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PostPosted: December 9th, 2020, 7:08 pm 
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Maybe I am wrong but using the formula
Ck = 1 / (2 * pi * f * Rk) 

I get
160 uf for 200 ohms
127 uf for 250 ohms

Is there some other formula I should be using?

ray


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PostPosted: December 9th, 2020, 7:40 pm 
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Roscoe,

I have not seen much in terms of designing a screen drive, except that you need alot of voltage swing at the screen compared with grid drive

1. You have the 6V6 setup in pentode mode. How much gain are you getting from the driver?
2. Looks like you are biasing the 6550 thru a bleeder resistor with some current running through the 5.1K resistor, correct?
3. It is interesting that you can direct couple with the screen at the plate voltage, that is an interesting feature of this setup.

Anyway, if you can provide a technical explanation of the circuit, it would demystify many of us (though I think Ray knows what is going on).

David


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PostPosted: December 10th, 2020, 12:13 am 
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As an aside -- 6V6s are not actually pentodes -- they're beam tetrodes -or- the Brits call them "kinkless tetrodes." They have forming plates tied to the cathode in place of the suppressor grid.

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PostPosted: December 10th, 2020, 8:30 am 
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SoundMods wrote:
As an aside -- 6V6s are not actually pentodes -- they're beam tetrodes -or- the Brits call them "kinkless tetrodes." They have forming plates tied to the cathode in place of the suppressor grid.


Beam tetrodes or beam power tubes were originally created to side step a patent on pentodes.


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PostPosted: December 10th, 2020, 9:11 am 
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Roscoe,

Have you considered using a gas regulator tube to stabilise the screen of the 6V6? I use that on with a 6688 driver (connected as pentode) on a 2A3 SET amp I built and from memory, it works very well. I believe I picked up the idea from Thorsten Loesch on one of the audio board.

Yeah, pentode vs beam tube (i.e. kinkless tetrode), got it. Regardless, the suppressor grid/beam plates are almost always tied to the cathode, so what is the difference?

David


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PostPosted: December 10th, 2020, 10:47 am 
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David McGown wrote:
Roscoe,

Have you considered using a gas regulator tube to stabilise the screen of the 6V6? I use that on with a 6688 driver (connected as pentode) on a 2A3 SET amp I built and from memory, it works very well. I believe I picked up the idea from Thorsten Loesch on one of the audio board.

Yeah, pentode vs beam tube (i.e. kinkless tetrode), got it. Regardless, the suppressor grid/beam plates are almost always tied to the cathode, so what is the difference?

David

Good point. When I had a amplifier that used tubes like that (based on the KT-88) with fixed-bias I could easily "roll" tubes like a madman. Each type had its benefits and problems when comparing the EL-34 (pentode) and the KT-88 (beam tetrode).

So -- what was one thing I learned? You CAN run a beam tetrode as a triode, but it is ill-advised to run a pentode that way. I don't recall the exact reason but it had something to do with the suppressor grid getting in trouble

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PostPosted: December 10th, 2020, 11:05 am 
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SoundMods wrote:

So -- what was one thing I learned? You CAN run a beam tetrode as a triode, but it is ill-advised to run a pentode that way. I don't recall the exact reason but it had something to do with the suppressor grid getting in trouble[/color]


I have no problem with running small signal pentodes like 6688. 7788, D3A, etc. as triodes, in fact, they are quite good and linear.

David


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PostPosted: December 10th, 2020, 11:43 am 
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David McGown wrote:
SoundMods wrote:

So -- what was one thing I learned? You CAN run a beam tetrode as a triode, but it is ill-advised to run a pentode that way. I don't recall the exact reason but it had something to do with the suppressor grid getting in trouble[/color]


I have no problem with running small signal pentodes like 6688. 7788, D3A, etc. as triodes, in fact, they are quite good and linear.

David


It's much easier with pentodes in which the suppressor grid is brought out on it's own pin rather than connected to the cathode internally... Have you tried using the screen grid as the plate? That works too if you can live with the screen dissipation limits...

Roscoe

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PostPosted: December 10th, 2020, 12:03 pm 
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Roscoe Primrose wrote:
David McGown wrote:
SoundMods wrote:

So -- what was one thing I learned? You CAN run a beam tetrode as a triode, but it is ill-advised to run a pentode that way. I don't recall the exact reason but it had something to do with the suppressor grid getting in trouble[/color]


I have no problem with running small signal pentodes like 6688. 7788, D3A, etc. as triodes, in fact, they are quite good and linear.

David


It's much easier with pentodes in which the suppressor grid is brought out on it's own pin rather than connected to the cathode internally... Have you tried using the screen grid as the plate? That works too if you can live with the screen dissipation limits...

Roscoe

It was actually a tube manufacturer's comment about running a pentode as a triode that I read about many years ago. I think it was Amperex.

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PostPosted: December 10th, 2020, 12:10 pm 
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David McGown wrote:
Roscoe,

I have not seen much in terms of designing a screen drive, except that you need alot of voltage swing at the screen compared with grid drive

1. You have the 6V6 setup in pentode mode. How much gain are you getting from the driver?
2. Looks like you are biasing the 6550 thru a bleeder resistor with some current running through the 5.1K resistor, correct?
3. It is interesting that you can direct couple with the screen at the plate voltage, that is an interesting feature of this setup.

Anyway, if you can provide a technical explanation of the circuit, it would demystify many of us (though I think Ray knows what is going on).

David
:character-oldtimer:

OK, here goes the Cliff Notes version... It's really a very simple amp, standard pentode operation of the driver, gain around 10 if I remember correctly. The output tube is just like a standard pentode output stage, except the control grid sees no signal. If you think about it, with no input signal at all, it operates exactly like any other self biased SE pentode operating stage. Plate at about 400VDC, screen grid at 250VDC, control grid at 0VDC, cathode at about 16VDC. The difference comes in where the signal is injected into the output stage... Take a look at a typical pentode characteristics curve:
Image
The screen grid really does isolate the plate from the rest of the circuit. In a triode, if you increase plate voltage and keep everything else the same, plate current goes up. In a P/T, changing only the plate voltage does essentially nothing (unless you go to ridiculous extremes) but if you change screen grid voltage, you do get a significant change in current. That's what I'm doing here, modulating the screen voltage as a way to control output current. Doing things this way eliminates the huge power consumption/heat generation common on most DC tube designs, as the cathode can be at the same reference above ground it would be with typical control grid modulation. The 5.1K resistor probably isn't necessary, but I wasn't comfortable just connecting the grid to ground (why do I type gride every time and have to go back and remove the 'e'?) on the first try...

This design does make some significant demands on the driver though, keep in mind that you have to swing a lot of voltage AND supply current to the screen. You're not going to pull this off with a 12AX7 for a driver;)

I should have the parts to complete the stereo pair Monday, then I can give the design a fair test.

Roscoe

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