UTC 3AX restoration

tardishead · Jun 25, 2015

Thanks Doug that's great
So I paralleled small caps to match the capacitors on the 2 units and they are sounding a damn sight closer now.
Almost identical in the top end.
I think the silver mica caps sound good but maybe a bit brittle. I figure as I have it apart I should try get the best sound possible. Any recommendations for really sweet NOS caps? I was going to try k40y9 for the 2 larger ones. What to use for the smaller ones?? Polystyrene? 10nf I can find in k40y9 but nothing smaller. I can find Vit Q down to 4.7nf but insanely expensive.
Slightly strange thing happens - when you have bass boosted max as you bring in the treble the bass gets attenuated a few db but not the other way round.
I thought it was supposed to be lossless!
I am testing with some neve line amps as gain makeup with no input output transformers for the EQ
I have the output of the eq terminated with 600r but nothing across the inputs
Also one unit has about 3db more bass when no treble is dialled and bass is max but they seem to be the same as more treble is added

emrr · Jun 25, 2015

Polystyrene on the highs might be worth a listen. k40y9, could series the caps for smaller values. That could be magic or mud, hard to guess. I'd replace the 510R with tight tolerance metal film, see if that helps with the differences. I'd expect some interaction, and a few dB interaction would not have been considered a problem then worth solving given the cost to do so. You have to go to pieces that only lived in labs or movie lots, way out of the price justification needs of broadcast or recording.

Love to see the low section if you've got it drawn, and how they both connect.

I've got a few dual gang odd-ball Daven that might serve for re-stuffs to get that reverse throw constant attenuation thing going on. This has got me thinking. Could go with some repurposed Altec Acousta-Voice filter parts. The use of 500/500 with 1K/1K is interesting and unexpected to me. They are probably also custom taper, and not exactly like an off the shelf T used for standard fixed-Z attenuation. The Acousta-Voice T's appear to be tapered differently to compensate for the L and C parts.

tardishead · Jun 25, 2015

I probably will take apart the low can as well and check the caps
It's peaking as well
What capacitors did they use for the high frequencies in old pultecs etc?
I might experiment with different caps in the bottom frequencies
They are 25, 50 and 100hz
The 100hz is absolutely brilliant. Really punchy ridiculous kick sound
But the other two frequencies are too low really to be useful

tardishead · Jul 14, 2015

Tried some polystyrene Suflex caps
Not as good as original sound. More blurry less sharpness.
I made up the 7nf cap from two 15nf caps in series. Is this a bad idea generally (more ESR?) does this soften the Q? Would it be better to have two 3.9nf caps in parallel to make up the value?