Strange old Tube compressor questions?

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AardvarkBry

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I just picked up this old tube compressor for almost nothing. It looks like it's pretty well made, and I'm hoping I can use it to add some interesting color. I found an old data sheet, and schematic, and I'm puzzled by how this thing functions. It says it has an 80k ohm input, and it can be used to either drive a 500 ohm line by adding a 1k ohm/2 watt resistor in parallel, or drive as low as a 3 ohm speaker by adding a 10 ohm/2 watt resistor in parallel.

I have 3 questions:
1. If I wanted to add a 600:50k, 1:10 input transformer, would I still need the 1k ohm resistor to drive the 500 ohm line level recording purposes? And would that be the best choice for an input transformer?
2. If you're driving either a 3 ohm speaker, or a 500 ohm line, does this change the input impedance?
3. How difficult would it be to add an adjustable time constant switch?
GC1-Schematic-and-info.jpg
 

hereforever

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C6/R16 are time constant components you could try modifying. Hard to read the schematic but looks like .25uF and 10M?

I can't really make out the voltages on the chart.

The 7B8 is the AGC tube - the negative bias created by the circuit to the right of the output transformer is what controls the automatic gain control ie. compression. Single ended compressor circuits like this are known to be thump machines but you could desolder one leg each of C6 & R7 put some components on alligator clip leads and see what sounds good?

Bandwidth on this is probably not great - the low values of the coupling caps suggest a HPF. Do you have REW? I would suggest running some tests with that to see where you are starting from.

What type of sources were you hoping of running into this?
 

AardvarkBry

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C6/R16 are time constant components you could try modifying. Hard to read the schematic but looks like .25uF and 10M?

I can't really make out the voltages on the chart.

The 7B8 is the AGC tube - the negative bias created by the circuit to the right of the output transformer is what controls the automatic gain control ie. compression. Single ended compressor circuits like this are known to be thump machines but you could desolder one leg each of C6 & R7 put some components on alligator clip leads and see what sounds good?

Bandwidth on this is probably not great - the low values of the coupling caps suggest a HPF. Do you have REW? I would suggest running some tests with that to see where you are starting from.

What type of sources were you hoping of running into this?
thank you for all the great info! .25uf, and 1.0m.

HPF is not ideal. I’m mostly a bass player, with a p-bass and flat wounds. I was hoping to get a little extra fuzzy, crusty, James Jamerson, Motown type mojo. I built a Wolf Box, so it would be a direct signal into my homemade OP6, then into the compressor, then into my homemade EQP1A, and then into an Ampeg 960.
 

AardvarkBry

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It says the input impedance is 80k ohms. I would like to add a 1:10 input transformer. I’m confused by the output load. It says if you add a 1k ohm/2 watt resistor in parallel to the input, you can drive a 500 ohm line as opposed to a 3 ohm speaker coil. Would the 1:10 transformer take the place of the resistor, would I have to still have the resistor, and/or would the value of the resistor change?
 

mjrippe

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It says the input impedance is 80k ohms. I would like to add a 1:10 input transformer. I’m confused by the output load. It says if you add a 1k ohm/2 watt resistor in parallel to the input, you can drive a 500 ohm line as opposed to a 3 ohm speaker coil. Would the 1:10 transformer take the place of the resistor, would I have to still have the resistor, and/or would the value of the resistor change?
That should read "1k ohm/2 watt resistor in parallel to the OUTPUT" to drive a 500 ohms line. Input transformer has nothing to do with output drive ;)
You might also try your bass straight into this box, without the DI.
 

hereforever

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That should read "1k ohm/2 watt resistor in parallel to the OUTPUT" to drive a 500 ohms line. Input transformer has nothing to do with output drive ;)
You might also try your bass straight into this box, without the DI.

+1 on both points

If you like the sound you could consider making the compression switchable on/off and/or making the threshold adjustable
 

AardvarkBry

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That should read "1k ohm/2 watt resistor in parallel to the OUTPUT" to drive a 500 ohms line. Input transformer has nothing to do with output drive ;)
You might also try your bass straight into this box, without the DI.
That makes a lot more sense! I couldn’t understand how the resistor on the input would effect the output that way! It’s funny they could make a mistake like that on their own instructions.

I like the idea of running a bass straight in. I would think the old time constant was probably pretty slow by today’s standard, which always lends itself to bass anyway.
 

mjrippe

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OK, I was curious about this device so I downloaded some more readable versions of the schematic. They actually DO show a 1k across the input - but that is coming from the output transformer of a ham radio receiver. It has nothing to do with the output of the GC-1 which can be wired straight to your next piece of gear as shown in the figure "Method 3".

Also, the stated frequency response is -6dB @ 100Hz, not designed for bass guitar :( I would start by upping C1, C3, C5 to .047uf
 
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AardvarkBry

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+1 on both points

If you like the sound you could consider making the compression switchable on/off and/or making the threshold adjustable
The manual says if you turn the sensitivity switch all the way counterclockwise, the input gets connected directly to the output. That sounds like a bypass.
OK, I was curious about this device so I downloaded some more readable versions of the schematic. They actually DO show a 1k across the input - but that is coming from the output transformer of a ham radio receiver. It has nothing to do with the output of the GC-1 which can be wired straight to your next piece of gear as shown in the figure "Method 3".

Also, the stated frequency response is -6dB @ 100Hz, not designed for bass guitar :( I would start by upping C1, C3, C5 to .047uf
wow, thank you for all that trouble. This is really helpful! I have several poly .047uf handy.
 

hereforever

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The manual says if you turn the sensitivity switch all the way counterclockwise, the input gets connected directly to the output. That sounds like a bypass.

If I'm reading the schematic correctly it looks like the switch bypasses the entire circuit. I meant an on/off switch for the sidechain to pins 3/4 of the 7B8. Basically this would turn it into a tube preamp/line amp.

wow, thank you for all that trouble. This is really helpful! I have several poly .047uf handy.

As Doug noted earlier increasing the coupling caps may induce thump. Try it and see what happens?

If it were mine I would definitely try some high impedance sources before going through the trouble/expense of adding an input transformer.

Hack away and good luck!
 

abbey road d enfer

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This compressor was designed to complement a radio receiver. It takes the output (loudspeaker or line), applies compression and drives another speaker.
Note 1 refers to the "receiver output". If the source is a line output (500 ohm), it needs to be loaded with 1k. If the source is a loudspeaker output, it must be loaded with 10 ohms.
The input of this compressor does not need to be loaded.
The input impedance is 80 k. A 10k:10k xfmr would be adequate here.
 

AardvarkBry

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This compressor was designed to complement a radio receiver. It takes the output (loudspeaker or line), applies compression and drives another speaker.
Note 1 refers to the "receiver output". If the source is a line output (500 ohm), it needs to be loaded with 1k. If the source is a loudspeaker output, it must be loaded with 10 ohms.
The input of this compressor does not need to be loaded.
The input impedance is 80 k. A 10k:10k xfmr would be adequate here.
Thank you. Am I reading method 3 correctly, that I can use the output as a line out/600 ohm? I assume “tuner, disc or tape recorder,” means line level? I am confused as to how it can drive anything from a 3 ohm voice coil, to a tape recorder line level. I was under the impression (perhaps falsely) that tube amps from this era were designing drive specific loads with their output tubes and transformers, otherwise you risk burning them out. Is it because the wattage is so low?
 

abbey road d enfer

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Thank you. Am I reading method 3 correctly, that I can use the output as a line out/600 ohm? I assume “tuner, disc or tape recorder,” means line level?
That's correct.
I am confused as to how it can drive anything from a 3 ohm voice coil, to a tape recorder line level. I was under the impression (perhaps falsely) that tube amps from this era were designing drive specific loads with their output tubes and transformers, otherwise you risk burning them out. Is it because the wattage is so low?
This amplifier supports unloaded operation because it is internally loaded by the 500r volume control and also because it uses a moderate amount of NFB.
 

AardvarkBry

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I just did the bare minimum, just to get it up and running, and safe, and man, I LOVE THIS THING! It's definitely got that warm crusty goodness I was looking for. Here's a little clip of what my bass sounds like, pushing the comp pretty hard, playing James Jamerson's bass line on Stevie's "For Once In My Life." It's probably missing a touch of low end, but I think the P-bass with flat wounds makes up for it. I think I can live with it. What do you guys think, should I try swapping the .047uf caps, or just leave it?
 

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AardvarkBry

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I’ve been running this compressor through it’s paces. It sounds great on electric bass, and upright bass. There’s a definite high pass filter, but the basses have so much low end that it doesn’t seem to diminish the sound. However, I tried diy on drums and vocals, and there’s a noticeable good chunk of bottoms end missing. You guys were talking about thumping being an issue if I expand the bandwidth. I’m not familiar with what thumping means? As a bass player thumping is a good thing, but I get the impression it’s an undesirable quality in this case. Should I swap in those .047ufs and risk the thumping, or just count my blessings this thing makes great bass compressor, and that’s it’s only function?
 

mjrippe

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"Thumping" in this case refers to an audible burst of low frequency noise when the compressor crosses the threshold. You can swap the caps and see if it is a problem, or leave it and use it only for bass. Although the high pass filtering seems counterintuitive, it may be removing some muddiness from your signal and leaving a clearer tone. Whatever works!
 

AardvarkBry

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"Thumping" in this case refers to an audible burst of low frequency noise when the compressor crosses the threshold. You can swap the caps and see if it is a problem, or leave it and use it only for bass. Although the high pass filtering seems counterintuitive, it may be removing some muddiness from your signal and leaving a clearer tone. Whatever works!
Thank you for the explanation. I noticed the thumping even with the HPF. I assumed that was what that probably meant. I went ahead and changed C1, C2, and C3 to .047uf, and added a 1:10 input transformer. It doesn’t seem to be thumping more than it was before, and it’s astonishing how much more bandwidth that’s added. Does the last remaining coupling cap .005uf C5, between the 12AX7 and the output tube, not affect the bandwidth in the same way? Would I be better off swapping that one out as well?

It’s got a magic eye indicator, and I’ve learned that as long as I set the compression to where the two ends don’t ever clap together at max gain, thumping isn’t an issue at all. This thing isn’t exactly a be all-end all comp that’s going to make me stop building this LA2A I’m working on, but it’s got a crusty stanky vibe to it that I really love.
 

AardvarkBry

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"Thumping" in this case refers to an audible burst of low frequency noise when the compressor crosses the threshold. You can swap the caps and see if it is a problem, or leave it and use it only for bass. Although the high pass filtering seems counterintuitive, it may be removing some muddiness from your signal and leaving a clearer tone. Whatever works!
Tell me if I’m being a pest, but I haven’t had any experience with tube circuits without cathode bypass caps. Would there be any point in adding some 10-25uf bypasses? Could that be detrimental? Are there any stages I should avoid them here?
 

abbey road d enfer

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Tell me if I’m being a pest, but I haven’t had any experience with tube circuits without cathode bypass caps. Would there be any point in adding some 10-25uf bypasses? Could that be detrimental? Are there any stages I should avoid them here?
There are two differences between by-passed cathode and un-bypaased cathode stages.
The un-bypassed has less gain and is more susceptible to noise injected by the AC filament voltage (or the ripple voltage of DC filaments).
By-passing the cathodes would increase significantly the overall gain of the compressor, maybe to the point of making it unusable.
 

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