Diff drive - Page 3 - 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 17th December 2008, 08:00 PM   #21
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Looks like RC-30 p.696 with CCS loads on triodes instead of pentodes
and without the direct plate to plate feedback (which might not be
great with your triode drivers anyway). Have you calculated some
real-life resistor ratios yet?

Plate feedback is very interesting with pentodes. I just read O.H. Schade
"Beam Power Tubes" and got inspired to play with pentodes (well
actually transmitting tetrodes...) and thought of plate feedback
directly to the plate of a pentode or drain of a MOSFET driver...
What didn't they think of 70 years ago???

Don
"Re-analysis (assuming no grid currents in the driver)"

The feedback from outplate to driver stage doesn't do anything
to lower the driver impedance... should be OK with receiving pentodes.

Cheers,

Michael
  Reply With Quote
Old 17th December 2008, 08:27 PM   #22
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
"What didn't they think of 70 years ago???"

Not much apparently. But it seems that only a limited portion of the ideas ended up in commercial amplifiers.

Hawksford EC was invented by a tube guy (Hawksford references his patent) but it only showed up in commercial SS amps after Hawksford expanded on it in his subsequent journal articles.

A very hot topic lately in power supply design called ripple steering is explained in Electronic Designer's Handbook by Landee, Davis and Albrecht in 1957 and patented over and over again as it was re-discovered 4 times afterwards.

The Harmonic Equalizer from WE tube days seems like an idea that was never fully developed after NFDBK was discovered. I think there is still some interesting potential there using the driver stage as the balanced mixer. (or an external mixer a'la S. Bench)

If cathode feedbacks can get rid of the odd harmonic residual of a diffl. stage, and so it appears, this should be primo stuff for Hi-Fi P-P amp design. While current feedback in SS ampls has a somewhat dubious high distortion rep., with triodes we have firm control of the gain using a CCS. Different animal altogether.

Don

post edit:

"The feedback from outplate to driver stage doesn't do anything
to lower the driver impedance... should be OK with receiving pentodes."

I think the feedback has to be lowering the driver impedance effectively too, because it will desperately try to get the output tube to behave accurately. But one can just use a low rp triode for the driver anyway.

It may also be possible to mix the two schemes (plate feedback to driver pentode and the cathode feedback to the triode driver) by using a driver pentode with crossed plate feedbacks to the driver screens and direct plate feedbacks via resistors to the driver plates. Plus the cathode feedbacks. The driver tube now acting like a triode and a pentode at the same time.
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 17th December 2008, 09:01 PM   #23
bigwill is offline bigwill  United Kingdom
diyAudio Member
 
Join Date: Dec 2004
Location: UK
I try to avoid too much feedback in my designs, and if I do use it, the loop is very short


I think the anode load CCS is the better of the two, as it forces the triode to be more linear and work with more gain

I must admit a lot of these replies are over my head a bit, so I'm not really able to partake in most of the discussion (though the replies are very much appreciated! ), my designs are pieced together with educated guesses and intuition into the way things might work
  Reply With Quote
Old 17th December 2008, 09:07 PM   #24
bigwill is offline bigwill  United Kingdom
diyAudio Member
 
Join Date: Dec 2004
Location: UK
I just realised "I try to avoid too much negative feedback" is such a clichéd phrase these days
  Reply With Quote
Old 17th December 2008, 10:10 PM   #25
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
"I just realised "I try to avoid too much negative feedback" is such a clichéd phrase these days "

I think the CCS in the plate version (post 21) should be reasonably buildable with its simplicity. The amount of actual neg. feedback of course depends on the selected resistor values for the feedback.

I myself don't like using huge amounts of feedback either usually. But since the neg. feedback equation shows that error reduces with increasing neg. feedback (ie, higher loop gain), I like to cheat and try to find some sort of anti-error signal somewhere so that the loop gain can stay low. Thats the idea behind the Hawksford EC, just compute the remaining error and subtract it out. But its theoretically equivalent to high gain, it just doesn't require actual device (tube) gains.

The WE Harmonic Equalizer also computes some anti-error stuff using a different approach (balanced mixer and common mode signals). So it gets by without high gain also.

The RCA RC-30 50 Watt design may be using the additional plate feedbacks (beside the cathode feedbacks) to take advantage of the slightly different transfer functions of plate feedback versus cathode feedback to null/tweak some distortion out. But since the driver tube and controlled output tube are in anti phase regarding current flow, this generally cannot produce wide range cancellation of distortion. (leading to odd harmonic residuals just like the diffl. stage at large signal levels)

To get tracking cancellation, the two devices need to be tracking current wise. So some extra cross coupled feedbacks (beside the cathode fdbks) are more likely to succeed. This would entail using some high value feedback resistors cross coupled between the output plates and the driver grids. Its not really an attempt to increase the overall feedback, just to partition some of it differently. This, of course, is a completely un-explored area. Maybe.., maybe..... the Citation II got it right?

Don
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 18th December 2008, 01:39 AM   #26
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Default Re: Diff drive

Overly complicated way to get local NFB. Some finals definitely need local NFB to sound good. Here's how I did that: Vixen Main Schemo.The feedback goes back to the grids of the drivers to avoid having the Crt of the 807s in parallel with the feedback resistors.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 18th December 2008, 01:49 AM   #27
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
A bit of help for those following trying to follow this thread.

The RCA RC-30 Handbook can be downloaded from Pete Millett's site.
http://www.pmillett.com/tubedata/RC30.pdf

The schematic from page 696 referenced above is this:
Apologies for the quality but best I could manage with the postscript snapshoot tool.


Cheers,
Ian
Attached Images
File Type: jpg rca50w.jpg (65.0 KB, 283 views)
  Reply With Quote
Old 18th December 2008, 03:01 PM   #28
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
another link to the RCA 50W schematic on Pete Milletts site (3.48 MB pdf, on pages 14 and 15):
http://www.pmillett.com/file_downloads/RCA_HiFi.pdf

a related design with just the cathode feedbacks:
http://mysite.du.edu/~etuttle/electron/elect35.htm
(also a similar design with doubled up 6V6GTs for 20 Watt is in the RCA RC-19 handbook, 1959, SP-20 amplifier)

and the AR Dynaco ST-70-C3 modification which used cathode feedbacks:
http://www.arcdb.ws/ST70C3/ARC_ST70C3_schematic.gif
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 3rd February 2009, 07:18 PM   #29
Jaap is offline Jaap  Netherlands
diyAudio Member
 
Jaap's Avatar
 
Join Date: Dec 2002
Location: Netherlands
Glass Audio 1996-8
Scott Reynolds; headphone amp for electret
Attached Images
File Type: jpg hphn2.jpg (61.2 KB, 166 views)
  Reply With Quote
Old 3rd February 2009, 09:36 PM   #30
diyAudio Member
 
richwalters's Avatar
 
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
Ian, several decades ago I built that inverse feedback RCA 50W amp but despite it's more complicated design, I could never get the performance to warrant the cost of the extra components, so I re-designed it as a basic p-p amp, which actually sounded better. I think the problem was the 6CB6 pentode drivers could have performed better as triodes, but that implies a complete recalculation of the feedback components. Not worth it.
To be fair, a well designed conventional design can produce a similiar thd signature.
DePalma's Audio Amplifier design of the symmetrical phasesplitter diff driver is the bit I'm working on: and this works well but needs refinement. Although 6SN7 triodes are used; pentodes can be used and the screen grids fed from o/p stage anodes.

Loads of ideas.

richy
  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
FS: Hitachi Router and Terk outdoor HD antenna, BD-Rom drive, Xbox HD Drive and more. Dougie085 Swap Meet 0 3rd May 2008 07:31 PM
diff between dipole and IB bser Multi-Way 6 18th November 2004 09:23 PM
Idler wheel drive vs. Belt-drive mig-ru Analogue Source 4 3rd December 2003 12:06 AM
Computer Question-Any Problems Transferring Office 97 From D Drive to C Drive kelticwizard Everything Else 12 22nd January 2003 03:34 AM


New To Site? Need Help?

All times are GMT. The time now is 08:44 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