another PRR vari-mu question..

the trimmer that goes between the cathodes...what do I aim for with it?..equal currentdraw through the tube halves, same voltage on anodes? or is it something more complex?
also... I'm just starting to build this one..am I crazy if I ignore the GR metering and just go with hanging VU's on the output?...I mean, knowing the GR is good, but the aim with this one is "glue" the tracks together, right?...not that it would be a big thing wireing it up with a switch and and opropriate resistor...

j

hi, I think that the goal of this trim is to balance the circuit, so yes, theorically, equal currentdraw and equal anode voltage witch is the same thing since your two 22k Ra are close enough in value (look at the "balance" pot in some guitare or hifi pushpull amps circuits). But I .might be wrong...
Perhaps in a more scientific way you could apply a 1khz at the input and mesure it at the output for symetry (= distortion cancelation).

I'm pretty sure the trimmer is there for something less complex, something called thump. mine's not completed yet, but I've read the instructions
from PRR's varimu construction page

Pot R32 should be set in the center initially. Once you have the compressor basically working, feed sharp transients through it at 3dB to 6dB peak compression. Listen through a tone control that blocks highs and boosts bass. Adjust R32 for minimum "thump" (DC bias shift) as the compressor goes in and out of limiting.

Click to expand...

you should have to adjust this when you change tubes, and perhaps periodically as the tubes age. you might want to put this on the front panel...
Kelly

thank's..
I didn't see PRR's own page about this one before you gave me that qoute and I did a search...only saw Kent's page.. have a tiny bit more to go on now :grin:
j

http://www.groupdiy.com/index.php?topic=22291&start=0&postdays=0&postorder=asc&highlight=

..it's alive...

..it's not perfect yet, but I'm working on it...
I had to modify the psu a little bit since I'm in Europe and can't use the dual primarys as PRR suggested.
the bypass doesnt work properly when running ballanced ( due to single pole switching ), but that's not a problem for me.
and I had to remove the internal rectifier from the cheap VU's to make them work in G.R mode, so now I'm building a small board with opamp based rectifiers/gain up from -10db, to make them work when used to show the outputs...and the bias LED is working double as power on indicator :grin:
thank's to Kent for the layout, it did things a lot easier, but I did have a small problem...on one chanel I had some ultrasonic oscillation..after scratching my head for a long time I traced it to one of the transformers. rotating it 180 degrees solved the problem, but why I dont know..it's a 1:1 transformer from the autostore...I'm a little uneasy about that...if it's a tolerance thing, what hapens when I need to re tube?..
...bottom line thou..it was an easy build and it sounds good...it's now in my rack...thank's PRR... :grin: :guinness:
http://www.aronnelson.com/gallery/main.php/v/Johan_001/PB080051.JPG.html
http://www.aronnelson.com/gallery/main.php/v/Johan_001/PB080047.JPG.html
click on the picks to get them bigger..
johan

Looking good so far, Johan. Good luck with getting it all finished up.

:thumb:
A P