Hi everyone,
I apologize if I haven't posted this in the right place but I'm new to this forum as poster although I've been reading here for years.
I've been interested in learning about varimu compression for the past few months and after staring at the fairchild 670 schematic for days decided to build something based roughly on the design. I ordered some 6ba6's and had some 10k:10k ct edcore transformers laying around. I've read how to wire up the 6ba6's to function as triodes by connecting the screen grid to the plate through a 100ohm resistor and the suppressor grid to the cathode. What I have so far is a pushpull configuration of 8 6ba6's with the four "push" tubes' cathodes sharing a 680ohm to ground and the "pull" tubes sharing a 680ohm to ground. The B+ (280vdc) of the tube is applied through the 10k output tranformers ct to the plates. I also built myself a little variable bias psu that goes from 0 to -24vdc that is being applied to the grids through the 10k ct of the secondary of the input transformer. The problem I'm having is that I'm seeing no variance in transconductance as I vary the inputs bias voltage. I can see with my scope that the audio is indeed riding on the negative bias properly but I figured I'd hear the gain change. I have a PA that I can use as the gain reduction circuit but for now I think I better figure out why I'm not getting any change in gain when changing the bias voltage to the grids. In the fairchild schematic I can see the cathodes of the tubes tie to ground through the 680ohm resistor and a 100 ohm balance pot but it looks like the cathodes also reference some negative voltage in the psu and I don't quite get it. As I understand it, remote cutoff triodes like the 6ba6 should lower in transconductance as the grids voltage goes more negative with respect to the cathode. Any help from you geniuses would be greatly appreciated!
I apologize if I haven't posted this in the right place but I'm new to this forum as poster although I've been reading here for years.
I've been interested in learning about varimu compression for the past few months and after staring at the fairchild 670 schematic for days decided to build something based roughly on the design. I ordered some 6ba6's and had some 10k:10k ct edcore transformers laying around. I've read how to wire up the 6ba6's to function as triodes by connecting the screen grid to the plate through a 100ohm resistor and the suppressor grid to the cathode. What I have so far is a pushpull configuration of 8 6ba6's with the four "push" tubes' cathodes sharing a 680ohm to ground and the "pull" tubes sharing a 680ohm to ground. The B+ (280vdc) of the tube is applied through the 10k output tranformers ct to the plates. I also built myself a little variable bias psu that goes from 0 to -24vdc that is being applied to the grids through the 10k ct of the secondary of the input transformer. The problem I'm having is that I'm seeing no variance in transconductance as I vary the inputs bias voltage. I can see with my scope that the audio is indeed riding on the negative bias properly but I figured I'd hear the gain change. I have a PA that I can use as the gain reduction circuit but for now I think I better figure out why I'm not getting any change in gain when changing the bias voltage to the grids. In the fairchild schematic I can see the cathodes of the tubes tie to ground through the 680ohm resistor and a 100 ohm balance pot but it looks like the cathodes also reference some negative voltage in the psu and I don't quite get it. As I understand it, remote cutoff triodes like the 6ba6 should lower in transconductance as the grids voltage goes more negative with respect to the cathode. Any help from you geniuses would be greatly appreciated!
It may be that the problem you have is that the grid bias is varying gm as it should, but triode gain is largely set by mu. Mu is set mainly by valve geometry, not bias. 'Variable mu' valves are actually more like 'variable gm' valves, so are somewhat misnamed.
To check this, add a fairly low resistance load to the transformer output (e.g. 1k?). This will reduce gain but should also restore the variable action. If not, check that you have actually build and tested what you think you built and tested.
To check this, add a fairly low resistance load to the transformer output (e.g. 1k?). This will reduce gain but should also restore the variable action. If not, check that you have actually build and tested what you think you built and tested.
Thanks DF96! I added a 1k and saw that it did in fact drop a few db when at -24vdc. I was able to get it to drop about 9db after some tinkering. Thanks a lot!
I have some questions about the side chain and what its actually doing in the fairchild. I've found a lot about it in regards to how the caps change attack and release times but the whole idea of driving a bridge rectifier and using that as a variable bias seems odd to me. I took a power amp with a 70v @ 50 ohms speaker output and instead hooked up a bridge rectifiers ac legs to the PA and hooked my scopes ground to the positive of the rect. and the probe to negative. As I understand it, the rectifiers flipping any positive portion of a wave over to negative and i'm looking at every frequency cut in half and doubled in frequency. Where I get lost is how you get away with feeding that into the remote cut-off tubes as a bias signal along with the audio. I try to imagine the rectified audio is sort of a dc bias that follows the envelope of the original audio but I must not be getting how the fairchild treats the audio as audio and the rectified side chain audio as an envelope follower that shifts the bias point. Am I on the right path? My brain hurts!
I have some questions about the side chain and what its actually doing in the fairchild. I've found a lot about it in regards to how the caps change attack and release times but the whole idea of driving a bridge rectifier and using that as a variable bias seems odd to me. I took a power amp with a 70v @ 50 ohms speaker output and instead hooked up a bridge rectifiers ac legs to the PA and hooked my scopes ground to the positive of the rect. and the probe to negative. As I understand it, the rectifiers flipping any positive portion of a wave over to negative and i'm looking at every frequency cut in half and doubled in frequency. Where I get lost is how you get away with feeding that into the remote cut-off tubes as a bias signal along with the audio. I try to imagine the rectified audio is sort of a dc bias that follows the envelope of the original audio but I must not be getting how the fairchild treats the audio as audio and the rectified side chain audio as an envelope follower that shifts the bias point. Am I on the right path? My brain hurts!
Presumably the rectified audio is also smoothed? The usual circuit is called an envelope detector (as used in AM radio), but it is more or less exactly the same circuit as a power supply - just different component values.
This is quite interesting 😀 About 35 years 🙄 ago, I worked on one of these. I was bringing it back to life and I had many of the same questions. I actually had a bet with my boss over these questions and I sure am glad he would rather teach me than take my $ 😀 Your on the right track!
Very nice unit, even today! I think I have the manual/schemo somewhere. I think I've also seen a SS clone on the web.
Very nice unit, even today! I think I have the manual/schemo somewhere. I think I've also seen a SS clone on the web.
I see. I guess the idea is that the signal from the side chain gets applied to the center tap of the transformer and is supposed to be phase cancelled out by the end of the GR stage but the envelope of the signal makes its way to the grids as DC and doesnt get cancelled out? I did get it working as far as compression goes but I get a lot of distortion from the side chain that stays whether or not there is audio passing through the GR part. I suppose my balanced stage isnt perfect and that's the cause? I'll keep tinkering. Thanks for all your help and encouragement guys!
DF96, looking up AM radio envelope detector circuits. Helps so much knowing what to google! Thanks again.
flg, That's really awesome to hear! Glad to know there is help here!
flg, That's really awesome to hear! Glad to know there is help here!
OK, I've gotten this monster to sound fantastic now. I have 600:15k and 15k:600 transformers around the 6ba6's now. The only problem now is a sort of crackling sound that I get from the power amp stage that drives the rectifier. I currently have this thing set up so that no audio passes through the 6ba6's but the gain reduction signal is still biasing the 6ba6's. This way I can hear all the artifacts that the power amp stage is introducing as it adjusts the bias to the 6ba6's. At first I thought it must be the famous "thump" sound that everyone talks about which is due to an imperfectly balanced circuit however this is more of a crackle and I have a feeling is might be the audio signal overcoming the forward voltage drop of the bridge rectifier (I can see on my scope that the crackle happens as soon as any gain reduction is applied. I don't currently have the ability to match my 6ba6's and I'm going to start measuring how accurately my edcor transformers are balanced but the audios gain reduction sounds beautiful. Also I've been using a 100w power amps 50ohm-70v output to drive the bridge rectifier and I'm not sure if its good to drive the grids of the tubes this way. I've drawn out a schematic of my rig and I'd be happy to record some audio samples if anyone has any ideas. Thanks!
Also the crackle goes away more and more as I increase capacitance in the time constant network but I figure the challenge is in making this beast sound good at the quickest attack and release times.
I've tracked down the cause of some of the pops and clicks that I am hearing. I know that the pops and clicks are coming from the the rectified bias voltage on the grids of the 6ba6's. The way I have it set up right now I have no audio passing through the GR stage but do have the bias voltage being generated and applied to the grids from the audio source. So its basically compressing nothing and I can hear just the artifacts.
I think it must be quite easy to end up with a lot of ******** in the audio if the GR stage is not perfectly balanced because the audio rides on the bias voltage and the bias voltage is a much higher voltage than the audio and will be cancelled out only if everything's perfectly balanced.
I don't have the ability to test the 6ba6's that I'm using at the moment but have enough to swap them around. The transformers I am using are edcor 15k:600's for both channels. What I could really use is some advice on what will most likely improve the balance of my pushpull circuit. tubes? transformers? cathode resistors?
If I wanted to test the balance of my transformers could I just pass a sine wave through the primary ct of the tranny and connect the primaries outer lugs together and see if I get anything coming out of the secondary? If so is there any way to better balance the transformer externally with resistors or caps?
Any help would be awesome. This thing sounds amazing aside from the clicks and pops. I took a picture of the circuit so far and will find a way to post it if that helps helping me haha.
I think it must be quite easy to end up with a lot of ******** in the audio if the GR stage is not perfectly balanced because the audio rides on the bias voltage and the bias voltage is a much higher voltage than the audio and will be cancelled out only if everything's perfectly balanced.
I don't have the ability to test the 6ba6's that I'm using at the moment but have enough to swap them around. The transformers I am using are edcor 15k:600's for both channels. What I could really use is some advice on what will most likely improve the balance of my pushpull circuit. tubes? transformers? cathode resistors?
If I wanted to test the balance of my transformers could I just pass a sine wave through the primary ct of the tranny and connect the primaries outer lugs together and see if I get anything coming out of the secondary? If so is there any way to better balance the transformer externally with resistors or caps?
Any help would be awesome. This thing sounds amazing aside from the clicks and pops. I took a picture of the circuit so far and will find a way to post it if that helps helping me haha.
you could put a resistor in each tube anode so you can see the current through every tube and then to sum each component, also you could seee the total through the bunch with another resistor in series of the individual plate resistors to the transformer primary, this resistors could be any beetween 22 to 100 ohms so you could sense the voltage and apply ohms law and check the current, also the individual plate resistors serve to distribute the currents so every 6ba6 could aport a fraction of the load current for stability isues, so all can take part of the job, and not only the stronger/newer ones.
Well, a year has passed and I've got the drip 670 Fairchild pcb's now as well as a 175b built and my frankenmu compressor. My obsession with varimu compression has only gotten worse. If anyone knows of some good info on how I could go about doing a solid state control amp based on the fairchild I'd really appreciate it. It seems like generating something along the lines of an 80v swing with a low enough impedance to drive the 6ba6's through the utc a-26's is difficult. Any references would be greatly appreciated.
I'm just spouting theory, but a long time ago, i wondered the same thing and thought that some sort of power MOSFET device could be used as a simple source follower to buffer the diodes that create the control voltage from the plates of the output stage. Those diodes draw current off of the plates of the output tubes, basically generating classic slew limiting in the output stage. If one put a follower between the diodes and the rest of the CV circuitry, the O/P stage plates wouldn't have to charge the CV cap directly, making the O/P signal a good bit cleaner.
Whether this is part of your goals or not, I think that a modern power MOSFET device can be used as a follower to handle a large voltage swing and provide a low impedance. In the old days, one would have used a 6080 triode as a cathode follower, or maybe something smaller like a 12BH7, but now, I think a MOSFET can do the same without the expense and size, or the large heater current requirement.
P.S. - re-reading your post, note that pulling some grids down to achieve gain reduction does not require much current or a low impedance drive. Even the grid resistors aren't in that path - it's just the leakage resistance and capacitance of the input transformer secondary that you need to drive (ideally). However, in a classic limiter, driving the control voltage cap (charging it up) which then drives the input transformer secondary is what requires current. That cap can be scaled down, but it can also be buffered by a MOSFET follower.
Best of luck!
Whether this is part of your goals or not, I think that a modern power MOSFET device can be used as a follower to handle a large voltage swing and provide a low impedance. In the old days, one would have used a 6080 triode as a cathode follower, or maybe something smaller like a 12BH7, but now, I think a MOSFET can do the same without the expense and size, or the large heater current requirement.
P.S. - re-reading your post, note that pulling some grids down to achieve gain reduction does not require much current or a low impedance drive. Even the grid resistors aren't in that path - it's just the leakage resistance and capacitance of the input transformer secondary that you need to drive (ideally). However, in a classic limiter, driving the control voltage cap (charging it up) which then drives the input transformer secondary is what requires current. That cap can be scaled down, but it can also be buffered by a MOSFET follower.
Best of luck!
anyone know if I could get away with using this as the control amp? Assemble Mono L150W FET Power Amplifier Board IRFP240 6 with Radiator | eBay
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