Tube PA Amp Remote Voltage Control Circuit

[tablebeast]

I have been modding Bogen PA amps into guitar amps for years. One of the first thing I do is rip out this "remote control" or "precedence" circuit. It has a lot of stuff going on that I don't need if I'm just doing a plexi type circuit or whatever. I like to gut and rebuild from scratch anyway.

But now I am curious about this type of remote control for channel switching/fading. Curious enough that I bought a Bogen SR-2 to check out what was inside it because I couldn't find a schem for it or the SR-4 units online. Unfortunately the SR-2 I got was heavily modified so I don't know what is stock and what is mod.

Anyone have any clue what is going on inside the remote circuits in these old PA tube amps? Is it just a pot and a resistor? And what is it actually doing? Lowering the voltage would seem to lower headroom as well. Could be interesting in a guitar preamp circuit I'm thinking, like a voltage starving effect. My best guess is that it uses a pot with a resistor in series across the B+ voltage rail to ground. The extra .22uf filter cap between the B+ where the control circuit wires up and the next two plate resistors in the RCR must be there to smooth out a pop if a shorting switch was used with the remote. Adjusting it looks like it raises and lowers the voltage, which in turn  should lower and raise the gain accordingly, but in a different way than adjusting the volume control. I don't have one of those amps on hand at the moment, so I am just guessing here.

I know it should be very, very simple. But Bogen has a few different things that can be used as a remote besides the basic 2 (SR-2) and 4 channel (SR-4) control units. Stuff like the RVC-2A and the LVP-1 seem like they are some kind of different circuit entirely. I'm really just trying to start a discussion about this kind of control circuit and how it can be deployed in new and interesting ways.

 

[PRR]

Not everybody grew up hiding out in the PA closet. Might be real nice to show/link wtf you are talking about.

 

[tablebeast]

Apologies, here is a schematic from my very first Bogen from the B generation, the CHB-35A

 

[tablebeast]

Whoa, I didn't see that you had already pulled and posted the relevant schem. Any insight PRR? You were always so helpful back in the day. You'd probably dig what I have planned for this type of circuit if it works the way I want it to.

 

[PRR]

> Lowering the voltage would seem to lower headroom as well.

This.

First: this is off-base for basic simulation. It is working in the far corner of 12AX7 curves (and pushed far into the far corner), and it is gridleak biased. I have not seen a good triode model with realistic grid current in this mode. I faked one up by assuming that grid electron interception is proportional to cathode current, but it can't be more than approximately right.

100mV peak input, with "REM" voltage down 10:1 at 16V, gives 1.7V peak output with 16% THD.

Note that the C-R coupling at a gridleak input will, on LOUD transients, charge-down and reduce gross overload after some dozens of mS. However the effect is small at the small levels expected in the Bogen.

Remember what these things were for. School Principal through a dynamic mike reading the Detention List. The peak inputs were few-mV, not 100mV. In fact the all-up input overload is 10mV peak. At that point we get like 2%THD, 0.157V out, gain like 15. (Gain actually does not fall much.)

But why wonder?? You can mock-up a variable supply voltage on any available preamp. If you get a 2-Watt (MIL-type) 100K potentiometer you can run it right across a 300V supply. (I have done that to vary a power tube screen as a "throttle".)

 

[tablebeast]

All good stuff to know! This is the kind of stuff I just can't find on the net, being so far of in the corner and all. Thanks for the advice.

Now, does it have to be a grid-leak biased circuit to work with the remote? I'm already familiar with those circuits and do not like them.

If some kind of smooth volume fading isn't really doable, what about a simple on/off switch? Wouldn't just shorting that remote connection to ground switch off the output to that channel with the filter cap controlling how fast it fades in/out?

 

[scott2000]

There's a mention of some remote volume control circuits here.....towards the bottom... Radio Constructor article....

http://electro-music.com/forum/post-320662.html

maybe some more stuff to look at......??

 

[tablebeast]

Thanks scott2000, there are some interesting things in that thread. Thanks for sharing. I'm trying to keep this project I have in my head as simple as possible. I will reveal some specifics here in the next few days as I get it from my head onto paper so I can get more feedback. For now, this switching/fading circuit is the part I have the most interest in and questions about.