Cathode feedback and DHTs - 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 15th February 2007, 06:24 AM   #1
jane is offline jane  Norway
diyAudio Member
 
jane's Avatar
 
Join Date: Jul 2004
Location: Norway
Default Cathode feedback and DHTs

Are there any considerations I should be aware of using DHTs and cathode feedback? The filament transformer will be non-decoupled in the signal path -- any chances for capacitive or inductive coupling between the push and pull side or strange frequency response?

At the schematic a filament transformer, with two center-tapped windings, is connected to x-ct-x and y-ct-y.
Click the image to open in full size.

Jan E Veiset
__________________
Life is hard - Then you die.
  Reply With Quote
Old 15th February 2007, 08:14 AM   #2
mobyd is offline mobyd  New Zealand
diyAudio Member
 
mobyd's Avatar
 
Join Date: Sep 2004
Location: Auckland
Jan, what software do you use to make those nice schematics ?
Mike
  Reply With Quote
Old 15th February 2007, 09:28 AM   #3
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
When I tried the same trick with 2A3 (didn't work very well), I used a transformer kit to make the 2A3 heater windings. The significance is that it was a split bobbin construction with the primary in one half and the other half left empty for the secondary. Thus, there was very low capacitance between the mains and the 2.5V 2A3 heater windings. You might want to consider the same idea here.
__________________
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 15th February 2007, 11:05 AM   #4
jane is offline jane  Norway
diyAudio Member
 
jane's Avatar
 
Join Date: Jul 2004
Location: Norway
Quote:
Originally posted by EC8010
When I tried the same trick with 2A3 (didn't work very well), I used a transformer kit to make the 2A3 heater windings. The significance is that it was a split bobbin construction with the primary in one half and the other half left empty for the secondary. Thus, there was very low capacitance between the mains and the 2.5V 2A3 heater windings. You might want to consider the same idea here.
Good point!
What went wrong with the 2A3s?

(@mobyd: I'm using MS Visio)

Jan
__________________
Life is hard - Then you die.
  Reply With Quote
Old 15th February 2007, 11:19 AM   #5
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
I think it was a poor output transformer. The moment I saw it I had my doubts - the winding window wasn't full. Anyway, trying to add cathode feedback (same proportion as yours) ruined the square wave response even though it reduced distortion.
__________________
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 15th February 2007, 01:27 PM   #6
Giaime is offline Giaime  Italy
diyAudio Member
 
Giaime's Avatar
 
Join Date: Oct 2004
Send a message via MSN to Giaime
Not so much gain available with DHT triodes, to use cathode feedback!

Most of them are already hard to drive, and increasing drive signal increases drive stage distortion. So, what's worse?

I've tried with 300B, and they become unity-gain devices

Linear, but useless.
  Reply With Quote
Old 15th February 2007, 02:43 PM   #7
jane is offline jane  Norway
diyAudio Member
 
jane's Avatar
 
Join Date: Jul 2004
Location: Norway
Quote:
Originally posted by Giaime
Not so much gain available with DHT triodes, to use cathode feedback!

Most of them are already hard to drive, and increasing drive signal increases drive stage distortion. So, what's worse?
PX25 is quite easy to drive, in this case ~70Vpeak at full power output with cathode feedback. The EL84 driver stage is tested and found very linear: 70V output with less than 0.1% THD, so I donít think the main problem will be lack of clean voltage swing from the driver.

Click the image to open in full size.

Jan E Veiset
__________________
Life is hard - Then you die.
  Reply With Quote
Old 15th February 2007, 03:46 PM   #8
Giaime is offline Giaime  Italy
diyAudio Member
 
Giaime's Avatar
 
Join Date: Oct 2004
Send a message via MSN to Giaime
How much power will you expect?
  Reply With Quote
Old 15th February 2007, 04:43 PM   #9
jane is offline jane  Norway
diyAudio Member
 
jane's Avatar
 
Join Date: Jul 2004
Location: Norway
About 2x15W

Jan.
__________________
Life is hard - Then you die.
  Reply With Quote
Old 15th February 2007, 05:23 PM   #10
rdf is offline rdf  Canada
diyAudio Member
 
rdf's Avatar
 
Join Date: Jun 2004
Location: big smoke
Hi Jan, a somewhat off-topic suggestion. I've been playing around with bias points for the triode-mode 6CW5/EL86, subbing for an EL84 in SE. At very near the operating conditions you're using the 3rd harmonic distortion component of NEC-branded version drops dramatically, multiple tens of dBs. I recall the plate current was more high teens than low twenties, otherwise plate load and B+ were near identical. You may be able to significantly better that 0.1% with an EL86 and get lower source impedance to boot. To my ear it also has a much more direct sound than the EL84.
  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
Cathode Feedback from Secondary for PP Amp gingertube Tubes / Valves 22 1st July 2009 11:45 PM
300B Cathode Feedback + Cathode Bias korneluk Tubes / Valves 13 7th January 2009 08:07 PM
Cathode Feedback How-To? sorenj07 Tubes / Valves 4 23rd October 2007 05:04 AM
DHTs and Cathode Bias 6f6 Tubes / Valves 4 28th March 2007 11:40 PM
Cathode feedback mark_titano Tubes / Valves 20 11th August 2005 01:56 AM


New To Site? Need Help?

All times are GMT. The time now is 07:12 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