GK-71 as a pentode.

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Alex Kitic
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#16

Post by Alex Kitic »

pre65 wrote: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.
Having just 600V will make the initial arrangement with the zeners easier.

As for the driver tube, you could rewire the socket for ECC81. This way you could use both halves as driver and CF, applying in full the RH813 schematics. I cannot advice a better solution than that.

Once you try it , you can always consider increasing B+ and applying the other necessary solutions. But first try my suggestion: it should be an eye opener.
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#17

Post by pre65 »

Right. Andrew and I had "brain fade" yesterday when talking about using the GK-71 as a pentode.

Re reading this thread has cleared my mind. :wink:

So, now to sort out some zeners to drop 200v. :)
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#18

Post by pre65 »

So, the 75v & 5W 20ma zener diodes are 1N5374bg

I string 3 together (3 x 75 = 225v) and connect to G2 straight from HT.

Is it that simple ? :?
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#19

Post by pre65 »

The diodes on Ebay were overly expensive, so I bit the bullet and ordered 10 from RS components.

They should be here tomorrow afternoon. :)
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Nick
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#20

Post by Nick »

pre65 wrote:So, the 75v & 5W 20ma zener diodes are 1N5374bg

I string 3 together (3 x 75 = 225v) and connect to G2 straight from HT.

Is it that simple ? :?
No, you need a load resistor to supply them or they will not be in operation for long.
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#21

Post by pre65 »

Nick wrote:
pre65 wrote:So, the 75v & 5W 20ma zener diodes are 1N5374bg

I string 3 together (3 x 75 = 225v) and connect to G2 straight from HT.

Is it that simple ? :?
No, you need a load resistor to supply them or they will not be in operation for long.
OK, I was sort of following Alex Kittics blog on the 813, and no load resistor was shown on that.

Can anyone point me in the right direction on this matter please ?
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#22

Post by pre65 »

I've been searching. Most articles seem to be about driver pentodes, but I found this.

"Effect of G2 on Pentode Operation.
On most designs you see g2 (screen grid) either connected to HT or with a resistor and a decoupling capacitor. As you increase the series resistor on g2, the voltage on g2 will vary inversely with Anode voltage due to inverse relationship of Ig2 and Ia below the knee. This is thus negative feedback and if the g2 is decoupled with a capacitor only affects DC. Without a capacitor the Audio or RF gain is reduced and signal is more linear also any hum or noise on supply rail has less effect on the load.
Up to about 1.5x the Anode load, the gain is indeed reduced and as a result the frequency response is better. Transient response is better. As Rg2HT is increased the gain of edges stays roughly the same as at the 1.5x load. But lower frequency gain continues to decrease. This means with transients there is severe "overshoot" About 50% when R on G2 to HT is about x3 or x4 the Anode load. So.. Add a decoupling capacitor? Well that gives terrible transient performance at larger signals! Maybe all pentodes behave like this? I dunno. Literature suggests up to x5 resistor for G2 load than Anode, but that doesn't work at all on these Russian tubes."

So, as the anode load is 5K, would a 15k - 20k load resistor be what I need ?
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#23

Post by Nick »

Ok, my mistake, they are between B+ and g2, I assumed g2 and ground. So no, no resistors needed.
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#24

Post by pre65 »

Nick wrote:Ok, my mistake, they are between B+ and g2, I assumed g2 and ground. So no, no resistors needed.
OK, thanks for keeping an eye on my progress. Much appreciated. :D
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#25

Post by Cressy Snr »

hi Phil,

The link below tells you all you need to know about screen grids, but were afraid to ask.

http://www.oestex.com/tubes/screens.htm
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Alex Kitic
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#26

Post by Alex Kitic »

pre65 wrote:So, the 75v & 5W 20ma zener diodes are 1N5374bg

I string 3 together (3 x 75 = 225v) and connect to G2 straight from HT.

Is it that simple ? :?
Yes, it's as simple as that.
I suggest using 5W zeners, and up to 75V.
Of course, you could use 2x75 and 1x56V, but 225V is probably a better option.

Keep in mind the maximum g2 voltage and do not exceed it.

Don't forget to use the LM317 for CCS!
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#27

Post by pre65 »

Thanks Alex.

Re the CCS, one step at a time. :)
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#28

Post by pre65 »

Wahey, Parcelforce have just delivered my zener diodes. :D
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#29

Post by pre65 »

Just to check.

Zener diode

Anode to G2

Cathode to power supply.
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#30

Post by pre65 »

It's up and running as a pentode.

I've got 401v on G2, so I shall insert another 75v diode soon.

It's a lot louder for the same volume setting.

Just playing some music to see how it sounds. :)
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