Gates M5514 Tube Limiting Amp Module Schematic and ???

Hello Folks,

I recently acquired a Gates tube module that is marked "M5514 Limiting Amp".  No schematics to be found so I drew it out, recapped it completely, checked the tubes and resistors and brought it back to life.  However it does not seem to be a limiter - just a 70dB preamp!  The output stage has something odd going on but I can't see how it would cause any limiting. 

I consulted with Doug (emrr) and another knowledgeable friend to no avail.  So here is a PDF with the drawing and my notes from testing.  If anyone has any ideas, please let me know!

Oh, and Doug suggested I redraw the output stage from scratch to double check.  I did so here, but nothing was amiss that I could see.  It is easier to read and might help...

The 0.1u+40Meg of V3b really has to go somewhere. My best guess is V1 pin 1, probably via T1 return or VR1.

However V1 being a real sharp cutoff type, this would be a rude limiter.

OTOH I can't figure a 70dB straight amplifier with a half-12AU7 output stage; input overload would be way too small.

Thanks for taking a look PRR. 

The unit appeared dead stock inside and I replaced the caps one at a time.  All terminals on VR1 and all transformer connections are accounted for.

It was not designed for broadcast use, so a rude limiter would still make sense.  However, nothing looks amiss at V1 either.  Of course the only reason it can run 70dB is because of VR1 attenuating any incoming signal before it gets amplified.  This might also explain the near constant THD+N regardless of input level.

I just re-tested the module and it appears that 1% THD+N is actually +16dB, not +17.  But still, it will go up to +28dB with lots of distortion and no signs of limiting.

These modules were part of a system used for the hard of hearing, so rude limiter makes a lot of sense. 

I suppose someone could have removed a connection back in the day because they needed a straight amp or the limiting sounded like crap.  Why else would they rectify the output, right?  So I guess I'll mess with V1 Pin 1 as PRR suggested and see what happens.  Updates soon. -Mike

Ok, here's the update.  Not soon on the lunar cycle scale but soon on the glacial scale and projects like this sometimes move glacially.

I messed around with the circuit a bit, taking a signal from [V3 Pin 6/7] through a high value resistor to the various grids of V1 but I did not see any limiting response, just some oscillations and other weirdness.  That 40M/.1uf has to be a time constant, but obviously someone modded this before it got to me and I can't retrace their steps.

So in the end I made this into a usable tube preamp.  The switch was repurposed to connect [V2 Pin 1] to either the output of V1 or the wiper of the 100k pot.  The second pole switched [V1 Pin1] from signal to ground.  With V1 bypassed the unit gave 40dB gain into 600 ohms and had a slight bass boost (around +1.5dB@100Hz).  With the first stage in it acted as described previously.  Into a high z circuit it can easily do +24dBm before clipping.  I set it up with a recapped GenRad tube PSU and sold it to a gentleman in TX.