Wilcox M57D1 tube limiter restoration

[abbey road d enfer]

Can you post exact schematics of what you do and expect to do?

 

[hereforever]

P1 cathode balance- value of 100 ohms? I just noticed that R3 ties to the cathodes, so not sure I've implemented this right
P2 plate balance- value of 1K
P3 attack
P4 release

I like the dual settings of Sta Level, BA6, and others, but kept it simpler here for now

 

[Heikki]

Looks to me C6 discharge path is P4 parallel to P3 + R18 || R19, ignoring the transformer resistance.

Adding the 100 ohm pot to cathode will change the bias a little bit and if it matters it can be corrected by changing R3. 1k plate balance pot probably doesn't do anything useful.

 

[abbey road d enfer]

Looks to me C6 discharge path is P4 parallel to P3 + R18 || R19, ignoring the transformer resistance.

Yes. I don't really understand their purpose, except maybe linearizing the impedance...? I made a simulation with and without them. Seems to me without is more correct.

 

[hereforever]

I tried removing R18/R19 in one of the units and frequency response/phase changed more pronounced with a hotter input signal/heavier limiting:


Unit 1 (R18/R19 left in place), -40dbfs frequency sweep


Unit 2 (R18/R19 removed), -40dbfs frequency sweep



Unit 1 (R18/R19 left in place), -20dbfs frequency sweep (heavier limiting)



Unit 2 (R18/R19 removed), -20dbfs frequency sweep (heavier limiting)

 

[zazoo]

I've finally had a couple of weekends to return to this project. In the meantime I came across another for sale and now I have a pair. The second unit has Thordarson input & interstage transformers and the same Stancor output transformer as the first.

The new unit is wired same as the print schematic with a minor addition- the T3 & T4 center taps are fed from a 10K resistor off the B+ line. I've wired my original unit to match this and I've been testing some modifications in the side chain. I made attack and release both variable and added a 'Speed' control inspired by the Sta-Level / Collins 356E / GE BA-9 circuits. I've been testing values for the capacitors and resistors by ear and how far I can push it until I get unstable behavior.

It's working well in this current configuration.

I also had a front panel made, mounted the units to rack shelves and used Molex connections to bring the circuit points I need out to the panel.

In measurements taken in Room EQ Wizard it seems to be fairly full frequency with a few db hump up around 12 or 15 kHz.

That looks amazing. Congrats. Do you have any recordings of it?

 

[zazoo]

I tried removing R18/R19 in one of the units and frequency response/phase changed more pronounced with a hotter input signal/heavier limiting:

View attachment 92601
Unit 1 (R18/R19 left in place), -40dbfs frequency sweep

View attachment 92602
Unit 2 (R18/R19 removed), -40dbfs frequency sweep


View attachment 92603
Unit 1 (R18/R19 left in place), -20dbfs frequency sweep (heavier limiting)


View attachment 92604
Unit 2 (R18/R19 removed), -20dbfs frequency sweep (heavier limiting)

How's do the high and low-ends sound on this? Based on these, it's pretty linear between 50Hz and 5KHz, but there's that weird wobble from 20 to 50Hz and that spike around 15KHz. How do those sound on actual music?

 

[hereforever]

That looks amazing. Congrats. Do you have any recordings of it?

Thanks. Haven't had any time to work on this recently, but will try to share some recordings/further tests on next pass.

I think r16 should be connected to the center tap of the GR transformer and the cap to the diodes, in a normal fashion. Then the transformer with the extra tube is actually an improvement over other typical designs, which take the signal for GR unbuffered from the output, such as the 176 etc.

My next move will probably be to wire the sidechain up as MaxDM suggests here.

 

[zazoo]

@hereforever Have you had a chance to do any recordings? I bought one too.

 

[hereforever]

I've still been fiddling with the side chain. Have not had too much time to dedicate on this as I have been working on actual music

I'm not very happy with the stock circuit for studio use but the transformers test well, so with some work I think this could be a cool unit.

Yours looks like it has the same transformers as my second unit. I thought this was a fairly rare piece of kit but I've seen at least 5 surface on Ebay/Reverb over the last year. Please keep us posted with any progress you make with yours!

 

[zazoo]

What are you unhappy with?
I plan to just use it as a blackbox and get what I can get from it for now and to learn what it is useful for, and eventually work with someone to make it sound more usable and versatile.

 

[hereforever]

Mainly I would like to get more control over the time constants and improve the frequency response as much as possible

 

[zazoo]

What bothers you about the frequency response?

 

[hereforever]

Well, ideally I would like a relatively flat response.

 

[Heikki]

Well, ideally I would like a relatively flat response.

You could remove the sidechain transformer and use capacitor coupling instead. For example copy from here how to wire the sidechain.

 

[hereforever]

You could remove the sidechain transformer and use capacitor coupling instead. For example copy from here how to wire the sidechain.

Thanks I've been looking at the UA 175 as well.

 

[emrr]

Well, ideally I would like a relatively flat response.

That means different things to different people. Did I miss a measurement? Many things pretty far from flat can fool the ear that doesn’t know. Many highly colored flat things seem not.

 

[hereforever]

That means different things to different people. Did I miss a measurement? Many things pretty far from flat can fool the ear that doesn’t know. Many highly colored flat things seem not.

Totally agree and hence "relatively"

 

[zazoo]

@hereforever I powered the Wilcox up with a variac today. It passes audio pretty well, definitely full-range, and I can hear its compression effect. But it definitely had some hum and buzz, so I decided to take a crack at replacing the caps. I got the caps with the right value, but I'm having a hard time locating the caps on the board. Since you've done this before, can you please tell me where c1, c2, c3, c4, c5, c6 are on the board?
Much appreciated!

 

[hereforever]

Here's the layout