Output Tube biasing for Toroidal Output Transformer - 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 25th February 2005, 04:33 AM   #1
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Default Output Tube biasing for Toroidal Output Transformer

Im in the design stage of a new PPP amp (4 off KT88) using Van der Venne Toroidal Output Tannies.

DC Balance of the output stage is vital. Rather than design a bias servo to adjust fixed bias I'm considering using active Current Sources in each cathode bypassed by Blackgate electrolytics.

Does anyone have opinion / experience with the pros and cons of this arrangement.

Since it is effectively cathode biased, does this mean I could use the higher Cathode Bias Rg values safely, and hence reduce requirements on the driver stage?

Cheers,
Ian
  Reply With Quote
Old 25th February 2005, 11:21 AM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
This arrangement will only work in class A. And if you're running class A, all you need is one current source. Yes, that will let you use the higher Rg values.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 27th February 2005, 10:04 PM   #3
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Default SY are you sure?

SY,
I don't think the Class A ONLY comment is correct. Are you absolutely sure?

I've got a little EL84 PP Ultralinear Amp - basically the design out of Morgan Jones' "Valve Amplifiers". Its running with a ring of two (BC557 and MJE340) 40mA current source in each cathode (i.e. as a direct replacement for the usual 270R resistor) with 470uF/50V Blackgate Standard Electrolytics across them. I haven't noticed anything objectionable with the sound - in fact this little 10W per channel amp is quite stunning. It must surely be transistioning into Class B (rail is +300V, bias voltage developed across the current sources ar idle is approx 10.5V). The reason for the post was to see if anyone had experience of scaling this up to Parallel Push Pull KT88s or similar.

Can you please respond with a "Class A ONLY is definitely correct" OR "On second thought ...."

Thanks,
Ian
  Reply With Quote
Old 27th February 2005, 10:11 PM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Well, in AB or B, the current isn't constant. Those EL84s are being run pretty close to class A, I'll guess.

Funny, I've gone in totally the opposite direction with red LEDs in the cathode to try to get close to a constant-voltage source.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 27th February 2005, 11:09 PM   #5
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Default Thoughts!!!

Well NO the current is NOT constant in Class AB BUT thats what the bypass capacitor is for.
I was mainly trying to get my head around things like what its recovery from overload would be like etc.

I guess I need to re-proiritize my hobby work and get my modelling program up and running - mostly conversion of various PSpice models to Spice 3F5 is what is required. That is to create the tube libraries.

I also spent some time on the weekend doodling on some scrap paper to see if I could adapt your SYclotron circuit to an output stage (using MOSFETS as the Source Followers). I think this can work - requires AC Coupling to the cross coupled grids unless full bias is developed across the MOSFET Source resistors (to the tube cathodes) BUT allows shunt feedback from the anodes to the MOSFET gates. Another circuit where my pencil and paper analysis is not sufficiently advanced to check what happens as one side cuts off, DC balance etc.

So - I guess its put away the soldering iron and fire up the PC for the next week or so.

Cheers,
Ian
  Reply With Quote
Old 7th March 2005, 02:03 PM   #6
diyAudio Member
 
triode_al's Avatar
 
Join Date: May 2004
Location: Westland
Blog Entries: 1
For several years I have a push-pull setting, cathode coupled, with EL34's in triode wiring. This means that the cathodes are floating. For biassing I have used a constant current source based on the LM7805. It has a 51 ohm load resistance. The IN-side has a set of capacitors (10 mu foil 63V, ERO 1613; 100mu electrolytic). The current is balanced to within 0,5 mA both sides. This means that the toroid will never saturate.
The transformer is a Métalymphy 110/110/13V toroid, a French firm, and the toroid were also used in some experiments described in the Audiphile magazine..

The amplifier is straight down to under 5 Hz. No feedback over the output. Driving is an ECL82. The high-end roll-off is about 100 kHz.

My speakers are QUAD ESL 57. Notoriously difficult to drive, with this setting the output impedance is calculated to about 1 ohm, without feedback. Not bad and in fact just what the ESL likes. Its has all the dynamics you like. The effect: the most gorgeous depth in the sound (well, one has to have something to cuddle
  Reply With Quote
Old 7th March 2005, 02:08 PM   #7
diyAudio Member
 
triode_al's Avatar
 
Join Date: May 2004
Location: Westland
Blog Entries: 1
The diagram is attached of my CC-PP with toroids.
Attached Images
File Type: gif cathode coupled el34 - gif.gif (7.7 KB, 367 views)
  Reply With Quote
Old 17th April 2005, 04:40 PM   #8
frugal-phile(tm)
diyAudio Moderator
 
planet10's Avatar
 
Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Blog Entries: 5
Default Re: SY are you sure?

Quote:
Originally posted by gingertube
EL84... 40mA... (rail is +300V, bias voltage developed across the current sources ar idle is approx 10.5V).
That is pretty much where we are running our SE EL84s and they are decididly Class A.

dave
__________________
community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
  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
3C24 in a SE output stage - output transformer impedance recommendation !!! aldovan Tubes / Valves 15 6th September 2008 10:18 AM
Adding secondary of output transformer into cathodes of output stage ? Eico ST-70 freddymac406 Tubes / Valves 3 21st April 2007 06:15 PM
My DIY Pre-Amplifier used 5692 and 45 Tube with output transformer cd149dc Tubes / Valves 3 3rd June 2006 04:34 PM
toroidal SE output transformer noyan Parts 10 11th July 2004 10:41 PM
toroid Transformer as output Transformer for tube preamp? tone Tubes / Valves 15 2nd February 2003 04:36 AM


New To Site? Need Help?

All times are GMT. The time now is 02:41 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