So I couldnt like wait a day for the proper tubes to arrive for a vari-mu circuit, so I thought I'd just jam a big negative transient down 12AX7's tailed pair throat, in that "cold clipper" position. 12AT7 and 12AV7 worked a little better, to where you could at least hear some compression really happening from the grid reference ties being driven negative by the envelope voltage. Which turns out to be the interstage potentiometer's ground connection now tied instead to the envelope voltage control.
Same DC envelope circuit essentially as Fairchild, only I'm using a spare 70V winding on the opt my amp happens to have to drive the full wave bridge. It only takes about 1/5 of the voltage that can make, so I'd have some confidence in a 16 Ohm winding being able to work also (25V) to generate a negative going envelope for upstream diff pair tube gain control.
I bought some 150V Zeners on the 12th still not here, probably lost in the mail. Re-bought different ones from a different vendor...
Same DC envelope circuit essentially as Fairchild, only I'm using a spare 70V winding on the opt my amp happens to have to drive the full wave bridge. It only takes about 1/5 of the voltage that can make, so I'd have some confidence in a 16 Ohm winding being able to work also (25V) to generate a negative going envelope for upstream diff pair tube gain control.
I bought some 150V Zeners on the 12th still not here, probably lost in the mail. Re-bought different ones from a different vendor...
After experimenting today, I've come to the conclusion that even a tube made for vari-mu or AGC circuits cannot work in an overdriven situation. You do get some gain variation, but not nearly enough to be effective as a compressor. Any such tube must go into the first position in the amplifier gain-chain, well, as depicted in all the compressor circuits I've seen and those I've posted here. A day wasted trying to put it in the cold-clipper position, aside from learning it cant go there.
Before I try a vari-mu tube as my input phase splitter, I'm going to play around with the Vactrol method of attenuation again. I also obtained some H11F1 parts; the LED - photoFET optoisolator. The FET part breaks at just 30V, while the photoresistor in the Vactrol is a bit more forgiving when it comes to the voltage you can put across it, in the high Z state. We'll see if I can make use of the H11F1 parts as well, considering solutions to their relative fragility wrt the voltage levels I'm seeing in these tube stages. Maybe, there's no way...
The cold clipper makes a nice "full wave rectified" looking signal across its 10K common cathode resistor and I'd like to use it to generate the DC control envelope for the LED part of these opto-isolators. One aspect I'd like to retain is having the effect not be dependent on an output to the speaker, as it would be when using a secondary OPT winding as a signal source. I noticed it doesnt take much current to the Vactrol LED to attenuate all the way to zero - heck just the DMM's diode test current will do it.
I have a gang of 3, 25 Ohm pots I pulled from some home theater passive surround box I snagged at the thrift store. I've wired all of them in parallel to make an ~8 Ohm resistive load, then connect the speaker to the wiper. Turned toward the ground side, it beats listening to my failing experiments with the speaker connected straight to the amps output. I'm going to have to find a place for that to mount permanently in my cabinet, along with a switch to disable the loading and let the speaker instead have 0 - 8 Ohms in series. It'll be great to simply reach under the bench and turn it down to a tolerable SPL, with the amp still running full blast and "properly" loaded.
Before I try a vari-mu tube as my input phase splitter, I'm going to play around with the Vactrol method of attenuation again. I also obtained some H11F1 parts; the LED - photoFET optoisolator. The FET part breaks at just 30V, while the photoresistor in the Vactrol is a bit more forgiving when it comes to the voltage you can put across it, in the high Z state. We'll see if I can make use of the H11F1 parts as well, considering solutions to their relative fragility wrt the voltage levels I'm seeing in these tube stages. Maybe, there's no way...
The cold clipper makes a nice "full wave rectified" looking signal across its 10K common cathode resistor and I'd like to use it to generate the DC control envelope for the LED part of these opto-isolators. One aspect I'd like to retain is having the effect not be dependent on an output to the speaker, as it would be when using a secondary OPT winding as a signal source. I noticed it doesnt take much current to the Vactrol LED to attenuate all the way to zero - heck just the DMM's diode test current will do it.
I have a gang of 3, 25 Ohm pots I pulled from some home theater passive surround box I snagged at the thrift store. I've wired all of them in parallel to make an ~8 Ohm resistive load, then connect the speaker to the wiper. Turned toward the ground side, it beats listening to my failing experiments with the speaker connected straight to the amps output. I'm going to have to find a place for that to mount permanently in my cabinet, along with a switch to disable the loading and let the speaker instead have 0 - 8 Ohms in series. It'll be great to simply reach under the bench and turn it down to a tolerable SPL, with the amp still running full blast and "properly" loaded.
Last edited:
Done!
It's hot to the touch, maybe 50 C after a few minutes of thrashing the amp with all 3 volumes at 10. If it fails, I'll use this other one, but have to get 6 ohms in front of it. I'm way up on the attenuation anyway, way down on the SPL. It's a Godsend - and I bet the neighbors would think so too. It's funny to hit that switch with it quiet as a chord is decaying, have it blast into a sustained feedback. Could almost put that into a Foot switch!
It's hot to the touch, maybe 50 C after a few minutes of thrashing the amp with all 3 volumes at 10. If it fails, I'll use this other one, but have to get 6 ohms in front of it. I'm way up on the attenuation anyway, way down on the SPL. It's a Godsend - and I bet the neighbors would think so too. It's funny to hit that switch with it quiet as a chord is decaying, have it blast into a sustained feedback. Could almost put that into a Foot switch!
Last edited:
I was able to coax the Vactrol back into operation today, using the 10K common cathode resistor of the so-called "cold-clipper" stage for a drive signal. It doesnt take much current to get the Vactrol moving its LDR around in value, ~300uA will shutdown the audio signal path I have it connect to, basically across one gang of the 250K "post" volume pot.
The tube of course has some quiescent voltage across that cathode resistor with no signal. I tried to trim that off some using an LED which has a larger forward drop than a regular diode. The diode has a 10K resistor in series with it, feeding a 0.5 uF cap, which discharges across a 2M potentiometer. The wiper of which connects to the Vactrol diode through a 180k resistor.
About mid setting you get the reverse envelope where as the guitar strings decay in amplitude exponentially, the Vactrol attenuated signal is doing the opposite; it's rising in amplitude. So the louder the guitar input is, the more squashed just one side of the drive to the output tubes - as it decays off the drive evens out in amplitude. A dynamic shift between SE and PP operation of the output stage.
The effect isnt so in your face either; it is more subtle than a full on compression.
So I am considering doing a voltage doubler for +/- DC off the existing grounded center tap 6.3V heater. To take it further, I really need op-amp circuitry to do the active analog envelope generation and processing to create a better drive signal for the Vactrol. Is there anything I'm unaware of attempting this power supply off the 6.3V heater - which has a grounded center tap already in place? i.e. ;
I'd be running a few op-amps off these voltages, input signal ground referenced, output signal floating (no current through ground). FWIW, I'm pretty sure my amp's heater winding can do better than 3A...
The tube of course has some quiescent voltage across that cathode resistor with no signal. I tried to trim that off some using an LED which has a larger forward drop than a regular diode. The diode has a 10K resistor in series with it, feeding a 0.5 uF cap, which discharges across a 2M potentiometer. The wiper of which connects to the Vactrol diode through a 180k resistor.
About mid setting you get the reverse envelope where as the guitar strings decay in amplitude exponentially, the Vactrol attenuated signal is doing the opposite; it's rising in amplitude. So the louder the guitar input is, the more squashed just one side of the drive to the output tubes - as it decays off the drive evens out in amplitude. A dynamic shift between SE and PP operation of the output stage.
The effect isnt so in your face either; it is more subtle than a full on compression.
So I am considering doing a voltage doubler for +/- DC off the existing grounded center tap 6.3V heater. To take it further, I really need op-amp circuitry to do the active analog envelope generation and processing to create a better drive signal for the Vactrol. Is there anything I'm unaware of attempting this power supply off the 6.3V heater - which has a grounded center tap already in place? i.e. ;
I'd be running a few op-amps off these voltages, input signal ground referenced, output signal floating (no current through ground). FWIW, I'm pretty sure my amp's heater winding can do better than 3A...
Last edited:
Thanks, wg-ski.
This afternoon, I took it for some exposure to a couple other players at this guy's detached garage studio. Double garage doors, one being an RV bay, tall ceiling, pretty big space inside. It was a good thing I cooked up the "power soak" accessory in my speaker cab, because straight to the speaker, this amp was just too loud. For those folks playing acoustics through compressors into a board / house PA system he has setup out there.
One guy was playing "aint talkin bout love" on his acoustic in a pretty commanding way, but he was a guest and I had the get together's host play through it first. Then his buddy played some slide, both using the host's Telecaster. They certainly seemed to like the amp's sound, the slide player commenting it had a good overdrive sound for electric slide playing.
I honestly couldnt tell if my "Vactrol compressor" part was even working, as currently driven from the clipper tube common cathode resistor. Turned the effect control up all the way, couldnt hear a thing change. Maybe my guitar with humbuckers that I developed the drive level with has a lot more signal than that Tele. Looks like I'm going to need a sidechain of op-amp signal processing to be able to compensate for a wide range of signal input and make sure I can drive that Vactrol attenuator "all the way down to off" for any reasonable electric guitar pickup input level / volume setting.
As they say, back to the drawin' board.
This afternoon, I took it for some exposure to a couple other players at this guy's detached garage studio. Double garage doors, one being an RV bay, tall ceiling, pretty big space inside. It was a good thing I cooked up the "power soak" accessory in my speaker cab, because straight to the speaker, this amp was just too loud. For those folks playing acoustics through compressors into a board / house PA system he has setup out there.
One guy was playing "aint talkin bout love" on his acoustic in a pretty commanding way, but he was a guest and I had the get together's host play through it first. Then his buddy played some slide, both using the host's Telecaster. They certainly seemed to like the amp's sound, the slide player commenting it had a good overdrive sound for electric slide playing.
I honestly couldnt tell if my "Vactrol compressor" part was even working, as currently driven from the clipper tube common cathode resistor. Turned the effect control up all the way, couldnt hear a thing change. Maybe my guitar with humbuckers that I developed the drive level with has a lot more signal than that Tele. Looks like I'm going to need a sidechain of op-amp signal processing to be able to compensate for a wide range of signal input and make sure I can drive that Vactrol attenuator "all the way down to off" for any reasonable electric guitar pickup input level / volume setting.
As they say, back to the drawin' board.
Last night, I put together a simple V-D power supply running off the 6.3V grounded center tap winding. Plan is to build a side chain of op-amp based signal derivation circuitry to drive the Vactrol. It's going to be my old, college daze op-amp full wave rectifier, where I graft a second order state space filter onto where the "stock" 1st order op-amp filter sits. This offers a few advantages -
Since the output of this is just a DC control signal, maybe 300 uA at the most, intended to drive the Vactrol or other LED based resistor, it's completely out of the path of the guitar signal's all-tube amplification. A true side chain, as long as I tap its input signal in an unperturbing way. Probably through a 1M resistor...
I'll post the schematic here, when I get it drawn up. Biggest problem? None of my solderless breadboards fit in the space I have behind the panel where I want to put the controls. A couple of my boards are missing and I cant find them anywhere. Was hoping to not have to wait another order / ship cycle just to begin.
- You can get a faster response, by setting the filter frequency and feedback just right; in the "critically damped" region.
- You can get a Tremolo by setting the feedback such that the 2nd order filter oscillates continuously without input.
- You can trigger decaying oscillations with your guitar playing, operating it in the "underdamped" region.
- You can use the LP output for an envelope with a LF warble on top, or the BP output for just a "triggered" warble.
Since the output of this is just a DC control signal, maybe 300 uA at the most, intended to drive the Vactrol or other LED based resistor, it's completely out of the path of the guitar signal's all-tube amplification. A true side chain, as long as I tap its input signal in an unperturbing way. Probably through a 1M resistor...
I'll post the schematic here, when I get it drawn up. Biggest problem? None of my solderless breadboards fit in the space I have behind the panel where I want to put the controls. A couple of my boards are missing and I cant find them anywhere. Was hoping to not have to wait another order / ship cycle just to begin.
Whelp, in order to get just what you need, now - you have to pay for it. I could have got 3 bigger solderless boards for less than the one that would "fit my application". $13 and a week to wait - so tempted to just run one of the larger ones I have across the table saw. Fit? I'll show you...
But I dont like cutting small things with embedded metal bits on that machine - I like cutting big things where I can handle the material 10's of inches away from that blade. I do have a small scroll saw that would do nice wobbly cut lines, as they dont put a fence nor a mitre guage on those to help me guide. There isnt even a good way to clamp a strip of something to the top of that machine's plate.
Cement half A to half B - but use only 4 op-amps. Now I have to wait another week for the electronic cement to come in...
But I dont like cutting small things with embedded metal bits on that machine - I like cutting big things where I can handle the material 10's of inches away from that blade. I do have a small scroll saw that would do nice wobbly cut lines, as they dont put a fence nor a mitre guage on those to help me guide. There isnt even a good way to clamp a strip of something to the top of that machine's plate.
Cement half A to half B - but use only 4 op-amps. Now I have to wait another week for the electronic cement to come in...
Got something small/thin to run across a table saw (Like PCB blanks)? Sandwich it between two pieces of scrap birch ply to make it rigid and easy to work with, then run it along the fence like you normally would. No sense in chipping it up or getting your fingers chopped off using the saw for things it’s not supposed to.
I got my solderless protoboard and mounted it using the standoff it came with between the tubes;
All the circuit does is take one of two output signals from the first tube - the LTP phase splitter - and full wave rectify it, while filtering the resulting envelope second order, then drive the Vactrol. The white wire is the Vactrol output I brought up so I dont have to turn the chassis over every time I want to look at the actively attenuated audio signal.
I made an offset circuit using an LED slightly illuminated from the (-) supply to approximate the drop across the Vactrol LED. So I can get current through it from a ground referenced positive going control voltage. The Vactrol LED cathode is connected to the cathode of the lit LED - a node about -1.7 V below ground. I certainly have space to do it in a better way, but, works for now!
The effect is definitely compressor like, with a bit of tremolo added on top if you like. I've listened to the amp swell back and forth between single ended and P-P so many times; so glad I have that power soak resistor on the speaker. It's a nice subtle effect, especially when the tremolo swells are well into the subsonic and match the rhythm of chord changes.
The controls are Tremolo frequency and the filter "Q"; still working on the range of each. Changing Q allows the circuit to operate between an ordinary compressor type effect to having decaying swells added on, to having a constant tremolo effect. Tremolo from SE to P-P, that is. I'll work on getting some scope captures posted next, as I develop the controls range. And my hands, playing electric guitar.
All the circuit does is take one of two output signals from the first tube - the LTP phase splitter - and full wave rectify it, while filtering the resulting envelope second order, then drive the Vactrol. The white wire is the Vactrol output I brought up so I dont have to turn the chassis over every time I want to look at the actively attenuated audio signal.
I made an offset circuit using an LED slightly illuminated from the (-) supply to approximate the drop across the Vactrol LED. So I can get current through it from a ground referenced positive going control voltage. The Vactrol LED cathode is connected to the cathode of the lit LED - a node about -1.7 V below ground. I certainly have space to do it in a better way, but, works for now!
The effect is definitely compressor like, with a bit of tremolo added on top if you like. I've listened to the amp swell back and forth between single ended and P-P so many times; so glad I have that power soak resistor on the speaker. It's a nice subtle effect, especially when the tremolo swells are well into the subsonic and match the rhythm of chord changes.
The controls are Tremolo frequency and the filter "Q"; still working on the range of each. Changing Q allows the circuit to operate between an ordinary compressor type effect to having decaying swells added on, to having a constant tremolo effect. Tremolo from SE to P-P, that is. I'll work on getting some scope captures posted next, as I develop the controls range. And my hands, playing electric guitar.
