AB763 WITH KT66 OT

[martin manning]

My impression is that that up to the 5Fx Tweeds, various loads / operating points were used by Fender for a 6L6 p-p pair. From the 5Fx series onwards, they seemed to move to the long term standard of 4k at 450V +/-about 30V, so I guess it's the 5F6a Bassman being referred to above :)

5881 implies 5x6 and 6x6 tweed Bassman ;^)

[ViperDoc]

I read those last few posts and then went and bought a set of steak knives. Now where did I put those damned potatoes....?

[pdf64]

...KT66 or EL34 ... 25W Pd max...

Dunno how relevant the following is to modern production tubes, but it should be noted that the EL34's 25W plate limit is under the design centre system, so it can be used directly, 'as is'.
Whereas the KT66's 25W plate limit is a 'design max', so is intended to be derated before being used by the equipment designer in such calculations.
To put them on an equal footing and make it an 'apples to apples' comparison, using the typical design centre/max derating available for the 6L6 and 6V6, the design centre plate limit of the KT66 is probably around 21-22W.
For more info on the design centre / design max thing, see http://www.one-electron.com/Archives/RC ... 0Tubes.pdf

...Can I take the "pentode operation" on a KT66 datasheet to simply mean "not triode", being as it is a KINKLESS TETRODE? ...

IIRC all the power tubes we use have been / were / are described by their manufacturer as pentodes. The difference being whether they're a 'beam pentode' or a 'pentode' (ie with a suppressor grid).
My take on the kinkless tetrode description is that it was just marketing BS, hoping to get those 'Johnny come lately' types some perceived advantage in the high end audio market.
KT66 seem to have been initially described in the 30s as 'output tetrodes', then in the 50s as 'beam pentodes', then in the 60s GEC decided they should be called 'beam tetrodes'.
https://tubedata.altanatubes.com.br/she ... k/KT66.pdf
https://tubedata.altanatubes.com.br/she ... k/KT66.pdf
https://tubedata.altanatubes.com.br/she ... 66_GEC.pdf
So my view is that is a nonsense for us to make a distinction between pentode and tetrode output tubes and their operation mode; it's perfectly correctly to describe them generically as being pentodes (qualified as being beam type or not if perceived to be necessary) and their operation modes as being pentode, distributed load / UL, triode or whatever, depending on how their g2 is used in circuit. ie there's no necessity to use the word 'tetrode'.

[martin manning]

My take on the kinkless tetrode description is that it was just marketing BS...

Alternate names like KT might have been used to avoid legal action by Philips, who held a patent on a "pentode."

[ViperDoc]

This is all amazing info, much appreciated, guys.

Would it be offensive if I went straight for dessert and asked which OT primary resistance you would choose for such an amp?

[didit]

6600 Ohms. Because ..

Best .. Ian

[martin manning]

Would it be offensive if I went straight for dessert and asked which OT primary resistance you would choose for such an amp? :mrgreen:

What is your goal here? A Deluxe Reverb with a KT66 output section? Are you planning on upgrading the power supply too?

[ViperDoc]

Yes, Martin, exactly that. And yes, the power supply would need to be adequate. I'm both interested in the math and logic behind it as well as trying to build one like that when I have it all figured out. I'm not there yet. I'm still waiting for parts (arriving tomorrow) to hopefully button up both PR builds I've got on my bench currently with your improved bias circuit. I've been reading lots of Blencoe's website material, very deep stuff.

I generally want to bridge the gap between "soldering-by-numbers" and proper design, obtaining proper voltages, output dissipation, bias, all of that. I would specifically like to know how and why someone would build a higher headroom AB763 amp without heading straight for the 50 watt realm. My only experience with KT66 is a JTM45 head I built with an OT intended for EL34s. It sounds great anyway. It is incredibly loud! Great tone. It's one of my first kit builds, but every kit/layout build has shown me how wrong they can potentially be, the Princeton Reverb "Zeus Bias" of late being a prime example.

The KT66 AB763 I saw the clips of I believe was billed at being around 35 watts, I just don't know exactly how and why that is. We've discussed lower relative screen voltages vs. power output here. I don't know if he used a 40-watt style Fender PT/OT/choke or a JTM45 version on that build. I'm sure there's lots to it. I'm just sifting through questions/thoughts while I wait.