Anyone cathode biased KT66's @ around 460V?

[azatplayer]

Ive got a heap of iron i wanna use up from early builds. Was thinking of an octal rocket. I have a 6K6 OT which would like KT's.
Ive did a double deluxe some time ago, had 4 x 6V6's, o tried it with just 2 KT's and that was around 370V, sounded pretty good.
I guess having a switch could be cool to go fixed/ cathode. Would like to keep it pretty simple tho.

[Andy Le Blanc]

The osram sheet says max 500v plate and 400v screen

and its ok to mix bias methods, a small amount of self bias and go the rest of the way fixed
a resistance in the cathode also gets you some degenerative feedback that can be
used in place of global feedback

[M Fowler]

My Orange Retro 50 clone has cathode bias and fixed bias.

It uses a DPDT switch to go from what Orange calls class A 30w -32v to class AB 50w. Nice noticeable difference between them.

150R/25w & 220uf/100v for cathode bias
47k bias trimmer

290-0-290 SS rectified.
10h 150mA choke
4k 60w OT

Mark

[azatplayer]

The osram sheet says max 500v plate and 400v screen

and its ok to mix bias methods, a small amount of self bias and go the rest of the way fixed
a resistance in the cathode also gets you some degenerative feedback that can be
used in place of global feedback

Thats intrigued me. What do you call small? 20-30% of typlical cathode values? Thats a pretty cool idea!

[TUBEDUDE]

Isn't the combined bias the main reason the early silverfaces were such tone killers?

[Andy Le Blanc]

Nothing new, mixed bias or combination bias, a un-bypassed cathode resistance
gets you bias and feedback, the bypass value can be "critical"
meaning you chose the value subjectively, a smaller value to shape the tone
instead of simply going for the most gain, the fixed grid bias brings the
power supply to your preferred current flow.

I got one on the bench now, mixed bias, the total bias will be set at 70ma..
this gets me a stable and known circuit voltages, irregardless of tube set

It has no global feedback but I put 4r on each cathode, not much for bias
but its a touch of degeneration we'll see how it goes, the bias is fixed otherwise.
I've seen 10 to 100 r in schematics.

To me its more important to maintain a known idle current for the rest of the
circuit voltages to design around, everything else is space for tweaking.

You could choose any % I guess, I've been using it unbypassed for the feedback

A lot of amp features center around bias , give a tone change
tube set mixing just swings bias, keep the bias the same and theres less
of a tone change
its the same with fixed/cathode bias switching, if the cathode bypass value
is large theres no feedback, just a different bias point

In those 100w silver faces theres a 5uf 25v cap going for cathode to cathode on each side
of the PP circuit, and then both sides are linked by a 100uf 6v non polar its neutralized,
The 50w are neutered side to side with just the 5/25 non polar
and then theres the cathode resistors degeneration on top of the global feedback,
because the caps aren't referenced to ground
and theres .002 caps to ground in the grid circuits...

Its just overkill... the feed back is too much with or with out it...
the phase inverter has plenty too even with out the loop...

I use the combination bias in amps that don't have any global feedback
and paraphase inverters that don't play well with a lot of global feedback
, simple way to tweak the tone color

[TUBEDUDE]

Sweet, that opens up a new alley for me to run down tonequesting. Thanx Andy!

[Tone Lover]

I think I am going to have to read that a few times to understand it.
Thanks Bill

[Weathered]

My Orange Retro 50 clone has cathode bias and fixed bias.

It uses a DPDT switch to go from what Orange calls class A 30w -32v to class AB 50w. Nice noticeable difference between them.

150R/25w & 220uf/100v for cathode bias
47k bias trimmer

290-0-290 SS rectified.
10h 150mA choke
4k 60w OT

Mark

Mark -

Do you happen to have a schematic on how that is done? I've been thinking about putting that in my new build, and can't quite figure out how to put the switch into my circuit.

[M Fowler]

Mark -

Do you happen to have a schematic on how that is done? I've been thinking about putting that in my new build, and can't quite figure out how to put the switch into my circuit.

Weathered: Send me an email shout.

Mark

[tweedeluxe]

i have run cathode biased kt66s in several 6L6 pa conversions at 400V up to 490V. the datasheet says to split the cathode Rs for this tube and use 10k grid stoppers.

Many of those PA amps used a voltage divider to feed the screens which to me sounds good both with 6l6s and kt66s. usually the total divider resistance is only around 40k-100kΩ which also stiffens up the power supply. you probably don't need to worry too much about dropping the screens this way though as many JTM45s run the plate and screens right at 460V with an even higher primary impedance.

[rp]

Do you happen to have a schematic on how that is done? I've been thinking about putting that in my new build, and can't quite figure out how to put the switch into my circuit.

Here's how I did it. Luckily I took notes for myself as I got tired of having to re-figure things out every time. Please double check my drawing, I tend to do notes late at night and fast. The photo OTOH is correct as it works

[Weathered]

That's exactly how I pictured it being done. Someone else can double check, as I still haven't built it, but logically it makes sense to me....