this vari mu can't be right

[pucho812]

downtime from work all week and my buddy brings  m a vari-mu to look at because he thinks it's acting funny.
Well he wasn't too far off, this things does all kinds of odd stuff. I am wondering if it is a tube related issue. All the  tubes tested out o.k. on the tube tester, th PSU is working fine with low ripple. Followed calibration instructions in the manual.

here is the odd
In order to see any gain reduction in compression mode I have to feed it a signal upwards of 19dBu as measured by the AP
I can get limiting happening with a +4dBu signal but I get a max output level of +12dBu  as measured by the AP
Also channel 1 seems way more noisy then channel two but I am guessing that is a tube thing?

Something seems quite off about this.... 

 

[mjrippe]

Tube testers only test tubes at a certain fixed point on their curve.  The unit may well be using a different point, or in the case of a vari-mu, section of the curve.  They also do not generally test for noise in the audio range.  Swap tubes and see what happens.

 

[bernbrue]

Would you tell us, which vari mu it is?
Bernd

 

[pucho812]

Would you tell us, which vari mu it is?
Bernd

starts with an M and ends with a anley.  but don't want to conjure up folks around here which is why I first left it out.

 

[ruairioflaherty]

C'mon guys.  EveAnna has been protective of her I.P. before, and rightly so IMO but she is not the devil.  Naturally I'm biased as she is a good friend but It's not in her interests to have a faulty product out there.  Of course, she would refer you to Paul at Manley service who is great but this is a DIY forum….

So first, which version is it?  The 5670, the 6386 or the T-Bar 6BA6?

As MJ says a tube tester isn't a whole lot of help in this instance.  Once we know what version we are looking at we can start getting a handle on expected voltages etc.

It sounds like a side chain issue, but more details will help.

 

[bernbrue]

Pull the sidechain tube (EAA91) and measure the amp only. Since you need an extremely hot signal to get compression, it is very likely that the sidechain tube is faulty. I've used the schematic of the DISA as reference, which is not that far away from your varimu.
Bernd

 

[emrr]

Attenuator?
Shorted cathode bias cap? 
Stereo versus mono behavior channel comparison? 
so many options.....so little known in TV land. 

 

[pucho812]

C'mon guys.  EveAnna has been protective of her I.P. before, and rightly so IMO but she is not the devil.  Naturally I'm biased as she is a good friend but It's not in her interests to have a faulty product out there.  Of course, she would refer you to Paul at Manley service who is great but this is a DIY forum….

So first, which version is it?  The 5670, the 6386 or the T-Bar 6BA6?

As MJ says a tube tester isn't a whole lot of help in this instance.  Once we know what version we are looking at we can start getting a handle on expected voltages etc.

It sounds like a side chain issue, but more details will help.

I agree and she is also a good frind, but do not want any unwanted attention around here is all.  Yes it has the t-bar mod.  I have a schematic but it does not have the t-bar mod on it. No biggie I am working it all out, just bouncing ideas off folks...

 

[bernbrue]

If it has the T-bar with two 6BA6 tubes as triodes you might look at the schematic of my sta level project to get an idea.
Bernd

 

[pucho812]

If it has the T-bar with two 6BA6 tubes as triodes you might look at the schematic of my sta level project to get an idea.
Bernd

will do