Resistors for ultralinear-to-triode conversion - 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 26th January 2007, 06:45 AM   #1
diyAudio Member
 
Join Date: Apr 2005
Location: Spain
Default Resistors for ultralinear-to-triode conversion

Hi

Iím planning to modify the output of my Copland CTA-501 valve amplifier from the standard ultra-linear to triode mode. Iíve searched on the net and it seems an easy modification, and fully reversible if the sound doesnít improve. The reduction of power (from 30 to about 15 w/ch) is no problem because I use a pair of easy to drive Spendor SP1/2 loudspeakers, and my listening room is only about 11 sq. metres.

Iíve attached to this post a ďbefore/afterĒ schematic of the changes.
1) Disconnect the screen grid G2 (valve pin 4) from the ultralinear tap in the output transformer, by unsoldering of the R33 and R34 resistors, or even cutting the pcb traces.
2) Join, at the valve socket, the valve pins 4 (G2) and 3 (anode/plate) through a 100 ohm resistor.
Regarding this 100R resistor, Iíve read that it must be non-inductive and rated at 0.5 or 1 watt, but Iíve not read any information about what voltage rating is needed. Most resistors are rated at 200-300 volts. Iíve found the Caddock MK-132 resistors, rather expensive, but small and of radial design (this will make easier the at-the-socket mounting under the pcb), non inductive and rated at 0.75 watts and 400 volts maximum.

My doubt is if the choice of this Caddock MK-132 is correct. Are its power and 400v voltage ratings high enough for this specific use? The anode voltage in the amplifier is 430 volts and the specified voltages in different points of the circuit are indicated in the attached schematic.
Have you any suggestion on alternative resistors?

Thanks in advance

Regards
Jose
Attached Images
File Type: jpg triode connection.jpg (27.0 KB, 320 views)
  Reply With Quote
Old 26th January 2007, 06:46 AM   #2
diyAudio Member
 
Join Date: Apr 2005
Location: Spain
This is the "before" original ultralinear schematic
Attached Images
File Type: jpg ultralinear connection.jpg (26.3 KB, 297 views)
  Reply With Quote
Old 26th January 2007, 06:57 AM   #3
diyAudio Member
 
Join Date: Aug 2006
Location: Berkshire UK
Default U/L to triode cxonversion

Hi Jose
1/2 to 1 watt will be more than adequate and ideally standard carbon type but in reality any resistor will do as all it is doing is connecting the screen grid g2 to the anode. Do not get paranoid about boutique components here it is not worth it The object of the resistor is to stop parasitic oscillations in the valve and to protect the HT line/rectifier if the valve goes screen grid to control grid short
As far as voltage rating is concerned I think you have the wrong idea here. Manufacturers voltage ratings are the maximum voltage a resistor can stand across it end to end NOT where it is in the circuit. As this resistor is 100R its end to end voltage will be negligible in the position chosen.

Regards

John Caswell
  Reply With Quote
Old 26th January 2007, 01:54 PM   #4
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: South Florida
The voltage rating is also related to the ability of the outer coating to break down. You can abuse this rating a bit if the resistor is mounted in free air and there are no other components near it, AND there is no possibility of human contact!

Quote:
1/2 to 1 watt will be more than adequate and ideally standard carbon type but in reality any resistor will do
I have learned the hard way that this is not true. I was using 100 ohm 1/2 watt metal film resistors from DigiKey. I had two resistors fail. Both died the same way. The sound instantly became weak and distorted in one channel, and the resistor was obviously fried. Both failed early (a few hours) in a new amp that was not abused (HiFi listening well below max power). B+ is high though, nearly 500 volts. The resistors are PC board mounted (spaced 1/2 inch above the board) with no other components nearby.

After the second failure, I switched to two watt metal film resistors from DigiKey and Mouser. I have built 5 amplifiers since then. They have all been through my standard abuse testing including an hour or so being fed with a guitar preamp operating in heavy clipping. No more blown resistors.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 26th January 2007, 02:03 PM   #5
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
One other thing to be cautious about- by changing from UL to triode, you've significantly increased the input capacitance. You will want to check the square wave response and redo any compensation to optimally balance bandwidth against stability.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 26th January 2007, 06:57 PM   #6
frugal-phile(tm)
diyAudio Moderator
 
planet10's Avatar
 
Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Blog Entries: 5
A more complex variation on the theme .... it separatly sets the DC & AC operating points.

This is one of many ways to do the same thing.

http://www.t-linespeakers.org/tubes/triode-trick.html

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
Old 28th January 2007, 12:47 PM   #7
diyAudio Member
 
Join Date: Apr 2005
Location: Spain
Hi

Thank you for your answers.

SY, I know that when triode mod is done, what I'll have will be a "new" EL34, with a behaviour different to what the designer had in mind and different operational parameters. Also, I have no access to measuring equipmente other than my DMM to fine adjust anything. However, as it is a well-known and safe mod and it is fully reversible,I'd like to try it and let my ears decide.

Just for playing safe (thanks, "tubelab.com"), I probably use the Mills MRA-5 resistors non-inductive wirewound (1%, 20ppm, 5W, about $2 each). They are 200v max, but, according to John Caswell, "as this resistor is 100R its end to end voltage will be negligible in the position chosen".
Thanks to all of you

Regards
Jose
  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
Difference bewteen ultralinear & triode mode? kevinkuang Tubes / Valves 80 8th January 2013 05:32 AM
Dynaco ST-70 Triode Conversion Techniques brosenau Tubes / Valves 34 4th November 2010 03:54 AM
6C33C to more standard triode conversion - any ideas? GlidingDutchman Tubes / Valves 2 9th July 2008 04:09 PM
UL to triode mode conversion on cathode bias amp-different cathode resistors needed? bigwill Tubes / Valves 6 30th December 2007 12:37 AM
Switching from ultralinear to triode Klimon Tubes / Valves 3 26th May 2005 11:50 AM


New To Site? Need Help?

All times are GMT. The time now is 08:24 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