Under rating of Vertical Sweep Tubes - Page 2 - 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 18th October 2010, 05:41 AM   #11
diyAudio Member
 
aardvarkash10's Avatar
 
Join Date: Mar 2007
Location: Auckland, NZ
Quote:
Originally Posted by mashaffer View Post
Just to be sure I understand. In the class AB case are you saying that the dissipation at idle can be well over spec?
Yes - largely because it will spend not a lot of time at idle (depending on how you configure obviously...)

Quote:
Originally Posted by mashaffer View Post
I was thinking that in class A max dissipation is at idle where in class AB the dissipation goes up with output.
While the instantaneous current may be very high in AB, the average will be at or below the spec'd value (again depending on your configuration). Since these are not sand items, instant doesn't matter, average does.
__________________
"Folks, you can't prove truthiness with information. You prove truthiness with more truthiness. In a process known as truthinessiness." - Stephen Colbert, The Colbert Report
  Reply With Quote
Old 18th October 2010, 02:10 PM   #12
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Huh... I guess I misunderstood the earlier comment on deep class AB. Class A runs at max dissipation all the time. So no up-rating possible. Optimum biased class AB runs at least dissipation at idle. (there is an optimum bias that gives least distortion and is usually relatively low current draw at idle). So it can take advantage of the crest factor to handle above rated power on peaks.

Deep class AB, I assume this means biased on for most of the cycle, would run hot. I wouldn't up-rate for that.

In theory, a class AB stage would again enter low dissipation at high current, since the voltage drop in the device would be low. But tubes won't work without some significant voltage drop, especially triodes. (pentodes can almost get there, but risk burning up the screen grid) Tubes are inefficient, they just get hotter the more power you try to get out of them.

Terman's book "Electronic and Radio Engineering" gives an example of optimum class AB (he calls it class B) biasing, using the projected cutoff method, on the bottom of page 355. (4th edition)
This involves sliding the two tube curves (one inverted) horizontally until their sum is closest to a straight line. He just uses the projected cutoff method to predict that point approximately.

Under biasing (deep class AB presumably) would overlap the conduction curves too much, causing "gm doubling" at low signals.
__________________
I want a Huvr-Board!!

Last edited by smoking-amp; 18th October 2010 at 02:38 PM.
  Reply With Quote
Old 18th October 2010, 03:21 PM   #13
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Late edit to above:

"So it can take advantage of the crest factor to handle above rated power on peaks."

Should be:
"So it can take advantage of averaging and the crest factor to handle above rated power on peaks."

Actually figuring out the amount of power handled however is complex due to the instantaneous tube power being the product of tube current and tube voltage drop. Then having to integrate that over a cycle, and then figure in the audio crest factor for non constant sine wave signals. And the crest factor varies depending on the signal source. Compressed FM radio having a low crest factor for example.
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 18th October 2010, 03:54 PM   #14
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
What if I told you what I observe in class A, that the max dissipation is at idle and it may decrease from idle to full signal? How can that happen?

From my analysis, the max dissipation of a class AB or B stage, for a sine wave workload, is somewhere in the middle of the output power, depending on the idle dissipation. Integrating the instantaneous volt*amps over the full cycle as Don points out. ("Instantaneous power" still doesn't make sense to me...)

Crest factor of music, even highly compressed music, is still relatively high compared with sine waves (CF of a sine wave is 1.414..) so I believe a class B or AB stage can be rated to play full volume music at clipping at a much higher level than a sine wave, based on max dissipation.

Here's an example of the crossover analysis assuming 3/2 power law, should be "gm-error":
Attached Images
File Type: jpg crossover-analysis.JPG (88.4 KB, 195 views)

Last edited by Michael Koster; 18th October 2010 at 03:57 PM.
  Reply With Quote
Old 18th October 2010, 04:30 PM   #15
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
"What if I told you what I observe in class A, that the max dissipation is at idle and it may decrease from idle to full signal? How can that happen? "

You are quite right. I was earlier forgetting to subtract out the output power for the class A case. For linear tubes, the current draw from the B+ is constant, so total power draw from B+ is constant. But power output goes up as V^2, so tube dissipation would then drop as the signal gets larger. Although practically it won't fall to zero since the tube needs some operating voltage. And typical tube distortion also makes the off going tube drag its feet about turning off when it should, which usually shows up as some increase in B+ current at max signal.

This would still not be useful for any tube up-rating purposes though, unless someone likes listening to max clipped waveforms all the time.

Near class B (ideal case, linear, no bias current) as you say, increases from zero tube dissipation at zero signal, to max tube dissipation at 1/2 max signal, and back down to zero tube dissipation at max signal. Practically though, the minimum tube voltage requirements keep it from coming back down to zero at max current. And idle current biasing keeps the low end diss. above zero too.

-------------------------

Crazy off the wall idea for the day:
Suppose the amp runs in class A mode, but has an aux. switching circuit that adds in a "square" wave at the tube side but subtracts it back out at the OT secondary side. The amplitude would be adjustable (a power D/A with limited bits) so as to bring the signal at the tube plates up to near max, to get minimum tube dissipation always. Efficient class A? I guess this would really be the same as digitally adjustable tracking B+ voltage. Then again, maybe not, the digital glitches cancel here. Nah, it still the same, the tube current will glitch with the changing voltages.
__________________
I want a Huvr-Board!!

Last edited by smoking-amp; 18th October 2010 at 05:00 PM.
  Reply With Quote
Old 18th October 2010, 06:52 PM   #16
diyAudio Member
 
rsumperl's Avatar
 
Join Date: Oct 2009
Location: Warren Center, PA
Greetings all,
So too many years have passed since school, what class of amp did the vertical amplifier run? I know the horizontal amps were class c.

Ray
  Reply With Quote
Old 18th October 2010, 07:14 PM   #17
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
"what class of amp did the vertical amplifier run?"

Apparently class A. I guess they had to linearly sweep both ways, where as the horiz. channel only had to linearly sweep one way? (not sure about that) Still you would think that an inductor could be linearly charged up then back down using just a switched square wave voltage? Maybe was due to the time interval (60 Hz) being slow, so would require a very big inductor? (avoiding 1000s turn winding on defl. yoke, such a big air gap for much inductance at 60 Hz)

See middle section of George's post:

6L6GC AB2 Amp

Some TV experts around?
__________________
I want a Huvr-Board!!

Last edited by smoking-amp; 18th October 2010 at 07:44 PM.
  Reply With Quote
Old 18th October 2010, 07:48 PM   #18
diyAudio Member
 
rsumperl's Avatar
 
Join Date: Oct 2009
Location: Warren Center, PA
Quote:
Originally Posted by smoking-amp View Post
"what class of amp did the vertical amplifier run?"

Apparently class A. I guess they had to linearly sweep both ways, where as the horiz. channel only had to linearly sweep one way? (not sure about that) Still you would think that an inductor could be linearly charged up then back down using just a switched square wave voltage? Maybe was due to the time interval (60 Hz) being slow, so would require a very big inductor? (avoiding 1000s turn winding on defl. yoke, such a big air gap for much inductance at 60 Hz)

See middle section of George's post:

6L6GC AB2 Amp

Some TV experts around?
Thanks for the link. I forget what tube it was, but one of the (beam) pentode had vertical amplifier data only.
  Reply With Quote
Old 18th October 2010, 07:54 PM   #19
работник
diyAudio Member
 
Rod Coleman's Avatar
 
Join Date: Jul 2004
Location: Warwickshire UK
Quote:
Originally Posted by Ty_Bower View Post
If I were messing around with sweep tubes, I'd keep the screen voltage low and beat the living snot out of the plate dissipation.
I think that is the best approach, too. The other thing to be careful of is the surface temperature of the glass. It's more important than at first blush, because the usual Barium-Aluminium getter cannot work normally at temperatures above 200 deg C.

I haven't tried it, but I imagine that the gun-type IR thermometers would give a good monitor of the temperature of the glass during varying overdissipation sessions.
  Reply With Quote
Old 18th October 2010, 10:05 PM   #20
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
"The other thing to be careful of is the surface temperature of the glass. It's more important than at first blush, because the usual Barium-Aluminium getter cannot work normally at temperatures above 200 deg C."

Getter begins outgassing?

-----------------------------

"I forget what tube it was, but one of the (beam) pentode had vertical amplifier data only. "

6HE5, 6JB5 etc?
These tubes are a bit of a mystery to me. They have nice clean pentode curves (low screen current distortion), and identical dataspecs to 6V6. (notably low gm, low DC max current, and high screen V) The 6V6 gets glam sound ratings, but its curves do show significant screen current distortion. Not really the same tube by the curves. Someone did recently think well of them (6HE5, 6JB5 ...) for a guitar amplifier they built. Called them 6V6s on steroids.

None of these look too impressive when triode wired though. Droopy curves on the curve tracer. But that is usual for small cathode tubes. Great triodes from pentodes seem to require a big flat cathode. 6CB5A a case in point. Over on the Asylum someone built a SE amp with this recently, and a group of listeners mostly gave it the winning vote versus a 300B amp.

I have to wonder what say 6LG6/6LB6 would sound like. Nice clean curves just like the 6HE5.... series, AND a big flat cathode. Well, I got a box full of 21LG6, guess I should try them out. Too many projects.
__________________
I want a Huvr-Board!!

Last edited by smoking-amp; 18th October 2010 at 10:20 PM.
  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
Sweep tubes and G3 rsumperl Tubes / Valves 3 5th October 2010 03:54 AM
Do Sweep tubes Glow Blue? wrenchone Tubes / Valves 26 20th July 2009 10:32 PM
Oddball Sweep tubes in PP mashaffer Tubes / Valves 8 4th June 2009 01:17 AM
Some interesting reading about Sweep tubes Vargas Tubes / Valves 2 28th April 2009 04:48 AM
Thoughts on horizontal sweep tubes? DaveCan Tubes / Valves 14 28th August 2008 09:24 PM


New To Site? Need Help?

All times are GMT. The time now is 10:37 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