26DQ5's vs 26HU5's vs 6KN6's and transformers

[rsumperl]

So, the moment of truth has arrived and Edcor wants my decision on which xfmrs to ship. I'm waffling between the 3.3k and the 4.2k. Can anyone shed any light? If I choose to go with 26HU5's would that change anything? The plate dissipation of the 26HU5 is 33w vs 24w for the 26DQ5. Plate voltage will be between 500-550v. There is a schematic in the forum using P-P 6KN6's with 340v on the plates and it's using 2k P-P.

Thanks,
Ray

[LAJ]

Because, if you have to convert to 6 volt tubes, the 6DQ5 will be easier to find.

[davorin]

With 3.3k OT, 500-550V plate voltage, 150V(stab.) screen voltage, You can expect ~100W or more, with pair of 6DQ5 (or 26DQ5), or pair 6KN6 (or 42KN6) tubes.
Drive at G1, and tune bias with scope and generator to minimum distortion, respecting maximum anode dissipation.
You can also use under similar condition 6CB5, 6HF5, 7867, 6EX6, 6KG6(40KG6), E(P)L505(xx509, xx519), with few watts less or more
I have no experience with 26HU5 (or 6HU5) tubes.

[rsumperl]

Is there any advantage to using 4.2K? Perhaps a little less power (which is ok) but less distortion?

Thanks

[davorin]

Yes, less power, but more distortion!
For 4.2k OT, You must go to 600-650V plate voltage (P~100W) and less distortion than amp with 500-550V plate voltage.
Here You have problem with plate dissipation at zero signal (tuning bias with scope and generator to minimum distortion!). Maybe go to 125V for screen voltage in this case, if You are do not need all 100W output power.

[rsumperl]

So, it sounds like 3.3k is the way to go. Do you agree? Why did you use 2k with your amp?

[davorin]

I have main transformer with 260VCT/0,6A, 40V/0,05A and 6,3V/7A at that moment(wounded for 4xEL360-PPP), and OT 2k/4-8 ohms(wounded for 4xEL34-PPP; "Marshall"-type), finished. For DC voltage 360V(no load!), or 340V(with load), and 2x6KN6 will better work 1.8k OT(~10% more power, and ~10% less distortion!), but I have not them.
I have ~15-20 tubes 6KN6, also 25-30 tubes 42KN6, and I wish to hear sound of that strong(real maximum plate dissipation for audio is much more than 30W!), and very warm tubes (19W is only heater for 1 tube!).
3.3k OT is very good choice for 500V-550V plate voltage, maybe best!