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Automatic bias board.
Automatic bias board.
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Old 30th November 2017, 07:17 PM   #1
kodabmx is offline kodabmx  Canada
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Default Automatic bias board.

Hey all.


I've just ordered one of these to see how it performs in place of CCS cathode bias with a toroidal PT as OPT.


Has anyone tried this?


Bias Supply Modul for four Tubes (Single pcb to Control 4 Power Tubes) | eBay
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Old 30th November 2017, 07:39 PM   #2
bigwill is offline bigwill  United Kingdom
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I'm sure it works fine - the real challenge with a bias servo however is how it behaves under dynamic conditions. Without seeing a schematic it is hard to predict how this will be. Additionally, if the amplifier has a global feedback loop, any servo (not necessarily the one shown) could potentially cause instability or at least LF ringing if its own time constant interferes (ie gets too near) with those in the amplifier itself.

Last edited by bigwill; 30th November 2017 at 07:42 PM.
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Old 30th November 2017, 07:50 PM   #3
kodabmx is offline kodabmx  Canada
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When it arrives I'll go over the circuit. Judging by the size of the components I'm guessing it varies the negative voltage to achieve a cathode current, rather than acting as a CCS.
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Old 30th November 2017, 11:17 PM   #4
jdarg is offline jdarg  United States
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Looks like a complete rip-off of the tent/vanderveen board without the bias supply PCB.
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Old 30th November 2017, 11:19 PM   #5
SpreadSpectrum is offline SpreadSpectrum  United States
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That's a dead ringer for Guido Tent's board which is based on the van der Veen circuit. It has an extremely long time constant and will not cause LF instability in a properly-designed feedback amplifier. It performs well with very little drift in bias even with very loud music signals with long, sustained bass notes.

ETA: I have one of Tent's boards in the amp in my bedroom.
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Old 1st December 2017, 05:16 PM   #6
crazyfrog is offline crazyfrog  Canada
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Automatic bias board.
Other related stuff on their website audioamp.eu
However I can't say if they're "Asian style imitator" or not.
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Old 1st December 2017, 05:38 PM   #7
SpreadSpectrum is offline SpreadSpectrum  United States
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Automatic dynamic control bias system for push-pull (PP) amplifiers. ...

This appears to be an exact board-layout match of the module Guido Tent sells, including the supply board. I sure hope this is being done in cooperation and isn't a case of someone ripping off everything including the layout.
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Old 1st December 2017, 05:40 PM   #8
jazbo8 is offline jazbo8
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Automatic bias board.
Pretty sure it’s the latter.
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Old 1st December 2017, 08:44 PM   #9
pavlikkkk is offline pavlikkkk  Czech Republic
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Automatic bias board.
You're right. Getting involved with a lot like Guido Tent. We designed the other timer (module center) and power supply circuit. Other times are also the time constraints of the module's rise. You are shorter. PCB design is custom.
Pavel audioamp.eu
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Old 2nd December 2017, 04:21 AM   #10
smoking-amp is offline smoking-amp  United States
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Apparently (from various descriptions) Pavel's board is using the 0 to 2X idle sensed cathode current control method (like Morgan Jones describes in his book), while the Guido-Tent board is using a narrow sensed current range around zero AC input signal (not sure HOW, since it doesn't have a grid input AC signal to monitor?). The PC board layouts are NOT the same anyway, if you look carefully.

Description of operation (method 2, I suppose)
Optimization of operating conditions for push-pull output stages of ...

Some discussion here:
Bias servo for class AB

However I see an issue in the -description- (method 2) on that audioamp.eu link, and also with the general 0 to 2X scheme description.

The class AB tube is operating at low to zero current range in the sampling region, so its gm is varying significantly. This means that Symmetric Vin operation around the zero Vac input region has -Asymmetric- current variation around the idle current that is to be measured. This would mean sampling should be from 0 to something LESS than 2X current to make up for the gm variation (to avoid "pulling" the integrated sense value upward).

While I see the audioamp.eu diagram has got it sampling from from 0 to 25 to 60mA, the Opposite correction! This may just be description semantics however. Just means the idle condition (Vac input = 0) is more like at 35 mA (not 25). One hopefully tests the idle current with an actual meter, using 0 Vac input signal (while adjusting the bias set pot on the circuit board).

My 2 cents...

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Last edited by smoking-amp; 2nd December 2017 at 04:44 AM.
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