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

Automatic bias board.
Automatic bias board.
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 2nd December 2017, 05:02 AM   #11
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Correction to the last line above.

Setting the idle current with 0 Vac input will actually see the dynamic operation occur at something Less than that set current, because of the gm variation pushing the average off center. (causing the sensed "2X" current to be larger than expected for a truly linear tube. So the circuit will reduce the bias current to compensate, by dropping the grid V slightly more negative) And that now dynamic idle current will vary some with signal amplitude up until the "2X" current limit is passed. Should still be OK if the integrator is slow enough.

Last edited by smoking-amp; 2nd December 2017 at 05:11 AM.
  Reply With Quote
Old 2nd December 2017, 06:46 AM   #12
pavlikkkk is offline pavlikkkk  Czech Republic
diyAudio Member
 
Join Date: Apr 2015
Location: Prag
Automatic bias board.
The capture of the cathode current is the same as Guido Tent. Measurement section for each tube measures the actual current around a very small window around the bias hence is not affected by any music signal (Guido Tent). The wiring diagram was on the Internet. When setting, the cathode current of one tube is measured. The current of the other three tubes is set automatically.
The setting is to measure the cathode current of one tube. The current of the other three pipes is set automatically. The signal on the grid tube has no effect on the setting. An audio signal may or may not be. However, it is good to check the signal at the output of the amplifier, such as an oscilloscope. However, the module is correctly set even without the oscilloscope.
Pavel audioamp.eu
apologize google translator :-)
  Reply With Quote
Old 2nd December 2017, 02:57 PM   #13
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Some link to the schematic?

From what I see, it is measuring cathode current across a 10 Ohm resistor, limited by 0.6V Si diodes. So the measurement interval is 0 to 60 mA. There is no actual grid signal input to determine the AC drive zero crossing. So it must be operating off the cathode current alone.

Tubes have a variation of gm with current, at 0 current they will have 0 gm. Instead of the rectangular integration expected from a linear amplifying device (with constant gm), you get a triangular integration from the gm non-linearity. The integrator output will vary the bias V until the triangle has equal area above and below the (new dynamic) DC set point. So it controls the bias V to get the average DC cathode current (with AC signal applied within the limited range) at about 25 mA.

So setting the unit up for 30 mA DC idle with no AC signal applied, will see a drop to 25 mA when AC signal is added. A long integration time will keep things more stable by averaging over the signal variation effects.

right ???

To do this more correctly, as Wavebourn mentioned in the DIYaudio link above, one would need a sample and hold function and the AC coupled grid drive signal input to determine the AC zero crossings to trigger the sample and hold.

.

Last edited by smoking-amp; 2nd December 2017 at 03:17 PM.
  Reply With Quote
Old 2nd December 2017, 04:55 PM   #14
SpreadSpectrum is offline SpreadSpectrum  United States
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Good to hear. I didn't spend a lot of time comparing the details (obviously) but the layout did look very extremely similar at first glance.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 2nd December 2017, 06:23 PM   #15
crazyfrog is offline crazyfrog  Canada
diyAudio Member
 
Join Date: May 2009
Automatic bias board.
Quote:
Originally Posted by SpreadSpectrum View Post
Good to hear. I didn't spend a lot of time comparing the details (obviously) but the layout did look very extremely similar at first glance.
My God! You put a lot of efforts! I can only say it's very appreciated. It crossed my mind to try such device in an amp eventually. But buying a "copycat board" wouldn't be my first choice. In this case, I think Guido's design kept attention from a "fan"
  Reply With Quote
Old 3rd December 2017, 01:47 PM   #16
pavlikkkk is offline pavlikkkk  Czech Republic
diyAudio Member
 
Join Date: Apr 2015
Location: Prag
Automatic bias board.
Default AB-Q

Quote:
Originally Posted by smoking-amp View Post
Some link to the schematic? <snip>
yes, you have described it exactly.
The image is from the output of the GRD module.

66d14533776e57a9a658de7db532321f.640x360.jpg
  Reply With Quote
Old 3rd December 2017, 02:41 PM   #17
pavlikkkk is offline pavlikkkk  Czech Republic
diyAudio Member
 
Join Date: Apr 2015
Location: Prag
Automatic bias board.
better picture :-)
AB-Q.png | Ulož.to

Pavel
  Reply With Quote
Old 4th December 2017, 01:00 PM   #18
Too much stuff is offline Too much stuff  Canada
diyAudio Member
 
Join Date: Mar 2011
Location: Fairport Beach (in Pickering), Ontario
Would this module work if the four tubes are for one channel such as a VTA M125?

What are the overall dimensions of the module? I didn't see that in the listing.
  Reply With Quote
Old 4th December 2017, 01:21 PM   #19
kodabmx is offline kodabmx  Canada
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
I don't see why it wouldn't work for 4 tubes, one channel. I assume you could also use this to bias 4 mono SE amps.

The dimensions are 100x57mm from audioamp.eu.

I've seen others talk about a 0 - 60 ma current window. Does that mean I can't use this module to set an idle current of 90ma? Or would I have to change the 10R sense resistors to 6 2/3R?
Thanks
  Reply With Quote
Old 4th December 2017, 02:01 PM   #20
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
I believe you can adjust the reference pot. on the board (center) to change the idle current. It will affect all four channels. The other way would be to put some shunt R across the 10 Ohm sense R (parallel summing). Could just add a shunt R from cathode to ground at the tube, I think. The latter might be better for high current TV sweep tubes, just to avoid smoking the 10 Ohm.

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

Pavel:
Looking at your posted waveform, I had imagined that the audio signal would produce some 2nd harmonic of itself in the correction waveform, from the Asymmetric tube gm around the current set point. But I don't see any obvious effect in the waveform. Is this a non effect? (and so no bias current drift between no AC or quiet and AC audio present?) In which case the scheme must be quite stable at holding the idle current. ( I was conjuring up compensation schemes for the gm variation..., like altering one of the limit diodes slightly by mis-match of Vf or adding a series Schottky diode)

.

Last edited by smoking-amp; 4th December 2017 at 02:31 PM.
  Reply With Quote

Reply


Automatic bias board.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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Dynamic Bias - Active Bias - Automatic/Self biased - Overview wanted tiefbassuebertr Pass Labs 22 8th May 2018 12:54 PM
DC Automatic Bias Control - Auto fixed bias Ceglar Tubes / Valves 21 11th October 2014 09:21 AM
Why Only Automatic/Semi-Automatic Bias? PsychedelicFish Tubes / Valves 4 26th October 2013 10:07 AM
Automatic Bias Control sliding bias litefoot22 Tubes / Valves 1 23rd April 2011 10:37 PM
Supressed bias amplifier, automatic variable bias system. destroyer X Solid State 2 20th September 2004 10:24 AM


New To Site? Need Help?

All times are GMT. The time now is 07:42 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