Modified PS design

[Paul Birkeland]

I'm going to try the 350v, 50mA, 5k OP to start.  Here is the ripple with the last LC stage removed. The voltage is up 17v or so.

Bias voltage?

[johnsonad]

-76v.  Sorry.

[Paul Birkeland]

What's the DCR of your plate choke?

[Paul Birkeland]

OK, 416 Ohms at 50mA will drop 20V.  You need another 75V for bias, then 350V on the plate.  This is 445V.  The stock supply makes ~470V with the 2A3 running and with 270 Ohms of power supply resistance.  You will have another 60 Ohms of DCR, which will drop almost another 5V, so you're at 465, and you need 445.  In actuality, you could use two PC-2 plate chokes in each power supply, simply back-to-back, to provide the voltage drop you need.  You should be just about dead nuts on, but I don't think the extra C stage will do much for you.  I think the plate chokes can be stacked on appropriate standoffs, though you may need a deeper base. 

-PB

[johnsonad]

Hi Paul.  I pulled my last post waiting to hear back from Mikey.  He says DCR is aprox 440 Ohms at 80H (a two volt difference from what you calculated).  I also got a little help from Jac in the way of a curve trace of my intended operating point using the EML 300B mesh tube and what the bias should be.  See attached photo.

I simulated the CLLC design by adding the henries and resistance and end up with the same 484v B+. Do I have the DCR of the transformer correct as my B+ is higher than the 470v that the schematic specs. If I change the DCR of the PT-4 to 60 Ohms then it comes out to 470v when using the stock values.  Attached is a photo of the ripple using a PT-4 DCR of 60 Ohms..

So am I still 20v high? I'm confused by your math and I know you are very good at it. 

[Paul Birkeland]

I'm partially going out of the manual for Paramount voltages, you also have to remember that the driver stage draws quite a bit of current.

[johnsonad]

Thanks! I was so focused on the output and forgot about the driver!

How would I wire the chokes in series?  It's something I'm not familiar with.  Am I too out of line to want to use a film cap in the final C?

It sounds like I'm ready to start this build and measure it out once running. Worse case if I need to drop more voltage I could change it to a CLCRC or a CRCLC and use the resistor to drop more voltage right? 

A couple of other questions.  I see a 1.5K cathode resistor, is that correct?  For the hum pot, is there much to gain from using a 10 turn 50 Ohm vs. the two 22 Ohm resistors and a 10 Ohm single turn or maybe a ten turn 10 Ohm? 

[Paul Birkeland]

Yeah, 1.5K is a good selection.

You can search for "series vs. parallel" wiring to answer the choke question. 

The two 22 Ohm resistors and the 10-turn 10 Ohm pot will give you the finest adjustment.

The film cap as the final C isn't a bad idea, I'd just focus more on the cathode bypass cap and the parafeed cap.

[johnsonad]

Many many thanks Paul. I will start with the CLLC design with lytics and possibly a film later on (there will be plenty of room for it).  I plan on using a 100uF/500v film cap (is this voltage rating too high?) as the cathode bypass cap and will use something nice for the parafeed.  I also plan on using Mikey's high nickel plate chokes also.

Iron should start arriving next week and I'll post here when I have some hard numbers.

[Paul Birkeland]

The 100uF/500V cap will work for the cathode bypass, though it will be physically gigantic.  A 100V film cap would work there, and would be much easier to fit.

-PB

[johnsonad]

Whew! Thanks!! I was going off of PJ's over voltage and based it off of the 450v used in the Paramount. 100v it is

[johnsonad]

Yeah, 1.5K is a good selection.

You can search for "series vs. parallel" wiring to answer the choke question. 

The two 22 Ohm resistors and the 10-turn 10 Ohm pot will give you the finest adjustment.

The film cap as the final C isn't a bad idea, I'd just focus more on the cathode bypass cap and the parafeed cap.

Paul,

The 1.5k cathode resistor value, how did you calculate that and what values did you use from the Paramount manual to get it?  I'm trying to learn to calculate it for myself at some point....

Regards,

Aaron

[Paul Birkeland]

75V of bias and 50mA of current, V=I*R, so 75V=0.050A*R, solve for R.

[johnsonad]

Thanks!  I knew it was Ohm's law but wasn't sure how to apply it.  I was reading an old Valve article and it seems a lot more complicated with other values added into it.  It was in reference to a direct coupled design and that may be where I was getting lost. 

Bias voltage, I will remember it now.

Regards,

Aaron