Tube compression circuit in a customized Roberts preamp

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CB Amp

Member
Joined
Jan 18, 2013
Messages
5
Hello,

I'm new on this forum, it seems to be a nice place ! :) And I need some advice, perhaps you can help me.

I have two Roberts preamp units in my workshop (from an old Akai reel to reel).
I wanted to use them to build 2 different Ampex preamps inside : an 601 and a 351, adding in/out transformers. And there is still enough room and tube sockets, so why not adding a tube compression circuit inside...
But the power transformer won't handle too much tubes. I can't build a LA2A or Vari Mu, I just have something like 500 mA left on the 6.3V windings.
So I'm looking for a very simple tube compression circuit, perhaps with 2 12AX7 (600 mA, ok...), an opto... Do any of you know such circuits ? I can't find anything except compression pedals for instruments.
Thank you !
 
I reply to myself...
Wouldn't it be possible to use only the compression part of the LA2A circuit (12ax7 and 6AQ5 tubes, and I'll add a trans for the heaters), and insert it in the Ampex preamp circuit, just before the gain control ?... The other 12ax7 and 12bh7 work as a preamp in the LA2A, so it should be possible to use only the compression part in other circuits.

But in this case, why not building a LA2A itself ?...  :eek: yes of course  ???
Anyway, for my knowledge I'd like to know if it could work. Any ideas ?

LA2A schem http://www.freeinfosociety.com/electronics/schemview.php?id=115
Ampex 351 : http://www.clarisonus.com/Archives/Ampex/Ampex351.pdf
And sorry, I just found service manuals for the 601 :
http://thehistoryofrecording.com/Schematics/AMPEX/Ampex-601-details.pdf
 
I have done something very similar to this recently. I refurbished a 1948 RCA PA amplifier for a client, adding a mic transformer and input/output XLRs as well as replacing all the caps and some of the PSU resistors. The client also wanted to add a compressor function to it. This would be based on the poor man's opto compressor I am developing and use the amps power output stage as the gain make up amp. To cut a long story short, the side chain consists of just a 6922 double triode wired as an SRPP driving the opto via a 1uF cap and 1K series resistor. If you set the gain make up for unity gain with no compression then, with compression on, the threshold is about -15dBu and the ratio varies from 3:1 to over 4:1 as the input level increases. The 6922 heater current is 375mA. The initial circuit of the poor man's compressor is here:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/PMTCL/PMTCLsc.jpeg

Cheers

Ian
 
Thanks a lot Ian. I think I will try this circuit.
With this, a T4B should also work as an opto, isn't it ? I'm not bound by the cost so I could use (build) one...

And another question : on the schem, you place this circuit just after the input preamp transformer (as in the LA2A)
In a typical studio configuration, we plug the mike in the preamp, and then the preamp goes into the compressor. So what would happen if we place this circuit just before the output transformer (at the end of the preamp) ? It would closer to a studio use.

Thanks !
Cyril.
 
CB Amp said:
Thanks a lot Ian. I think I will try this circuit.
With this, a T4B should also work as an opto, isn't it ? I'm not bound by the cost so I could use (build) one...

A T4B would work fine. By the way there is an error in my corcuit. The LED should be across the opto LED i.e. on the other side of the 1K resistor.

And another question : on the schem, you place this circuit just after the input preamp transformer (as in the LA2A)
In a typical studio configuration, we plug the mike in the preamp, and then the preamp goes into the compressor. So what would happen if we place this circuit just before the output transformer (at the end of the preamp) ? It would closer to a studio use.

Thanks !
Cyril.

The only problem with that is you have no method of adjusting the make up gain but other than that it will work fine.

Cheers

Ian
 
I've built a half dozen LA-2As into these chassis. I put the T4bs on an octal socket mounted on standoffs, and simplify the circuit with the Stereo and Limiter Response knobs omitted. Just strip the existing output transformer and choke and the tagboard down to bare, rewire the heaters to share the load (keep the hum balance pots in place), take the Cayacosta layout and mod it a bit and you're there. Power transformer and the tube socket lineup make this a great fit.
 
By the way there is an error in my corcuit. The LED should be across the opto LED i.e. on the other side of the 1K resistor.
Yes thanks, I noticed this  ;)

The only problem with that is you have no method of adjusting the make up gain but other than that it will work fine.
I thought I could add the whole circuit (with the two 6922) at the end of the preamp circuit, with a DPDT bypass (comp on/off). But I'll perhaps have to add a little tranny to feed the heaters...

I've built a half dozen LA-2As into these chassis.
Yes, these pres are very usefull pieces !
 
CB Amp said:
I thought I could add the whole circuit (with the two 6922) at the end of the preamp circuit, with a DPDT bypass (comp on/off). But I'll perhaps have to add a little tranny to feed the heaters...

Ah, I see. You confused me when you asked what would happen if you put it just before the output transformer - I thought you meant connect the side chain to the transformer primary. What you suggest will work fine. If there is an output stage with about 20dB of gain then you can use that as the gain make up and you only need to add the side chain.

Cheers

Ian
 
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