Thordarson 22R07 Bias Supply Question.

[FuzzFaceBuilder]

PT is a Thordarson 22R07. HV winding is 350-0-350 @ 200mA.

Help me understand this bias supply circuit.

Looking at the picture below, one side of the HV goes to the .05 cap, 27k to ground, and diode. Voltage divider and negative DC.

Burned(?) 15k to ground is supposed to be the RC low pass network, with NO ripple cap?

Pot and resistor to ground are variable voltage divider to bias resistors.

Did I get this right?

Also, red/yellow wire is grounded to chassis in the pic. Shouldn’t the CT be floating for this bias supply to work?


https://flic.kr/p/2jy1Uku

2EBE7A00-DB8C-4FEE-8B4E-845C87F2864B.png

Thanks!

[sluckey]

Did I get this right?

I think so.

Also, red/yellow wire is grounded to chassis in the pic. Shouldn’t the CT be floating for this bias supply to work?

No.

First time I've ever seen this circuit. What amp? Got a schematic?

[FuzzFaceBuilder]

It’s a Mosrite amp built in the 60’s by Alexander Dumble. No schematic yet. Dynaco A431 OT, EL34’s, verbatim Dynaco power amp, Fender preamps and tremolo. Just studying the photos to understand what’s going on. Cleans were supposed to be HUGE, amazing sounding.

2C3C513A-9B08-4034-967D-5E360717D4B4.jpeg

C8309D34-327E-4213-9F8C-3CEB367F86F8.jpeg

[sluckey]

Looks similar to an old Sunn. I'd really like to see a schematic if/when you get one.

[thetragichero]

^^^^ my thoughts exactly

[FuzzFaceBuilder]

Looks similar to an old Sunn. I'd really like to see a schematic if/when you get one.

Working on a schematic for this one. From the 470k mix resistors on it’s exactly the driver section of a Dynaco Mark III/Sunn 100S amps! 6AN8A driver tube and all.

[sluckey]

I'm very familiar with the Mark III. Still wonder about why use a cap coupled bias supply input when a simple resistor of proper value (100K to 220K) would work just as well. If that amp were on my bench I would replace that cap with a "Class X" cap, or even better, replace the cap/resistor with a 1 watt resistor.

[FUCHSAUDIO]

The bias supply is common in many amps. Ampeg liked that supply a bunch... https://schematicheaven.net/ampegamps/b25b.pdf

Nothing special here...

[Ten Over]

Whereas Ampeg frequently used a two-resistor voltage divider to lower the HT voltage for bias use, Mosrite used the impedance of a capacitor at 60Hz and the resistance of a resistor for the voltage divider. If you think that it is difficult to determine in advance what the resulting bias will be using all resistors, you should try it with a capacitor and resistors with the phase angle shifts and things. I can guarantee you that the values were selected by trial and error.

[FUCHSAUDIO]

Another suggestion is (assuming room), you can take a small transformer with a 12 or 16 volt secondary, and hang it off the heater line. You'll get 50 or 60 volts out of the primary, which you can rectify and make a nice, isolated, stable bias source. No charge, you're welcome