Control Voltage cabling in tube limiters

[tardishead]

Hi guys
I am finishing a Fairchild style comp for myself after making a few units for friends over the years which have all worked extremely well.
Typically when setting up my own unit I came across a problem I have not experienced before.
Basically AC audio signal is appearing on the control voltage line after the bridge rectifier.
It is particularly present from low bass to 1khz and then attenuates up to nothing at 10khz.
It completely messes with the limiting effect and there is audible distortion on the faster settings.
On the time constant switch when I switch in larger capacitors only then the AC bleed is flattened.

Its been doing my head in for a few days. I have rechecked my build over and over paying particular attention to the CV line after the control amp transformer and to the grids of the vari mu tubes.I was convinced I had made a mistake.
Now I did mention that I have built this design and works perfectly. 3 times before.
And I suddenly realised that the only difference with the current one is that all previous units have had the in/out transformers mounted on the back of the unit with holes connecting wiring to the inside electronics. On this one I mounted the control amp transformers on the INSIDE for aesthetic purposes. They are open frame Edcor types.

I suddenly thought could the CV bleed be coming from the control amp transformers themselves.
My CV wiring to the Time Constant switch on the front panel is not screened as I have never found a need to do this before. But like I said the CA transformers I have always previously mounted the on the outside.
Could the CV wiring be acting as a large aerial and picking up the field of those transformers?
Could this be the source of the problem?

Or is the positioning of the control amp output transformers on the inside of the unit (pretty close to the signal amp input circuitry and transformer) more of a obvious problem?

I was going to try wiring the cv with screened cable but wanted to just get some insights before I did so.
Is normal unbalanced ok?
Or should I use twisted pair?

 

[abbey road d enfer]

Hi guys
I am finishing a Fairchild style comp for myself after making a few units for friends over the years which have all worked extremely well.
Typically when setting up my own unit I came across a problem I have not experienced before.
Basically AC audio signal is appearing on the control voltage line after the bridge rectifier.

This is absolutely normal.

It is particularly present from low bass to 1khz and then attenuates up to nothing at 10khz.

Yes. Rectification ripple is more pronounced at low frequencies.

It completely messes with the limiting effect and there is audible distortion on the faster settings.

That's a quite normal artefact of compression.

On the time constant switch when I switch in larger capacitors only then the AC bleed is flattened.

That's to be expected.

I suddenly thought could the CV bleed be coming from the control amp transformers themselves.

I can't imagine one second this explanation.

My CV wiring to the Time Constant switch on the front panel is not screened as I have never found a need to do this before.

Not surprising.

Could the CV wiring be acting as a large aerial and picking up the field of those transformers?

No.

Could this be the source of the problem?

Look somewhere else. Are you sure something is not discharging the rectifier? It just takes a leaky cap to change drastically the release time.

Is normal unbalanced ok?

Yes

Or should I use twisted pair?

No need for it.

You should compare the control voltage with one of the working units, you'll probably find they are different. Maybe the time-constant cap is not of the right value...

Can you post the schemo?

 

[tardishead]

Thanks Abbey
yes of course there will be ripple on the CV. Sometimes my brain panics and creates all kinds of stupid possibilities.

It completely messes with the limiting effect and there is audible distortion on the faster settings.
That's a quite normal artefact of compression.

Yep I realise this but my unit is crapping out to distortion a lot earlier than normal. I could normally smash drums on the fastest setting at massive amounts of GR and still sound good.

I must have some seriously unmatched tubes.
I understand that unmatched tubes will create thumping but I have never witnessed this amount of IM distortion before. On both left and right channels as well.
I think its time to build a tube tester rig and really find out whats going on.
I have a lot of tubes from completely different manufacturers.

I can null the thumping with the cathode balance pot but cannot get rid of this harsh distortion in deep gr. Sounds very much odd order harmonics.
Would seriously unmatched tubes do this??

 

[abbey road d enfer]

I must have some seriously unmatched tubes.
I understand that unmatched tubes will create thumping but I have never witnessed this amount of IM distortion before. On both left and right channels as well.
I think its time to build a tube tester rig and really find out whats going on.
I have a lot of tubes from completely different manufacturers.

I can null the thumping with the cathode balance pot but cannot get rid of this harsh distortion in deep gr. Sounds very much odd order harmonics.
Would seriously unmatched tubes do this??

It could be, but I think you would experiment distortion even with moderate GR. Are you positive about the time-constants. A leaky could shorten drastically the release time.

 

[emrr]

Have you replaced the side chain rectifier?  At least with tube rectifiers, they can be the culprit.  Probably a failure mode with SS as well. 

 

[tardishead]

Yes checked the side chain amp, TC network and rectifiers.
Measured the tubes 6BC8 with a  basic test rig
turns out that only one double triode out of 20 had a reasonable balance.
In the past I was able to buy batches of 10-20 made by the same manufacturer and found some very suitable tubes.
I bought a batch of 10 by GE and they were all rubbish.
I read at the undertone audio site that he was struggling to find suitable 6BC8s for his Unfairchild.
They are quite hard to find in batches and also they are more expensive now. I think buying any more at the moment would be risky.
I think I will have to check out the Russian 6N5P tubes.

I don't think thats the only problem I have
I had a quick look at the control amp and I think I am getting overshoot - probably from too much negative feedback hey?