GK71 Output Transformer

[tubedog]

I have a pair of GK71 waiting to be used in an SE amplifier.
I would like to know if any members of the forum have used the GK71 in an SE in triode mode and what they found was the best output impedance for the output transformer and what voltages and bias were used?

[kk84]

Hello,

In my construction I`ve used 3k primary resistance transformer with UL tap on 20% from B+. Circuit run`s nice in pentode mode (Va = 1kV, Ia = 125mA, Us2 = 400V, Us1 = -48V, I reached an 42W on output still operating in A1 class. Connecting G2 to UL tap of tr. power drops down to 35W but sound quality .... huh .... Of course in triode mode internal resistance of tube drops drasticly, so I sugest to use 5k primary res. tr.

[tubedog]

KK84

Thank you for your reply. I was planning to use the GK71 in triode mode. I found GK71 triode mode curves and calculated load lines for 5K, 6K and 7K. I have plotted them on the triode curves.
I calculated the Rp at 1283 Ohm, the mu at 6.22 and the gm at 4.8 mA/V. The 7K load line gives the lowest 2nd harmonic distortion (around 2%) and looks like the best candidate for triode mode.
The 3K OPT you tried seems very low for pentode mode. If the sound was good than the GK71 is a very forgiving tube.

However designing an SE output transformer with 7K impedance with good specs with a lot of DC current and sufficient insulation, proved more difficult than I anticipated. I thus decided to go for 100 mA, 800V and a bias of -130V.

What amplification stage and driver did you use in your amp?
How much drive voltage did you put on G1 in pentode mode?
Any problems with grid current, requiring a cathode follower?
Did you experience any hum problems with the filaments of the GK71 (rectified)?

[tubedog]

KK84

Thank you for your reply. I was planning to use the GK71 in triode mode. I found GK71 triode mode curves and plotted load lines for 5K, 6K and 7K. I calculated the Rp at 1283 Ohm, the mu at 6.22 and the gm at 4.8 mA/V. The 7K load line gives the lowest 2nd harmonic distortion (2%) and looks like the best candidate for triode mode.
The 3K OPT you tried seems very low for pentode mode. If the sound was good than the GK71 is a very forgiving tube.

However designing an SE output transformer with 7K impedance with good specs with a lot of DC current and sufficient insulation, proved more difficult than I anticipated. I thus decided to go for 100 mA, 800V and a bias of -130V.

What amplification stage and driver did you use in your amp?
How much drive voltage did you put on G1 in pentode mode?
Any problems with grid current, requiring a cathode follower?
Did you experience any hum problems with the filaments of the GK71 (rectified)?

Tubedog

[Bandersnatch]

Originally posted by tubedog
KK84


The 3K OPT you tried seems very low for pentode mode. If the sound was good than the GK71 is a very forgiving tube.

Tubedog

Td,
Try drawing the load line at 3k slope. Pentode likes a steep load line I've found. Still, 3k and 100 mA only leaves you swinging +/- 300V before you cut it off. I have an amp running the slightly smaller 4E27 in PP with 10k a-a, 40% U-L taps to g2 and 20% taps for E-Linear with B+ of 550V. Those tubes are pinned like 813, and my filament TX can be rigged for 10V instead of V easily. Sooooooon....813's. The bottles certainly look neat.
cheers,
Douglas

[kk84]

Hi.

Thank you for your reply

No problem, seems We both are working at familliar projects

I was planning to use the GK71 in triode mode

In the basics principles of my desing I`ve planed triode mode, but radically low Vg2 changed my mind. There was an option connecting g1+g2+g3 and drive them all together - failed caused by no avaliable for such connection curves

I calculated the Rp at 1283 Ohm, the mu at 6.22 and the gm at 4.8 mA/V

I understand that plate resistance that U calculated is according to mentioned work conditions.

The 7K load line gives the lowest 2nd harmonic distortion (2%) and looks like the best candidate for triode mode.

The best load for triode is Rp x 2. In Your case 7k / 1k283 = 5.46 ratio ... little bit too much but damping factor should be nice.

The 3K OPT you tried seems very low for pentode mode

Not really. Best optimal load resistance for pentode connection is 0.04 - 0.15 of Rp in static work point. If in my case Rp @ 1000V/125mA is about 24k Ohms...

What amplification stage and driver did you use in your amp?

First stage is based on classic aplication of EF86 with B+ 300V, the second stage is SRPP with E88CC.

How much drive voltage did you put on G1 in pentode mode?

56Vpp or 40Veff - still working in A1

Any problems with grid current, requiring a cathode follower?

I wont let the Ig1 flow. First of all g1 is coupled with driver by cap, second Ig1 cause large signal asymetry and thus increase distortion, so I`m not planing to drive this tube in A2.

Did you experience any hum problems with the filaments of the GK71 (rectified)?

I`m supplying GK71 filament with 20V~, no rectification, no regulation.

[quikie22]

Seems a little late... but how did this GK-71 amp turn out? Any problems/issues? Any schematics?

the reason I am asking, I am curious to know how it sounds compared to the GM-70.

[tubedog]

quikie32

The amp has still not been build. I ordered 2 pieces of custom made 7K SE OPT's, but they did not meet my expectations at all. Not to say that I was disappointed. I have decided that I will now rewind the OPT's myself. I plan to do that very soon and will keep you updated on the progress of the project.
I have collected a number of schematics and can post them if you are interested.

[quikie22]

Ok Tubedog. Please post the designs. I am trying to build a GK71/GM70 amp. Was wondering if the GK71 could be trioded and run like the GM70.