Pros and Cons of this technique?

[G]

When I built my SE EL84 amp I used one resistor to bias both output tubes ala "Zen triode amp". What are the advantages and disadvantages of this technique. The manufacturer of the Zen makes it sound like the thing to do. It "locks in the stereo imaging". I had planned on doing this in my new SE EL34 amp but I would like some unbiased opinions first. Thanks.

G

[dhaen]

I would like some unbiased opinions

very good, but they'll never work
If they're matched, fine. If not, you'll probably lose more than you gain.
Of course your stereo image'll be rock steady if you use grid bias; matched or not

[G]

Hi dhaen. I assume you mean a fixed bias? No the amp will use a cathode bias.

G

[45guy]

As I understand it, a shared cathode resistor will cause a bit of regenerative feedback affecting only the portion of the signal that the 2 channels have in common. In other words, it cancels a bit of the center image making things sound a little bigger. I would never say that it "locks in the stereo imaging", in fact some say that this approach is messing with the information in the source material and it's a step backwards in terms of fidelity. I'm not that picky though...I like the effect that it can give.

Tom

[fdegrove]

Hi,

I would never say that it "locks in the stereo imaging", in fact some say that this approach is messing with the information in the source material and it's a step backwards in terms of fidelity. I'm not that picky though...I like the effect that it can give.

I don't as it isn't true to the source, it's akin to crossfeeds in some headphone amps although to a much lesser extent.

Cheers,

[G]

I'm sorry dhaen I'm a little slow today. That was funny!

G

[G]

" I'm not that picky though...I like the effect that it can give. "

It seems to work well in my SE EL84 amp. Of course I have no reference to compare it to though.

G

[dhaen]

Gavin,

I had assumed that you would bypass this resistor with a suitable cap. My remarks were based on this.*
Of course if you don't bypass, then all the above posters' comments are true.

*I'll explain what I mean if that is the case

[G]

Yes. I will be using a 1000uF bypass cap across the cathode resistor.

G

[dhaen]

Ok. So bypassed, there'll be no signal mixing as such.
The reason I believe it's better for the stereo image, is that if a transient pushes some grid current on one valve, the bias of both will be affected equally. Remember, the gain varies with DC changes (a little). This just might be enough to destabilise the image.