KT 66 Rocket and seperate Bias resistors

[Markusv]

Hi All

I seem to remember someone mentioning that PP KT 66's, when Cathode Biased, should each have their own seperate Cathode resistor.
Am I understanding correctly?

What would be the reason for that?

Secondly, I have seen posts mentioning that Shuguang KT 66's need to be biased hot for best performance
What is hot? How much over 70%?
I have 340 on my plates in the KT 66 Rocket

Sincerely,
Markus V

[soma_hero]

The datasheet says to use separate cathode resistors, but doesn't say why.

http://thetubestore.com/kt66reviews.html

This site says at least 70% plate dissipation.

[Markusv]

Soma

Thanks for that.
Perhaps this is a Hi-fi practice? (the seperate resistors?)

I'll try and read some more and report back if I find something.

BTW I bought my tubes from the same place and the delivery was really fast!- less than 24 hrs
The one tube is physically larger than the other. About 1/4 inch taller and a little fatter
Do you think such inconsistencies are common?
Sound great though

Markus V

[tweedeluxe]

One datasheet I read said to use separate cathode Rs with KT66s unless there is no chance that the anode will ever reach maximum dissipation.

Or something like that. In my KT66 Rocket, the tubes are running at about 75% max dissipation, and it sounds geat. (305 on the plates, 400ohm ea Rk).

[Markusv]

Thanks for the insight

Currently, I run my tubes with a shared 220R cathiode resistor at about 340 on the plates

The tubes seem happy and so am I . No redplating even at max volume although I have only done that for short periods
The Shuguangs seem to manage their heat dissipation well with the huge glass surrounds and they sound great
I popped a pair of el34's in for an hour or so (while keeping an eye open for red plates etc as I kept cath R value the same. Wow!- sounds beautiful and quite different. Turns it into a different beast I would say, although I can't say I prefer it over the KT66's neccessarily.

Markus V

[tweedeluxe]

Some more relevant KT66 datasheet stuff that I wrote down:

-Pair of valves working at max ratings should be mounted no less than 3.5" inches apart center to center (mine are 3").

-Tp prevent parasitic oscillation, a 100-300ohm R should be mounted on the screen grid tag (I used 220), and a control grid resistor of 10k-50k is recommended (I used 5k6 like a JTM45).

[Andy Le Blanc]

That spacing guide line is something I haven't stumbled across yet. Which
data sheet did you find that in?

I had a long fight with myself on a recent project whether or not to go with
separate cathode resistors or one common resistor, and finally went with one
shared bias resistor. Separate resistors were very nice clean, but I liked to
push the amp, and the common bias resistor seemed better there.

[fishy]

KT66 Datasheet. Spacing is mentioned towards the end

[Andy Le Blanc]

found it ...... thanks

[tweedeluxe]

Separate resistors were very nice clean, but I liked to
push the amp, and the common bias resistor seemed better there.

So going with a single Rk made for better distorted tone? that's very interesting.

Was the clean tone actually better in your opinion with dual Rks? Think it's worth adding a switch to my KT66 build to A/B, or was there not a substantial difference?

[Andy Le Blanc]

Yes, give it a shot. Discussion like this instantly places us into the realm of
subjective opinion. There is always a huge gulf between projects and the
ears listening to them. I found that there really was a difference in how my amp presented
its self, over all, how it responded when pushed into full clip, and for the
applications to which I use my amp it suited my ear. At the very least you
will have an option to change the "texture" of the amps response.

[Zippy]

So going with a single Rk made for better distorted tone? that's very interesting.

Was the clean tone actually better in your opinion with dual Rks? Think it's worth adding a switch to my KT66 build to A/B, or was there not a substantial difference?

Interesting, indeed, and simple enough to implement an A/B test: Install dual cathode resistors and tie them together with a SPST switch.

[tweedeluxe]

Yeah since mine is already running 2 Rks I migh as well give it a try huh?

[Zippy]

Low voltage, low current... Yes, please do.

If there is an audible difference, it would be interesting to tie the two points together via a resistor and watch what is passing between them.

[RJ Guitars]

Good thread... I'll try not to louse it up!!!

I didn't use two cathode resistors but only because I had not done enough homework... I'm curious of the results.

Tube spacing... I had that wrong too. I used an Express chassis to build my prototype in and the output tubes are pretty close together.

I've had a few people ask about a 66 Rockster kit... I want to do it, maybe in a combo format. I'll move the tubes apart and plan on two cathode bias resistors...

As I said, this is a good thread and I think these are all good issues to sort out...

thanks guys.

rj