Convert CT full wave bias supply to bridge recto supply (and drop voltage)

[ChrisM91]

I am building Pete Millet's DCPP "Engineer's Amp". I am not using the stock PT specified in the schematic.

Rather than a PT with a 50V winding with a center tap (50-0-5), I have a transformer with a 130V winding (no center tap) (130-0).

I want to convert the stock bias supply (see attached) to be used with my 130V (no center tap) transformer.

I was thinking since no CT on my transformer I would just use a bridge recto. Then a 100V zener diode to bring the voltage down.

Stock bias supply puts out (1.414 x 50V ~= 70V) where the new winding would put out (1.414 x 130V ~= 180V). So I figure I have to drop at least 100V and the easiest way to do with would be with a zener. Thoughts?

Side note: what current does the bias supply itself actually use?

 

[euro21]

The "C-" load is relatively constant: 17mA of LTP cathode CCS + about 3mA bias part (per channel).

If the input DC voltage is -for example- 180V vs. 70V (110V difference), and load is 20mA,
try to increase R12 from 100R to
110V/0.02A= 5500R.

You can use R12= 2k and R12a (between C8-C16) = 2k5.

The correct calculation of R12+R12a is (at 20ma load!):

Rresult = (DC voltage of C1 - 60V)/20mA

If you stabilized the bias voltage and B+ not stabilized (only smoothed), the AC line fluctuation will causes bias point fluctuation too.
Both or neither must be stabilized/not stabilized.

 

[ChrisM91]

Sorry what do you mean when you say,

The "C-" load is relatively constant: 17mA of LTP cathode CCS + about 3mA bias part (per channel).

The bias supply (C-) is supply the output tubes fixed bias supply. The other tube's have cathode biasing with CCS loads. So I am confused how the non power tubes come in to this.

If you stabilized the bias voltage and B+ not stabilized (only smoothed), the AC line fluctuation will causes bias point fluctuation too.
Both or neither must be stabilized/not stabilized.

I understand line flucuations but I am confused at what you mean.
The B+ and screen supplies are regulated, the bias supply is not regulated.


DCPP "Engineer's Amp". schematic

 

[euro21]

See the attachment.



The "C-" is the negative supply of the LTP (17mA CCS) and the bias part.
---------------------

http://www.pmillett.com/DCPP.htm

Pete wrote:
"
The power supply

The power supply uses silicon rectifiers and a MOSFET ripple filter for the B+ - no expensive and heavy chokes here. The screen voltage of ~150V is generated from the raw HV supply with a simple MOSFET regulator."

The B+ not regulated, only the screen voltage.

 

[ChrisM91]

Hey euro21, I thought about your replies some more and now understand all your points, thanks!

Though I think for the dropping resistor it would be (17mA + 3mA)x2 = 40mA as the bias supply (C-) is common to both channels. So 110V/40mA = 2.750k
No?

 

[kevinkr]

I believe the bias required is 20mA per channel, what is the current rating of that 130V secondary?

 

[ChrisM91]

130V @ 100mA

 

[kevinkr]

That should be sufficient for the 40mA. Since you have so much voltage margin to work with I would start with an R rather than a C (even better would be an inductor) to reduce the peak current drawn from the winding. You could use something like an RCRC filter here. Fine tune the first R for the required output voltage under load.

 

[ChrisM91]

Is that RCRC input filter for the bias supply in addition to the RCRC input filter in the already existing design?

 

[kevinkr]

No, I did not realize it already had an RCRC based bias supply filter. Should have figured that Pete would have done that probably for the same reason I just suggested it. 😀