audio-talk

Ah yes, I remember. Has anyone done the test with/without the VCT yet ?

Are there any potential problems on using 100R instead of 22R ?

I've got 1 monoblock modified so far (with 2 x 22R 50W) and the mounting plate gets quite hot.

No much of a problem. You will get a bit more degeneration with the unbipassed resistance, but its unlikely to be a issue.

I hadn't spotted this problem before, but Izzy Wizzy has pointed out that the G2 maximum voltage rating for the GK-71 is only 400V :
http://www.goldenmiddle.com/files/GK71.pdf
Which seems a bit mean given the 1.5kV limit on the anode.
This, if correct, would rather limit the available power ( in A1 ) when they are used in triode mode.
I would estimate, given i ran 465V B+ and reckoned about 3W output, that with 400V from cathode to G2/anode and 80V bias, hence B+ 480V, you might only get 4 to 5 W output.
Not that this wouldn't sound good - from my experience even 3W was very nice and had great bass - but not enough to drive 'normal' speakers.

I do wonder if this rating is real though - it might be worth finding out if anyone in Russia has used them with less concern !

Most big pentodes have that sort of limitation. For example EL34, Va=800, Vg2=500.

Not sure there is a official answer other than it seems to work ignoring the value if triode connected and a limiting screen resistor is used.

Agreed, the voltage limit set for screen in normal use. Triode Connexion not usually applicable.

If only for triode mode, has anyone tried tying the screen grid to the signal grid?

Paul Barker wrote: ↑Tue Jun 11, 2019 1:47 pm Agreed, the voltage limit set for screen in normal use. Triode Connexion not usually applicable.

It's one of those try it and see. Some burn up the g2 as it goes over disapation. Some leak too much into the grid. And some are fine. It's what made me choose the 813 but even its g2 v max is way below the anode at 1100v. Also depends on the application and how hard it is being run. Keep in mind max g1 resistance too.

Cheer,
Stephen

Nick wrote: ↑Tue Jun 11, 2019 12:12 pm Most big pentodes have that sort of limitation. For example EL34, Va=800, Vg2=500.

Not sure there is a official answer other than it seems to work ignoring the value if triode connected and a limiting screen resistor is used.

My GK71 amp (6n1p /dc/ 6n6p /Tx/ GK71 in triode mode) runs with a Va of around 550v , cathode biased at around -75v. g2 is connected to the anode with a humble 100ohm resistor..... what sort of value of limiting resistor would you suggest, please Nick?

I did briefly check online what sorts of Va folk were running trioded GK71s with and there seemed to be a few running at higher voltages than mine....not that that means it's OK of course...

I went for a web trawl and found this Russian site talking about GK71. In Google translation it is quite funny but the general idea seems the GK71 sounds great, some like it better than the GM70. Some have run it up as a triode around 900V which is very interesting.

Cheers,
Stephen

what sort of value of limiting resistor would you suggest, please Nick

100R seems about ok to me.

izzy wizzy wrote: ↑Tue Jun 11, 2019 8:05 pm I went for a web trawl and found this Russian site talking about GK71. In Google translation it is quite funny but the general idea seems the GK71 sounds great, some like it better than the GM70. Some have run it up as a triode around 900V which is very interesting.

Whoops, forgot the link
http://audio-db.info/AudioDB/BazaPrakti ... it/GK/GK71

My GK-71 amp is in essence the Nicholas Chua GM70 circuit with a few additions to make the GK-71 act as a triode.

http://diyparadise.com/nikcgm70.html

And here's mine for posterity.....

No prizes for originality.....I started off with two 6n6ps which sounded very nice, but changed V1 to a 6n1p for a touch more gain, and played with the biasing until I thought it was about right.
Ideally I think I could have upped the 320v and rebias and run a bit more current through V2 but never got round to it, until the regulators blew at Owston and I err had to....
Looking at my fake centre tap for the GK71 filaments I may be overworking the heater supply a bit, I might increase the resistor values (8r5) a bit.

Good to know you're getting along OK with G2 at 625V