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Pentode pre/compressor (warning nube attempt at circuit drafting) --suppressor grid
Pentode pre/compressor (warning nube attempt at circuit drafting) --suppressor grid
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Old 3rd March 2014, 06:24 PM   #11
smoking-amp is offline smoking-amp  United States
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The remote cutoff pentodes would normally use DC variation on grid 1 for gain control. 6BA6 however does show grid 2 (not 3) control on its data sheet (last graph), but using large voltage variation:

http://frank.pocnet.net/sheets/093/6/6BA6.pdf

A more suitable type tube are the dual control pentodes, which have considerable grid 3 effect:

6BV11 http://frank.pocnet.net/sheets/123/6/6BV11.pdf
6GY6 http://frank.pocnet.net/sheets/135/6/6GY6.pdf
6HZ6
6AS6
6DB6
6DT6

and then there are ones that have dual - dual control grid 3's:

http://www.diyaudio.com/forums/tubes...ml#post2083661

6LE8 http://frank.pocnet.net/sheets/135/6/6LE8.pdf
6BU8 http://frank.pocnet.net/sheets/093/6/6BU8.pdf
6KF8
6MK8
6GS8
6HS8

and then there are the Beam Deflection Tubes (which can actually do "linear" multiplication):
6AR8
6JH8 http://frank.pocnet.net/sheets/093/6/6JH8.pdf
6ME8 http://frank.pocnet.net/sheets/049/6/6ME8.pdf

Last edited by smoking-amp; 3rd March 2014 at 06:35 PM.
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Old 3rd March 2014, 07:36 PM   #12
Blue Jinn is offline Blue Jinn  United States
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Quote:
Originally Posted by smoking-amp View Post
The remote cutoff pentodes would normally use DC variation on grid 1 for gain control. 6BA6 however does show grid 2 (not 3) control on its data sheet (last graph), but using large voltage variation:

http://frank.pocnet.net/sheets/093/6/6BA6.pdf

A more suitable type tube are the dual control pentodes, which have considerable grid 3 effect:

6BV11 http://frank.pocnet.net/sheets/123/6/6BV11.pdf
6GY6 http://frank.pocnet.net/sheets/135/6/6GY6.pdf
6HZ6
6AS6
6DB6
6DT6

and then there are ones that have dual - dual control grid 3's:

http://www.diyaudio.com/forums/tubes...ml#post2083661

6LE8 http://frank.pocnet.net/sheets/135/6/6LE8.pdf
6BU8 http://frank.pocnet.net/sheets/093/6/6BU8.pdf
6KF8
6MK8
6GS8
6HS8

and then there are the Beam Deflection Tubes (which can actually do "linear" multiplication):
6AR8
6JH8 http://frank.pocnet.net/sheets/093/6/6JH8.pdf
6ME8 http://frank.pocnet.net/sheets/049/6/6ME8.pdf
Cool. I'm assuming that 6GY6/6BV11 would be better suited than 6BA6?

Something I don't exactly understand is how the thump from introducing the DC control voltage into the signal path is mitigated by a balance control on the cathode resistance. The common control grid on several of the tubes mentioned would obviate the need for an input transformer, but would the opposite phase cv on the suppressor/beam deflectors mitigate the thump? That doesn't seem to be any different than apply it at the control grid. Or would fixed bias make a difference?
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Last edited by Blue Jinn; 3rd March 2014 at 07:49 PM. Reason: corrected schematic (input transformer)
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Old 4th March 2014, 02:05 AM   #13
smoking-amp is offline smoking-amp  United States
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For a balanced P-P type stage, the balanced application of the DC control voltage at turn on would cause a common mode current shift on both sides which would not make it thru the output xfmr. There might still be a small disturbance, even when balanced via the cathode trimmer, due to the gain change, but that should be negligable compared to a DC level shift in one phase.

For single phase designs, you probably have to put up with the thump, since it will mimic a real signal. Maybe it can be applied slowly enough to be subsonic.

The 6BV11 would seem ideal for a balanced P-P type gain control stage using g3 control.

Something like the 6LE8 could theoretically apply the control V to g1 and the differential audio to the two g3's, to get balanced P-P operation, but I suspect the g3 dynamic range is not good enough or linear enough for the audio signal. (try it and find out, it might work OK, worked good enough for TV color anyway)
So (if not) then one would have to use two 6LE8s with diffl. audio on g1 - g1 and control on all the g3's. A single 6LE8 really makes more sense as a manual volume control, with a POT controlling the g3 levels, and single phase audio on g1. The 6LE8 is kind of a low cost BDT. I've seen them on sale for $0.50.

The real BDT are sufficiently linear to put the diffl. audio on the deflectors (g3 equivalents) and the control V on the g1 for current control. But the dynamic range of the deflectors is close to the audio input level and may limit its use at low current (low gain), since the dynamic range collapses as the current is reduced. So you are likely forced to use two of them with diffl. audio on the g1's and control on the deflectors. These BDT tubes are somewhat tricky to use besides, since they are sensitive to external magnetic fields, and may need demagnetizing initially. Typically get Mu metal shields placed around them.

(the 6LE8 should be largely immune to magnetic fields however, since it operates just like a normal dual control grid tube, excess currents get absorbed by g2 rather than deflected)

Last edited by smoking-amp; 4th March 2014 at 02:35 AM.
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Old 4th March 2014, 03:15 AM   #14
smoking-amp is offline smoking-amp  United States
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The 6JH8 and 6ME8 BDTs may not have the problem with dynamic range collapse on the deflectors at low current after all (ie, at low gain set by g1). (if you can believe the datasheet curves anyway)

Earlier attempts to make a preamp with a volume control used the 6AR8 I think, which does appear to have the dynamic range problem on its datasheet curves (see below). Looks like the 6JH8 and 6ME8 also have a full +/- 30 V dynamic range on the deflectors, so these could handle typical audio inputs easily.

http://frank.pocnet.net/sheets/093/6/6AR8.pdf
http://frank.pocnet.net/sheets/093/6/6JH8.pdf
http://frank.pocnet.net/sheets/049/6/6ME8.pdf

Still, these are much more difficult tubes to use. Great if you like to experiment though and have plenty of patience. And for all those 300B SET ultimate linear snobs out there, these BDT tubes actually ARE linear to 1st order (on the deflectors), no stinkin' 3/2's power law here, like triodes. These were the ONLY really linear devices ever......

Hmmm.., got a whole box of them cheap, maybe I should make a highly parallel BDT output stage. Braggin' rights.....

Last edited by smoking-amp; 4th March 2014 at 03:38 AM.
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Old 4th March 2014, 07:36 PM   #15
Blue Jinn is offline Blue Jinn  United States
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Quote:
Originally Posted by Printer2 View Post
Don't you need a center tap transformer for the input?
No, the CV is going to the suppressor grids not the control grids. Got this mixed up with the PRR at that comment.
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Old 4th March 2014, 08:58 PM   #16
Blue Jinn is offline Blue Jinn  United States
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@SmokingAmp: Do you think this is a viable circuit? It seems simple enough to do on a breadboard, replacing the 6AL5 with rectifier diodes , leaving the attack and decay resistors fixed as in the orig side chain, and tying the screen to plate on a 6BY6 (would prefer to stick with 9 pin) Also, use the auto noise transformers in the same manner as the PRR (No center tap on the input or to the triode, which would require finding an appropriate plate resistor) One thing I'm not sure of is the 1M resistor and pot on the triode (**the one in the signal path not the one in the side chain**) as a means to control the makeup gain. Something tells me there is a problem with doing that after the transformer.

I don't have Spice to try and simulate and I am truly an amateur, but the concept here seems pretty straight forward enough. (Another practical problem for me is how to actually test it--- and to adjust freq response)

The feedback I got at GDIY suggests using the suppressor would work for gain reduction,although I don't understand that either. Again if I am understanding this correctly, putting the cv on the control grid acts like negative feedback to reduce gain? So how does putting a negative voltage on the suppressor grid accomplish the same goal. Or am I am missing something.
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Last edited by Blue Jinn; 4th March 2014 at 09:27 PM.
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Old 4th March 2014, 09:31 PM   #17
Blue Jinn is offline Blue Jinn  United States
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Here is the revised schematic, for some reason it disappeared. Can only edit for 30 minutes. . .also meant 6GY6.
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Last edited by Blue Jinn; 4th March 2014 at 09:57 PM.
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Old 4th March 2014, 10:02 PM   #18
smoking-amp is offline smoking-amp  United States
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The suppressor grid reduces gain (or transconductance actually) when it is pushed negative. It causes some of the cathode current stream to be returned to the positive screen grid. The devices are characterised on their datasheet with a constant +V on the screen grid. Connecting the g2 to the plate will cause that gain control to vary in effectiveness with signal level (plate V) I would think. Probably make for some distortion. You could try it and see, I'm guessing it will distort considerably when triode configured.

You need some kind of CT ground reference on the input xfmr to reference the input grids to ground (so they see the bias voltage on the cathode)

No idea how effective the 6BA6 was in the original circuit versus the developed control voltage. I would guess the 6GY6 or other dual control tubes will be more effective, so some downward adjustment of the control V level may be required.

That ground connection on the 1st pot in the control filter looks like it is shorting out the voltage. Ground probably just goes to the bottom wire.

Last edited by smoking-amp; 4th March 2014 at 10:11 PM.
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Old 4th March 2014, 10:26 PM   #19
Blue Jinn is offline Blue Jinn  United States
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Quote:
Originally Posted by smoking-amp View Post
The suppressor grid reduces gain (or transconductance actually) when it is pushed negative. It causes some of the cathode current stream to be returned to the positive screen grid. The devices are characterised on their datasheet with a constant +V on the screen grid. Connecting the g2 to the plate will cause that gain control to vary in effectiveness with signal level (plate V) I would think. Probably make for some distortion. You could try it and see, I'm guessing it will distort considerably
OK. Now I get it. So constant screen voltage. I've been reading about that, a resistor off the b+ bypassed with a large capacitor. The RCA book tables (for resistance couple amplifiers) had fairly large screen resistors.


Quote:
You need some kind of CT ground reference on the input xfmr to reference the input grids to ground (so they see the bias voltage on the cathode)
PRR uses a couple of resistors to 'create' a center tap on the input transformer.

Quote:
No idea how effective the 6BA6 was in the original circuit versus the developed control voltage. I would guess the 6GY6 or other dual control tubes will be more effective, so some downward adjustment of the control V level may be required.
Interesing, there is that one circuit I posted earlier which uses 6BA6 in a strange configuration. PRR called it wacky, and found it to be less useful for thump correction than a push pull. Ebay has a nice prototype board with places for three 9pins.

I misdrew the filter, the connection is in the wrong place, I'll double check the ARRL book but I am pretty sure the original (single sided draft I posted first) has it correct.
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Last edited by Blue Jinn; 4th March 2014 at 10:50 PM.
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Old 5th March 2014, 12:04 AM   #20
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You might want a small cap from screen grid to cathode on the 6GY6 or whatever to hold the Vg2 constant. The high value screen grid resistor is commonly used to drop the voltage from B+, but it doesn't hold Vg2 constant against current change unless the signal is really small.
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