807 screen regulation

[Klimon]

Hello,

I'm planning to build an 807 beam tetrode amp, basically the RH807 (www.tubeaudio.8m.com) but with a different power supply, other bias point for the output stage (around 370V, 60mA) and regulated screen supply à la Gordon Rankins 807 SE (Sound Practices 1992). Rankin uses a single shunt regulator with 0d3 and 0c3 in series, shared by both channels. I would like to use a separate shunt for each channel (because the power supply is already split in two for B+ and B++). I'm not sure though if the current draw of one screengrid (or is it the shunt that draws the current?) is enough for good regulation by the VRs. Rankin writes the shunt draws 20mA, my output stage draws the same current as his so both of my shunts should draw 10mA. Can anyone confirm this guesstimate? It would be a pity to do all this extra work for a good screen supply when in the end the regulators wouldn't draw enough current to do a good job. The VRs I'll be using are good from 5mA and up.

Cheers,

Simon

[arnoldc]

Have you looked up the screen current requirement from the 807 datasheet? My tube files are all offline right now, but you take it from the web.

FWIW, I built the Gordon Rankin 807 and I like it, but I busted my 6072 which caused me to retire the amp.

[mach1]

Klimon,

Simple solution - you just change the value of the dropping resistor so that the total static current (that drawn by 807g2 while idling plus that drawn by the Vreg tubes) is about 25mA.

0D3 + 0C3 means that V reg = 250V

If your B+ = 370 your required voltage drop = 120V

Resistior value = 120/.025 = 4800R

4k7 should be close enough

As the g2 current demand increases on peaks, the current passing through the Vreg tubes will drop by the same amount while still maintaining the required voltage.

pm

[Miles Prower]

Originally posted by Klimon
Hello,

I'm planning to build an 807 beam tetrode amp, basically the RH807 (www.tubeaudio.8m.com) but with a different power supply, other bias point for the output stage (around 370V, 60mA) and regulated screen supply à la Gordon Rankins 807 SE (Sound Practices 1992).

If you're going to be playing around with different Q-Points, better get this first: STC 807 Application Report

Rankin uses a single shunt regulator with 0d3 and 0c3 in series, shared by both channels. I would like to use a separate shunt for each channel (because the power supply is already split in two for B+ and B++).

You could also use an active regulator. Anything would beat that unbypassed series dropping resistor the RH 807 uses. That's very bad.

[Shoog]

You could also use an active regulator. Anything would beat that unbypassed series dropping resistor the RH 807 uses. That's very bad.

The question is, will regulating the screen greatly improve the performance. Old Alex says in the original article that he heard very little difference between the triode mode and the pentode mode. He extensively simulated this circuit. The plate to plate feedback seems to be extremely effective in smoothing out anomilies in response.

Shoog

[Klimon]

The question is, will regulating the screen greatly improve the performance. Old Alex says in the original article that he heard very little difference between the triode mode and the pentode mode. He extensively simulated this circuit. The plate to plate feedback seems to be extremely effective in smoothing out anomilies in response.

I think it will make a difference, a resistor-tied screen, whether it is triode or pentode connected, is a compromise. Alex didn't compare a shunt regulated screen supply with a conventional one. Only that test would be the relevant for this issue, not triode-pentode comparison.

Shoog, Arnoldc, mach1, Miles: thanks a lot for your input, I'm ready to take the plunge --- those VR tubes do look nice

Simon

[Shoog]

Its not really possible to speculate to what extent Alex actually modelled different types of screen supply - he did tend to imply elsewhere that the setup he suggests was tried against more elegant screen supplies and it came out well.

Early on when I was putting together my parafeed version of the RH807 I tried a dedicated screen supply. I had to drop it eventually because of a number of factors. The short time I tried it the sound that was coming out was brighter. It was however working with a smaller amount of Plate to plate feedback so this issue may well have disappeared with more feedback applied.

Just though you may have been interested in that snippet of info.

Shoog

[Miles Prower]

Originally posted by Shoog
The question is, will regulating the screen greatly improve the performance. Old Alex says in the original article that he heard very little difference between the triode mode and the pentode mode. He extensively simulated this circuit. The plate to plate feedback seems to be extremely effective in smoothing out anomilies in response.

Shoog

Given the single-ended, Class A1, topology, it may not make that much difference. However, with AB1, and a screen current that varies between 5.0mA at no signal and 16mA at max output, I find it difficult to believe that letting the screen voltage wander all over the place would be good for the sonics, even with local NFB. Still, with just a series dropping resistor, any pentode will start to display a remote cutoff type characteristic. Unless it's a low level stage where the current doesn't vary all that much, or something like a Class C unmodulated driver/frequency multiplier stage that draws a consistant current (and where distortion doesn't matter anyway) then I do like to nail down that screen voltage so that it doesn't wander. In the case of the RH 807 design, use a voltage divider, at least.

"Alex says in the original article that he heard very little difference between the triode mode and the pentode mode."

Well, that's the whole point behind local NFB: to make the dynamic plate characteristic more triode-like, and less pentode-like, as well as reducing the effective r(p) seen by the primary of the output xfmr.

"The plate to plate feedback seems to be extremely effective in smoothing out anomilies in response."

Not a part of my design philosophy. I want to minimize anomilies in response before the NFB (global or local) is connected. Include just enough NFB to improve an already good sounding design instead of covering up for your open loop design mistakes. If going the extra mile by including screen voltage regulation is part of that, then, so be it. I'm not designing for the "Big Box" people, and have no need to make the design as cheap as possible to produce.

[Klimon]

Just though you may have been interested in that snippet of info.

Thanks Shoog, I'm definitely interested... just think I'm sold already for the shunt regulators I'll mail Aleksandar to ask him about his screen experiments. BTW: how does your rh807 parafeed perfom? Did you compare with the regular version?

Simon

[Shoog]

I never compared it to the regular version. I did it this way because I had suitable toroidal transformers as outputs.
Mine sounds extremely good, with excellent bass response and all the SE characture you could want. I was running it with a 700V supply and a TT21(KT88) CCS plate load. At the moment it is in semi retirement as I built a 6080 PP amp with transformer phase splitter which out performs it by a long way. Also I just couldn't live with the terrible efficiency. I am currently thinking about rebuilding it with plate chokes, for efficiencies sake. I will also include a cathode CCS (LM317 based) and a few other improvements. I will probably bypass the screen supply myself as this becomes a more realistic option with the lower supply voltage.

Don't get me wrong on the screen supply - it is obviously sensible to do the best you can to get the best result, and common sense suggests that the results will benefit. My point is that its not always best to second guess the designer on these details. It would be an interesting experiment to build the shunt screen supply and then substitute a resistor for comparison. Wouldn't be difficult and would be educational for yourself if no one else. The output Tubes behaviour is completely transformed by the use of plate to plate feedback and it cannot be absolutely assumed that what works with a triode or pentode will work in a RH style circuit. Experimentation is required.

Shoog