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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 16th August 2014, 01:12 AM   #31
Blue Jinn is offline Blue Jinn  United States
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The guys at GroupDIY, didn't see (or didn't mention) distortion as a factor but thump in the lower frequencies, and suggested push-pull to compensate for that. (The ham radio would be limited to voice freq, bass "thump" wouldn't be a problem.)

I have been taking a look at the links at radiomuseum.org. I admit, I have just figured out triodes, so the curves for pentodes I'm still learning, but I *think* I get the non-linearity, between the valve-wizard site and some other documents I have on pentodes.Also, as for distortion, if the gain reduction were kept moderate (like the 12AU7 PRR "vari-mu") could it still be kept in the linear range of the curves?

I am going to spend some time looking at that other info.
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Old 18th August 2014, 07:47 PM   #32
Blue Jinn is offline Blue Jinn  United States
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@vinyvamos: understand with the EF50; good find to have something like that to experiment. A guitar pre/overdrive with a bit of compression may not be such a bad thing.

The original ARRL side chain has always intrigued me. I've another rough sketch, with a 12AU7 for the make up gain and a 12AX7 driving the side chain. (12DW7)

There is also this schematic, which seems a similar idea, using the suppressor for gain control in a single ended configuation.
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Last edited by Blue Jinn; 18th August 2014 at 08:16 PM.
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Old 18th August 2014, 08:16 PM   #33
Merlinb is offline Merlinb  United Kingdom
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Suppressor-grid compression by Amos and Johnsone:
https://dl.dropboxusercontent.com/u/...n_AGC_Amos.pdf
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Old 18th August 2014, 08:25 PM   #34
Blue Jinn is offline Blue Jinn  United States
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The discussion about 3/4 the way on page 415 seems to validate the approach. A poster at homerecording.com made the point about the amplification before the 6AL5.
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Old 22nd March 2017, 11:36 PM   #35
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Default necroposting, sorry.....

Unsurprisingly, by now the link has gone stale. Is it possible to re-post the article?
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Old 29th December 2017, 02:06 AM   #36
jfetter is offline jfetter  United States
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Yes, please put up the pdf here so is no longer lost.
i too would like to see it.

thanks
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Old 30th December 2017, 05:33 PM   #37
flysig is offline flysig  United States
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I, too, am interested if the article is still available somewhere. A search has not found it for me.

Fred Nachbaur has an interesting circuit on his page which is worth looking at. He taps off of the speaker output to charge up a passive circuit which is then connected to the control grid of the first preamp valve.

The preamp valve is a remote cutoff pentode 6BJ6. Using diodes and capacitors his compressor circuit captures peak voltages which then feed back to the grid circuit to adjust the gain of the pentode.

Here's the compressor circuit. "Li'l 4x4" Compressor Option Since it taps off of his speaker output, it would need to be modified to be suitable for whatever amp/preamp it is being used in. I am modifying it to feed back from the first stage of the preamp itself, to provide compression prior to a second stage which can be driven to distortion. But the theory should still apply.

Here is his preamp section. Note that it is the older version with a different valve. He describes in the compressor section what changes to make to component values. "Li'l 4x4" Tube Amplifier - How It Works

Here's a link to his full schematic except for the compressor. Amp, preamp, and power supply. http://www.dogstar.dantimax.dk/tubes...s/4x4large.gif

Note that the power amp is unconventional! +- 140 volt power supplies, with push-pull throughout 3 stages! The preamp is run off of just +140V and ground, without a negative supply.
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Old 30th December 2017, 05:59 PM   #38
jazbo8 is offline jazbo8
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Pentode pre/compressor (warning nube attempt at circuit drafting) --suppressor grid
The article is available at American Radio History: Documents from the History of Radio TV and FM
broadcasting and Electronics

Look for Wireless World 1951, Oct. issue.
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Old 30th December 2017, 09:01 PM   #39
flysig is offline flysig  United States
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Thanks jasbo.

Here's the direct link for those interested: http://www.americanradiohistory.com/...ld-1951-10.pdf

The article begins on page 33 of the pdf as it displays on my browser. The magazine shows it as page 415.
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Old 31st December 2017, 03:26 PM   #40
jfetter is offline jfetter  United States
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Here are selected pages. Thanks for the link.

Interesting in that implies sharp cut-off pentodes may be better for compressors.

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