GK71 Output Transformer - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 12th August 2008, 07:43 PM   #1
tubedog is offline tubedog  Mauritius
diyAudio Member
 
tubedog's Avatar
 
Join Date: Mar 2008
Default GK71 Output Transformer

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?
  Reply With Quote
Old 21st August 2008, 01:54 PM   #2
kk84 is offline kk84  Poland
diyAudio Member
 
Join Date: Aug 2008
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.
  Reply With Quote
Old 21st August 2008, 04:11 PM   #3
tubedog is offline tubedog  Mauritius
diyAudio Member
 
tubedog's Avatar
 
Join Date: Mar 2008
Default GK71 triode curves

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)?
  Reply With Quote
Old 21st August 2008, 10:02 PM   #4
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
Default Re: GK71 triode curves

Quote:
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
__________________
the Tnuctipun will return
  Reply With Quote
Old 21st August 2008, 10:14 PM   #5
kk84 is offline kk84  Poland
diyAudio Member
 
Join Date: Aug 2008
Default Re: GK71 triode curves

Hi.

Quote:
Thank you for your reply
No problem, seems We both are working at familliar projects

Quote:
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

Quote:
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.

Quote:
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.

Quote:
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...

Quote:
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.

Quote:
How much drive voltage did you put on G1 in pentode mode?
56Vpp or 40Veff - still working in A1

Quote:
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.

Quote:
Did you experience any hum problems with the filaments of the GK71 (rectified)?
I`m supplying GK71 filament with 20V~, no rectification, no regulation.
  Reply With Quote
Old 31st August 2011, 11:06 PM   #6
diyAudio Member
 
Join Date: Sep 2009
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.
  Reply With Quote
Old 4th September 2011, 01:21 PM   #7
tubedog is offline tubedog  Mauritius
diyAudio Member
 
tubedog's Avatar
 
Join Date: Mar 2008
Default building GK71 amp

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.
  Reply With Quote
Old 28th December 2011, 04:44 AM   #8
diyAudio Member
 
Join Date: Sep 2009
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.
  Reply With Quote
Old 7th June 2014, 02:18 PM   #9
diyAudio Member
 
Join Date: Jan 2014
Try 1250V Power Supply and 1,2kOhms cathode resistor. Plate resistance should be 5 to 6kOoms. The tube needs 185V clean driver peak, so a 300B would be the best driver. A good alternative would be a 6ck4 or a AD101 if available. 60watts in SE that provide a rather smooth and warm sound.
  Reply With Quote
Old 8th June 2014, 12:39 PM   #10
Junm is offline Junm  Philippines
diyAudio Member
 
Junm's Avatar
 
Join Date: Dec 2008
Location: Dau
Send a message via Yahoo to Junm
more than 2 years thread...anyway still interesting info.....thanks,..
any schema to share?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Someone help: SE shematic with GK71? bezobraznia Tubes / Valves 30 24th April 2011 12:17 AM
GK71 B+ turn on delay kk84 Tubes / Valves 4 2nd February 2009 07:31 PM
3C24 in a SE output stage - output transformer impedance recommendation !!! aldovan Tubes / Valves 15 6th September 2008 10:18 AM
Adding secondary of output transformer into cathodes of output stage ? Eico ST-70 freddymac406 Tubes / Valves 3 21st April 2007 06:15 PM
Can An Output Transformer Change A Voltage Amp's Output Impedance From 0.1 To 47 Ohms kelticwizard Everything Else 11 25th March 2007 05:17 AM


New To Site? Need Help?

All times are GMT. The time now is 03:13 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2