What is the theoretical advantage of direct heated triodes? - Page 21 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 4th December 2012, 12:30 PM   #201
TheGimp is offline TheGimp  United States
diyAudio Member
TheGimp's Avatar
Join Date: Aug 2009
Location: Johnson City, TN
The filament is magnetized, reverse the direction of the ac and demagnetize it.
  Reply With Quote
Old 4th December 2012, 12:39 PM   #202
diyAudio Moderator
TonyTecson's Avatar
Join Date: May 2003
Location: Palatiw, Pasig City
i was told that the amp was fine except for the annoying little bit of hum, and that a new tube cured it, i don't know what to say....the only thing that is clear to me was that the old tube has been in service for about 4 years till he replaced it....
planet10 needs your help: Let's help Ruth and Dave...http://www.diyaudio.com/forums/plane...ml#post5010547[B
  Reply With Quote
Old 4th December 2012, 12:41 PM   #203
diyAudio Member
Rod Coleman's Avatar
Join Date: Jul 2004
Location: Warwickshire UK
Filament breakage failures (partial or complete) were very frequent in the case of Svetlana and older Sovtek DHTs, and some others as well. I'm not sure why they failed to achieve what Western Electric and Marconi-Osram seemed to achieve near faultlessly, for decades.

If you choose to run filaments from ac (not recommended for any DHT, but still..) - please take one precaution: don't run a 1.2A filament from a 6A rms winding. If you do, the filament will actually suffer a 6A+ startup pulse - the cold filament resistance is well below 1 ohm in a 300B. It takes little imagination to see that the breakages of the filament are most likely to happen during this violent cold start.

The latest version of the EML DHTs have altered their filament design, to give a very slow rise in current, which I suspect is aimed at avoiding this exact problem.

With proper current limited dc, filaments are protected from turn-ON surges.
  Reply With Quote
Old 4th December 2012, 12:45 PM   #204
kmaier is offline kmaier  United States
diyAudio Member
kmaier's Avatar
Join Date: Jul 2007
Originally Posted by Tony View Post
afaik, ac filaments, and he had the amp for almost 4 years now....i suspect cathode depletion, can i be correct?
Sorry, but again not enough information. Too many unknown variables...

If he's had the amp running for 4 years (assuming reasonable on time) then there are some number of operating hours on the tubes. Cathode depletion would be a far stretch, but again, not enough detail. No idea what the operating point is for the 2A3, which affects overall useful life. No idea of the condition of the original 2A3 (NOS, used, etc.) 4 years ago. Did the hum just start suddenly or build up over time?

If AC filaments are being used, what is the topology for nulling the filament hum? Is there a balance pot? Was the replacement tube a simple replace or was there any adjustment required (aka, hum pot)?

If he's running some newer production 2A3 tubes (meaning not a real NOS unit from the 50's or earlier) then it's anybody's guess... could be gas, a cathode coating flake that's touching a grid wire, some other internal contamination, etc..

Regards, KM
... just because you're paranoid, it doesn't mean they're not after you...
  Reply With Quote
Old 4th December 2012, 12:55 PM   #205
diyAudio Moderator
TonyTecson's Avatar
Join Date: May 2003
Location: Palatiw, Pasig City
the guy was supposed to send me his 2A3 set after i cured his 6SN7 line amp of hum, needless to say i haven't seen the amp nor the construction details.......then i got a call from him the other day telling me that he replaced the 2A3 tube with a new one and that the hum was gone, i did not press him for more details.....i will ask him again if and when he calls me...
planet10 needs your help: Let's help Ruth and Dave...http://www.diyaudio.com/forums/plane...ml#post5010547[B
  Reply With Quote
Old 5th February 2013, 09:13 AM   #206
diyAudio Member
Join Date: Sep 2006
I use both dh and idh valves, and I find very little difference,

The main valves I use are pre 1935 designed British triodes,

I have used many of the sub two watt IDH triodes as output valves and they give nothing to the px's except power, the likes of mesh 41mp and the 41mpx, ac/p1 etc, (not the later types) They were designed as output valves, but also make top notch ..possibly the best drivers I own,
As those same valves get newer production, they don't sound quite so well, but still very good compared to most of the modern stuff,
As an earlier comment stated is about the metals they are made from.
I have some 600 volt military transmitter versions of the px25 and they don't sound just quite as good as the audio versions, as they use different metals, they have the sound of transmitter valves.....as in missing the finer harmonics in the recording compared tho the audio version. I have both the Osram and Mazda versions of these.

My understanding is both the cathode coating and the quality of how it was applied was/is critical to the emissions of the valves.
I have it on good authority that some of the early pentode a where built on the same lines and provide exceptional detail. I have a box of early (pre 35)ac/pens that will be tested one day

My understanding is the original mono plate 2a3 /6a3 was a lesser copy of the original british px4

post war production is to be avoided.with many valves. Just hear the difference between the wartime vt75/kt66 vs the post war kt66 or a globe px25 and the later straight sided version. its not subtle with the right kit.

Last edited by steve shiels; 5th February 2013 at 09:18 AM.
  Reply With Quote


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
Direct heated tubes with centre tap on filament mig-ru Tubes / Valves 12 18th July 2016 07:27 AM
1AG4 Direct Heated Pentode, Output TheGimp Tubes / Valves 11 18th February 2012 09:19 PM
Best low-signal direct heated triodes? gentlevoice Tubes / Valves 7 5th May 2008 03:03 AM
Any theoretical or sonic advantage to a sealed versus T-line enclosure?? jjkozlow Subwoofers 18 26th October 2005 03:17 AM
cathode connection in directly heated triodes metebalci Tubes / Valves 2 21st March 2004 08:58 PM

New To Site? Need Help?

All times are GMT. The time now is 04:10 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2