Re: Rare tube compressor: Lomo 600U-17 ( Ломо 600У-17 ) - Need info!
« Reply #20 on: January 09, 2017, 06:49:30 AM »
Ok, so I need to switch the comp in pos 4 or 5 and ajust these is order to have the less amount of control voltage (so DC? ) in the audio path..?

Anyway, I will recap it prior to keep working on it.

I'll also take some time to redraw the schematics as I realise it may contain some mistakes, and is not convenient to read at all. Way less convenient than the pixel version I have of the original one. I'll redraw it and will add the correct values..

I'll also have to build a proper power supply!

Here are the tubes power requirement:
250VDC @ 5,5mA
6,3V @ 330mA

Right now they are replaced by a pair of EF93 (drop in replacement, but maybe I am wrong? )
250VDC @ 4,2mA
6,3V @ ? mA
Datasheet :

150VDC @ ? mA (can't quite figure it out from the datasheet?)
6,3V @ 385mA

When I have figured out the missing values, I guess the ideal PSU will be 250VDC and 6,3VAC. I'll need to add the power consumption for each tube and that will give me the minimum amount of mA the PSU will have to deliver.
Am I correct so far?
Sorry if dumb questions, I have never designed PSUs for tube circuits... I know there is also a load that I will have to calculate in order to figure out the voltage drop of the PSU.


Re: Rare tube compressor: Lomo 600U-17 ( Ломо 600У-17 ) - Need info!
« Reply #21 on: January 18, 2017, 09:39:49 AM »

Today, a very kind man from the Lomo Museum near St Petersburg sent me the schematics I have been looking for for nearly a year! Many many many thanks to him, if he reads this.

So here it is, the long awaited schematic for the Lomo 600U-17 tube compressor.

If any of you have an idea of how to add a gain reduction meter,  that would be of great help.
But now, I am going to start by recapping it properly (some caps values are different than in the schematics).

It was designed in 1962, my unit is from 1964 if I believe the date stamps on the electrolytics capacitors!


Re: Rare tube compressor: Lomo 600U-17 ( Ломо 600У-17 ) - Need info!
« Reply #22 on: January 18, 2017, 10:03:12 AM »

If any of you have an idea of how to add a gain reduction meter,  that would be of great help.

From page 1:
You could probably shunt the 120R at the "ratio" switch with a DC-reading moving-coil meter. Maybe just a 1mA or so..? your case, across the 150R resistor..

Jakob E.
..note to self: don't let Harman run your company..


Ok thanks! Just across R5 while keeping it connected?


..note to self: don't let Harman run your company..


Thanks Jakob, I'll experiment with that!!

I have recaped the comp completely. I now need to build a proper power supply :)

I have a few questions regarding the general wiring:

What is the purpose of R42 (5,1K)?

I need help with the pinout...  I know A5 is the HT input, A6 is the 6.3V DC input (B6 is the other side of the twisted heater wires but its connected to ground). Until now I was feeding 6.3VAC between A6 and B6 so maybe that's why the ground cable was getting hot  ::) ...?
What I am unsure about is the input and output, or more precisely where I should hook up my hot & cold leads.. A8 seems pretty obvious for the input hot (except about that mistery R42 resistor), but what about cold? To ground, or to A7?
For the output, A1 seems obvious too, but then it is unbalanced?

Here is a schematic for the power supply... It clearly shows the heater supply is rectified.. Also, I don't know what are ШР-1 and ШР-2 on bottom right, but that's not really important...


ШР-1 and ШР-2 are sockets
Ш - штырь (pin) Р - разъём (socket, connector)
ШР-1 input, ШР-2 - output
« Last Edit: January 21, 2017, 04:55:42 PM by ungifted » - parts for guitar/studio gear


It looks like:
A5 is B+
A6 (B6) is filament
A8 (B8) - is input (via bypass switch)
A1 (B1) - is output
All B pins are connected to ground.

R42 goes to pos 1 of the П-1 switch must be something like resistor in feedback circuit ("off" position).
(where 1 - is "Off", 2 - 20dB to 10dB, 3 - 16dB to 10dB, 4 and 5 marked as balance "К" and "Э" don't know what it is) :)
« Last Edit: January 21, 2017, 05:24:49 PM by ungifted » - parts for guitar/studio gear


Thanks ungifted! So the compressor, despites its input and output transformers is unbalanced... that explains a lot :)
Still don't really understand that R42 and it seems the input signal goes thru it no matter what... maybe it's some sort of input pad?

I also have the full user manual but it's in russian from top to bottom, and I guess it explains how to set up the balance functions properly... I'll see if I can have someone translate it for me and report back!

Can't wait to try it in proper conditions!


I can see the manual and rty to translate it. - parts for guitar/studio gear


It's designed so -4bB (0.5V) input signal is passed untouched, Over 0.5V can be compressed 20 to 10 (or 16 to 10)

In troubleshooting section mentioned R42 - it allows to pass 0.5v input signal untouched.

Unbalanced, 600R
Attack - 1.2 milliseconds
Release - 500 milliseconds

Balance K and Balance Э are for settings: balancing in cathode or grid circuit (R3, R7) for lower noise,

Add active load to output 500R, switch to balance K (in this case to the center tap of input trafo applied 0.1V, using R3 try to get the lowest output signal level.
Add active load to output 500R, switch to balance Э (in this case to the center tap of input trafo applied 0.7V, using R7 try to get the lowest output signal level.
« Last Edit: January 22, 2017, 02:23:02 PM by ungifted » - parts for guitar/studio gear


Awesome!! Thank you! I'll get to that power supply and will keep you guys updated!!
Many thanks again for your kind help everyone!


I have build a PSU and ... the beast is alive!! It is very violent and squashy so I'll build an input and output attenuators! I'll also bring the threshold on the front panel. Sometime a slower attack time would be great, I'll see if I can figure out a way to add this.
It has a quite a lot of gain but is dead silent, which is great (maybe DC heater circuit helps!)

Jakob, I tried using a 1MA DC meter across the 150R resistors, and it's just pegged like crazy all the time so I quickly disconnected it (vintage simpson meter I was afraid to blow)

Thanks everyone for your help, and stay tuned for the racking process & pictures


So I played around with some T Pads attenuator in front and after Edcor 600:600 transformers, and it completely messes with the in & out level, and really has no use...

How can I figure it out the optimal input and output impedance for the circuit design? That way I can find the best way to attenuate the signal?

Thanks a lot!


I decided that since the comp is unbalanced and I use short cables, I'll use it that way.
I still need to figure out a way to wire passive attenuators at the input and output.
Is there a way to attenuate a unbalanced signal without messing with the impedance?


Without going into tedious figuring--- use a 10K or 25K pot.

The schematic has an apparent error.

Assuming 250V supply, this thing should have Zout under 300r but won't swing over 40V into 10K load, distort badly (and low-cut) below 1K load.

Assuming now-typical 10K inputs following, a 10K or 25K pot will be decent loading on the Lomo and decent source for the following gear. The 40V peak into the pot is so much-much more than it likely wants to give that THD will be small.

If you must drive true 600r load, you need a 10K:600 transformer, and expect low output.


If 1mA meter goes over full-scale, drive it from a 1K trimmer across the resistor in question...

Jakob E.
..note to self: don't let Harman run your company..



Sorry for the delay, I have a very poor internet connection at the moment.
I have played with 10K pots as input and output attenuators and it works just fine. These two with the comp threshold allow a lot of different options and flavors, just what I needed! Thanks a lot.
> I have noticed that on many tube comps, R1 & R2 on the Lomo are a dual gang pot input attenuator (Federal, RS124, STA Level, etc). Do you guys think I should wire my input attenuator here, instead of before the comp? It would allow me to hit the input transformer harder, no matter how I pad the input signal.
Re the meter, I have used a 200ohms trimmer accross a 1mA meter, it allows a more precise "0 VU" setting. I have absolutely no idea how much GR is going on, but the needle moves and gives an indication of how hard the comp is working so that's enough for my needs I guess :)
Last think I'd like to figure out, is how to tweak the timings of the comp. Right now it is very fast. A slower attack & release may be welcome some time. I have tried comparing the Lomo 600U's input push pull with the Federal's input as I have already tweaked one, but I am having a hard time figuring out what couple of resistor / capacitor I need to play with on the Lomo.




I am very sorry about the late late answer, things have been kind of crazy these last few years.

I am back on the old beast, in the process of recap and redesigning a power supply because last time I used it (over a year ago) it was making regular clicking noise like a cap had gone bad somewhere.

I am also designing a proper rack at the moment.

It is kind of hard to determine how much GR is going on, so I think I will drop the idea of adding a GR meter. I was thinking of maybe replacing it with a glowing bulb, if I find a way to have it off when the compressor is not working, and then light stronger and stronger when compression is happening.
If I understand correctly, a light bulb reacts to both current and voltage to shine, whereas the meter only reacts to current... ? I wish I was able to strap a light bulb instead of the meter and call it a day haha

To be followed ! Many thanks again for your support