Audio drive to g2 connected to g1 via 15K0 - 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 21st October 2010, 06:49 PM   #1
diyAudio Member
 
Join Date: Oct 2010
Default Audio drive to g2 connected to g1 via 15K0

The KT88's I have were in a modulator for a transmitter.

Audio drive from a centre tapped transformer, was fed to g2 on the KT88's, and g2 was connected to g1 via a 15K0 resistor.

What connection type is that? Thanks.
  Reply With Quote
Old 21st October 2010, 07:16 PM   #2
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
That is zero bias Class B screen grid drive. If g1 is positive then the valve is turned on hard, but the current flow is determined by g2 voltage. The tetrode is being used as a low mu triode. The 15K resistor restricts g1 current, as it is not designed to dissipate much power. g1 only goes a few volts positive. As g1 is much nearer the cathode than g2, if they had the same potential then g1 would take most of the current and get very hot.

The aim here is efficiency rather than ultimate fidelity. When the KT88 first came out it was just another big beam tetrode, and not so astonishingly expensive as it is today, so it was used for all sorts of normal everyday applications.
  Reply With Quote
Old 21st October 2010, 07:29 PM   #3
diyAudio Member
 
Join Date: Oct 2010
Yes, I see no bias arrangement at all on the circuit diagram that someone drew out.

I'll probably rebuild the modulator but not with the GEC KT88's - a bit too good for the purpose. Possibly a modern KT88 pair.

Anyway, thanks, I was almost not believing my eyes as to the connection, not ever having come across that before.
  Reply With Quote
Old 22nd October 2010, 09:58 PM   #4
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
What connection type is that? Thanks. .......That is zero bias Class B screen grid drive.
I have seen a similar circuit using 807's. I built it and played around with it several years ago. The value of the resistor affects the distortion a lot.

Quote:
The tetrode is being used as a low mu triode.
Actually the Mu is rather high. My experience reminded me of an 811A. I have seen some circuits like this from the WWII era for modulator or PA amp applications. A few of us have experimented with circuits like this using solid state components to improve the distortion. No highly successful designs have resulted yet though.

G1=G2/mu Scaled Drive Strawman Design
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 23rd October 2010, 09:44 AM   #5
diyAudio Member
 
Join Date: Oct 2010
Looking at the instructions I have for the AM transmitter, I see it does mention that the tubes are working "in class B zero bias". Not seen that before.

And curiously, the tubes mentioned are 807's.

But, the modulator deck had a pair of KT88's instead of the 807's. Maybe someone just put in KT88's for the 807's.
  Reply With Quote
Old 23rd October 2010, 06:28 PM   #6
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by richard8976 View Post
Looking at the instructions I have for the AM transmitter, I see it does mention that the tubes are working "in class B zero bias". Not seen that before.
Paralleling G1 and G2 (and G3 for pents with actual suppressors) is something that is done a lot to make high-u, RF, pseudotriodes. It helps uncomplicate designs by eliminating the screen supply. I've seen it done with some of the bigger horizontal deflection VTs as well. In that case, distortion is not a factor since you eliminate the nasty harmonics with one or more LC tuners, bandpass filters, antenna tuners/"matchboxes", and the like.

It's not something I'd do for an audio amp where the consideration was fidelity as opposed to efficiency and lotsawatts. You have all the same problems that the proper, high-u RF triode finals have: a grid current drive problem, the high r(p) that reduces speaker damping, the possibility of increased h3 and higher order harmonics.

I'd use the KT88's (or 807's) the way they were intended: as pents, and clean up the harmonics with local NFB and/or gNFB. If you want triode audio finals, then consider types like the 6CK4, 6BX7, 6AS7, or one of the other vertical deflection or series pass types. If you need more output, consider 845's.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  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
G1=G2/mu Scaled Drive Strawman Design Michael Koster Tubes / Valves 89 11th July 2010 01:59 AM
HK 'Deuce front end on G2 drive output stage? GordonW Tubes / Valves 0 9th June 2008 02:22 PM
6JN6 screen-drive (g2 driven) plate curves? GordonW Tubes / Valves 26 23rd April 2008 01:56 PM
Anyone know the g1-g2 capacitance of 6AU6? ray_moth Tubes / Valves 10 29th August 2004 07:19 AM


New To Site? Need Help?

All times are GMT. The time now is 09:41 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