -250vdc, rather than -60 or -70)?.

Why not copy the Bogen schem, with the trim pot as I described?

[Steve Conner]

I'm not surprised that won't work, there's no DC path for the diode current.
There, fixed.
Edit: Oh, but it still won't generate any bias when you're in standby, so there'll be a big surge of plate current when you flip the standby switch. A NOS VINTAGE RARE cookie for anyone who can figure out how to fix that.

[pdf64]

Not a perfect fix, but if current in-rush suppressors (NTC thermistors) were fitted in series with the ground returns of the power tube cathodes, then at least some cathode bias / current limiting would occur at startup.
It might be difficult finding parts with just the right spec though.
The old 'brimistors' in AC50s would be perfect.
Perhaps have a 3 position standby switch, arranged off/cathode/fixed, which switched in some suitable cathode resistors in the middle position, then shorted them in the 'fixed' position.

[Steve Conner]

I think a better plan would be to rearrange the circuit so that it still generates bias when the standby switch is open. *ahem* single pole standby switch

[pdf64]

Damn - that cookie is just playing hard to get!

[j-stylez]

Which tap should I put the SPST switch on, the top part in my drawing? So I simply remove the bottom part of my dpdt switch in my drawing?
Because I thought that would still leave me in the same situation.

So I guess I could use SPST switch after the voltage doubler, but then I would first have to find a 600v dc rated switch...

And MWJB, I actually didnt look at the right bogen schematic. The 50 was so fuzzy that I kept looking at the others.

[Steve Conner]

Yes, just delete the bottom pole of your standby switch. Then the bottom end of the PT winding goes back to the centre tap of the two series capacitors, and that completes the circuit for your bias tap. The way you had it, with the standby switch open, the bottom end of the winding was just dangling in the breeze.

Re the 600V DC rating: If you get a double pole switch and connect the two sections in series, that doubles the voltage rating. Twice the contact gap means twice the arc breaking capability. So I suggest you do that even if you're breaking AC.

[j-stylez]

Time to celebrate, the bias circuit and voltage doubler now work. The whole power supply section seems to be working fine. Thanks again for all the help so far (and more to come )

1.) Bias

Plate voltage of the 5881's is still +535V, my tubes are Sovtek 5881WXT.
Now to the biasing: went through first power up, and took some voltage readings...

Voltage across 1ohm bias resistors are:

3mV at -75V bias

15mV at -65V bias

1 ohm resistors are 5% wirewound, the voltage across them differs slightly. I think its still within margins, for the second measurement (15mV) it was 0.6mV difference between the tubes.

I couldnt go lower than -65 with my bias combination, so I added another 68k in parallel to the 56k before the bias trimpot, because it was a bit hard to reach the 65k and replace it by a lower value. I will replace the 68k with a higher value to give me correct sweep once I've found the voltage that gives me the maximum plate current I want to be able to dial in.

22mV and 24mV at -60V bias

The difference has increased, but I believe its mostly due to my 2 not-so-1ohm resistors.

At 535 plate voltage I would be at 0.023A*535V = 12.3 Watt
Maximum dissipation is 23 Watt for the 5881, so I was planning to go up to 15W/535V = 0.028A

2.) Noise / Shielding

I definately have to get some shielded wire in the preamp section. I dont have much left, so I will replace the signal path to the first preamp tube by shielded wire first. There is quite a bit of noise.
Also, I read its a good idea to lift the 68k resistors from the turretboard and attach them directly to the input jacks. Would you recommend this?
Or, could I run only one wire for each of the two channels to the turretboard and make the connection of the 68k's there for each channel to further reduce noise? I dont plan to use inputs 2 of both channels at all.

Im running 12ax7 in v1, because my 12at7 / 12ay7 havent arrived yet. (out of stock :/ )

3.) Filament 6.3V heater center tap

Does the filament center tap need to be grounded? I wasnt sure about this. I am thinking that I might be getting hum because its not grounded?

[MWJB]

1) Bias - as you increase the plate current (yes 28-30mA per tube sounds fine) your plate voltage should drop. The nominal difference in plate current you have is not a concern.

2) 68k on the input jacks woks fine in millions of amps.

3) Either ground the filament centre tap, OR fit a 100ohm resistor from each leg of the filament secondary to ground (virtual centre tap). Usually the most convenient place to implement this is at the pilot light.