Converting Apache clipper to limiter?

[Ed-VA3ES]

I'm considering converting the Apache clipper circuit into a limiter (along with the 12AU7 tube).  Before I continue, does anyone have any ideas, or thoughts? I don't want to reinvent the wheel.

[w3jn]

Could be done.  I'd use the 6AL5 to rectify some of the audio and feed the DC voltage back to a 6BA6 remote cutoff pentode which would be perhaps the 2nd audio stage.  I believe old buzzard compressor/limiters used 6L7 multigrid toobs for this purpose. 

Probably easiest to do this outside the xmitter though.

[WA1GFZ]

I was thinking of doing that to the limiter in the V2- CDC
I figure all you need is the right cap, reisitor and diode to adjust the attack and decay times

[Ed-VA3ES]

I'm now considering using a disused Waters compreamp, or clipper/limiter, installed in the unit.   OTOH, a self contained, repeateable mod would be useful for publication.

[WU2D]

Ed,

I think there are enough sockets to do it with a 6AL5 and one other socket. You would need a double tube with a remote cutoff pentode which would become the Vari-Mu compressor stage. The triode section could be the input stage. Tubes like the 6GH8/6U8/6DX8/6EA8 and 6GW8 come to mind.

Here is the idea - I have attached a schematic.

Mike WU2D

[w3jn]

AFAIK the only triode/remote cutoff pentode in a 9 pin miniature is the 6AZ8.  All of the above tubes are sharp cutoff and will not work for this application.

[WU2D]

I didn't design it - he calls for a Sharp Cutoff.

A remote cutoff tube would have more dynamic range, possibly resulting in lower distortion. For this simple circuit and many others, sharp cutoff types including the 6AU6 and 6BA6 are used as gain control tubes.

Hey it is a simple circuit that would fit in the Apache and it still leaves two sockets free, one for a 12AX7 preamp and one for something like a 6CG7 or 12AU7 Phase Splitter driver. Who wants to breadboard it?

Mike WU2D

[w3jn]

Ed,

I think there are enough sockets to do it with a 6AL5 and one other socket. You would need a double tube with a remote cutoff pentode which would become the Vari-Mu compressor stage. The triode section could be the input stage. Tubes like the 6GH8/6U8/6DX8/6EA8 and 6GW8 come to mind.

Here is the idea - I have attached a schematic.

Mike WU2D

That schematic may work FB as it uses the suppressor grid as another control grid, but the 6HZ6 is a sharp cutoff pentode.  The 9 pin miniature triode/pentode combinations generally don't have the suppressor grid taken to a separate pin; all the ones I've seen are internally connected to the cathode so this circuit would be pretty much impossible to build with one of the toobs cited above.

[Ed-VA3ES]

 Very interesting ideas guys. Thanks.   Not quite sure what you all mean by "it still leaves two sockets free".    Which tubes? 

If I can free up tube sockets, I'll build it in, otherwise, I may use a mini-box  and an appropriate plug, and plug in to an existing socket.   

[WU2D]

Wow - using the suppressor grid. The beautiful thing about the AM forum is that we have guys like John who really digs tubes and he heads off many problems.

The Apache has two modulator sockets (octals) and three 9 pin sockets and one 7 pin (the 6AL5). What I meant by "leave two sockets free" is that you really do not need V8 - you have PLENTY of gain. So lets consider that V8 and V10 are available.
That is exactly what would happen if you did the popular W3SCC Apache mod (attached).

The original question was basically if you could make the clipper into a limiter or compressor? V8, the 12AU7 and V10 the 6AL5 deal with the clipping. I figured this is where you wanted to put the compressor. These sockets are empty when you do the W3SCC modifications.

To do this however, we are back to trying to find a suitable tube for the compressor, which John says may not be easy (to find in a 9 pin double tube).

We could keep the 12AU7 in the V8 socket and use half of it for the input amplifier triode. Then we could put an appropriate miniature tube in the 7 pin socket - one with a suppressor grid like the original 6HZ6 from the diagram or a remote cuttoff type like 6AZ8 like John recomended or even a semi-remote cuttoff type like 6JH6,(try em all) and use a couple of diodes for the AGC rectifier.

Mike WU2D