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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 18th February 2014, 12:02 AM   #1
Blue Jinn is offline Blue Jinn  United States
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Default Pentode pre/compressor (warning nube attempt at circuit drafting) --suppressor grid

I posted this on GroupDIY as well. I was thumbing through an old ARRL handbook, which had a 6AL5 based side chain for a speech compressor. It looked cool. the input was 'parallel' with the grid of the last triode, before the power stage, and the input to the 'suppressor grid of the first stage' noting that it required a circuit with a pentode input. I'm not a designer by any means, and I googled the heck out of this. Did come across discussions using a 6AL5, and also the "Li'l 4x4" which uses a similar concept, except not with the suppressor grid. I have no idea how to calculate the gain reduction using that method (I generally have no idea how to calculate a lot of things, which is why I lurk here too trying to learn things) but I wanted to take a crack at it anyway to see if anything made sense.

I took the pentode part numbers straight out of the RCA tables for a 6AU6 which seemed close enough to a 6BA6 (what I read suggested you use a remote cutoff pentode) the triode parts from Zugster's triode gain calculator, and the side chain straight out of the ARRL book. (Except I put in larger value pots in place of the fixed resistors for the attack/decay.) I would guess all of that would need some adjustment, and I am wondering if I need all that gain with the pentode as well, I am assuming I could strap the screen grid to the plate, and perhaps no bypass the cathode resistor?

I didn't find anything like this googling, so I am pretty sure I am barking up the wrong tree here, but thought I'd put it up for criticism anyway. Again, I haven't attempted to do any proper math here, and I have no clue how to calc the gain reduction. Also, I am not 100% certain there should be a pot on the input to the triode. The book called for a "single plate to push pull grids" interstage transformer, so I just took a swag at 1:2.

All input including "you don't know what you are doing, go back and study some more" are welcome
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File Type: pdf pentode_pre_comp.pdf (230.7 KB, 255 views)
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Last edited by Blue Jinn; 18th February 2014 at 12:19 AM.
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Old 18th February 2014, 01:25 AM   #2
Miles Prower is offline Miles Prower  United States
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You'd be better off using the Dogstar option. Making the suppressor grid negative is just asking for trouble, as this will likely lead to instability. The suppressor also has much less influence on the gm than does the control grid. It'll work much better if the control voltage is applied there.

Si diodes also have much lower forward voltages and so don't need their own preamp stage.

As for figuring votage gains, this can be determined by reading off the gm from the "Transfer Characteristics" given in the spec sheet, than determine gain:

Av= gmRL.

Another possibility is to use a pentagrid converter as a voltage gain control amp.

In any case, be prepared to do some empirical adjustments to the basic design.

That output doesn't look right. Either connect the primary of the OPT in series with the plate of the final, or capacitor couple. As it stands now, the primary of the OPT shorted out the 100K plate load resistor.
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Old 18th February 2014, 07:58 AM   #3
marcelop is offline marcelop  Argentina
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Some time ago I wanted to do a compressor without the regular methods (vari-mu, opto, etc.) My idea was to attach the anode of a triode dc amplifier to G2 of a small pentode with low G2 current so the triode grid voltage defined the pentodes screen voltage and hence Gm and gain of the stage. The signal chain would be: The pentode with controlled G2 voltage-->triode output stage and in parallel a triode buffer that fed the signal rectifier 6AL5 (just as in the schematic you posted basically)--> to the triode whose anode was connected to the pentodes G2. Best results are achieved with fixed biased on the pentode or LED cathode bias for constant g-k voltage as the screen voltage varies (cathode bias with very big bypass cap might also work but not as well)

I donīt know if I made myself clear, anyhow the circuit worked pretty well for the prototype but I never completely tested it. I think it is a good idea since the screen grid of pentodes is pretty linear as opposed to the intrinsically nonlinear (ie distortion) grid curves of vari-mu devices.
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Old 18th February 2014, 08:31 AM   #4
jazbo8 is offline jazbo8
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Pentode pre/compressor (warning nube attempt at circuit drafting) --suppressor grid
On Dogstar's site, there is also the "Dogzilla" amp, which contains a compressor function which might be worth a look:

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 18th February 2014, 10:38 AM   #5
DF96 is offline DF96  England
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Quote:
Originally Posted by Blue Jinn
I took the pentode part numbers straight out of the RCA tables for a 6AU6 which seemed close enough to a 6BA6
I'm not sure what you mean by 'part numbers'. Do you mean component values? The 6AU6 and 6BA6 have the same pinout (IIRC) but apart from that are different. The variable-mu characteristic of the 6BA6 only applies to its control grid (g1). The suppressor grid (g3) will be like any other pentode.
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Old 21st February 2014, 06:32 PM   #6
Blue Jinn is offline Blue Jinn  United States
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Thanks everyone. I've been looking at this a bit further, and also came across a few other compressor circuits using a similar side chain, one with the cv applied to the control grid as suggested here, another applied to the suppressor on a pair of 6BA6's in a strange configuration. I had a computer hardware failue, so a lot of those schemos I need to retrieve from another site, but will post soon. Also, Ian at GDIY noted that single ended is problematic as well, so I've resketched in a push pull input, which will also post. The PRR varimu (which is on the pending build list) has been very helpful in understanding some of the concepts.
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Old 21st February 2014, 07:21 PM   #7
Blue Jinn is offline Blue Jinn  United States
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Here are those other two examples. Love the eye tube in the 6BA6 one.
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File Type: jpg RADELIMITER.jpg (355.9 KB, 458 views)
File Type: jpg vm comp.jpg (151.0 KB, 445 views)
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Old 28th February 2014, 06:56 AM   #8
Blue Jinn is offline Blue Jinn  United States
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Here is an updated draft, based on some of the suggestions, and the other fragments posted earlier. Scanning and then fixing a mistake with Gimp for some reason was a hassle this time.

I've not done calculations as to the resistor values on the pentodes and I'm not entirely sure about the interstage. This is still a "proof of concept" for me, and a learning tool. I've read other problems (noise) with pentodes, so I'm not sure of the practical application either.
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File Type: jpg 6AL6-pent-2.jpg (72.2 KB, 407 views)
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Old 2nd March 2014, 03:56 AM   #9
Printer2 is offline Printer2  Canada
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Don't you need a center tap transformer for the input?
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Old 3rd March 2014, 04:50 PM   #10
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?
Yes, I didn't catch that mistake. Thanks!
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