PRR varimu power supply question ?

Hi there,
I'm trying to build the PRR variµu limiter, i've start with the PSU of course and i measure -17.52/+18.6 (supposed to be -16/+16) 100V, 1.54V & 17.48 instead of 12V, these measures were made with no load.
I'm a newbie, and before going further i wanted to resolve this problem before building the main board.
I've used 2 toroids to get 115V at the output of the second one cause i got them handy.
Anyone to help me. :?: :?: :?:

If there is no load it's possible to measure as high. If you built it according to PRRs spec you should be fine.

:thumb:thanks ciminosound, do you think that the tube heaters will drop the voltage from 17 to 12V because of its current consumption ?
Have you built this unit ?if yes how does it sound ?

Hi,

Curious: Are you using PCBs?

> do you think that the tube heaters will drop the voltage from 17 to 12V because of its current consumption?

DIY 12AU7 compressor

There is a 16 ohm resistor in the line to the heaters. (If you can't find 16, try 15 or 20 ohms.) With no load: zero current times 16 ohms is zero volts, 17V-0V=17V. With the 0.15A load of one 12AU7 heater wired for 12V operation: 0.15A*16Ω= 2.4V drop, 17V-2.4V= 14.5V. This is higher than than the 12.6V rating, but a 12AU7 will live for years at 14.5V.

So go ahead and clip-lead it to a 12AU7's heater. Measure now. In fact, unless you used very big transformers and a filter cap much bigger than 4,700uFd, you will have another couple of volts of sag. If you wind up anywhere between 11V and 13V you will be fine, 10V will work (slightly lower gain and maybe a bit more distortion) and 14V will work good, certainly long enough to find a few more ohms to put in series with the 16 ohms.

Looking again after several years, I see I wrote "0.3A". Maybe I was thinking stereo. 0.3A*16Ω= 4.8V, and 17V-4.8V is 12.2V. With a little sag in transformer and cap, this could end up nearer 10V. Again, measure under load and fudge the "16Ω" resistor to get 11V to 13V at the heater. It does NOT have to be 12.60V, it only has to be "in the area of twelve and a half volts". We're not making an atomic bomb, just cooking a few electrons.

The -17.52/+18.6 is a wee bit (0.12V) above the 36V rating of most chips, but they won't blow-up, and again the actual voltage under load will sag enough to be fine.

I don't know why your positive voltage is higher than your negative voltage. If loading the heater connection makes the "12V" sag to almost nothing, you may have forgotten the transformer center-tap.

Hi fazeka, i use Veroboard for this project.

PRR, thank you very much for your answer, now i'm quite confident for the continuation, i'll put some pics when finished if there is some interest.
This place is really :thumb: :thumb: :thumb: i always find anwers and learn more each time.

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