Need Help: Figuring out replacement output transformer value for tube clipper

[mjrippe]

Hi Folks,

I have an odd CAA tube clipper "limiter" on my bench with a burnt out primary on the OPT.  All the original iron is Thordarson, but the OPT part number T-49919A is a house number.  Google search only tuns up a similar unit that sold on ebay.

I have drawn out the schematic for the output stage (below).  It is a bit convoluted, but I believe everything is correct.  The value of L1 *may* be off as it was moving around a bit on my Sencore LC75.  Measured with a handheld Tenma LCR meter I got a steady 0.79 or so.

So point "C" is the B+ straight from the rectifier - about 265 volts with the bad transformer in place.  Point "B" goes to the input stage plates and point "A" is audio from the input stage.

The output should be 600 ohms, obviously there is a center tap on the original transformer but it is not really needed.  My guess from working on similar CAA/FAA stuff would be a 10K primary, but I'm not sure if R19 is an indicator of that or a red herring!  Basically the output stage is just weird enough that I can't get my head around it. 

So the question is - what primary impedance, and roughly how many mA of DC should I expect?

 

[ruffrecords]

R18 is very puzzling as it forms a pot diver from the HT with R17, the cathode resistor of the output 6SN7. It might be a way of raising the cathode voltage without using a large cathode resistor. 250V or so across 25K is 10mA . A quick look at the data sheet suggests the operating point is a little over 5mA which is typical for a 6SN7 single ended topology. It might be worth checking you have traced that part correctly.

The 6SN7 has a plate resistance in the region of 10K at typical cathode currents so a 10K:600 transformer is probably in the right ball park.

Cheers

Ian

 

[mjrippe]

Thank you, Ian.  I just triple-checked and the drawing is correct.  However R17 is measuring around 750 ohms or 10% out, so that will be replaced.  I will look for an output transformer with 10K:600 and 15mA DC handling just to be on the safe side.

 

[emrr]

Agreed about R18 raising cathode voltage as a combo of cathode and fixed bias, I haven't looked long enough to see if this approach makes sense due to the clipping method. 

I suspect it along with the zobel and the choke are also a tailored EQ for band-limiting purposes.  That's just a casual guess. 

 

[mjrippe]

Thanks for the input, Doug.  As I'm sure you know, these things were pretty bandwidth-limited for speech use so that makes sense.

 

[ruffrecords]

A 15mA transformer should be fine. 15mA at 265V is  above the permitted plate dissipation of the 6SN7 so if you end up with that much plate current the tube won't survive long!

Cheers

Ian

 

[mjrippe]

Hi Ian, I was thinking overkill might be good since the original OPT lost it's primary winding   I just scored a Freed transformer that was removed from some Navy gear.  The specs are 8k single ended Primary at 20ma class a current to 2 600 ohm secondaries.  It is rated +.3, -.5dB 250-3500 hz.  Hopefully this will do the trick.