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Automatic bias board.
Automatic bias board.
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Old 4th December 2017, 06:00 PM   #21
pavlikkkk is offline pavlikkkk  Czech Republic
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Automatic bias board.
and what to use Schottky? The window would smalle. I do not know about germanium diodes in SMD design :-)
About symmetry window ..
I think the window should be symmetrical. the course of the saw moves linearly around the "synchronized swimming" reference position.
by changing the voltage of the reference source, reference is set on inverted input UC6 / 10 with function 10mV / mA.
I certainly agree with placing 10 Ohm cathode resistors. Due to thermal loss on reistors I recommend a max. Current of 200mA per cathode.
If someone wants more, they need to be removed from the board and resistors for higher power and connect to the plinth tubes.
Another option is to use resistors with a lower resistance value, such as 5 Ohm. The heat loss is reduced by 4x, but the 10mV / mA equation is no longer valid. but the accuracy of the setting is reduced by 50%. In that case it would be necessary to use cathode resistors with an accuracy of 0.5%.

Thanks
Pavel

Beyond the GRD output is a decoupling resistor (about 100k). When you measure the audio signal at the end tube's inlet, it is totally clean, signal integrator or harmonics are not there.
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Old 4th December 2017, 06:20 PM   #22
pavlikkkk is offline pavlikkkk  Czech Republic
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Automatic bias board.
Quote:
Originally Posted by Too much stuff View Post
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.
yes, the AB-Q module can be used to control four tubes in one channel.
The module has dimensions 100x57mm.
Pavel

Quote:
Originally Posted by kodabmx View Post
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
The module will work for four tubes. The module is a trimmer that can easily set the cathode current of the tubes 0 - 200mA. Cathode resistors are on the module.
Pavel
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Old 4th December 2017, 11:59 PM   #23
kodabmx is offline kodabmx  Canada
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Thanks for the clarifaction, Pavel.
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Old 8th December 2017, 02:06 AM   #24
smoking-amp is offline smoking-amp  United States
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A problem with these type of auto-bias boards has come to light.

If one tube draws more screen current than the other, then the currents to the OT are no longer balanced. This is because the cathode current is sensed by the board, not the plate current. Some provision is needed to differentially adjust the bias set points for each tube in a pair (while measuring plate current with a meter(s)). At present, there is only one bias adjust pot for all four tubes.

I don't think gm matched tubes check for this issue either. Screen grid alignment with the grid 1 wires affects the amount of screen current drawn. This is a separate issue from grid 1 gm matching.

Last edited by smoking-amp; 8th December 2017 at 02:09 AM.
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Old 8th December 2017, 07:59 AM   #25
pavlikkkk is offline pavlikkkk  Czech Republic
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Automatic bias board.
Default AB-Q

This problem can not occur. Each cathode section works independently. The set reference level is connect to each section independently. The bias on grids is set independently of other grids. The unique advantage of this AB-Q is that you can use both old and new tubes without pairing them. The cathode current of all tubes will have the same value according to the reference voltage setting. This has been proven many times when mounting dozens of modules!
Pavel audioamp.eu
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Old 8th December 2017, 12:57 PM   #26
smoking-amp is offline smoking-amp  United States
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I agree that the cathode currents will all be set the same. The problem (for the OT) is that the plate currents are [cathode currents - screen currents]. If the screen currents are not identical, then the plate currents are no longer identical. The OT is not going to like the resulting DC current offset.
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Old 8th December 2017, 02:19 PM   #27
pavlikkkk is offline pavlikkkk  Czech Republic
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Automatic bias board.
the basic requirement of setting the same currents of both PP branches (without audio signal) is met. Output transformer is not saturated.
Pavel audioamp.eu
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Old 8th December 2017, 03:15 PM   #28
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by smoking-amp View Post
I agree that the cathode currents will all be set the same. The problem (for the OT) is that the plate currents are [cathode currents - screen currents]. If the screen currents are not identical, then the plate currents are no longer identical. The OT is not going to like the resulting DC current offset.
This is true, but in reality probably not a problem - think how many amps are manually biased via a measurement on a cathode resistor anyway!

In a UL EL34 amp (for example) with an HT of say 430V, the screen current is around 5mA, assuming the unintentional differences between valves is on the order of a couple of mA, not much to worry about (unless you have finicky cores on your OPTs!)

Last edited by bigwill; 8th December 2017 at 03:19 PM.
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Old 9th December 2017, 08:04 AM   #29
pavlikkkk is offline pavlikkkk  Czech Republic
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Automatic bias board.
Circuit checks the current symmetry of both PP OT branches. These streams are the sum of the anode and screen grids tubes (Kirchhoff). And that is fulfilled. The distortion in the tetrod beam caused by the difference in the streams of the screen Grids will occur anyway. I agree that the difference in screen grids streams in UL connections is very small.
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Old 7th February 2018, 07:29 PM   #30
kodabmx is offline kodabmx  Canada
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So I wanted to let you all know that I've got this module up and running and it works flawlessly. I will definetely purchase another one for a different project. It allows the easy use of toroidal power transformers as OPTs, and just on the cost difference between those and Hammond proper tube OPTs the board more than pays for itself. Combine that with "set it and forget it" or the fact you can pair a 6L6 with a KT88 in a pinch to still have output and you're gold!
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