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Zero bias

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Posted on July 29, 2020 at 20:39:57
vetmedrobert@gmail.com
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 If I ground the grid(0 voltage)of a tube and put an input transformer in front of it , will it work? Let's assume that it will operate into positive grid voltage. Forgive me if this has been covered here before.

 

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RE: the new breadboard is earning its keep, posted on July 29, 2020 at 20:52:52
Jim Doyle
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 I know the 211 tube has a zero bias operation point that people have played with that is accessible with ordinary plate voltage and parts. This article makes mention of it but then the author goes off the rails to use the SV811-10 rather than the 211.

http://www.radanpro.com/Radan2400/Pojacala/Bob%27s%20Single%20Ended%20SV811-10%20Amp.htm

 

RE: the new breadboard is earning its keep, posted on July 29, 2020 at 21:09:31
vetmedrobert@gmail.com
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 There is no transformer coupling in the article you mentioned. Can an input transformer drive a tube with 0 bias?

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 19:33:54
Tre'
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I thought with grounded grid operation the input signal goes to the cathode and just because the grid is grounded doesn't mean there is no bias voltage. Bias voltage is the difference between the grid and the cathode.

If the cathode is at +20 volts and the grid is grounded (0 volts) then you have -20 volts bias for the tube. That is to say the grid is -20 volts negative with respect to the cathode.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 19:45:13
vetmedrobert@gmail.com
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 Thanks for clarifying that. In your schematic does it make any difference if the cathode is bypassed?

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 19:53:34
Tre'
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 Yes. You can not bypass the cathode resistor because the input signal is going to the top of the cathode resistor. The bypass cap would ground all of your signal.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 19:56:01
vetmedrobert@gmail.com
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 Hopefully the last stupid question, why such a large capacitor at the input?

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 20:01:37
Tre'
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 In that example the cathode resistor is a small value. If the capacitor is going to pass all the low frequencies the capacitor has to be large.

20uf into 1k ohms gives a -3db down point of the high pass filter, created by the two, of 8Hz. I would use a 40uf cap.



Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Here's an article about grounded grid amplifiers, posted on July 30, 2020 at 20:08:11
Tre'
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 .
Have Fun and Enjoy the Music
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RE: the new breadboard is earning its keep, posted on July 30, 2020 at 20:09:52
vetmedrobert@gmail.com
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 Makes sense, and the voltage is low, the issue for me becomes decent quality caps at that value. No electrolytics obviously.

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 20:26:12
rivervalley817
 filtering with gain?

with regards,





 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 20:56:04
Tre'
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 That are filters, low pass and high pass, through out any audio system.

The trick is to keep them out of the audio band.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 20:56:36
Tre'
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 what circuit are you thinking about?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 20:59:12
Jim Doyle
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 I don't think an input transformer could do it because the core would saturate from the rising grid current during the positive phase of the signal. This is not like a other A2 designs - i.e the 811A where the grid current is being drawn even at idle. As a result, you should be abler to do most of this with a normal interstage tranny like the typical 5K jobber from Hammond or Lundahl.

Its worth a try.... The 211 will do it at 400ish volts. Zero bias triodes are typically an RF transmitter issue. The 805 will certainly do it. Everything else comes in a forced air base and ceramic bottle....

 

RE: the new breadboard is earning its keep, posted on July 30, 2020 at 22:43:43
rivervalley817
 thank you

of course I knew that you would say that

just wanted to afford you the opportunity to reaffirm what should be a basic design paradigm ... I remember Victor K. addressing the 'secret' to viable power designs a few years back re: tooling digital amps

form follows function subject to implementation

it's the gospel of aviation and ... never mind, just thought of the F35

it's late here so g'night ...

personally, I'm at the point where good is good enough

best regards,





 

RE: the new breadboard is earning its keep, posted on July 31, 2020 at 05:30:17
dave slagle
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 One other thing that is important to remember about grounded grid (cathode drive) circuits is that the input impedance is quite low so they can be rather difficult to drive. The input impedance is the cathode impedance in parallel with the cathode resistor and assuming the tube in Tre's sketch above is a 12AX7 you would have around a 600Ω cathode impedance in parallel with a 1000Ω cathode R for a 375Ω Z-in. Applying the 10:1 golden rule of source Z to load Z you need to drive the input from a <50Ω source.

Broskie (Tubecad) does an excellent job of laying it out and is a must read if you are considering this topology.

The only place i am tempted to try this this to drive an tube that must handle both A1 and A2 drive but that is for another thread.

dave

 

RE: the new breadboard is earning its keep, posted on July 31, 2020 at 10:25:54
vetmedrobert@gmail.com
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 I would like to try the 843 in some capacity. It seems to be an indirectly heated version of the 10/vt25. I have a few that I got a few years ago but never got around to trying. There is one data sheet that at least has plate curves.

 

yes, thank you. nt, posted on July 31, 2020 at 10:28:05
Tre'
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Have Fun and Enjoy the Music
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RE: the new breadboard is earning its keep, posted on July 31, 2020 at 23:16:34
Tre'
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 Why grounded grid. Are you planning to run into A2?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on August 1, 2020 at 04:40:11
dave slagle
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 as an aside on this... how does the transcendent grounded grid pre obtain its published 50K input Z?

dave

 

RE: the new breadboard is earning its keep, posted on August 1, 2020 at 08:40:52
vetmedrobert@gmail.com
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 I did not realize that grounded grid topology was a "thing". A2 was what I was proposing and that obviously will not work with an input transformer not gapped for the current. Still I have learned something new.

 

RE: Here's an article about grounded grid amplifiers, posted on August 1, 2020 at 15:02:34
PakProtector
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 There is a small group of power tubes that can do very nicely with grounded grids. Zero bias.

Feed the cathodes from an IT, and A2 is easily available with no step change in input impedance when the grid swings from positive to negative.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

A2 Brute', posted on August 1, 2020 at 18:02:59
elblanco
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 .

 

RE: the new breadboard is earning its keep, posted on August 1, 2020 at 20:03:19
Tre'
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Location: So. Cal.
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 I believe that grounded grid is used so the when the tube runs into A2 the input impedance doesn't suddenly drop (and demand current).

By driving the cathode instead the grid the input impedance is already low (so you need a drive stage that's up to the task) but it doesn't drop lower every time the output tube enters A2 from A1.

Or at least that's what I thought grounded grid was all about.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on August 1, 2020 at 20:04:07
Tre'
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Location: So. Cal.
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 I'd love to see a schematic.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on August 2, 2020 at 00:36:09
Stephen R
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 Google it.

 

RE: the new breadboard is earning its keep, posted on August 2, 2020 at 07:15:25
Tre'
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Location: So. Cal.
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 The input stage is not GG. It's a CF with the cathode resistor returning to a negative supply and a 50k volume pot in the grid circuit. The second stage is a GG, active loaded, mu follower sort of thing.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

found it. nt, posted on August 2, 2020 at 07:16:06
Tre'
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 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on August 2, 2020 at 11:17:04
dave slagle
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Posts: 5430
Location: NYC
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 Yeah... saw it. the second stage isn't technically a grounded grid is it? The 100K / 20K voltage divider from the output applies feedback to the grid.

Looks to me like a differential phase splitter with the signal from the non-inverting output :-)

dave

 

you're right. So how is it called a gg? nt, posted on August 2, 2020 at 11:19:39
Tre'
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Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: you're right. So how is it called a gg? nt, posted on August 3, 2020 at 08:18:26
elblanco
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 from Broskie at Tubecad

"When we get to the actual chapter on tube circuits, we find Rozenblit, once again, mislabeling the cathode-coupled amplifier a grounded-grid amplifier. Does he not get the same amount of angry e-mail that I receive? To be more generous to Mr. Rozenblit, the cathode-coupled amplifier is a compound circuit, the wedding of a cathode follower to a grounded-grid amplifier, so his label is half right.'

I know the grid of second triode is not grounded, but it receives no input other than what looks like feedback via a voltage divider.

 

RE: you're right. So how is it called a gg? nt, posted on August 3, 2020 at 17:47:32
Tre'
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Location: So. Cal.
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 I'm just surprised that the input stage of a preamp called a "ground grid preamp" is not a ground grid stage. The second stage is whatever it is.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: the new breadboard is earning its keep, posted on August 12, 2020 at 05:20:46
PakProtector
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Posts: 12364
Joined: May 14, 2002
 Hence the benefit of driving it with a transformer secondary. Idle current will be an issue if going SE then. Going PP side-steps most of that. But it still needs some power input; as in low impedance, and significant voltage swing.

Best I have come up with for biasing is a variac on the primary of the plate Iron to adjust B+ since I don't want any resistance between the grid and ground.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

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