Adding panpots to tube Raytheon console
« on: December 05, 2017, 12:51:51 AM »
Hi all,
I'm a new member here, but i've spent many hours reading this forum over the years. Great source for info, thanks!
I'm starting the process of rebuilding/converting a vintage Raytheon RC11 tube broadcast console into a 9x2 tube mixer. It originally is a "dual" mono console, each channel has a switch to send the audio to the program or the monitor amp, or off (center position).  I want to replace these switches with a NewYorkDave style panpot, to convert the program/monitor amps to "left/right". I am not sure what impedance/resistor values I should be shooting for. Each mic preamp has a plate to line transformer feeding a 500ohm daven attenuator, and then a 470ohm mix resistor feeding the mix bus. Input for the program/monitor amps are 500ohm line to grid transformers.
Any ideas on the best value for the panpots? I would like to remove the 470ohm mix resistors (and switches) and replace them with the NYD panpot circuit.
Attached is a schematic for the RC11.
Thanks!!!
Tyler



ruffrecords

Re: Adding panpots to tube Raytheon console
« Reply #1 on: December 05, 2017, 03:54:38 AM »
This may be difficult. This is a typical 'constant impedance' mixer. When an input is routed to a bus it is fed through a resistor to the bus. When it is deselected it is replaced by a resistor of the same value across the bus so you cannot just change the value of the series resistor as in the NYD pan circuit. Also, the existing switch system connect and input to one bus OR the other. A pan pot need to connect the signal to both buses and I am not sure if you can maintain the constant impedance to both buses as the pan operates and also not overload the channel amp.

It probably can be done but it is not as straightforward as you might think.

Cheers

Ian
« Last Edit: December 05, 2017, 04:15:39 AM by ruffrecords »
www.customtubeconsoles.com
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

emrr

Re: Adding panpots to tube Raytheon console
« Reply #2 on: December 05, 2017, 06:04:24 AM »
What Ian said.  Any impedance change from any channel will make overall gain and mix swim up and down with the change.   

I honestly don't think it's worth the work for the limited value gained, and the value hit on the console either, unless the console is truly trashed.  You may be taking value off for the next guy down the road.  All the guys I have set up with consoles like this want them as original as possible. 

And, the console isn't dual mono with 2 of the same output amp either, unless you're telling me the output transformers are the same, and only the output tube configuration is different.   You can do that with two different amps, but it's not an entirely matched sound or setting.

If the switches have enough contacts, you can go LCR, try to find Altec 250SU or T3 documents.  Those are the only old consoles that even did that, everything else was mono or dual mono. 

When panning was done then, it was a Daven or Langevin type ladder pan pot, essentially a dual Ladder designed to maintain impedance no matter the position.  No stock console was ever built with a full array of those, nor with even a few.  Customs may have had one or two on a patchbay for custom assignment.  You could probably do a simple version of that as a best case, maybe 5 position.   Ton of work. 
« Last Edit: December 05, 2017, 05:40:41 PM by emrr »
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: Adding panpots to tube Raytheon console
« Reply #3 on: December 06, 2017, 02:45:04 PM »
Thanks for the thoughts guys.
I could sacrifice one of the mic preamps as the "center" make up gain stage amp like Altec 250SU... and setup the switching for L/C/R.
Here's another idea: The output transformers for the channel amps have a dual secondary. I could split up that secondary into two seperate outputs, and add in 7 more 500/500 ohm davens, and have L/R volume controls for each channel. However, this would change the impedance reflected to the 6J7 tube, and lower the output gain..... so I dont know how well that work. Plus i'd have to source those davens. Thoughts?
emrr, you are correct that the the two output amps are slightly different, one has output wired as triode and the other as pentode, and have different output trannys. The specs for gain and frequency response in the manual for the two different amps are very close however, so I am not going to worry about it. That will add a little extra "flavor" into the mixes!
I have not been able to find NewYorkDave's .pdf about his panpot design. Does anybody have it? I would like to read it.
Thanks again,
Tyler

ruffrecords

Re: Adding panpots to tube Raytheon console
« Reply #4 on: December 06, 2017, 04:27:00 PM »
I guess the big question is how many pan positions do you need. If you need a continuous pot then it is not likely to be possible. If you just want LCR and a step each side between these two then it might be possible with some rotary switches.

Cheers

Ian
www.customtubeconsoles.com
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

emrr

Re: Adding panpots to tube Raytheon console
« Reply #5 on: December 06, 2017, 05:19:43 PM »
Here's another idea: The output transformers for the channel amps have a dual secondary. I could split up that secondary into two seperate outputs, and add in 7 more 500/500 ohm davens, and have L/R volume controls for each channel. However, this would change the impedance reflected to the 6J7 tube, and lower the output gain..... so I dont know how well that work. Plus i'd have to source those davens. Thoughts?

That's a better idea than many, if you're drilling the face.  Sourcing those Davens currently at least is cheaper than the parts/labor of wiring up pan pots that'll work.  Those preamps are 30dB at most, maybe less.  I don't think you take much of a hit doing that.  If I follow the ratio correctly, splitting up the windings changes it to a pair of 125R windings, so each then is driving a lighter load.  I think it mostly comes out in the wash.  You can then lose all the switch loading since it's no longer needed, and the switches. 

The Altec LCR switching plan means replacement of some of those switches too, there aren't enough contacts on all of them.   I think I'd add another center  buffer amp somehow rather than giving up an original preamp channel.  Only a slight hit on Fil/B+ requirements. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

PRR

Re: Adding panpots to tube Raytheon console
« Reply #6 on: December 07, 2017, 12:52:39 AM »
I was thinking: cut the middle out, route out to a $200 8/2 line mixer and back.

Re: Adding panpots to tube Raytheon console
« Reply #7 on: December 07, 2017, 12:21:00 PM »
Thanks again for the replies.
I have found one 500/500 Daven locally. I will try splitting the output tranny on one channel and wire up a daven for each side. Will report back on how it works....
-Tyler

Re: Adding panpots to tube Raytheon console
« Reply #8 on: December 07, 2017, 12:35:56 PM »
PRR, that's certainly a solution that would work.... but the whole point with this console is to not use transistors or opamps... all tube. :)

emrr

Re: Adding panpots to tube Raytheon console
« Reply #9 on: December 07, 2017, 01:17:35 PM »
....I refrained from suggesting 990C buffers.....
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde