Plate current and red plate problem - Page 2 - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

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 12th November 2012, 01:39 PM   #11
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
It looks like you're switching to a combination of cathode bias and fixed bias, yes?

I was just asking why you didn't go with full cathode bias... it's fine that you do this, was just asking what the idea behind it was to not go to full cathode bias... (or whatever ratio you picked)

Regarding the NFB, as you increase the NFB you also drop the gain, so this reduces the effective *drive*, dropping the clipping, so this can account for the change in tone you are hearing. To know how it sounds different you'd want to adjust the input level to match the change in gain so that the peak signal level on the output remained essentially the same... but no matter, as long as it *sounds good* it's fine now.

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 12th November 2012, 01:52 PM   #12
diyAudio Member
 
Join Date: Mar 2011
But it is a full cathode bias. Resistor and capacitor switched in and the fixed bias circuit gets grounded.

My point about the NFB was not to increase it, but decrease it for more distortion. Down to less than 1% of the signal getting into the PI...
  Reply With Quote
Old 13th November 2012, 12:34 AM   #13
jjman is offline jjman  United States
diyAudio Member
 
Join Date: Jan 2009
OT shunt method can be bogus if the meter has relatively significant resistance itself. Even respected models of meters can have too much internal resistance for accurate results with OT shunt.

Anybody remember the way to use the meter to test the meter for the internal resistance figure?
  Reply With Quote
Old 13th November 2012, 12:48 AM   #14
diyAudio Member
 
Join Date: Mar 2011
I probably damaged mine. I've had a few 'oh sh$%s' when I forgot to switch the probes and dial back to voltage. Once the fuse blew into pieces of glass. It's a decent meter, a Velleman, so I thought it should be able to handle it. Oh well, I learned now...

And, Gerald Weber swears by the OT shunt method in his books, says the guy who made Trainwreck amps used it-his amps sell for 30K plus now-and this link: Bias FAQ claims "This is the way many pro techs measure plate current."

So that's why I chose this method. I will be using the cathode bias method instead...
  Reply With Quote
Old 21st November 2012, 01:41 AM   #15
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
If ther are no current sensing resistors between cathode and 0V then I use the voltage drop across half OT primary method. With power off and after PS capacitors have discharged then use your multimeter to measure the resistance of each half of the OT primary, the 2 sides can be signficantly different in resistance. Then with the amp at idle CAREFULLY measure the DC voltage drop across each half primary and the anode current is then easily calculated from Ohms law.

Of-course if it is my own amp I always mod it to have a 1 or 10 Ohm resistor between each cathode and 0V, measuring the voltage across those is always the SAFEST method of determining tube (anode + screen) current.

I have never had much faith in the shunt method, as stated above results can depend upon the internal resistance of the meter.

Cheers,
Ian

Last edited by gingertube; 21st November 2012 at 01:43 AM.
  Reply With Quote
Old 21st November 2012, 02:37 AM   #16
diyAudio Member
 
Join Date: Mar 2011
Ian, that sounds interesting. How do you measure half the OT primary? Where does the red and black probe touch?
  Reply With Quote
Old 21st November 2012, 03:14 AM   #17
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
With Power OFF and Caps discharged, then measure DC resistance from the primary centre tap of the OT to the anode connection on each side, won't matter which way round the multimeter probes go for these 2 measurements.

Then with amp at idle (no signal) measure the DC voltage drop from OT centre tap (multimeter red lead) to each anode lead (multimeter black lead). CAUTION: both these points are at high voltage even though the voltage difference you will read will likely be only a volt or two (HINT: clip the red lead onto the Centre tap connection before applying power so you are only moving one probe around). Calculate the current in each side by taking the voltage drop for that side divided by the resistance for that side.

Hope this makes sense.

Cheers,
Ian

Last edited by gingertube; 21st November 2012 at 03:18 AM.
  Reply With Quote
Old 21st November 2012, 05:14 AM   #18
diyAudio Member
 
Join Date: Mar 2011
Thanks, that sounds easy, I'll try it tomorrow...
  Reply With Quote
Old 21st November 2012, 05:51 AM   #19
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
And just a thought. You have two tubes in parallel on each side, and you are taking measurements that ought to be for two tubes, and you then infer that half of that reading is for each tube. SO 56ma means 28ma per tube.

But the reading alone does not guarantee the two tubes are sharing equally. If for some reason one tube was not conducting fully, its mate might be drawing more current, so the total mighgt look good, but one tube woould red plate while its mate idles along cold.

Not saying it is happening here, but never rule it out. If in doubt, pull one tube at a time and see if you get 28ma for a single tube. (or whatever reading is appropriate.)
  Reply With Quote
Old 22nd November 2012, 04:10 AM   #20
diyAudio Member
 
Join Date: Mar 2011
That seems to work pretty good. My OT has both sides the same resistance, maybe a .2 ohm difference. One is 38.4 the other is 38.6. Is that normal?
  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
Bassman 50 red plate and bias TheGimp Instruments and Amps 14 26th September 2012 12:44 PM
Red plate KT88 h00hbt Tubes / Valves 32 13th September 2012 12:47 PM
Plate Current VS Plate Voltage Graphs Captn Dave Tubes / Valves 2 13th February 2009 08:57 PM
Why does red plate spell early death? Ty_Bower Tubes / Valves 19 28th September 2008 03:37 AM
Heathkit W-5M Red Plate Issue Trout Tubes / Valves 7 28th May 2006 02:18 AM


New To Site? Need Help?

All times are GMT. The time now is 04:49 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