Go Back   Home > Forums > >

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

Audio Research amp design question
Audio Research amp design question
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
Old 9th December 2020, 01:54 AM   #11
peterVsi55 is offline peterVsi55  United States
diyAudio Member
 
Join Date: Aug 2020
[sigh] I have much to learn, apparently.
  Reply With Quote
Old 10th December 2020, 02:11 PM   #12
peterVsi55 is offline peterVsi55  United States
diyAudio Member
 
Join Date: Aug 2020
This is a quote from the Audio Research book "Making the Music Glow", by Ken Kessler, regarding the M300 mono amplifier:

"Most notably, the output transformer design was distinguished from previous designs by featuring three center-tapped primary windings: one for the plat supply, one for the screens and one for the cathodes."

"J Gordon Holt explained in his Sterephile review of the M300: 'The output tubes are both plate and cathode coupled to the output transformed, an arrangement which allows for higher coupling efficiency, smaller windings ratios, and reduced DC magnetization of the transformer core (since current flow is opposite through the cathode and anode windings)'.

"Adds Chris Ossanna, 'Along with the higher efficiency of the output transformer, the use of 8 6550 output tunes in quad push-pull operation allowed the primary impedance ration of the transformer to be only 1,250 ohms from primary to secondary, instead of the usual 5,000 of a typical power amplifier.'"
  Reply With Quote
Old 10th December 2020, 02:59 PM   #13
Alex M is offline Alex M  Europe
diyAudio Member
 
Join Date: Apr 2001
Location: Hampshire, UK
ARC seem to have used more than one arrangement for cathode feedback. In the Classic range the cathodes were indeed connected to ground via the secondaries of the output transformers:

Click the image to open in full size.

As John Atkinson notes in his review of the Classic 60, this introduces a small (up to 30mV) DC offset at the speaker terminals.

Alex

Last edited by Alex M; 10th December 2020 at 03:08 PM.
  Reply With Quote
Old 10th December 2020, 04:15 PM   #14
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Quote:
'The output tubes are both plate and cathode coupled to the output transformed, an arrangement which allows for higher coupling efficiency, smaller windings ratios, and reduced DC magnetization of the transformer core (since current flow is opposite through the cathode and anode windings)'
Unfortunately, CFB winding does NOT reduce DC magnetization in the OT from each tube. If it were true, then all SE designs would be done that way. Easy to check. AC signal is just the same as more or less DC signal. It has to increase/decrease the magnetization or nothing would go thru the OT.

Or one can use the Broskie technique of moving the tube "thru" the CFB winding until it is below it like a conventional configuration. Just a transposed position series arrangement. The phasing of the two winding sections is such that both contribute to magnetization the same way for the same currents thru the tube.

----------------------------------

An additional note about CFB windings. For P-P arrangements, the two CFB windings must be coupled equally (accurately) to the secondary since they are being used for N feedback. Any error here means permanently damaged output signal symmetry. Many P-P OTs, not intended for CFB, will not have equal leakage L from two such windings, unless a symmetrical split bobbin wind-up is used.

.

Last edited by smoking-amp; 10th December 2020 at 04:33 PM.
  Reply With Quote
Old 10th December 2020, 09:33 PM   #15
painted is offline painted  Luxembourg
diyAudio Member
 
Join Date: Jun 2012
DC magnetization would be the same, indeed. The anode winding will be reduced (to 90%?) while the cathode winding is added (10%?). Is there a penalty in parasitic capacitance / inductance or is the same BW possible as with the odd OT?
  Reply With Quote
Old 12th December 2020, 01:32 PM   #16
peterVsi55 is offline peterVsi55  United States
diyAudio Member
 
Join Date: Aug 2020
I think you misread what the quote from the book says. The cathode winding is *separate* from the primary winding, and wired in reverse to the anodes.





Quote:
Originally Posted by smoking-amp View Post
Unfortunately, CFB winding does NOT reduce DC magnetization in the OT from each tube. If it were true, then all SE designs would be done that way. Easy to check. AC signal is just the same as more or less DC signal. It has to increase/decrease the magnetization or nothing would go thru the OT.

Or one can use the Broskie technique of moving the tube "thru" the CFB winding until it is below it like a conventional configuration. Just a transposed position series arrangement. The phasing of the two winding sections is such that both contribute to magnetization the same way for the same currents thru the tube.

----------------------------------

An additional note about CFB windings. For P-P arrangements, the two CFB windings must be coupled equally (accurately) to the secondary since they are being used for N feedback. Any error here means permanently damaged output signal symmetry. Many P-P OTs, not intended for CFB, will not have equal leakage L from two such windings, unless a symmetrical split bobbin wind-up is used.

.
  Reply With Quote
Old 13th December 2020, 01:21 AM   #17
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
If you remove the tube and connect the CFB and plate winding taps back together there, the full winding would have no turn reversal in it.

So one can mentally reduce the turns in one section (with the tube re-inserted) while increasing the turns in the other section with no effect on operation, except on the amount of N Fdbk. Putting all the turns into the plate winding and zero turns into the CFB section returns it to a conventional grounded cathode set-up. None of the variants cancels DC magnetization for a single tube. They are series equivalent as far as DC is concerned. Likewise for AC.

Last edited by smoking-amp; 13th December 2020 at 01:35 AM.
  Reply With Quote
Old 13th December 2020, 02:50 AM   #18
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
It seems to me that for push pull output transformers . . .

When the DC plate currents are equal, their magnetic fields cancel.
(if the turns for plate 1 = turns for plate 2).

When the DC cathode currents to the cathode feedback windings are equal, their magnetic fields cancel.
(if the turns for cathode 1 = turns for cathode 2).

Of course for single ended, or for a single tube, the magnetic fields do not cancel, agreed.

And for Pentode and Beam Power tubes, Plate current does Not equal Cathode current (close perhaps, but not equal).
the Cathode current includes the Screen current, the Plate current does not.

This is a push pull circuit.
Now what was that about magnetic fields not cancelling?
That is why I always advise designers/builders to make sure to provide for a way to match the DC currents in the push versus the pull windings.

The key to being able to use smaller laminations is that the algebraic Total of All Ampere Turns cancels, regardless of how you do that.

Let's try and not confuse the Newbies.

Last edited by 6A3sUMMER; 13th December 2020 at 03:01 AM.
  Reply With Quote
Old 13th December 2020, 03:05 AM   #19
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Yep, P-P DC is fine.

Was just finding an issue with the article quote, which might be out of context in some way anyway.

----------------------------------------------------------------
Quote:
Is there a penalty in parasitic capacitance / inductance or is the same BW possible as with the odd OT?
CFB should provide a nice improvement in winding capacitance, since two ends of the windings are at AC ground, which could be placed near the secondary. 50% CFB would seem optimum.

If the CFB portion IS the secondary, then primary turns get reduced and the coupling increased. Also helpful.

Last edited by smoking-amp; 13th December 2020 at 03:24 AM.
  Reply With Quote
Old 14th December 2020, 12:05 PM   #20
Chris Hornbeck is offline Chris Hornbeck  United States
diyAudio Member
 
Chris Hornbeck's Avatar
 
Join Date: May 2011
Location: Little Rock
50% CFB is sometimes called the Circlotron, the McIntosh or the Norman Crowhurst variation on the McIntosh. Folks looking to explore will find these interesting. Partial G2 loading/feedback is also possible, as used in the QUAD. All date to the Truman or Eisenhower administrations in USA time parlance. Lots of cool ideas.



YOS,
Chris
__________________
"A marveilous newtrality have these things mathematicall and also a strange participation between things supernaturall, immortall, intellectuall, simple and indivisible, and things naturall, mortall, sensible, componded and divisible." - John Dee
  Reply to this post

Reply


Audio Research amp design questionHide this!Advertise here!
Thread Tools

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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
The Audio Amateur #2 1984, Audio Research D-150 MortenD150 Tubes / Valves 32 18th December 2020 01:35 PM
Repair question Audio Research D100B muizel Solid State 1 8th February 2010 12:20 PM
Audio Research SP-5 recap question bacoburn Solid State 5 18th October 2009 02:48 AM
Any website reference for diy Audio Research REFERENCE amp and Manley Studio amp bmpa Tubes / Valves 4 19th April 2004 06:11 AM


New To Site? Need Help?

All times are GMT. The time now is 06:54 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Copyright ©1999-2021 diyAudio
Wiki