EL84-PP - Why just 5W? - Page 3 - 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 21st October 2005, 08:14 PM   #21
frugal-phile(tm)
diyAudio Moderator
 
planet10's Avatar
 
Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Blog Entries: 5
Quote:
Originally posted by Giaime
I think that 150ohm is right for a COMMON resistor, not just one for tube.
Just as a ref... Dyna SCA35 uses 95 common for all 4 tubes (ie 190R for a pair/380R for a single tube)

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 21st October 2005, 09:06 PM   #22
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
The SCA35 also used a higher plate voltage, so it needs more cathode bias.
__________________
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 22nd October 2005, 12:18 AM   #23
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Most pushpull cathode biased EL84 amplifiers I have seen used cathode resistors of > 200 ohms on a 300V supply. A 150 ohm resistor with 21V across it equates to 140mA which is excessive for a pair and is equivalent to over 20W dissipation each.. They are rated at 12W dissipation, although in triode connection you may get away with as much as 14 - 15W.

It may be that your plate voltage is substantially higher than the 300V indicated in your schematic, this would result in the problems you are observing.

Normal cathode bias voltages are typically around 12-14V which leads me to suspect high plate voltage again.

Check the coupling capacitors for leakage, if you see more than a couple of hundred mV across the grid resistor they are leaky and should be replaced. (Do this with the tubes removed so that grid current does not confuse the result.)

Try changing that 150 ohm resistor to about 250 to 300 after verifying that the coupling caps are ok.

The SCA35 I owned had 7189's installed in it from the factory and they are a little more rugged than the EL84/6BQ5, they bias slightly differently than the EL84 as well, although the SCA35 and ST35 do definitely over run them.

Kevin
  Reply With Quote
Old 22nd October 2005, 12:28 AM   #24
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
The data sheet indicates 130 ohm for a shared cathode resistor, 270 ohms for separate ones.
__________________
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 22nd October 2005, 03:35 AM   #25
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Hi SY,
Yes I have seen those datasheets, my experience with EL84 has been different. In a lot of cases those values seem to work well in situations where the plate voltages are quite low, say 250V. In many other instances the recommended values have the tubes running well beyond their static ratings. Vintage ones may behave differently, but truthfully I haven't seen much difference.



Kevin
  Reply With Quote
Old 22nd October 2005, 03:57 AM   #26
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Well, you bring up a point which is pretty much in accord with what I'm on about. Is there a problem with the amp or is it out of spec tubes? If the former, it's straightforward to fix. If the latter, Danko ought to ask his tube vendor for a refund rather than have to redesign his amp to accommodate defective parts- it IS possible to buy in-spec EL84s.

The JJs I've been using seem to conform pretty well to the original spec. I can't comment on other new production, but I do note that Morgan Jones built 5 different stereo pairs of the Bevois Valley amp with 270 ohm separate cathode resistors and 300V Vak.
__________________
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

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
5W EL84 drive EL84 SE Audio_idiot Tubes / Valves 31 31st May 2012 08:38 PM
El84 Pp Tobruk Tubes / Valves 1 1st May 2007 12:05 PM
EL84 PP Diy. Pls help me!!! BigOx Tubes / Valves 24 11th June 2006 06:09 AM
el84 help inquisitive Tubes / Valves 14 26th March 2005 07:19 AM


New To Site? Need Help?

All times are GMT. The time now is 05:03 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