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 16th October 2007, 01:03 AM   #1
6f6 is offline 6f6  United States
diyAudio Member
 
Join Date: Oct 2006
Default Class B Triodes

Looking through my tube manuals, I have discovered many interesting and often neglected tubes. Amongst these are Class B ("enhanced mode" in ss terminology) triodes such as the #53, #79, 6y7g. Have any of you guys ever built a push-pull amplifier using such devices? Do you think that a small forward bias might help clear up any distortion caused by Class B operation? Just curious.

Cheers
6f6
  Reply With Quote
Old 25th February 2013, 12:46 AM   #2
diyAudio Member
 
Join Date: Jun 2011
Class 'B' audio is not so much done properly with heavy distortion, rather it comes as the result of not planning for the grids to pull current when driven into the 'B' region.

One just cannot 'cheaply' couple via RC network topology and achieve the juice necessary for low distortion operation.

Look to beefy cathode followers, or interstage transformers capable of the necessary Watts.

Many very sweet-sounding AM broadcast transmitters used class 'B' modulators and got a typical overall THD of 5% or less at full (100%) modulation with sine-wave tone tests.

Hard tubes correctly balanced & biased were always a prerequisite.

Good 'B' interstage iron was never cheap, and is tough to source today. The step-up ratio is most critical in selection as well. Best to consult your transformer company and discuss it with them...as power will be flowing in those secondaries. Think phase shift, you don't want any.

The payoff is prolific power from places you never thought, even more than AB2.

Dennis

ps...1635 dual triode. Sleeper.
  Reply With Quote
Old 25th February 2013, 02:58 AM   #3
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
For superb performance at modest cost, NOTHING comes close to PowerDrive at dealing with a positive control grid current regime. Thank you George Anderson, AKA Tubelab!

If my finances ever permit, I intend to build an "El Cheapo" that employs Class "B2" #46 "finals". PowerDrive figures highly in my cogitations. The example circuitry provided by the data sheet uses a good deal of voltage gain, followed by a step down, phase splitting, interstage trafo. That sort of "iron" is damned expensive and constrains the use of NFB. PowerDrive "blocks" between the 12AT7 LTP anodes and the O/P tube grids is low cost and makes loop NFB easy.
Attached Images
File Type: gif EC big.gif (38.8 KB, 240 views)
__________________
Eli D.
  Reply With Quote
Old 25th February 2013, 03:04 AM   #4
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
you can also touch bases with SY, his screen grid drive amp is also an enhanced triode amp....3 or 4mA cathode current at idle......but i suppose this is not for the newbie to do....that is why i have'nt done it yet....this year maybe....
  Reply With Quote
Old 25th February 2013, 03:07 AM   #5
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
Quote:
Originally Posted by 6f6 View Post
Looking through my tube manuals, I have discovered many interesting and often neglected tubes. Amongst these are Class B ("enhanced mode" in ss terminology) triodes such as the #53, #79, 6y7g. Have any of you guys ever built a push-pull amplifier using such devices? Do you think that a small forward bias might help clear up any distortion caused by Class B operation? Just curious.

Cheers
6f6

tubes are very hard to cut-off, if you bias the tubes more than 10% of plate dissipation, then you enter classAB territory....others more knowledgeable than me can correct me if i am wrong.....
  Reply With Quote
Old 25th February 2013, 07:10 AM   #6
hpeter is offline hpeter  Europe
diyAudio Member
 
hpeter's Avatar
 
Join Date: Feb 2013
Quote:
Originally Posted by 6f6 View Post
Looking through my tube manuals, I have discovered many interesting and often neglected tubes. Amongst these are Class B ("enhanced mode" in ss terminology) triodes such as the #53, #79, 6y7g. Have any of you guys ever built a push-pull amplifier using such devices? Do you think that a small forward bias might help clear up any distortion caused by Class B operation? Just curious.

Cheers
6f6
high mu types ?
  Reply With Quote
Old 25th February 2013, 10:06 AM   #7
diyAudio Member
 
the_manta's Avatar
 
Join Date: Nov 2006
Location: Munich, Bavaria
I'm working on something like that right now. Have a box of 50 x 6Н7С (russian version of 6N7GT). The goal is to get 10W out of it and only use one other tube. (12Ж1Л as voltage amplifier - CCS loaded with open loop gain about ~2000 and heavy feedback) Both tubes can be bought for less than 1$.
When this is done, it will be the daily rocker. No worries about tube prices anymore.
Had a few problems with the current driver. Need to deliver a max of about 30mA grid current for 80Vpp. Will use a floating OpAmp with driver for the next tests.
the 6N7GT is working in a 8K-Rpp load. See attachment. I'm looking to reduce the current even more. I don't need that much power here
Next thing to do is an small SMPS for it.
Attached Images
File Type: gif 6N7_loadline.gif (147.3 KB, 195 views)
__________________
Терпенье и труд все перетрут
  Reply With Quote
Old 25th February 2013, 02:13 PM   #8
tricomp is offline tricomp  Denmark
diyAudio Member
 
Join Date: May 2004
Location: Jutland
Quote:
Originally Posted by Conelrad View Post
Class 'B' audio is not so much done properly with heavy distortion, rather it comes as the result of not planning for the grids to pull current when driven into the 'B' region.

The payoff is prolific power from places you never thought, even more than AB2.
The grid only draws current when it's in the positive voltage region.
This is what's designated Class (x)2, where (x)= A, B or AB
When you speak about the 'B'-region, you're talking about what negative grid voltage is sufficient to just cut-off the plate-current. Well, almost cut-off, a fraction may still be left.
In this state you'll not experience grid-current in any case.

About the second statement:
I don't think so. If AB2 is done right you'll pull A LOT of power from the tubes and at low distortion as well. You may use FET's or Cathode-Followers as DC-coupled drivers to achieve this in real-life.
The difference between B1 and AB2 is huge.
Now, if you were speaking of B2 it would be another matter but the price to pay is distortion.

rgds,

/tri-comp
  Reply With Quote
Old 25th February 2013, 02:18 PM   #9
tricomp is offline tricomp  Denmark
diyAudio Member
 
Join Date: May 2004
Location: Jutland
Quote:
Originally Posted by the_manta View Post
I'm working on something like that right now. Have a box of 50 x 6Н7С (russian version of 6N7GT). The goal is to get 10W out of it and only use one other tube. (12Ж1Л as voltage amplifier - CCS loaded with open loop gain about ~2000 and heavy feedback) Both tubes can be bought for less than 1$.
When this is done, it will be the daily rocker. No worries about tube prices anymore.
Had a few problems with the current driver. Need to deliver a max of about 30mA grid current for 80Vpp. Will use a floating OpAmp with driver for the next tests.
the 6N7GT is working in a 8K-Rpp load. See attachment. I'm looking to reduce the current even more. I don't need that much power here
Next thing to do is an small SMPS for it.
Please explain why you expect to get away with a load-line so far above maximum plate-dissipation.
How much could you exceed this recommended dissipation-level and still be safe?

rgds,

/tri-comp
  Reply With Quote
Old 25th February 2013, 04:28 PM   #10
diyAudio Member
 
the_manta's Avatar
 
Join Date: Nov 2006
Location: Munich, Bavaria
The modulation of a single tube in terms of voltage or current is half-sinusoidal. In terms of power it is proportional to the voltage or current squared.
When your idle operating point is at zero watts plate dissipation, then your maximum modulation can go as high as 4 times the max-dissipation of the tube. (e.g. 100W for an EL34). This works because the mean value is still below or equal 25W. Thermal inertia of the plate material guarantees the rest. This was done successfully millionth times.

The attached picture shows this effect. The area below the curves is equal, but the highest point of the red constant curve (Class-A) is only 1/4 of the blue.

As I said, I want to go down with my idle operating point to lower the idle dissipation. Interestingly I'm already lower than the datasheet states.
Hope that helped a bit. There is tons of literature which describes this operation principle.
Attached Images
File Type: gif class-b.gif (17.4 KB, 147 views)
__________________
Терпенье и труд все перетрут
  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
Anyone know of some ridiculously powerful triodes for class A useage? Test Electrix Tubes / Valves 71 30th June 2009 01:11 PM
FS: two new 811 triodes Tekko Swap Meet 0 17th November 2008 11:27 AM
More triodes kmaier Tubes / Valves 4 4th July 2008 09:22 PM
LPT with low triodes? Fuling Tubes / Valves 9 4th December 2006 11:48 PM


New To Site? Need Help?

All times are GMT. The time now is 06:15 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