a better pentode for less! - 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 25th August 2003, 03:18 AM   #1
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Talking a better pentode for less!

I have found a way to make an excellent pentode from some real "junk" tubes. Look at the plate curves for any of the dual control pentode tubes (6LE8, 9KC6, 6BV11, 6MK8,6BU8,....)and they are very rounded and high distortion due to screen current siphoning off plate current at low plate voltage. These tubes have a close-meshed G3 (suppressor grid) instead of the very open spaced G3 usally found in normal pentodes. (Usually used for control/modulation purposes in the dual control case) The screen grid G2 wires normally cause electrons passing by to be deflected slightly off course due to the screen grid potential. The open mesh G3 of normal pentodes at zero volts still lets most of these get by to the plate, but the close mesh G3 of the dual control tubes deflects a lot of these back to the G2 screen grid causing the high screen grid currents. G3 is normally operated at zero volts (relative to cathode) for convenience and to repel secondary emission electrons from the plate back to the plate. Most normal pentodes have G3 internally connected to the cathode as well. If one puts about +12V to +15V on G3 (relative to the cathode) of the dual control pentodes (which have an isolated pin for G3), then the screen deflected electrons still get by to the plate. Secondary emission suppression is hardly affected by 15V less difference between plate and G3. The plate curves square up into the best beam pentode curves you have ever seen. Screen current drops significantly (compared to 0V on G3) at low plate voltage. Current into G3 is miniscule. The plate resistance of these tubes is very high due to the screen grid and the close meshed G3 both acting as shields. They should make excellent high impedance current sources. These dual control tubes often go for $.50 when on sale.

Don
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 25th August 2003, 04:42 AM   #2
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
6BZ6 is another I think...
Damn thing takes as much heater current as a 6V6. Yikes!

Nope, that's not it.. GE 'sheet says .3A.. must be another type then.. oh well...

Oh, and I agree with your post.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 25th August 2003, 06:23 AM   #3
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
Default Re: A better pentode for less!

Quote:
They should make excellent high impedance current sources.
That's an interesting post smoking-amp. I looked up a data sheet, and see where you're coming from. I then wondered where one could use one of these - given that the output capacitance is a bit higher than the competition. The ideal use would be as a CCS for a pair of balanced cathode followers. If the drive to the cathode followers is balanced, then the sum of the audio currents is balanced, so the cathode current in the dual anode CCS valve would be constant. The fact that the CCS's output capacitance is a bit high wouldn't matter, because of the low impedance at the cathode of those cathode followers, and that capacitance would be swamped by the load capacitance anyway.

Good find! I may have to do some experimenting.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 26th August 2003, 01:00 AM   #4
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Some more dual control tubes: 7 pin: 6GY6, 6GX6, 6HZ6, 6AS6, 6DB6, 6DT6, 5725, 6187, 6954
9 pin: 6LE8, 6BU8, 6KF8, 6MK8, 6GS8, 6HS8 (these have dual plates too)
octal: 6888
compactron: 6BV11,12BV11 (two dual control units per tube)
compactron mixed: 6AD10, 6AL11, 6BF11, 6BY11, 6G11, 6T10, 6Y10?, 9BJ11 ,12AE10, 13V10, 18AJ10 (one dual control unit and one power pentode per tube)

The 9 pin tubes are dual control and dual plate. I have been using the 6LE8 with the plates tied together to act as one tube,
but as EC8010 suggests, they can be used as dual current sources separately, and balanced signals fit nicely.

I have been looking at the specs on output plate capacitance of the dual control tubes to compare with other tubes. Just from the presence of more wires in the G3 grid I would expect more output capacitance to plate. Surprisingly though, it depends on what you compare with. For example, the 9KC6 has identical structure to a 12HL7 tube except for G3, the 9KC6 has 3.0 pf and the 12HL7 has 6.0 pf. This is probably the case for beam pentodes which have a lot of sheet metal in their beam forming G3. The 6LE8 has 4.2 pf per plate section for a total of 8.4 pf, while a 6HB6 (nearly identical cathode and grid 1, a beam pentode) has 8.0 pf. However, the dual control tubes are derated about 50% in power due to the high screen grid current they usually draw. With G3 at +12V to +15V, the screen current drops significantly, so some of this derating can probably be rolled back.

If one compares the dual control tubes at their given watt rating with similarly rated tubes though, then output capacitance is more like 2x. Since the dual control tubes were used at high frequencies as demodulators typically, I would be surprised if output capacitance is really that much of an issue, especially at audio frequencies. But in any design, output capacitance, stray wire capacitance and next tube grid input capacitance must be considered when calculating max slew rate.

Don
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 7th August 2008, 07:44 PM   #5
Schunda is offline Schunda  Argentina
diyAudio Member
 
Join Date: Oct 2005
Please can anyone post a schematic or an example with a practical application of these tubes?

Regards.

Sch +Z
  Reply With Quote
Old 7th August 2008, 08:55 PM   #6
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
I haven't seen any schematics using these oddball tubes. But a few guidelines can be used to convert them to conventional tube versions. First, you have to find the optimum g3 voltage that minimizes g2 current at the operating point chosen. Typically +12V to +32V works on g3. (too much + g3 and the current starts going to g3 instead, NG) This is low current at g3, so a resistive divider can be used from B+ to derive this, or a zener setup.

A primitive curve tracer setup using a scope can be very illuminating while adjusting g3. The plate curves will square right up like a normal beam pentode when g3 bias is optimum.

Next, measure the new g2 current and plate current at the chosen operating point with g3 bias applied. You will find that g2 current has dropped considerably from the datasheet figure (given at g3 = 0). And this means that more current is arriving at the plate than before. (the + g3 bias prevents electron returns back to g2).

This effects two things. The previous large g2 power dissipation will drop, and this is usually what was limiting for the design sheet data. The 9KC6 for example is datasheet rated at 7 Watts plate dissipation, and the similar 12HL7 and 12GN7 is rated for 10 Watts. You should still observe the 1 watt (12HL7) limit for the screen grid when exceeding the 7 Watt limit.

Second, the g1 transconductance increases with g3 pos. bias since more current is making it to the plate instead of being intercepted by rebound at g2. The 9KC6 should increase it's g1 transconductance from its 24000 datasheet figure to more like 36000 (12GN7 territory). Remember, that gm scales as plate current to the .666 power (practical, not theoretical) when comparing different operating points or different tubes. Rp varies as -.666 power with current. I don't think the g3 bias will much affect the rp figure at the same current like it did for the g1 gm though.

So net effect is the 9KC6 becomes close to a 12GN7. A 6LE8 becomes close to a 6HB6 (although the 6LE8 does not have the same large plate structure as the 6HB6, so I would stick to the same sum: 2+2+2 = 6 watts for total screen grid + plates dissipation). Similar upgrade adaptations can be figured for the other dual control tubes.

Some further tweeks are available with these tubes. Normally a pentode has an expanding 3/2 power rule for current versus g1 voltage up until saturation. By lowering the + g3 DC bias from its optimum, this expansionist rule can be bucked some by diverting some current back to the g2 grid. (Keep an eye on g2 dissipation though). So some linearization (really counter distortion) can be produced much like in UL with a high g2 voltage. A distortion analyzer is called for to look for an optimum.

For g2 drive applications (not common for these small watt tubes though) one can try connecting the g3 grid to the g2 to get more g2 gm effectively.

Don
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 8th August 2008, 01:34 AM   #7
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
I haven't seen any schematics using these oddball tubes.
Member Pete Millett published an amplifier design in the AudioXpress magazine recently. It used a dual control pentode with positive bias applied to G3. The article can be found here:

http://www.audioxpress.com/magsdirx/...millet2903.pdf
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 8th August 2008, 02:27 AM   #8
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
Millet's article mentioned nothing of G2 current reduction.
That much at least, is very significant new observation.
  Reply With Quote
Old 9th August 2008, 01:01 AM   #9
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
For g2 drive applications (not common for these small watt tubes though) one can try connecting the g3 grid to the g2 to get more g2 gm effectively.
Many sweep tubes have G3 pinned out seperately for application of a positive voltage. 30 to 50 volts is applied to eliminate Barkhausen Oscillation. These are beam tetrodes, but some experimentation is in order when I get back home.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 9th August 2008, 08:27 PM   #10
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
Quote:
Originally posted by tubelab.com


Many sweep tubes have G3 pinned out seperately for application of a positive voltage. 30 to 50 volts is applied to eliminate Barkhausen Oscillation. These are beam tetrodes, but some experimentation is in order when I get back home.
So this is the 'snivets' that the data sheets talk about? cooool!
cheers,
Douglas
__________________
the Tnuctipun will return
  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
All pentode PP amp? ray_moth Tubes / Valves 7 31st January 2007 12:52 AM
Anyone using an 813 in pentode? Svetlana Tubes / Valves 7 30th August 2006 11:01 PM
A small no-fuss pentode/pentode SE amp Tubes4e4 Tubes / Valves 13 1st January 2006 05:16 AM
Big pentode help! massimo Tubes / Valves 0 13th February 2005 10:44 PM
Pentode AB1 or AB2? Fuling Tubes / Valves 85 22nd January 2005 10:42 PM


New To Site? Need Help?

All times are GMT. The time now is 02:45 PM.


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