GK-71 op points

JamesD · Post by **JamesD** » Sun Jun 23, 2013 2:19 pm

Just having some fun looking at the GK-71 in triode mode... It is a bit of a beast.

Sensible op point seems to be 600V at 100mA at about -85V on the grid and a 3750R load. Plate power is 60W and output is 12Watts at less than 1% distortion - if the curves can be believed... Rk is 850R at 8.5W so a 25W resistor needed...

Interestingly beast mode is 800v at 120mA at about -123V on the grid and a 5K loadline. Plate power is 96W and output power is 25Watts at less than 1% distortion... Interestingly Hammond do a transformer with this spec for less than £100!!!
Rk is 1025R at 14.5W so a 50W resistor needed... psu is about 935Volts for this one - at least its less than 1KV!

The lower power settings swings to zero volts on the grid but the beastie is apparently well behaved for grid current so little flows although I'm sure some does... Beastmode stays away from zero volts so should be fine to assume no grid current...

Graph attached.

James

andrew Ivimey · Post by **andrew Ivimey** » Sun Jun 23, 2013 3:00 pm

more bangs for the buck than GM70 then!

Paul Barker · Post by **Paul Barker** » Sun Jun 23, 2013 3:18 pm

It may pay to buy a bunch while they are prevalent. 813's have gone up considerably in the time these have remained the same. We saw them at the time these things were new to us as a poor man's alternative but it seems we may have misjudged them.

What with that GU81M coming out of the woodwork aswell. We never knew of it's existence before.

I could pop my GU81M in the 833a amp quite easily.

Might leave the GK71 to others.

My projects are unfolding as

1/ convert 6b4g PP amp into GU50 (it looks like my 4 cost me $26, better value than GK71)

2/ make a low powered push pull globe 45 amp with Nickel output transformers. (can't do that until I restore my ability to wind transformers which may be a long distant dream)

Maybe a sensible output SE amp can stay on the cards using existing transformers. Perhaps it will be GK71 but it won't come ahead of the above.

andrew Ivimey · Post by **andrew Ivimey** » Sun Jun 23, 2013 4:40 pm

"I could pop my GU81M in the 833a amp quite easily."

I was thinking along these lines. It appears that GU81ms are only supposed to last for 1000 hours - why!?! then the filaments go 'boinggggg'

Paul Barker · Post by **Paul Barker** » Sun Jun 23, 2013 5:27 pm

andrew Ivimey wrote:It appears that GU81ms are only supposed to last for 1000 hours - why!?! then the filaments go 'boinggggg'

Crazy, I missed that significant matter.

Probably 1,000 hours would cover me for life anyway, as unlikely to use it at home.

IslandPink · Post by **IslandPink** » Sun Jun 23, 2013 5:28 pm

Nice info James , very encouraging . Looks like mu is ~4.75 as mentioned before. This is the one to try first ( for me ) as there's some useful power out even for a 500V supply , and it'll be OK on the Slagle transformers which are 3k and gapped for about 90mA .

Andrew, is that 1000hrs from user experience at sensible dissipation, or from the data sheet for full output in transmitter usage ?

andrew Ivimey · Post by **andrew Ivimey** » Sun Jun 23, 2013 5:30 pm

Well its only a guess but I have read it in two different places. My guess is transmitter usage (full on).

Paul Barker · Post by **Paul Barker** » Sun Jun 23, 2013 6:00 pm

IslandPink wrote:Nice info James , very encouraging . Looks like mu is ~4.75 as mentioned before. This is the one to try first ( for me ) as there's some useful power out even for a 500V supply , and it'll be OK on the Slagle transformers which are 3k and gapped for about 90mA .

Andrew, is that 1000hrs from user experience at sensible dissipation, or from the data sheet for full output in transmitter usage ?

good point. My friend in Scarborough (who now repairs Apples for a living) that used to maintain the Pye relay stations shod with parallel push pull 212 100v line driver amplifiers told me the valves lasted about a fortnight. But they had the anodes white hot all the time. The amplifiers were the size of a 7cwt van, I forget how many there were in our area, maybe he said 4 or 5 amplifiers. They received the BBC radio and then relayed to paying houses around Scarborough who payed for it. So you had a 100v line input transformer at your home.

Whereas I haven't had any of my 212's burning white hot I cannot say they die every two weeks either, and have no reason to expect they will not last my lifetime at the sensible dissipations we use.

Of course when the Relay stations were no longer necessary because the radio transmitting into our area was improved, he never kept a single valve or anything. PRobably in the 1960's everyone thought these old 212 amplifiers the size of a small van would have no further usefulness. He didn't even keep a single valve. Scrapped the lot. All I have is his experience. Definatly they cooked them until the anodes glowed white. None of your red spots, white hot!

Nick and I cooked the graphite GM70 very hot that one day at Eggborough, we tret it like it was a 212 and didn't it come on song? cook off a few of those graphite impurities getting it white hot and the valve sings!

IslandPink · Post by **IslandPink** » Sun Jun 23, 2013 11:31 pm

I wonder what you put on the anode of these things ? - looks like a pin rather than a proper cap.

andrew Ivimey · Post by **andrew Ivimey** » Mon Jun 24, 2013 7:23 am

crocodile clip!

Paul Barker · Post by **Paul Barker** » Mon Jun 24, 2013 7:50 am

Yes crock clip, or you take the brass part out of a terminal strip, after removing screws they drop out.

Over sleeve the connector with a few layers of shrink wrap it is a pretty professional job. the heat of operation finishes the shrinking process.

i connect to the side pins in eimac valves this way but be careful in the eimac application the glass easily breaks when the pins come out of the side. I imagine these GK71's are a lot more rugged, especially because the position is likely a strength position of the glass envelope. the side is not strong.

Post by **Nick** » Mon Jun 24, 2013 8:13 am

Though its worth noting that the grid pins on eimac tubes are meant to be used with a turned heat sink connector. The rated grid and anode dissipation on them is only valid if the correct finned connectors are used. The same may be true for these.

IslandPink · Post by **IslandPink** » Mon Jun 24, 2013 10:44 pm

Hopefully that sort of thing ( " down with this sort of thing"

) will not be needed unless we run them near full power.
It seems that if the anode structure of the GK-71 is very similar to the GM70 , and the dissipation is similar, then hopefully heat-finned connectors are not crucial .

Ps. I have just bought a pair of GK-71's . I have been painted into a corner by all this talk of their characteristics .

JamesD · Post by **JamesD** » Tue Jun 25, 2013 11:29 am

Great! So you will have copper GM-70 and graphite GK-71 to compare with 300BXLS goodness

I've been costing a Beastmode amp for the GK-71 and it gets a bit expensive to do a 1KV amp properly!

Maybe I'll settle for the 12Wrms version....

BTW I've been looking at GU-50 triode curves - awfully close to 300B curves...

James

simon · Post by **simon** » Tue Jun 25, 2013 1:14 pm

Well I think I'm convinced

. I was going to use a pair of 5k OPTs with GM70s, so these should be a decent compromise to try both in the same amp. Same filament requirements, similar HT, driver should be close enough. What's not to like?!

audio-talk

GK-71 op points

#1 GK-71 op points

#2

#3

#4

#5

#6

#7

#8

#9

#10

#11

#12

#13

#14

#15