• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

dual power supply

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#61
NickKUK said:
I'd advise you todo some modelling to understand what happens with tube failures etc. Last thing you want is 1KV at the speaker jack or the B+ being dragged negative (with polarised electrolytic on the B+).

Voltage across the tube. Not "plate voltage". (I apologise if I miss read that point)

I had tube CCS'd cascode frontend like the M60 designed. +320V and -320V supplies with ~155V across each tube. The Issue I had is that I then needed two heater power supplies one elevated to +50V and the other referenced to -150V. It doesn't matter where the tube sits, relative to itself it still sees 160V across it. You're probably better positioned with a directly heated tube.

Current play thing is a hybrid circlotron - so it has four power supplies (9 in total) and due to both sides being within 5V at any point of the waveform output, I can use a single heater power supply and a single tube. It doesn't change if it's a tube or SS - it's the voltage across not simply "at the plate", even more so with a circlotron's floating system.

Caps - keep them across the the B+ and return of the same supply. If you need more voltage then caps in series and then use balancing resistors. The same for series resistors to get you up to higher voltages.

Although I am concerned with the question and 1KV mention in the same sentence.. (maybe I've missed something)

EDIT: JC beat me to it.
Click to expand...
Yes, I understand all the above and congrats on your circlotron.
I am committed to my design if I can understand the process of bias resistor adjustment with dual supplies. Thanks for your help so far.
 
#67
What is cct?
Yes, there was a plan then; there is a plan now: to get this thing working and within specs.
And I am very careful with these voltages (thank you for asking). I am well acquainted with the Vacuum Tube Amplifier Maker's motto: "Electricity is not my friend." And, "Nice Sounding Amp"
 
#69
Thanks for the article. It was helpful. Now I understand your question a bit better.
My initial idea which directed me to a dual supply was not anything like using a dual op amp. It was very simply that I could use better, smaller capacitors in the amp, as well as other mods/refinements in the amp that could not otherwise handle higher voltages.
 
#70
jcalvarez said:
I have experimented with dual power supplies, and it works. But the easiest option to use lower voltage capacitors is to connect them in series.
Click to expand...
It has been a while since we last corresponded. Since then I have had some successes with the dual power supply with tube amps, but I want to understand the process a bit better. Can you help with some of it given a theoretical situation? To keep it simple:
B+ = 100v
B- = 100v
plate resistor v. drop= 15v
what should the cathode resistor be so that the current through the tube = 5ma?
what will the voltage be at the plate?
Thanks in advance.
 
#71
I would say your case is a strange one, very little drop on the plate resistor. But it makes no difference, as many said before, the calculation for the bias resistor is the same, based on the curves and desired current. Easier to illustrate with a simple circuit, two implementations, one with +/- 100V supply, the other with 200V. The bias resistor is exactly the same:

1661984028574.png
 
#72
Thanks for your response. You should note that my "case" is in fact hypothetical (as I said in my initial post) because for me, the numbers are irrelevant. At this stage, I want to understand the process and the theory behind the process.
Thanks for the schematic. I would build it if I had the time.
Can you be more specific and address the theoretical aspects of the problem?
 
#73
lotsmorebetter said:
Has anyone had any experience with using a dual power supply for tube amps? If so, did it work? If not, why not? I ask because It seems to me (at least in theory) that it would bypass the problems of having to use hv caps if one could split up the b+ into a negative leg and a positive leg.
Click to expand...

You’ll also bump into heater limitations.

That’s done all the time - often to bias or to provide a zero voltage level to minimise dc offset on capacitors etc. (that’s for coupling caps)

A cap/diode/resistor will see the voltage that’s applied to it. Irrespective of reference.
A +320V and a -320V will have 640Vdc between them. A cap between then sees 640Vdc.

The point here is the current flows in loops (for decoupling filter caps). Merlin did a nice piece on this.
 
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#75
lotsmorebetter said:
Thanks for your response. You should note that my "case" is in fact hypothetical (as I said in my initial post) because for me, the numbers are irrelevant. At this stage, I want to understand the process and the theory behind the process.
Thanks for the schematic. I would build it if I had the time.
Can you be more specific and address the theoretical aspects of the problem?
Click to expand...
I'm struggling to understand what the problem is. Which theoretical bit is not clear?
 
#77
jcalvarez said:
I would say your case is a strange one, very little drop on the plate resistor. But it makes no difference, as many said before, the calculation for the bias resistor is the same, based on the curves and desired current. Easier to illustrate with a simple circuit, two implementations, one with +/- 100V supply, the other with 200V. The bias resistor is exactly the same:

View attachment 1086391
Click to expand...
It has been a while since I have had the time to look closely at this schematic. It has helped me with my original question. Thanks again for it. I have another question related to this schematic: given that all tubes use the bias set by the cathode resistor to control the flow of electrons, how does a tube with B+ and a B- control the bias? With the same cathode resistor?Doesn't it have to go to ground in order to do so?
Thanks in advance.
 
#78
It has been a while since I have tried to finish this amp because I am still having problems with it.
I have attached the front end of the schematic with ideal voltages / the actual voltages.
Since my last correspondence I have done many things to this amp but mostly they were to get V1 working properly, which it is. V1 is set up correctly: I get a/c up to the grid of V2, but no amplification from V2, no sound and the 100w cathode resistor burns up. It is a 100W resistor!
Also, I checked the current at the plate of V2 and it would seem that no current is flowing even though the plate resistors are good (checked 2x, replaced x1).
Then I swapped out the 550ohm resistor with a huge variable resistor (75W) on the cathode of V2, but to no effect: It too go very hot.
I believe that the tubes are good (they are both new).
So I am stumped.
If you need a schematic of the p/s I can provide that too.
 

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