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 7th January 2009, 12:10 PM   #1
diyAudio Member
 
korneluk's Avatar
 
Join Date: Jan 2005
Location: South Florida
Default 300B Cathode Feedback + Cathode Bias

Folks,

I ran across this schematic online and was wondering if there are any drawbacks to using both cathode bias and cathode feedback at the same time.

Click the image to open in full size.

The schematic can be found here:

http://www.tubebuilder.com/schematic2.html

TIA,

-- josť k.
  Reply With Quote
Old 7th January 2009, 12:36 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
4% THD at 3W? Now that's some fine design.
__________________
"The way to deal with superstition is not to be polite to it, but to tackle it with all arms, and so rout it, cripple it, and make it forever infamous and ridiculous."- H. L. Mencken
  Reply With Quote
Old 7th January 2009, 01:01 PM   #3
diyAudio Member
 
Join Date: Apr 2002
Location: Cayman Islands
Quote:
4% THD at 3W? Now that's some fine design
.....but its all second harmonic so its sooooo pleasing to the ear, and, because its so pleasing you don't even think about its in-efficiency
__________________
None of us are leaving this world alive. Enjoy it responsibly as you may.
  Reply With Quote
Old 7th January 2009, 01:03 PM   #4
diyAudio Member
 
korneluk's Avatar
 
Join Date: Jan 2005
Location: South Florida
Quote:
Originally posted by SY
4% THD at 3W? Now that's some fine design.
Those figures caught my eye as well and made me wonder about the cathode feedback implementation.

This technique was used in push-pull amplifiers to reduce the plate resistance and the distortion level, so I am trying to figure out what went wrong with this design.

-- josť k.
  Reply With Quote
Old 7th January 2009, 01:21 PM   #5
jlsem is offline jlsem  United States
diyAudio Member
 
jlsem's Avatar
 
Join Date: Jun 2003
Location: Dallas,TX
The output tube is only being run a 15.7W plate dissipation so the 3W 4% distortion would be pretty normal for a single ended amp of this size. I don't see any cathode feedback in this circuit. I do see an RF choke in the cathode circuit in lieu of a grid stopper to prevent parasitic oscillation, though.

John
__________________
When an LP gets scratched it still makes sense; when CDs crack up they go completely senile and gibber incontinently. - Hugo Williams
  Reply With Quote
Old 7th January 2009, 01:24 PM   #6
rtsang is offline rtsang  Hong Kong
diyAudio Member
 
Join Date: Aug 2005
Default Only around 250V anode-cathode voltage

The 300B is running in the specs of a 2A3

Plate dissipation is only around 15.5W
So even without cathode feedback, the amp could only make around 4W, assuming 25% efficiency.

Is it a good way to run an output tube in such a low plate dissipation (<60%).
I used to have a Sun Audio 300B amp that runs a 300B like that, never sounded good to me. But the driver stage of that amp also have problems, so I am not sure if the driver or the output stage was the culprit.

Raymond
  Reply With Quote
Old 7th January 2009, 02:21 PM   #7
diyAudio Member
 
korneluk's Avatar
 
Join Date: Jan 2005
Location: South Florida
Quote:
Originally posted by jlsem
...I don't see any cathode feedback in this circuit. I do see an RF choke in the cathode circuit in lieu of a grid stopper to prevent parasitic oscillation, though.

John
It is not an RF choke but rather a separate winding off the output transformer being used for feedback.

-- josť k.
  Reply With Quote
Old 7th January 2009, 03:03 PM   #8
jlsem is offline jlsem  United States
diyAudio Member
 
jlsem's Avatar
 
Join Date: Jun 2003
Location: Dallas,TX
I stand corrected although the schematic is a bit ambiguous.

John
__________________
When an LP gets scratched it still makes sense; when CDs crack up they go completely senile and gibber incontinently. - Hugo Williams
  Reply With Quote
Old 7th January 2009, 04:58 PM   #9
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
I don't think the distortion is being generated in the output stage...

Michael
  Reply With Quote
Old 7th January 2009, 05:15 PM   #10
45 is offline 45  Italy
diyAudio Member
 
45's Avatar
 
Join Date: Dec 2008
Location: United Kingdom
Quote:
Originally posted by korneluk


Those figures caught my eye as well and made me wonder about the cathode feedback implementation.

This technique was used in push-pull amplifiers to reduce the plate resistance and the distortion level, so I am trying to figure out what went wrong with this design.

-- josť k.


I will never understand why people have to make things complicated more than necessary....
I guess that a 300B (run like that) in a Loftin-White circuit performs better!

IMO, the first wrong thing in that schematic is the input tube. The 12AT7 in SE mode (both RC and transformer coupled) is good only if you don't need too much swing. Let's say, it is good for driving EL84's.
In this case the swing is low as well but the static load is 10K only and the step-up transformer will have a lot of stray capacitances....with a 40K secondary, I think the 12AT7 will see some nF's!
Then have a look at the characteristic curves of the 12AT7 and it will be clear it is working in a quite non linear mode...
I think much of the distortion comes from it and at the output you will see a lot of 3rd harmonic because of some harmonic cancellation ( basically 2nd harmonic cancellation).
Harmonic cancellation can help, however it works (sounds good) only if it is a small effect, in my experience.

Cheers,
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
300B SE cathode feedback to grid? alan_982002 Tubes / Valves 3 29th March 2010 01:04 PM
Changing grid leak bias preamp to cathode bias bereanbill Tubes / Valves 8 28th March 2009 05:26 PM
Stray hum pickup *preamp* cathode R-bias vs. battery bias & 12AU7vs.5687 awedio Tubes / Valves 2 19th April 2008 01:30 AM
UL to triode mode conversion on cathode bias amp-different cathode resistors needed? bigwill Tubes / Valves 6 30th December 2007 12:37 AM
Best way to connect 300B cathode Ralph Tubes / Valves 12 3rd February 2003 10:39 AM


New To Site? Need Help?

All times are GMT. The time now is 06:59 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