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Old 14th November 2017, 02:30 AM   #2101
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Printer2 View Post
I wonder if you can modulate a sharp cutoff tube using the screen?
George will know, from his screen-drive experiments...

Varying the screen grid (bias) voltage definitely changed the gain in my 6JW8 preamp stage, but I didn't try experimenting with feeding it a control signal for compression. I was just trying to find a good DC operating point.

I expect it will take a lot more modulation voltage at G2 compared to G1. Maybe that's why Nachbaur used the scheme he did?

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Old 14th November 2017, 03:11 AM   #2102
thoglette is offline thoglette  Australia
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Originally Posted by Gnobuddy View Post

I would have thought modulating G1 with DC would cause all sorts of thumping and bumping noises at the anode,
There's a pile of "vibrolux" type circuits that do pretty much the same, but usually by hitting the cathode - e.g. this Supro (EL34 world)

I've also seen compressors (e.g. EF86 based by Bruce Vicknair ) with Vcontrol on G1 and the input on G2

Ken Stone did a Russian "rod" pencil pentode based unit using G2 for Vcontrol.

Using remote cutoff pentodes as VGAs is a technique as old as the hills: if you can think of it, someone's probably tried it.

More than one way to skin a cat.
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Old 14th November 2017, 03:55 AM   #2103
thoglette is offline thoglette  Australia
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Too late to edit. I remembered that Roger Modjeski's article on 6DJ8s points out that there's also variable mu triodes out there. Often as variants of existing triodes (e.g. 6DJ8 vs 6ES8 ) where the only difference is the way the grid is wound
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Old 14th November 2017, 06:24 AM   #2104
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by thoglette View Post
There's a pile of "vibrolux" type circuits that do pretty much the same
I've seen "bias wiggle tremolo" before. In this case, there is at least the advantage that the tremolo waveform is more or less a sinusoidal one, and its frequency is well below the lowest guitar frequency, so you have some chance at high-pass filtering it out via small coupling caps. (Though I have still read a lot of complaints about thumping sounds when tremolo is turned on.)

I would think a compressor would be more of a headache, considering that you might want fairly short attack times - which, by Fourier transform, equate to correspondingly higher frequencies in the control signal.

(This might be a good time to mention that there was a period in my twenties when I failed miserably at multiple attempts to design a well-behaved guitar compressor. The least-bad ones were optical, using an LDR as the gain control element. Since the LDR is just a resistor, and can be fed a purely AC signal, there are no DC changes to "thump"!)

I have a feeling Nachbaur played electric guitar rather differently than many of us. He seems to have enjoyed playing classical guitar pieces by Sor and Carcassi et al, on his electric guitar, using extremely clean tone. Perhaps this type of playing (no sharp pick attack transients) helped cover up some compressor thumping?

Quote:
Originally Posted by thoglette View Post
More than one way to skin a cat.
Indeed, but the good audio compressor circuits I've run across all have some sort of circuit topology designed to keep the compressor control signal itself from showing up at the same circuit node as the audio output (using balanced bridges, etc). Pretty much the exact opposite of putting the control signal on G1, along with incoming audio!

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Old 15th November 2017, 01:40 AM   #2105
Tubelab_com is offline Tubelab_com  United States
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George will know, from his screen-drive experiments
Changing the voltage on the screen grid of a pentode will affect the current through the tube, and thus vary the Gm. This will change the tube's gain without having to disturb the DC voltage on G1. The amount of gain change depends on the tube's characteristics, but I doubt that a very large change can be affected by this method alone. Multiple stages will be needed.

There are a few "dual control" pentodes around that are probably more suited for something like this. They look like a regular pentode, but a second control grid is substituted for G3. There are several of these but the only number that pops into my head right now is the 6AS6. I have a box full of these and have been tinkering with them an a vacuum tube version of a Moog style ladder VCF. These have been successfully used in audio compressors.

Quote:
Using remote cutoff pentodes as VGAs is a technique as old as the hills
A common sharp cutoff pentode has a control grid wound as a spiral wire of constant pitch. A remote cutoff pentode has the grid wound with a variable pitch, usually tightly wound near each end, slowly changing to a wider spacing in the middle. This way as the voltage on G1 is increased from cutoff, only the center of the tube is being used. The fine pitch part remains cut off. As the grid voltage is brought less negative more of the cathode area is active, and less is cutoff. These tubes were intended for variable gain RF and IF amplifiers in radio and TV sets, but do find themselves in some audio applications, including compressors.

The original Fender Champ 5C1 used a 6SJ7 sharp cutoff pentode for the input tube, and grid bias is developed by the grid leak method. Driving the snot out of a 5C1 with my trusty germanium fuzz box would send the grid voltage negative enough to prove that blocking distortion can indeed happen in an input stage. This was in the 1960's.

I discovered a pseudo cure could be obtained by plugging a 6SK7 remote cutoff pentode into the input tube's socket. I would soon learn that the real cure was to eliminate the grid leak bias by using a cathode resistor and a bypass cap. I still liked the sound of that amp with a 6SK7 when pounded with my DIY fuzz tone.

There are plenty of common tube types that exist in both sharp cutoff and remote cutoff versions. Look at the IF strip in any radio or TV set. The most common pair is the 6EJ7 sharp cutoff, and 6EH7 remote cutoff. Going back in time, the 6AU6 sharp cutoff, 6BA6 remote cutoff......6SJ7, 6SK7.......6J7, 6K7.

Quote:
Often as variants of existing triodes (e.g. 6DJ8 vs 6ES8 )
Yes, this same technique was applied to triodes, usually for TV tuner duty. The dual triode was usually configured as a cascode amplifier with signal applied to the lower grid and AGC voltage applied to the upper grid, although both could be applied to the lower grid as well. Again these found their way into audio compressors.

The most famous is the Fairchild 670 which now sells for STUPID money. It used the 6386 tube which was originally designed for military aircraft radios. NOS versions of this tube sell for $200 and up.....way up! I have a few well used ones.

You can find the Fairchild schematics online. The key to using any of these variable gain tubes in audio is not only the thump, pump, or other control voltage effects, but the nonlinearity of the tubes themselves. When you build a tube that has a variable gain, it is inherently nonlinear. Some tubes are worse than others.....much worse. The good ones require a rather large change in control voltage to effect a change in gain, thus the audio signal will be a smaller percentage of the total voltage across the tube. The good tubes tend to have a constant radius to their "bent" transfer curve, thus creating only even order harmonics, predominantly 2nd. The key here is to use a totally balanced design to cancel out those harmonics. The Fairchild is balanced and transformer coupled throughout, and multiple stages are used to get the required control range.

The VGA cell is an integral component of any compressor. It can be a simple LDR, or a complex vacuum tube or FET circuit. I will eventually get to tinkering with the VGA, after I conquer the VCF, both for music synth reasons. I have several tubes to test and some 6386's to compare them with. I need to get some reasonably priced magnetics, probably Edcor, to try out some Fairchild type circuits. Either way, it's not going to happen soon.

BTW, the LDR makes for a wicked resistive element in a Moog ladder filter. Weird delayed release though.
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