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Old 9th Aug 2022, 1:30 am   #21
Boulevardier
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Default Re: Quad 303 High End boards - Dada

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Originally Posted by Radio Wrangler View Post
MJ15003 looks like a very good idea. By going to a much higher voltage part, the Ft is brought down to something closer to an early 2N3055.

I’m getting a bit confused here! From previous related threads, I’d got the impression that the 2N3055 with original 0.8 Mc/s Ft was essential for the 303’s stability. We now seem to be saying that higher Ft transistors will be OK because that Ft reduces at higher voltages. I seem to remember that Dada also used to say that the rarity of original 2N3055s is a problem for the stability of the 303 output triples (that advice is no longer on their website).

So, is it right that this problem with newer 2N3055s with their higher Ft has become a non-problem because more modern transistors than the original 2N3055s will work stably?

I have been meaning to try out BDY20s in this circuit (Ft of only about 1 Mc/s and still available from Cricklewood) thinking that this might be the only way to “tame” the triples.
Mike

Last edited by Boulevardier; 9th Aug 2022 at 1:36 am.
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Old 9th Aug 2022, 5:14 am   #22
Radio Wrangler
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Default Re: Quad 303 High End boards - Dada

No, I was trying to say that modern bipolar power devices with voltage ratings in the 2N3055 league all now have too high an Ft for stability in the Quad triplets, even ones with 2N3055 written on them, unless they were made many decades ago.However, even modern high voltage transistors have to be made with processes which limit their Ft as a side effect. So although their voltage ratings may be gross overkill, the reduced Ft is what's needed to keep those triplets stable.

They're still a bit quicker than early production 2N3055, but not as much quicker as everything else seems to have become, so the spacing between the pole of the power devices and the poles of the other two devices is maintained.

It would also be possible to rewire the 303 to use straight-forward darlington output stages (maybe 3-devices in Darlington configuration). The triplets were a way of getting round the lack of good PNP power devices in the 1960s. The bias arrangement would need changing to suit, and it would no longer be a 303, but it could be made to work stably. The triplets were an attempt to simulate complementary pairs of super-transistors. We no longer need to simulate them, we have them nowadays. The risk with this approach is that the power transistor Ft may be so high that RF instability could occur due to coupling between the wires from the board to the heatsink.

Overall, I prefer the Dada approach with the high voltage transistors.

David
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