PRR Vari-Mu PSU problem - need help

Hi everyone,

I'm building the PRR Vari-Mu on variboard using Kent's updated schematics/PCB layout:
http://www.conditionedresponse.com/DIY/C5/index.html

Since I live in europe I've got to use two transfomers, one 230-12 for the heater, bias, opamp supply and one 230-24 in reverse for the B+.

The B+ voltage is alright (reading ca +104 V DC), the bias seems fine (ca. +1.5V, though it sometimes drops as low as 1.3), but I'm getting much lower values which change after some time for the rest, namely

-12,2-13,5 V DC at the -16V rails
+ 1.3V DC at the + 16V rails
+1,7-1,3 V DC at the 12 V heater supply

I'm also reading some AC on those rails now and then (it's not there all the time).

I've checked everything 10 times now and even changed the rectifier for the 12V AC supply, but cannot get behind the problem.

I'm using a 4.700uf cap instead of the 5.000uf, but also soldered in a 300uf cap in parallel for testing, which didn't change anything.

Could this be related to a faulty cap?

Also, the LED glowed up a little at a time, but now doesn't, it's getting 1,38 V DC...

Any help would be much appreciated, I'm at a loss now...

Thanks alot!
Gregor

hi Gregor,
I started to build this project two months ago. First i'm sorry to post a response in your topic without info that could help you :? .
But do you use the audios transfos from radioshack?


thank you

it appears that the opamps, heaters etc. are powered from a center-tapped 24 volt transformer secondary--is that what you are using?

warpboy, I'm using Pikatron xformers on the input and Edcor on the output, both 600-600 line level. I think it was either Kev or Kent who wrote that the radio shack transfomers take away some bass.


shabtek, I'm using a 230V-12V torroidial transformer (with two wires each for primary and secondary) for the 12V stuff. A second (laminated steel core) transfomer rated 230V-24V is connected backwards to produce the B+ voltage. Both will be mounted outside, on the backside of the case to avoid hum.

the plan calls for 120,120,12,12
120= primary
120=b+
12,12= +/-,bias,filament

I know, but it should work the way I did it, shouldn't it? I cannot use the transformer originally suggested since I live in 230V-land. And there is 12V-12V AC present at the rectifier from which the +12V,+-16V, +1.5V lines are derrived... Am I missing a ground or something here?

I'm still not sure if you are using a 24v center tapped transformer secondary (2x 12volt secondaries in series, with the node where they join connected to 0v)...
it sounds like you have a single 12v secondary, connected in a manner different than the plan...
this may work if the xfmr has a high enough VA rating, but you would save a lot of trouble by getting the part spec'd

here is a thread that shows a way to get +/- (bipolar) rails from a single secondary
http://www.groupdiy.com/index.php?topic=6469

what you need is a 230:12+12 transformer, with two 12VAC secondaries tied in series, with the center tap grounded, to get your bipolar +/- 16VDC, LED bias, and your 12VDC heaters. look at how the schematic shows the power supply (apart from the B+).

then to get the B+, you need an "international" power transformer with dual 115VAC primaries in parallel, so that you can go from the 24VCT of the first transformer back up to 115VAC.

so:

230:12+12 > 12+12:115

these could even be two of the same transformer, just wired differently.

ed

Thanks shabtek and edanderson!

But I still don't understand why I cannot use my transformers. What is the center tap actually good for? From my - simple - understanding, I'm feeding 12V AC into the 'non-B+' part of the circuit, the rectifier converts this to roughly +/- 18V DC, which is then knocked down by the respective resistors and filtered by the caps... I'm no EE, I guess I'm missing something important here...

If the center tap is needed, would it suffice to just get one 230:12+12 for the +/-16V, bias and heater and use my 230:24V in reverse for the B+? Because B+ is working fine in my current assembly...


Thanks again!
Gregor

think about this for a moment, and you will see why it cannot work...

from a very simplified view... you put in 12VAC and you want to get out +/-16VDC. AC voltages are commonly specified as RMS values, meaning that the actual peak to peak (from the topmost peak of the wave to the bottommost trough) voltage is about 1.4 times higher. the difference between the most positive and most negative value you can get out of 12VAC is 12 * 1.4 or roughly 17V. compare that to the difference between +16VDC and -16VDC, which are separated by 32V, and i hope you will see why this cannot happen, at least not without quite a bit more circuitry.

the center tap is necessary as a way to ensure that the plus and minus voltages you get out are equally distant from some reference. in this case it is tied to ground so you get +/- 16 rather than say, +20/-12.

ed

Thanks again!
Just one more question: Can I use 1N4148 or BZX85C24 instead of the D1N4402 in PRRs power supply circuit to which you posted the link?

Gregor

i don't know what bzx85C24 is, but what you want is a power supply diode, not a small signal diode like 1n4148. the 1n4002 (and the rest of that series) are common parts here in the US.

PRR's split supply from one winding that shabtek linked to should work, but note that the way the diodes is hooked up is different from the 12AU7 comp power supply.

i shouldn't have said in my earlier post that a single 12VAC winding "cannot" work in this circuit, because obviously it can... if you use a different rectifier configuration than the original circuit. it isn't really "more" circuitry (less actually), but a significantly different circuit, with its own advantages and disadvantages.

ed

Thanks again guys, I got a new transformer (center tapped) and it's working properly now!

Happy holidays!

Gregor