Pentode Control Gain by Screen Grid

[opacheco]

Hi,

I would like to know if I could take the control gain in pentodes using a variable voltage in the SCREEN grid instead to Control Grid??......I have a pair of very old preamps units using a pentode input stage but they have a lot of Gain and microphonics. I readed that I can change the Gain of this pentode stage feed  lower DC voltages than this stage are using.

Any comments will be appreciate!!!

Thanks
Opacheco.

 

[opacheco]

Hi,

I would like to know if I could take the control gain in pentodes using a variable voltage in the SCREEN grid instead to Control Grid??......I have a pair of very old preamps units using a pentode input stage but they have a lot of Gain and microphonics. I readed that I can change the Gain of this pentode stage feed  lower DC voltages than this stage are using.

Any comments will be appreciate!!!

Thanks
Opacheco.

ANY COMMENTS!!

 

[shabtek]

post a schematic...what about a triode-mode switch

 

[PRR]

> ANY COMMENTS!!

YES!!

Try it ! ! !

You sure could have tried it quicker than waiting for other people to put their brains in gear.

 

[PRR]

> I readed that I can change the Gain of this pentode stage feed  lower DC voltages than this stage are using.

Simply changing G2 voltage will upset DC bias faster than it changes gain.

Reducing G2 voltage and then modifying other DC parameters to keep a happy DC bias will probably increase gain. 2-cent proof: Output clipping of a happy pentode is a large fraction of the supply voltage. Input signal needed to clip the output is roughly Vg2/Mug2. Low Vg2, low signal needed to slap the output to the rails.

Clip out the G2 cap. That will reduce gain.

Then short G2 to Plate. Gain will go way down.

Don't use pentodes as mike input stages. They have advantages and disadvantages. In the modern studio, you just do not need pentode inputs.

 

[rotation]

Don't use pentodes as mike input stages. They have advantages and disadvantages. In the modern studio, you just do not need pentode inputs.

Hi,

I thought about this before. Imo biggest disadvantages about using pentodes as mike input is their noise and price because we need to select them. But what are other disadvantages?

 

[Wagtbtoobz]

IMHO , none  : Let's look at the 60' recording stuff : EF86 everywhere , or EF804 in TFK ... I've tried 6BR7, 6J7, 1603 and prefer them so far    6j5, ECC83 are good in lesser gain stages  . 

Do you want to reduce gain ? use semi remote cut-off pentodes : you can get tons of them for cheap !
cheers  Pierre

 

[rotation]

Well, another disadvantage could high Rp, maybe also G2.
So, is it better to use two triodes instead of one pentode if we look at this from design point of view. Also, it seems strange that EMI, Telefunken and others didn't come to this conclusion for their designs.
To me, the only strange choice was EF86 for power amp input stages. Example is Mullard 5-20 and others.
PRR (and others), can you please tell us more about this claim?

Miha

 

[Wagtbtoobz]

But less Miller effect with pentodes if you want high gain on hi-z xformers ... don't worry TFK engeneers knew their job   

 

[opacheco]

Thanks a everybody for your comments!!!...I will post a schematic in order to get a better idea.

Opacheco

 

[opacheco]

The Schematic Here!!!....Basically the problem is the Noise and very high microphonics in the 6AU6 Tube, I think that reducing its gain this will be solving but this circuit have a feedback brach(DC and AC too!). I tried to change the type of 6AU6 and I got a lower noise and Microphonics but that is very high until.......The 6AU8 Tube do not have a Microphonics at all!...


Any sugestions??

Opacheco.

 

[opacheco]

Sorry here the Shematic

Opacheco.

 

[PRR]

Yow! The minimum gain is 800, 58dB. Add the popular 1:10 input transformer, it has 78dB gain!!!

The maximum gain is roughly 10,000-40,000, 80dB-92dB, plus another 20dB in the input iron.

That's just too much gain.

> The 6AU8 Tube do not have a Microphonics at all!...

The 6U8? It would have to be 200 times worse than the first tube for its noise to be heard at all.

> this circuit have a feedback brach

Reduce 200K to 20K.

 

[opacheco]

Yow! The minimum gain is 800, 58dB. Add the popular 1:10 input transformer, it has 78dB gain!!!

The maximum gain is roughly 10,000-40,000, 80dB-92dB, plus another 20dB in the input iron.

That's just too much gain.

> The 6AU8 Tube do not have a Microphonics at all!...

The 6U8? It would have to be 200 times worse than the first tube for its noise to be heard at all.

> this circuit have a feedback brach

Reduce 200K to 20K.

PRR,

Thanks for your comments,
Opacheco.

 

[bwithwings]

http://www.cgs.synth.net/tube/vca.html

Another idea - use G3 instead of G2!

I have tried this, and it works nicely.   There *is* CV feedthru, read the page I linked, it explains some steps the designer went to to mitigate this.  You *could* match a pair (good luck) and use a transformer to negate the CV, but the amount of matching you'd need is pretty massive!

I have a guitar amp where I used G3 voltage as the gain control for the amp.  It works fine.  Not a huge amount of gain change available, can't remember how much.  And the bias does shift around a lot so linearity will be affected.  But, I wasn't worried about distortion or bias change - it's a guitar amp  ;-)

The author claims he 'rediscovered' this effect - guys on tube guitar amp boards bring this up every now and then, and have been for as long as I've been using the internet!

HTH...
Darren

 

[bwithwings]

Oh yeah - looking at this after I posted - the author has the heater with one leg attached to ground.  I usually hang 100R+100R across the 6.3VAC and reference the centre to a few volts above ground, like eg. the top of R7.  Or if you have one, use the 6.3VAC winding's centre tap.  I forget the theory, but it reduces hum from the heater a *lot*.

Um...   good luck, if you have a power supply handy it would only take an hour or so to throw together 

 

[opacheco]

Oh yeah - looking at this after I posted - the author has the heater with one leg attached to ground.  I usually hang 100R+100R across the 6.3VAC and reference the centre to a few volts above ground, like eg. the top of R7.  Or if you have one, use the 6.3VAC winding's centre tap.  I forget the theory, but it reduces hum from the heater a *lot*.

Um...   good luck, if you have a power supply handy it would only take an hour or so to throw together 

bwithwings

Great Ideas!!! I will read at all!...Thanks a lot

Opacheco

 

[PRR]

> Another idea - use G3 instead of G2!

Same main problem: the plan Opacheco has shown us is a feedback amplifier. Gain is controlled by the feedback resistor ratio, not the tubes, or not until the tube is de-biased so far you might as well remove V4 and go right into V5.

I made a mistake. With the 300 ohm pots turned to maximum gain, the 200K resistor does nothing, but the 400K||1.25Meg resistors are still applying NFB. Minimum gain is still 800, 58dB, but maximum gain is 2,000 or 66dB (plus typically another 20dB in the input transformer).

Changing 200K to 20K reduces min gain to 38dB (a useful value in modern studios), but max gain is still 66dB, a long span from 38dB. With the simple linear pot it may be hard to set low gains like 40dB and 45dB. If 66dB (+20dB) is more than you could need, also change the 400K to maybe 100K. That gives min=38dB, max=54dB (58dB-74dB with input transformer).

Getting the 35dB-55dB gains you need around loud instruments, without trouble from microphonics, really requires a re-design. The tubes could be re-wired as Triode, two triodes is plenty of gain and less working parts to shake and make bad sound. The input cathode resistor could be larger, perhaps 1K with no cathode capacitor. Then the 400K could be replaced with 500K audio-taper pot.

By the way, there is no "DC feedback". Although there is DC at some points on the NFB network, the amplifier has no DC gain (5.6nFd cap blocks DC).

That's a lot of output capacitors. The 8uFd must be high voltage, so it could be expensive to use a bigger cap. Instead the NFB loop corrects any bass-loss through the 8uFd. The NFB network would leak DC from V4 cathode to output. The 24uFd blocks that, and because it is only 1V here the 24uFd can be smaller/cheaper than a high-voltage cap. But then why the 10uFd cap? Perhaps to compensate bass loss in the 24uFd cap. But only if the load is low impedance. And the cathode follower will not drive a very low load impedance. While it seems expertly designed, I don't understand some of these choices.

 

[opacheco]

> Another idea - use G3 instead of G2!

Same main problem: the plan Opacheco has shown us is a feedback amplifier. Gain is controlled by the feedback resistor ratio, not the tubes, or not until the tube is de-biased so far you might as well remove V4 and go right into V5.

I made a mistake. With the 300 ohm pots turned to maximum gain, the 200K resistor does nothing, but the 400K||1.25Meg resistors are still applying NFB. Minimum gain is still 800, 58dB, but maximum gain is 2,000 or 66dB (plus typically another 20dB in the input transformer).

Changing 200K to 20K reduces min gain to 38dB (a useful value in modern studios), but max gain is still 66dB, a long span from 38dB. With the simple linear pot it may be hard to set low gains like 40dB and 45dB. If 66dB (+20dB) is more than you could need, also change the 400K to maybe 100K. That gives min=38dB, max=54dB (58dB-74dB with input transformer).

Getting the 35dB-55dB gains you need around loud instruments, without trouble from microphonics, really requires a re-design. The tubes could be re-wired as Triode, two triodes is plenty of gain and less working parts to shake and make bad sound. The input cathode resistor could be larger, perhaps 1K with no cathode capacitor. Then the 400K could be replaced with 500K audio-taper pot.

By the way, there is no "DC feedback". Although there is DC at some points on the NFB network, the amplifier has no DC gain (5.6nFd cap blocks DC).

That's a lot of output capacitors. The 8uFd must be high voltage, so it could be expensive to use a bigger cap. Instead the NFB loop corrects any bass-loss through the 8uFd. The NFB network would leak DC from V4 cathode to output. The 24uFd blocks that, and because it is only 1V here the 24uFd can be smaller/cheaper than a high-voltage cap. But then why the 10uFd cap? Perhaps to compensate bass loss in the 24uFd cap. But only if the load is low impedance. And the cathode follower will not drive a very low load impedance. While it seems expertly designed, I don't understand some of these choices.

Firts,
Thanks a lot for your analysis!!!....This unit Don't have any input transformer at all!!...Why do you thinks so?.
This unit is designed for a linear response from 10Hz to 2000Khz +/-0.5Bdb!!..Amaizing, do you?
and was used with 50Ohms load.

"The 24uFd blocks that, and because it is only 1V here the 24uFd can be smaller/cheaper than a high-voltage cap. But then why the 10uFd cap? "

I believe, the 10uf is for DC blocking from the 300Ohms resistors conected to the V4 Catode and the V4 Catode and V5 Triode Catode are DC conected by the 1.25MegaOmhs(I very little DC current).

By the way how do you make this kind of calculations gains???....do you use a specific network analysis like Feedback Amplifiers Method??

Thanks a lot again.
Opacheco.

 

[opacheco]

I forgot to note,

When I touch the Valves in order to know where the microphonics are, the only one tube presenting a ring of high frequency disturb is the V4. The V5 don't have this kind of noise when I touch it.....and....When I send the 3.15Mega Ohms(the input of V5 to Ground!!) to Ground the microphonics GO OUT and I can listen a I little Hiss noise from the V5 I suppose!!!...

Opacheco.