Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki 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

6L6GC AB2 Amp
6L6GC AB2 Amp
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 13th June 2017, 02:05 PM   #681
Sodacose is offline Sodacose  United States
diyAudio Member
 
Sodacose's Avatar
 
Join Date: Feb 2006
Location: mke, wi
Interesting. Do you have any discussion of this on your blog?

Some kind of servo was also something that I realized might work but I never did any research. Followers and CCSs are easy enough for a tube addict to understand while servos are a bit more out in the weeds
__________________
WTF Amps || /r/diytubes/
  Reply With Quote
Old 13th June 2017, 05:14 PM   #682
SpreadSpectrum is offline SpreadSpectrum  United States
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
I never wrote about it since it wasn't really my idea and it works as advertised.

The only thing you really want to watch out for with bias servos is making sure that you don't unintentionally create LF instability in the amp. I set my LF roll-off for 2Hz for my amp and set the HF roll-off of the servo to be much lower than that. You really just want to set it as low as practical. You don't want to have to wait 10 minutes for your amp to bias itself when it powers up or anything. I usually shoot for 30 seconds to 1 minute, then things work out so that there is very little bias drift even on prolonged transients and I don't have to wait too long to listen to music. I've not had any LF instability problems.

There are a few threads here that discuss the Van der Veen circuit, gingertube has a nice sketch he has posted. Alternatively, Guido Tent has a nice module that he sells that uses the circuit. It includes the bias supply and everything. You just supply the AC input. I bought one of his modules. If you end up building your own circuit, feel free to ping me and I can help with some small improvements.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 13th June 2017, 05:20 PM   #683
Sodacose is offline Sodacose  United States
diyAudio Member
 
Sodacose's Avatar
 
Join Date: Feb 2006
Location: mke, wi
Cool, I will go thread hunting. I'll look it up in the Self books I have as well. Haven't really explored those chapters much yet.
__________________
WTF Amps || /r/diytubes/
  Reply With Quote
Old 13th June 2017, 05:43 PM   #684
SpreadSpectrum is offline SpreadSpectrum  United States
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Good luck! Here's a recent thread discussing servos: Bias servo for class AB
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 15th June 2017, 04:04 PM   #685
Sodacose is offline Sodacose  United States
diyAudio Member
 
Sodacose's Avatar
 
Join Date: Feb 2006
Location: mke, wi
Here's an interesting post from JB:

https://www.tubecad.com/2005/April/blog0044.htm

The first diagram seems applicable.

I may be approaching the servo thing from the wrong direction though. I'm thinking that I want to match the tube's DC grid voltage to a DC reference by sampling the source voltage for the MOSFET and adjusting the gate accordingly. This would keep the tube's grid bias constant but doesn't take into account the current through the tube.

It looks like a lot of the servo applications use a sense resistor across the cathode of the tube and then adjust the grid voltage to keep it constant. I'll read through that AB thread to see if I can get any clues. I'm wondering how well any of this would work with A2 operation though.
__________________
WTF Amps || /r/diytubes/
  Reply With Quote
Old 15th June 2017, 06:31 PM   #686
Tubelab_com is offline Tubelab_com  United States
diyAudio Member
 
Tubelab_com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
All bias servos have limitations.

As stated there is an issue with time constants, the servo must be slow enough to ignore bass, but fast enough to keep up with a drifting or runaway output tube.

The issue that I have found with most "servos" is that they do not work well when the amp is called upon to amplify music, or some other signal with a low peak to average ratio, or crest factor. Consider the worse case, cranking the amp to full power with a continuous sine wave.

The AVERAGE cathode current will be the same as the "maximum signal plate current" seen in the tube manuals for full power operation. The servo will not know that the amp is fully cranked, and assume that the bias is way too high and attempt to reduce it, probably to its maximum negative voltage capability.

As soon as the full power sine wave is removed, the amp will be biased in cutoff, and will take seconds to recover.

OK, we don't listen to sine waves, but many people listen to highly compressed bass heavy music, which will have the same effect to a lesser degree.

A good "active bias controller" (for lack of a better name) will look not only at the cathode current of the output tubes, but also at the amp's own signal input. It must react quickly and sample the output tube's cathode current only when the amp is not being driven above a fairly low threshold. The amp's circuitry must be quick enough to recover from overload so that the bias is not still settling from an overload condition when the cathode current sample is taken. The amp designed here is good in this regard.

I started down this road using a dsPIC chip about 10 years ago. That work got published in Circuit Cellar magazine, but has vanished from their web site.

I have recently rekindled the whole concept using a more modern controller for a new amp project.

I have been talking about building the "Big One" for years, but I haven't actually built one. Sure I have seen test amps on my bench that recorded power levels in the 200 to 500 watts, but I still don't have a "Big One." Do I need one, no, but I'm going to build one anyway....I have all the parts, and I'm not getting any younger.

I have been quietly testing stuff to decide what path to take, but some things have already been figured out. The driver board that was designed in this thread is a possible contender, but so are some others.

I'm am aiming for 1 kilowatt of power (500 WPC) in a package that I can carry up the basement stairs, and power from a single US spec wall outlet (1800 watts). Yes, these are conflicting, and stringent requirements.....but anything this powerful MUST have some kind of "watchdog" controller. WHY?

Have you ever seen lightning in a bottle......how about a full blown thunder storm! Well, I created one today. This happened just testing tubes, but when you are playing with 600 to 750 volts at over an amp, stuff can go wrong quickly. This was just a bad tube.

I will start a new thread to cover the design and build of this amp. It will not take place overnight, and there will probably be some periods of inactivity, but it is time to start building the BIG ONE!

Teaser picture....there will be more.
Attached Images
File Type: jpg BottledLightning_3_x.jpg (596.9 KB, 258 views)
__________________
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
  Reply With Quote
Old 15th June 2017, 07:07 PM   #687
SpreadSpectrum is offline SpreadSpectrum  United States
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Quote:
Originally Posted by Tubelab_com View Post
The issue that I have found with most "servos" is that they do not work well when the amp is called upon to amplify music, or some other signal with a low peak to average ratio, or crest factor. Consider the worse case, cranking the amp to full power with a continuous sine wave.
In my testing with the Van Der Veen bias circuit, bias drift was small when I cranked the amp up to ear-splitting volumes with modern compressed bass-heavy music. I mean, there was some small drift that I could observe on a scope but it was really not enough to be concerned about. I was actually kind of surprised at how little drift there was.

While not perfect (as a signal threshold-detecting bias controller would be), the Van Der Veen clipper bias servo works quite well in practice, certainly orders of magnitude better than a cathode bias scheme. The response of the RC filter is already really slow and the clipper circuit slows its response further to audio frequency transients.

I'm not trying to say that it is superior to what you are proposing, just trying to point out that servos can be made that perform quite well.

I do a separate monitoring circuit that also monitors cathode currents and shuts the amp down if the average gets too high. I plan on expanding that to monitor all supply voltages, but especially the bias supply.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 15th June 2017, 07:16 PM   #688
SpreadSpectrum is offline SpreadSpectrum  United States
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Quote:
Originally Posted by Sodacose View Post
I'm wondering how well any of this would work with A2 operation though.
A2 will just be some additional cathode current whenever the control grid goes positive. It is still dwarfed by the variations in cathode current that are already happening.

It is all a trade-off. You make the response of the servo slow so that current peaks in transients have minimal effect. However, you also don't want the servo so slow that you have to wait 10 minutes to listen to music when you power the amp up.

In the last amp I made, I set the response so that it would bias up in about 20 seconds. In my testing, this 50 Watt amp (using fairly efficient speakers) was turned up to as loud as I could stand. I noted about 1% deviation in bias with bass-heavy music. I wasn't concerned about that amount of bias drift.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 16th June 2017, 10:07 PM   #689
Rootz is offline Rootz
diyAudio Member
 
Join Date: Jan 2010
Quote:
Originally Posted by Sodacose View Post
Here's an interesting post from JB:

https://www.tubecad.com/2005/April/blog0044.htm

The first diagram seems applicable.

I may be approaching the servo thing from the wrong direction though. I'm thinking that I want to match the tube's DC grid voltage to a DC reference by sampling the source voltage for the MOSFET and adjusting the gate accordingly. This would keep the tube's grid bias constant but doesn't take into account the current through the tube.

It looks like a lot of the servo applications use a sense resistor across the cathode of the tube and then adjust the grid voltage to keep it constant. I'll read through that AB thread to see if I can get any clues. I'm wondering how well any of this would work with A2 operation though.
I recently did design something like this. IIRC Spreadspectrum used almost the same circuit although without the last two npn's/ccs loaded emitter follower. The emitter follower is an idea of Gingertube. I haven't seen the original circuit by Menno van der Veen, so don't know for sure how close this is. Bias stability depends on the 'slowness' of the circuit, which is set by the cap/resistor combination in front of the second opamp. Values shown should provide stable bias on severe overload conditions whilst settling fast enough after startup.
Click the image to open in full size.
  Reply With Quote
Old 16th June 2017, 10:10 PM   #690
Rootz is offline Rootz
diyAudio Member
 
Join Date: Jan 2010
Oh well, never mind. I see Speadspectrum already mentioned the thread about the servo's here...
  Reply With Quote

Reply


6L6GC AB2 AmpHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Red Hot 6L6GC oldmanStrat Tubes / Valves 26 29th May 2009 12:53 AM
Need help with 6l6GC choices BlackUnikorn Tubes / Valves 19 5th August 2008 01:39 PM
6l6gc SET, my first DIY alexmoose Tubes / Valves 3 27th April 2006 03:00 AM


New To Site? Need Help?

All times are GMT. The time now is 12:56 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki