Understanding a tube PSU

Hi all

I am working right now on a bass preamp (which has a small EL84 5W power amp section) and i am still a bit clueless about the psu. I need bipolar 15V supply for the ic's as well as a B+ voltage and the heater voltage. I am thinking about going the route Jakob already did in his projects (using 2 toroids, one backwards).

Here is the schematic for the preamp:

http://mitglied.lycos.de/InfernalDeath/swr_interstellar_overdrive1.gif

So now i try to understand the psu in the schematic. Let's see:
We got 220V incoming, after the rectifier we should have about 310V. So i looked at the datasheet of the 12AX7 and there is says that a 12AX7 draws 8mA. Looking the same way a EL84 seems to draw 48mA. That makes 104mA together. So i guess that's the current pushing through R65 which is a 270r resistor. By ohms law this makes a voltage drop of 28V through the resistor. Now first question. How do they come to 250V from 310V through this resistor ?
So even if we have 250V behind the resistor, through the resistor R66 (12K) we have again voltage drop of 96V (assuming that only the 8mA gets consumed through R66). So the 12AX7 seems around 154V ?

I am quite sure i have quite some errors in my thinking. Could anybody please be so kind to explain to me how this psu is working ? I need to adept it for my project and i want to make sure to not blow something up before connecting :wink:

As always thanks for reading.

Flo

D 8 looks wrong

Q1 is a kind of cool use of a transistor in a tube amp section.

Is the real transformer a 220 CT?


You are thinking max current rating, a tube can work lower than that.

The schematic might be wrong on purpose to mislead.

Look up a power supply made with a CT transformer and two diodes. Then look at the voltage rating of the caps c28,29,30 they are 250VDC.

Fixed bias output tube the -15 supply needs to be clean. GOOD DESIGN will have the center of the pot close to what the circuit should be set at.

So look up the curves of a EL84 at -7.5VDC. Then think about how hot the tubes will run.

There might be even more to ? in the schematic.

The plate current of any tube depends on plate voltage and grid bias. "8mA" is meaningless without also specifying the other two. Look at the plate curves on the datasheet. Under average conditions, in average circuits the plate current for a 12AX7 is more in the realm of 1mA or so per section.

From the two-rectifier configuration of D1 and D2, it makes sense that the transformer high voltage winding would be 220-0-220 with the centertap grounded. Since only one ground connection from the power transformer is shown, the high voltage and low voltage windings must be sharing a common centertap terminal.

If the AC input to the diodes is 220V, the DC at the first filter will be 1.414(220-0.6)=310V. Since the voltage at the second filter is 250V, that means the combined current draw of V1, V2, V3 and Q1 is (310-250)/270=222mA.

Ohm's Law is a beautiful thing :wink:

BUT a couple of things don't make sense. First of all, the filter caps are only rated at 250V. Also, 222mA for a couple of EL84s at +250V is way too much. What I suspect is really going on, is that the transformer is actually 110-0-110 (or 220VCT), the voltage at the first filter is around 154 volts, and that "250V" shown at the second filter is a misprint.

5W from el84x2?

For low freq. work thats seems like a vaste of vaccum.....

Dave That was what I was hinting at(maybe I don't hint well).
I did not want to do all the work for Flo. I was thinking 110 0 110 and about 150VDC like you posted (it might be easy to say that 2 should be a 1 someone in drafting made a mistake). I tried to hint at the low - bias voltage. At -7.5, B+ would need to be low.

I think I read this is more an effect box so lower tube voltage might be a part of the design

I would guess the schematic has other errors, maybe on purpose, I stopped looking.

Thanks for all the answers. Hmm so the schematic seems to be wrong. Yeah i noticed the 250V value of the caps. But i thought that the typo could be at the caps, maybe that they will be 350V rating.
But all the things you guys said makes sense.
Yeah this preamp is supposed to be quite the distortion effect box. That's mainly the reason i am building it. So it also makes sense that the tubes get hit with a quite small voltage.

So i will look for 2x110V primary transformers for bringing the voltage up from the 15V.

Thanks again for all the answers. They really helped ! :thumb:

Flo

[quote author="Gus"]

Q1 is a kind of cool use of a transistor in a tube amp section.


[/quote]
Wow, that is interesting, BJT PI --I haven't seen that before.

-Eric