Pros and Cons of this technique?

[G]

I see. So you are saying that using one resistor is not a good thing? Just mant to make sure I'm on the same page you are. I'm not being cheap. I can just as easily(well almost) use two resistors and two caps on the output stage. I was just curious if there was some advantage to using one. On my first amp I used just one because I was gullible and believed what I read on the internet. It turned out ok though. I think I will go with two on this amp. Thank dhaen and Frank and 45guy et al.

G

[Brett]

Originally posted by G
I see. So you are saying that using one resistor is not a good thing? Just mant to make sure I'm on the same page you are. I'm not being cheap. I can just as easily(well almost) use two resistors and two caps on the output stage. I was just curious if there was some advantage to using one. On my first amp I used just one because I was gullible and believed what I read on the internet.

I don't like the sound when using common cathode bias resistors. But for the cost and effort involved, why don't you just try it for yourself? That will give you a more definitive answer.

[Brett]

Originally posted by 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.

Why not just build an amp, or size it correctly to the speaker/room/usage, so that it never goes grid positive?

[45guy]

Hi G,

Given dhaen's explaination, I guess the Decware folks are correct. I had not thought about it in terms of .

If you want to hear this effect without much fuss, here's a little trick you can try. Find a pot that's 1/2 the value of the individual cathode resistors on your driver tubes. If you are lucky it will be a standard value, like 1K. Tie the cathodes together to the top lug of the pot, tie a bypass cap (2X the capacitance of your individual caps) to the wiper, and ground the bottom lug and the cap. Unless you are pulling a lot of current in those drivers, a 1W pot should do. Now fire her up and rotate your pot. You will hear the effects of this very clearly. You will also loose some bass response, but this is just for grins anyway. I did this with a simple 6SL7/2A3 amp and it was actually pretty cool to play with.

I do agree with Frank that this is an effect and it's not something that I would want that couldn't be defeated, but it is fun to play around with. Remember the Polk speakers from the 80s that used cross-fed drivers? That's what this kind of reminded me of when I tried it.

Who says audio shouldn't be fun.
Tom

[G]

I don't understand what you are saying Brett. I did try using just one on the first amp I built. I like the sound of the amp and I am using very efficient speakers so I have never had to "push" the little fella hard. I would like to use individual cathode resistors and caps on my next one though. I plan on using a 2 watt pot in series with each cathode resistor so I can make the bias voltage the same for both tubes whether they are well matched or not. I was just curious as to whether I was giving up something substantial by using two resistors instead of one common resistor. As you say though. I will never know unless I try it. Thanks for the reply.

G

[Brett]

Originally posted by G
I don't understand what you are saying Brett. I did try using just one on the first amp I built. I like the sound of the amp and I am using very efficient speakers so I have never had to "push" the little fella hard. I would like to use individual cathode resistors and caps on my next one though. I plan on using a 2 watt pot in series with each cathode resistor so I can make the bias voltage the same for both tubes whether they are well matched or not. I was just curious as to whether I was giving up something substantial by using two resistors instead of one common resistor. As you say though. I will never know unless I try it. Thanks for the reply.

G

(assume you're referring to post 12)

All I meant was, for the cost of another couple of resistors and caps, and a few minutes with the soldering iron, you can try it both ways and see which one you prefer. You could even put a switch in during testing. Whether anyone else likes it is moot, as it's your system.

Common cathode R's seem to trangulate the soundstage, with the apex in the centre and receding away. but worst of all, it seems to vary depending on the level and spectrum, and I find it very disconcerting. But as others have mentioned, they like the effect, and you might too. IMO, the only real application for any sort of crossfeed (which I think this is) is perhaps in headphone amps when listening to non-binaural sources.

[G]

Gotcha. Thanks Brett.

G

[Positron]

For at least a couple of reasons off the top of my head.

1) There is feedthru, even with a bypass capacitor. And it is frequency dependent. It changes the frequency response in that more of the signal feeds thru in the bass region, giving a feel of more bass. At high frequencies, like 10kc (depends on brand, size etc.), , the cap self resonates and then becomes inductive, bleeding the channels together.

2) There is feedthru even in the mids, so stereo separation suffers. This is quite evident when comparing to monoblocks.

3) An electrolytic cap has a notorious reputation for grunge, smearing the sound. It is a good way to increase the bass by falsely "bloating" this region.

4) If one channel clips, it is bound to change the bias on the other channel. Hopefully, the time constant won't be short though.



Just some thoughts of my testing and experience.

[Brett]

Can't disagree with what you've added there Steve.

[G]

Hi Steve. As it stands now this is the design I'm going with. I just ordered the last parts today. Should be built by Thanksgiving.