Madison Fielding Rcvr -- Filament Trick - 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 19th March 2015, 03:01 PM   #1
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
Default Madison Fielding Rcvr -- Filament Trick

I have had this Madison Fielding 440 Receiver forever -- and found one at a thrift operation. It was manufactured by Crosby circa 1961. It is interesting that they run the filaments for the preamp section through the cathodes of the output stage which is set at 24V. I can't imagine that this is a bit below optimal for the ECC83/12AX7 input tubes:
Attached Images
File Type: png MadisonF_Output_SM.png (45.3 KB, 163 views)
  Reply With Quote
Old 19th March 2015, 03:56 PM   #2
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
Originally Posted by jackinnj View Post
I have had this Madison Fielding 440 Receiver forever -- and found one at a thrift operation. It was manufactured by Crosby circa 1961. It is interesting that they run the filaments for the preamp section through the cathodes of the output stage which is set at 24V. I can't imagine that this is a bit below optimal for the ECC83/12AX7 input tubes:
I'm lost where is the 24V connection for the preamp tubes?
Perhaps I'm missing something..WX..
I see what looks like a fixed bias supply?

Regards
M. Gregg
__________________
What is the sound of one hand clapping?

Last edited by M Gregg; 19th March 2015 at 04:01 PM.
  Reply With Quote
Old 19th March 2015, 04:01 PM   #3
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
Quote:
Originally Posted by M Gregg View Post
I'm lost where is the 24V connection for the preamp tubes?
Perhaps I'm missing something..WX..

Regards
M. Gregg
The heaters become the cathode resistors when hot. Then they pick off 12v. to feed back up to the grids to create the 12v. bias needed for the outputs.

20

Last edited by 20to20; 19th March 2015 at 04:08 PM.
  Reply With Quote
Old 19th March 2015, 04:13 PM   #4
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
Originally Posted by 20to20 View Post
The heaters become the cathode resistors when hot. Then they pick off 12v. to feed back up to the grids to create the 12v. bias needed for the outputs.

20
Thank's

OK got it..

Bit of a pig when the output stage fails but its really a preamp heater..LOL

Regards
M. Gregg
__________________
What is the sound of one hand clapping?

Last edited by M Gregg; 19th March 2015 at 04:17 PM.
  Reply With Quote
Old 19th March 2015, 05:53 PM   #5
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
I wonder if they ever considered that this would modulate the heaters of the preamp stage!
  Reply With Quote
Old 19th March 2015, 06:45 PM   #6
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
Quote:
Originally Posted by jackinnj View Post
I wonder if they ever considered that this would modulate the heaters of the preamp stage!
The heaters are only about 80 ohms to ground when hot. And that is behind the 420K grid leak where the grid signal is. There is no concern about signal crossovers to anything. They would act like any other cathode resistor and provide a small degenerative feedback. They didn't even bother to bypass them which could have easily been done.

20
  Reply With Quote
Old 19th March 2015, 08:38 PM   #7
gofar99 is offline gofar99  United States
diyAudio Member
 
Join Date: Jan 2007
Location: Arizona USA
Eico did that in their dual 35 WPC power amps. Ran 112AX7s in the cathodes of the EL34s. They did have rather large filter caps across the heaters though. A simple way to get DC heaters without a separate supply. I am sure other companies did the same.
__________________
Good Listening
Bruce
  Reply With Quote
Old 19th March 2015, 09:41 PM   #8
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
Quote:
Originally Posted by gofar99 View Post
Eico did that in their dual 35 WPC power amps. Ran 112AX7s in the cathodes of the EL34s. They did have rather large filter caps across the heaters though. A simple way to get DC heaters without a separate supply. I am sure other companies did the same.
A Silvertone model 29 P-P 6BQ5 uses 1 12AX7 and a 10R in series w/ a 50uf bypass. It has 15v. on the K dropped through the R. It puts the hot heater at 90R if it's carrying the whole 150ma.
  Reply With Quote
Old 19th March 2015, 10:01 PM   #9
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
My Fisher X-101-B used the heaters for four 12AX7s as the cathode resistor of the output tubes.

I modified the amp to have four individual bypassed current regulators as cathode resistors because I kept having 7591s red plate due to less than perfect matching. Now they are all happy. I don't like designs that damage tubes if they aren't perfectly matched.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 21st March 2015, 12:51 AM   #10
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
The HH Scott 99 mono integrated amplifier used the same scheme and might have been one of the first to do so back around 1954..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  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: SACD Players (Yamaha S1800BL, Denon 1930CI, Sony S9000ES) and Onkyo TX-SR508 Rcvr zxgravediggerxz Swap Meet 9 17th December 2012 06:44 AM
Lightbulb trick on filament circuit? Rundmaus Tubes / Valves 9 24th January 2011 03:31 PM
Tube mod a HT rcvr? mashaffer Tubes / Valves 7 4th February 2008 01:21 AM
Parallel vs. series filament supply - or another filament idea engels Tubes / Valves 11 13th May 2007 01:06 PM
Pioneer rcvr/eros Q bbksv Multi-Way 1 14th December 2003 07:33 PM


New To Site? Need Help?

All times are GMT. The time now is 11:52 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2