Mixed bias for balancing unmatched tubes in PPP? - Page 2 - 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, 07:47 AM   #11
Ian444 is offline Ian444  Australia
diyAudio Member
 
Join Date: Nov 2008
Location: Brisbane QLD
My idea of four mosfet followers will not work, don't know what I was thinking...
Ken's idea looks simple and good. I think this is what he is describing athos56:
Attached Images
File Type: gif PPP bias scheme.GIF (12.2 KB, 110 views)
  Reply With Quote
Old 21st October 2010, 12:56 PM   #12
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
Thats it exactly, my PC crashed last night, else I would have drawn...
  Reply With Quote
Old 21st October 2010, 03:37 PM   #13
athos56 is offline athos56  United States
diyAudio Member
 
athos56's Avatar
 
Join Date: May 2006
Location: Yakima, Washington
Thanks guys!
  Reply With Quote
Old 22nd October 2010, 07:20 AM   #14
athos56 is offline athos56  United States
diyAudio Member
 
athos56's Avatar
 
Join Date: May 2006
Location: Yakima, Washington
Any idea what kind/size of pots to use in those locations (100, 1k, 25k, 100k)? And I assume that that's a ubiquitous 5mm red led?

Athos
  Reply With Quote
Old 22nd October 2010, 09:25 AM   #15
45 is offline 45  Italy
diyAudio Member
 
45's Avatar
 
Join Date: Dec 2008
Location: UK
Quote:
Originally Posted by athos56 View Post
So some might know I'm designing a 6B4G PPP amp. The bias scheme I have now can balance the bias between the both sets of output tubes (on either side of the opt) but not individual tubes (on one side of the opt). So would a resistor and pot from each tube cathode to ground allow me to fine tune each tube in concert with the fixed bias?

Athos
As far as I have seen it is not necessary and doesn't bring advantages. I have done it for a 2A3 PSE and there is no practical difference in terms of THD, IMD and sound between individually biased 2A3's (each one with its own cathode follower driver) and both 2A3's biased by a single cathode follower.
Therefore in the PPP case a nice dynamic balance between the two sides of the PP is by far the most important thing (i.e. getting the best square wave response at low frequency, 40 Hz for instance), IMHO. This usually requires a small gap in the OPT to take into account small differences in terms of DC currents, too.

45
  Reply With Quote
Old 22nd October 2010, 03:39 PM   #16
athos56 is offline athos56  United States
diyAudio Member
 
athos56's Avatar
 
Join Date: May 2006
Location: Yakima, Washington
Quote:
Originally Posted by 45 View Post
As far as I have seen it is not necessary and doesn't bring advantages. I have done it for a 2A3 PSE and there is no practical difference in terms of THD, IMD and sound between individually biased 2A3's (each one with its own cathode follower driver) and both 2A3's biased by a single cathode follower.
Therefore in the PPP case a nice dynamic balance between the two sides of the PP is by far the most important thing (i.e. getting the best square wave response at low frequency, 40 Hz for instance), IMHO. This usually requires a small gap in the OPT to take into account small differences in terms of DC currents, too.

45
Interesting, thanks for the real world experience. I would like to keep the overall complexity down. What OPTs did you use?

Athos
  Reply With Quote
Old 22nd October 2010, 05:08 PM   #17
45 is offline 45  Italy
diyAudio Member
 
45's Avatar
 
Join Date: Dec 2008
Location: UK
Quote:
Originally Posted by athos56 View Post
Interesting, thanks for the real world experience. I would like to keep the overall complexity down. What OPTs did you use?

Athos
Lundahl 1627.
It is basically the same transformer for both PP and SE except the gap.
If you use the SE version for a PP the headroom will be the same if no DC unbalance however if (more likely) you have some DC unbalance having that gap is a lot better. The lower primary inductance is not a problem at all. Actually, if you have a gap, you will get nearly the same low end response from a fraction of 1 watt to the max Pout while usually the low-end cut-off is a function of the output level in un-gapped OPT's and inductance drops quickly in presence of DC unbalance. This because, without a gap, you have a larger variation of the core permeability as function of the AC induction (which in turn is proportional to the signal) and DC unbalance.
In many PP OPT's without gap you can re-arrange the core with a small gap. How big the gap depends on the specific case but you can find the final solution trying different sizes (small fraction of mm, usually).

45
  Reply With Quote
Old 22nd October 2010, 07:39 PM   #18
cerrem is offline cerrem  United States
diyAudio Member
 
Join Date: Jun 2005
Location: San Diego, CA
I agree with 45....This is important consideration of using gap...but you still need to be sure you have enough inductance to make it down to lowest frequency of interest...
The whole issue with using balancing pot for the DC current has it's limits.... Keep in mind that the output transformer may be happy when it has canceled DC flux but an even bigger issue is the skew of the transconductance across the Push-Pull halves may be even more of an issue.... For example you have an output stage with -42V on one side and -50V on the other side needed to balance the DC....now you will have one side clipping sooner than the other.... The AC imbalance can be corrected for, provided you have enough headroom in the gain of the feedback loop for error correction...

Chris
  Reply With Quote
Old 22nd October 2010, 09:55 PM   #19
athos56 is offline athos56  United States
diyAudio Member
 
athos56's Avatar
 
Join Date: May 2006
Location: Yakima, Washington
Quote:
Originally Posted by 45 View Post
Lundahl 1627
Well that's not going to happen . The $50 buck Edcors are where it's at...
  Reply With Quote
Old 23rd October 2010, 06:50 AM   #20
45 is offline 45  Italy
diyAudio Member
 
45's Avatar
 
Join Date: Dec 2008
Location: UK
Quote:
Originally Posted by athos56 View Post
Well that's not going to happen . The $50 buck Edcors are where it's at...
That was just the answer to your question. You can have a gap on the Edcor as well. You just need to find the practical solution which means enough inductance together with enough headroom in case of DC unbalance. You can start trying with 0.05-0.1 mm.
In your case you also have 8 power tubes to play with and I am pretty sure that if they measure within 10% one from any other for current and mutual conductance it will be easy and straightforward to make pairs so that you can find dynamic balance with little adjustment together with good DC balance using a single bias network for each pair.

45
  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
mixed mode bias TUBESMAN Tubes / Valves 4 6th October 2010 12:50 PM
ppp el84 fixed bias schematics jogas Tubes / Valves 17 31st August 2009 01:02 PM
mixed bias TUBESMAN Tubes / Valves 2 15th January 2009 01:19 AM
Will this work? "Mixed" 300B PPP IT ppereira Tubes / Valves 4 24th April 2004 05:29 PM
Balancing output tubes by changing heater voltage. Circlotron Tubes / Valves 8 10th February 2003 11:15 AM


New To Site? Need Help?

All times are GMT. The time now is 09:17 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