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Birkemeier audio compressor
Birkemeier audio compressor
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Old 8th August 2017, 07:17 PM   #11
Thomasha is offline Thomasha
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Any idea of how to adapt it to use 5672 subminitures in a pedal?

My idea was to use a second one and rectify the signal and in some way make a control for the first tube screen grid. Seen some schematics with triodes, totally different from this.
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Old 10th August 2017, 01:30 PM   #12
jfetter is offline jfetter  United States
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Birkemeier audio compressor
I have several hundred 5702 subminature pentodes just for this compressor project. They work just peachy at -+24vdc. In fact every small signal tube I've tested works great with ~50V potential. The bias is fixed on all of those tests. I start out with full spec sheet voltage and work down. Does the 5672 have any internal connections?

The control/sidechain circuit is a separate project I'm working on. Yes a simple rectifier would work but with limited performance.

As printer2 mentioned, the sidechain is tricky.

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Old 10th August 2017, 05:06 PM   #13
Thomasha is offline Thomasha
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The main problem that I have with the 5672 is the direct heated cathode. Kind of difficult to work with the bias.

Another problem is that the suppressor grid is connected to the cathode, this only leaves the screen grid to fiddle with.
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Old 12th August 2017, 01:48 AM   #14
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by Thomasha View Post
The main problem that I have with the 5672 is the direct heated cathode. Kind of difficult to work with the bias.
What I've seen in the HiFi world is building a current source to drive the heater and then use that current (through the cathode resistor) to set the bias point.
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Old 12th August 2017, 05:32 AM   #15
PRR is online now PRR  United States
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> to set the bias point.

This is gone way off-topic.

In a compressor VCA, the "bias point" is set by the sidechain, as demanded by the amount of gain reduction needed at the moment. Direct-heat is a minor detail. Generally the type of tube curve is critical, and it is futile to wish to use this or that tube.

jfetter -- the Birkemeier patent claims the Plate Current is "constant" as the control voltage varies (for some unspecified P and G2 resistances). Is that the case?

It matters, because conventional AVC/VCA worked single-ended "thump". Gain is varied by causing a large change of plate current. In audio limiters, the attack time is equivalent to a low bass thump. Some compromise must be made. The high-end is push-pull to cancel thump, but transformer balance and heft are critical. Non-transformer drive and recovery are either less happy or more complex. A VCA with very low current shift over a large gain-range could be super-interesting.

I'm not in a position to breadboard or I would.

I'm not at all sure the SPICE models I have model Pentode G2 current even loosely, and G3 not at all, but both seem critical here, so I'm not banging on SPICE.
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Old 12th August 2017, 07:59 AM   #16
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by jfetter View Post
I threw this circuit together and it is 100% functional as advertised.
Congratulations on your experiments and posting results

Yes, pentodes are natural born compressors and a strong reason for guitar amps still being made using such an "obsolete technology"
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Old 13th August 2017, 02:59 AM   #17
jfetter is offline jfetter  United States
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Birkemeier audio compressor
Quote:
Originally Posted by PRR View Post
> to set the bias point.

jfetter -- the Birkemeier patent claims the Plate Current is "constant" as the control voltage varies (for some unspecified P and G2 resistances). Is that the case?

It matters, because conventional AVC/VCA worked single-ended "thump". Gain is varied by causing a large change of plate current. In audio limiters, the attack time is equivalent to a low bass thump. Some compromise must be made. The high-end is push-pull to cancel thump, but transformer balance and heft are critical. Non-transformer drive and recovery are either less happy or more complex. A VCA with very low current shift over a large gain-range could be super-interesting.

I'm not in a position to breadboard or I would.

I'm not at all sure the SPICE models I have model Pentode G2 current even loosely, and G3 not at all, but both seem critical here, so I'm not banging on SPICE.
Its still on the breadboard and yes need to understand more about it .
The dc shift on anode is indeed low. Had to think about it some more. The low-z control is maybe a hint. It almost operates as though the signal is summed and shunted on g1-g3. I have analyzer software but this ckt is still in functional test.
After its fully understood will use auto test pulse, dist and sweep plots.

I use a manual bridge dist analyzer and scope during early circuit vetting.
Then as more refined move to pc.
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Old 13th August 2017, 07:02 PM   #18
Hermanni is offline Hermanni  Finland
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The dc shift on anode is indeed low.
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How low is it compared to audio signal?
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Old 14th August 2017, 02:46 AM   #19
PRR is online now PRR  United States
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The control voltage can be low impedance-- we got opamps.

What range of CV is "useful"? When too negative the tube cuts-off and output is too small to use.

When you sweep the CV from zero to that low limit, how much does the plate voltage change? (Mention the B+ too.)
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Old 15th August 2017, 02:27 AM   #20
jfetter is offline jfetter  United States
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Birkemeier audio compressor
Quote:
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The control voltage can be low impedance-- we got opamps.

What range of CV is "useful"? When too negative the tube cuts-off and output is too small to use.

When you sweep the CV from zero to that low limit, how much does the plate voltage change? (Mention the B+ too.)
Ill check as soon as can get a moment. Thats an easy test but can't do it till Wednesday.
I remember it was at least minus 12v for cutoff.
Thats why said minus 10v. B+ was 140 but its not critical.

This is an EH ef86.
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