GK-71 as a pentode.

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pre65
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#1 GK-71 as a pentode.

Post by pre65 »

Next planned step on my GK-71 amp was to beef up the power supply using the 360-0-360v toroids currently powering the 833a amps. Target HT in choke input mode would be about 600v, but a cheat cap could raise this a bit. If I go cap input then 950v HT is possible.

So, it might also be the opportunity to try GK-71 in pentode mode, and perhaps as Alex suggested also increase the current a bit ?

I'm reasonably happy with my linear PSU for the filament heating, and if at all, my next step on that would be one of Andrew Ls modules. For various reasons I'm not into this HF AC method.

If Alex has the time, perhaps he could advise specific instructions on how to connect everything together so a thicko like me can understand. :lol:
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Alex Kitic
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#2 Re: GK-71 as a pentode.

Post by Alex Kitic »

pre65 wrote:Next planned step on my GK-71 amp was to beef up the power supply using the 360-0-360v toroids currently powering the 833a amps. Target HT in choke input mode would be about 600v, but a cheat cap could raise this a bit. If I go cap input then 950v HT is possible.

...

If Alex has the time, perhaps he could advise specific instructions on how to connect everything together so a thicko like me can understand. :lol:
I guess you are not a thicko at all! Why would you call yourself that, to make other people who imagine to be knowledgeable happier?!

As for the GK71 in pentode mode, I can only suggest you to re-build or modify your amp as an RH813 - with GK71 basing and of course heater arrangement (whichever type you choose).

If you are not interested in going all the way towards this project, suffice to say that you can connect the g2 to B+ by using a 5W zener diode of adequate voltage characteristics. If the GK71 cannot take more than 700V Ug2, this means you will need to pay more attention to this value, depending on your B+.

You can regard your 720V CT transformers as plain 720V transformers (leaving the CT unused). For the RH813 schematics you will need approximately 350mA AC current for the whole amp (220mA x1.62 = 356mA). If your 720V secondaries are less powerful than that, you can connect the two in parallel: just check the phase before making the connection!

Of course, there are other people who can give better advice... why ask me?
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Paul Barker
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#3

Post by Paul Barker »

Maximum G2 Voltage is 400v, as is the 813.

The danger is proximity to the control grid and the filament. The Anode is much further from those structures.
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#4

Post by pre65 »

So, if the G2 needs to be (for example) 250v less than the anode, does that mean I need a 250v zener ?

Told you I was thick. :wink:

Anode will be approx 650v and G2 is max 400v (thanks Paul).

Or, thinking about it, do I need a 400v (or slightly less) zener ?

I've never used zeners in my amps. :)
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#5

Post by Paul Barker »

250v because it is in this case holding the G2 250v less than the anode. IT goes from the B+ not from the anode.

If you put the zenner at the anode you are in triode mode, if you put it at the B+ you are in Pentode mode. For Pentode mode the g2 voltage must not change with signal.

What you may not realise is the anode shifts potential all the time with the AC behaviour, the G2 will do the same if it is attached to the anode, by a potential difference but nevertheless it shifts with AC behaviour. As soon as it shifts with AC it is no longer behaving as a pentode but as a triode.
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#6

Post by Paul Barker »

Alex, Phil is a very brave person who jumps in with both feet. Not thick, but not fully aware of all the consequences to what he is doing. But he asks us first and we put him straight. He is a great guy.
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#7

Post by Paul Barker »

Yes you need a 250v Zenner. All the Zenner knows is that at the cathode (the end with the ring around it) there is 650v (which is from your B+ not your valve anode) and it will not allow the voltage at it's anode (which goes to your valve screen grid) to be any different from 250v less, so that will be fixed in this case at 400v.

If you had a mental abheration and decided to put a zenner from ground to the G2 you would need a 400v zenner, and also a resistor from the G2 to the B+ sized to pass the zenner current and the maximum screen grid current you might require which you get from the valve data. Or to put it another way go with what Alex is telling you, he has pioneered this approach. Yes there are other approaches but this is working well for Alex, copy him. Leave the experimenting until after you have built a successful clone.
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#8

Post by Paul Barker »

You will by now have realised that the nomenclature for diodes Zenner or otherwise appears back to front compared to valve terms. An obstacle which has tripped up many no doubt.
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#9 Re: GK-71 as a pentode.

Post by Mike H »

Alex Kitic wrote: I guess you are not a thicko at all! Why would you call yourself that, to make other people who imagine to be knowledgeable happier?!
English humour. :D

Self derogation type, making fun of oneself.

A prize example I haven't seen him use again lately, he says he has only 2 brain cells, 'A' and 'B' ~ except when 'B' is on holiday :lol:

And who sends a postcard home. :lol:
 
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#10

Post by Paul Barker »

When I was a paediatric nurse, at hand over (that time when you slag off any nirses not there, all Doctors, all hospital management Government Trustees etc, and one other thing, what was it now? oh yes, parents) it was common to say "if she had a brain cell it would be lonely.

If you want a good description of the average single mother of a child requiring admission to hospital, just watch Jeremy Kyle.
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#11

Post by Alex Kitic »

Paul Barker wrote: Maximum G2 Voltage is 400v, as is the 813.
Maximum g2 potential is 400V, but for 813 it is 1100V. This is the main difference between those tubes.

Thus output power from the GK71 will be slightly lower and you will need to apply much higher B+ for the same output power performance. Probably 1000 or 1100V.
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#12

Post by pre65 »

From previous use of the intended PSU (on GM70 some years ago) the HT in choke input will be around 600v.

Output power in triode mode is sufficient, so as long as pentode mode is not less, all will be well.

The driver valve is a high gain pentode in triode mode - 12HG7 (12GN7)
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#13

Post by Alex Kitic »

Paul Barker wrote:250v because it is in this case holding the G2 250v less than the anode. IT goes from the B+ not from the anode.

If you put the zenner at the anode you are in triode mode, if you put it at the B+ you are in Pentode mode. For Pentode mode the g2 voltage must not change with signal.
The most important difference is the AC path: by connecting g2 to the B+, be it through a zener or resistor or otherwise, you provide a different AC path, which makes it a pentode. If you connect g2 to the anode it behaves like a triode due to the same AC path.

In the first option, 650V B+, you will need a zener diode string of 250V. I suggest that you use 2x 75V and 2x 56V, for a total of 262V. You would probably be safe enough even with 3x 75V (225V). I suggest the zeners should be 5W types.

The second option with the 720V transformer(s) will require a different approach. If you are going to get as high as 900 or 1000V (for full power) dropping voltage across zeners becomes less practical. I suggest using the VR tubes string set for 400V approximately both for the driver tubes and g2. Further, an obvious source of rectified voltage would be the CT, you should get half B+ "for free": this you can further regulate with VR tubes for levels between 300 amd 400V.
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#14

Post by pre65 »

With the 720v toroids I could get 950v in cap input mode, but I am going to go choke input, so HT WILL BE 600v plus or minus a few volts.

At the moment the driver valve has a CCS in the anode, but that would not do if it were wired as a pentode.
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#15

Post by Alex Kitic »

pre65 wrote:From previous use of the intended PSU (on GM70 some years ago) the HT in choke input will be around 600v.

Output power in triode mode is sufficient, so as long as pentode mode is not less, all will be well.

The driver valve is a high gain pentode in triode mode - 12HG7 (12GN7)
With an input cap, B+ will probably rise above 800V. With just 600V power is much lower, probably between 15 and 20W. Even less in triode mode. Thus you can rest assured that output power in pentode mode is going to be up to 2x what you are getting in triode mode.

What is the data for 12GN7 in triode mode? Mu, transconductance? You could probably try imagining it's the ECC81, and temporarily forget about the CF tube... 440k Rfb from anode to anode, and 100k or maybe 200k as anode resistor for the driver tube.

I must check this driver tube: using it in pentode mode might be the next thrill for you to experience.
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