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Autobias install questions

Hi all,

I'm going to install the AB-Qi modules in my homebrew monoblocks and have some ideas/questions I want to discuss. They are currently cathode biased.

First off my design uses only 2 coupling capacitors but has 4 output tubes. I'm intending to bias the tubes as pairs ala Audio Research because I'm loath to add another pair of caps into the signal path to individually bias the tubes.

The AB modules run the cathode resistors on the board and have you remove all connections. Ok fine, but in my case with 2 tubes biased together I want to have the cathodes tied to ground with 1R resistors, giving me an easy test point for current for the 2nd tube which will NOT be connected to the AB module.

If I'm visualizing the circuit correctly having 1R to ground won't affect the voltage feedback to the AB module.

So two questions result: am I ok to have the 1R cathode resistors in place with the AB module? And should I just bite the bullet and add more coupling caps to bias individually? It sounds so lovely as it is that I am really hesistant to mess with the signal path.
 
How I'm planning this:
 

Attachments

  • SS2 Amp Schematic Mod 1.jpg
    SS2 Amp Schematic Mod 1.jpg
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A. If you need to use 4 output tubes per channel, and only one autobias module per channel, then you need to do 3 things:

1. Get an Extremely Well Matched Quad of EL34s for each channel (2 sets of matched Quads for Stereo).
2. Connect the 'pull' cathodes together, that also parallels the 'pull' 1 Ohm cathode resistors. That becomes a single 0.5 Ohms sense resistor.
The bias module will give the bias for each tube, just the same as if the tube was a single tube with a 1 Ohm resistor (but only if those tubes are Extremely Well Matched).
3. Repeat # 2 for the 'push' cathodes.

B. Or, if you do not want to connect the 'pull' cathodes together; and do not want to connect the 'push' cathodes together, then you will need 2 autobias modules for each channel.
(4 auto bias modules for Stereo).

C. Or, if you add add 2 coupling capacitors per channel, you have to use 4 individual fixed-adjustable grid bias potentiometers for each channel.
That is a negative power supply, and 4 potentiometers (8 potentiometers for Stereo).
Who wants to do all that???

D. You really should purchase an Extremely Well Matched Quad for each channel. Otherwise, not only the bias may be a problem, the transconductance may not match either (Unequal Gm? Call that a gain mismatch, and a signal current mismatch).

Your choice.

Good Luck.
 
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I missed something when I looked at your original schematic.

It is very late here, but I do not see a good solution for changing the design from 2 EL34 per channel, to 4 EL34 per channel.

The autobias module may or may not work with no grid return resistor, but if you use 220k, the driver tube now sees 220k in parallel with 220k = 110k.
There is too much that is unknown about the autobias circuit.
Caution, whatever you do, be sure that each EL34 cathode has a 10 Ohm resistor to ground.
 
The autobias module may or may not work with no grid return resistor, but if you use 220k, the driver tube now sees 220k in parallel with 220k = 110k.
There is too much that is unknown about the autobias circuit.
Caution, whatever you do, be sure that each EL34 cathode has a 10 Ohm resistor to ground.
The 10R cathode resistors are on the AB board, hence why they recommend "floating" the cathodes (they aren't, it's just on the board).
 
The auto-bias board will work with 220-270k grid leaks without changes. You are correct in using separate bias for each EL34. I have used these. Make sure the -ve rail is filtered as normal as there is little rejection of the negative supply to the bias outputs on the board. As it servo bias there's less chance of runaway with higher grid leak resistors as as long as there's enough negative supply. The AB board has the cathode resistors on the board. It should give you 40ma per EL34 which can be adjusted. You could consider KT77 if your having problems with reliability.
 
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Some designers do not like cathode self bias, with an individual resistor and a bypass capacitor for each tube.
They do not like the B+ voltage loss due to the self bias voltage. Just design a higher B+ voltage to make up for it.
And . . . They hate the idea of using capacitors that bypasses each self bias resistor (Those are capacitor Integrators).

Well, the autobias circuit in Post # 15 has . . . guess what? It has a 47uF 25V INTEGRATOR Capacitor.
I thought designers hated integrator capacitors.
I guess we are back to needing to use integrator capacitors (Deja vu all over again)!
 
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Never overlook the options for capacitors.
There are Lots of Non-Electrolytic 47uF 25V capacitors (or 47uF at more than 25V).
Modifying a tube amp? Then modify the autobias board, and change what I assume is a 47uF electrolytic to a much better capacitor.
 
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