GHR Audio

Would 6BA6 tubes work ? Read that they are not fully equivalent to 6K4P, something to do with internal resistance.

Testing // matching / calibrating tube gain

This should help find tubes for better matching.
.
1st step is to trim SC CV so that it rests at (i.e. slightly above) required SC out junction, effectively calibrating the sidechain CVs. Then:


"If you need some (minor) unity gain calibration -- and ...

Thanks for the high-precision values for the Gain rotary. I soldered it, using up to two hand-matched resistors in series for a few values. Easy enough. The steps are spot on.

Otherwise I have been playing with pots to assess whether log (A) wouldn't suit me better.

THR A1k sure feels more ...

And thanks for the walkthrough regarding calibration. I have to get the computer up and running. I changed machine and it's still awaiting complete reinstall.

What software did you use to get the THD graphs of the other units? Or is that excel fed by hand?

If RV3 and RV4 are 1k instead of 500 ohm it will change the threshold range.

Yes, 1k gives deeper threshold. Biur 500R is enough, I think, at least for now.

GAIN rotary
I looked at the gain rotary today. Could you please share ideal resistor values, meaning not rounded to E standard? I checked ...

For diode string, I used 1N4448.

Matching of diodes is not required, they should average out. Still, from a batch of 100 diodes, I sorted them using a simple DM diode tester. Ended up using the 64 ones that measured closest to one another.

Also all resistors in parallel positions across channels ...

For GR indication, I ran a cable on underside of board from sidechain CV1 out to onboard J13 (CV1 VU meter driver input). Stuffed R93 and R94 with resistors equivalent to VU meter resistance. Looking good for now.

Flicker LED indicator is onboard.
VU meter driver is onboard.
LED meter would require ...

For RATIO I installed B10k for now.

But a bit weird maybe, cos with ratio dialled to zero, when THR is dialled in, unit starts limiting ever so slightly...

Got it:
I was driving the input rather hot. Cool, useful function ! For the 1k on the ratio string to ground.
Something like LED starts ...

For unity gain (J9 to J10), it sits somewhere around 5K.

For quick test so far:

Didn't have 4004, so I installed UF4007 for D73 and D74 sidechain bias -- should be OK, no?

I trimmed the SC until LEDs are barely visible lit up in the dark.
Then measured after R33, yielding 4V62.
Then measured directly at the diode junction, yielding 2V2x.
I finally ...