KT66 Bias in AstroDust Duo ?

[TheOtherOne]

I have a RedPlate Astrodust Duo that I would like to try a pair of KT66's in, but I'm not sure where to set the bias.

The chassis suggests 72MV for 6L6GC and 46MV for 6V6GTA.

Do I need to open the chassis and take measurements to bias this correctly, or can it be inferred from this data?

Thank You!

[pompeiisneaks]

First off, do you know this amp has the heater current capacity to handle KT66's? They need 1.27A per tube and 6L6 only need about .9A. If you pull too much on the heater winding you could burn up the Power Transformer

Second, you'd need to know the voltage that the amp runs at so as to calculate an optimal rating for the bias setting. The manufacturer has set some recommended ones per what they know the 6L6 and 6V6 tubes would use, and how they'd operate, but since they've not specified KT66, you'd need to open up the amp, and measure the plate voltage on the power tubes as is to see. This is dangerous if you've never opened a tube amp as it will be likely 350-450VDC in the amp when powered on. you also need to keep the speaker connected and the volume/reverb volume to 0 to do the bias measurements as well.

Don't kill yourself if you've never opened an amp. It may be wiser to contact the manufacturer's support site and ask if they know if KT66's will work and what their recommended bias setting is.

~Phil

[10thTx]

I would follow Phil's advice and contact the guy that made the amp and ask about the PT current and biasing.

From the Tube Store reviews of KT66 tubes:

Tungsol KT66 - Not your average KT66. In fact it’s more like an extremely rugged 6L6GC in a KT88 bottle. Heater current is the same as a 6L6 and the biasing is more like a 6L6. You might want them just for the look but the tone is all there. Big and open with very solid lows and smooth treble response. The mids are well balanced and don’t go to the extremes of sounding scooped or nasal. Like all the Tung Sol branded tubes, the build quality is top notch. The over sized base has the metal band you usually find on a KT88 tube, and the large bottle helps dissipate heat if you like to run your tubes hot or if you have cathode biasing in your amp. If your amp has “bear trap” tube retainers they will have to go because of the size of the tube's base but also the bear trap clamp will ground the metal base to the chassis and cause a malfunction. If your amp requires the tubes to pass through a hole in the chassis for socket insertion you’re also out of luck. Home audio is where I would use these bottles since they are really functional art objects. Bottles like this should be on display. If you have a Marshal head I think you might find them to be an alternative worth exploring. They will give you great tone but since they draw similar current as a 6L6 you will draw 1 amp less heater current in a 50 watt head and 2 amps less in a 100 watt head. Your power transformer will run cooler and you will get closer to the 6550 tone that many Marshall enthusiasts love

With respect, 10thtx

[TheOtherOne]

I did hear back from their builder.
He said KT66’s work well in this amp and ‘typically have a plate voltage of 465’.
Clearly, I’m no expert but I have built a handful of amps, and am aware of safety concerns.
I was thinking there may be a formula to predict the bias current on tube type C if you have values for tube types A and B, however that’s likely a thought out of laziness.

[pdf64]

It’s reasonable to regard KT66 as part of the 6L6 tube family. Not quite as beefy as 6L6GC, perhaps akin to 5881 in regard of plate dissipation but with much higher voltage capability.
From the info provided, 72mA cathode current per pair (72mV at the test point) should be fine, maybe being it back to 60-65mV if you want to be more conservative.
Personally I just set all of the big octals to 30mA idle cathode current (60mV on your test point); plate dissipation is a limit, I don’t perceive a valid technical case for using it as an operating point.

[pompeiisneaks]

I'm glad to know more about the KT66's, I've not used them before. The datasheet I used may have been for a very different variant since it showed slightly higher heater current etc.

Thanks!

~Phil

[Colossal]

I'm glad to know more about the KT66's, I've not used them before. The datasheet I used may have been for a very different variant since it showed slightly higher heater current etc.

Thanks!

~Phil

Great sounding tubes, and especially in single-ended mode, IMO/E.

[pdf64]

I'm glad to know more about the KT66's, I've not used them before. The datasheet I used may have been for a very different variant since it showed slightly higher heater current etc.

It’s the current production ’tung sol’ that are the oddball; all real kt66 have higher heater current, as you noted. Rather than being a genuine attempt at producing a tube that accurately replicates vintage kt66, the tung sol is most likely just one of their regular 6L6 types in their kt88 bottle and 6550 base.

[pompeiisneaks]

I'm glad to know more about the KT66's, I've not used them before. The datasheet I used may have been for a very different variant since it showed slightly higher heater current etc.

It’s the current production ’tung sol’ that are the oddball; all real kt66 have higher heater current, as you noted. Rather than being a genuine attempt at producing a tube that accurately replicates vintage kt66, the tung sol is most likely just one of their regular 6L6 types in their kt88 bottle and 6550 base.

Oh lol that's kinda funny/odd. Yeah I think i had up some older version of the KT66.

~Phil

[pompeiisneaks]

Yeha in fact I just pulled up the current JJ ones and they do show 1.3mA which is a lot closer to the 1.4mA I saw on the other datasheet I got. So I would be sure to check that any current or NOS version will work. (They said yes, without any caveats so you're probably good).

~Phil