SE With shared Rk in output

[mashaffer]

I have seen a couple of SE power amplifiers with a shared Rk to bias the output stages. Obviously a cost cutting measure but what is the downside of such an arrangement?

[DigitalJunkie]

Worse channel separation is the first thing that pops into my mind.

[pmillett]

You need matched tubes, otherwise they will potentially bias themseves at very different levels.

The resistor must be bypassed well, or you get interaction between channels (like bad seperation).

Pete

[mashaffer]

The reason I ask is that I am planning on building a 6LU8 SE UL similar to Kegger's but with switchable NFB and UL/Triode modes. I think I have just about everything I need on hand except that I don't have the appropriate cathode resistors. I do however have and 1800 and 470 ohm that I can parallel which should give me about the right bias if I run both cathodes through them.

I thought I might try prototyping it that way to see how it goes but if the sound quality would be seriously compromised then it wouldn't be worth the trouble. A little loss of separation is probably not a problem.

[Sheldon]

Originally posted by mashaffer
A little loss of separation is probably not a problem.

Records wouldn't sound good if it were.

Sheldon

[bigwill]

Slight loss of separation is one of the most over-exaggerated problems IMO.

Edit: In fact, wouldn't a shared Rk would have a slight differential effect creating a stereo "widening" effect?

[Dave Cigna]

Originally posted by mashaffer
I thought I might try prototyping it that way to see how it goes...

Sounds to me like you got the best advice from yourself.

Seriously, you already have the parts, how hard would it be? (How much time have you spent participating in this thread?)

You can try separate resistors later ...

[dr._sleep]

The Decware Select SE84-CS uses a single cathode resistor (5 watt 100 ohm) and cap (1000uf) for both SV84 or EL84 power tubes. I believe it widens the sound stage vs.using 2 resistors and 2 caps.

dr._sleep

[mashaffer]

Little bit of a Duh here. I just realized that my PT has a 45V bias tap. I could just use fixed bias. I will have to do a little bit of reading up on it as I have not really looked at that option seriously before.

The tap is just a tap rather than a separate winding. I suppose that the load current is so small that interaction with B+ is theoretically insignificant. Also due to low current I imagine that filtering is not too difficult. I have a lot of SS diodes and bridges around as well as low voltage caps for filtering and diode snubbing so this should be very doable with parts on hand. I also get to save my power resistors for B+ filtering and screen connection for UL and triode mode. Figuring on 470ohm for the screen.

I worry a little bit about getting a hard SS kind of sound but will just have to try it and see how it goes. Has anyone ever used fixed bias for both output stage and VAS?

[Sheldon]

Originally posted by mashaffer
I worry a little bit about getting a hard SS kind of sound but will just have to try it and see how it goes. Has anyone ever used fixed bias for both output stage and VAS?

I'm not sure what the connection with hard sound is. but yes I have two amps with fixed bias on the input and output stages.

Sheldon