Hello
Can i have some assistance validating my calculations for a PP quad KT88. I couldn't locate any suitable curves for KT88 with Vgs at 330 V. I ended using a copy of a copy of some unverified KT90 curves.
Of course if ive made an error somewhere please let me know too.
Can i have some assistance validating my calculations for a PP quad KT88. I couldn't locate any suitable curves for KT88 with Vgs at 330 V. I ended using a copy of a copy of some unverified KT90 curves.
Of course if ive made an error somewhere please let me know too.
I'm trying to do the same thing you are, only with a quad of 6l6gc's per channel, so while I cannot comment on the curves used, I'll comment on the method as I am learning this now also.
1. Are you planning on using a 4200 ohm OPT for the quad KT88's? For a quad you would probably want an OPT of 2100? Normally this would work to a 4200 load line, am I correct in this thinking?
2. For class A, don't you use the intersection of the first load line with the second one to calculate the max swing of Class A; in this case the intersection point would be at 220 volts, also see Fig.1 here:
loadmatch4-pp-beamtetrodes
3. After looking at both curves, it seems one is drawn one way and the other differently?
1. Are you planning on using a 4200 ohm OPT for the quad KT88's? For a quad you would probably want an OPT of 2100? Normally this would work to a 4200 load line, am I correct in this thinking?
2. For class A, don't you use the intersection of the first load line with the second one to calculate the max swing of Class A; in this case the intersection point would be at 220 volts, also see Fig.1 here:
loadmatch4-pp-beamtetrodes
3. After looking at both curves, it seems one is drawn one way and the other differently?
Last edited:
So using a quad (p-pp) AB UL stage of 450V with fixed bias (Kt88 or 6550) each tube biassed to roughly 75mA quies; (135mA full load), one should get 110W o/p with a 2K A-A 43% UL tap. Vg = approx -45V; varies between tubes.
This is exactly what I get. Many consider this as hard driven.The quies current could be reduced, but with some o/p trannies the thd will rise at the higher audio frequencies.
With the identical setup. and going further down with a low B+ to 260V;each tube biassed at 95mA quies; 110mA full load with same 2K A-A o/p tranny one gets 25W with Vg approx -15V. I consider this AB nearish class A performance. The rest depends on the power supply to deliver the current swings.This is a long life setup for modest power.
Interpreting the graphs is an approximate business: doing ones own charts will display considerable fluctuations.
Well known in the business,The JJ KT88 has a different bias point compared to standard 88's and 6550's.
In general, using an output tranny with a higher A-A impedance for a specific set of tubes will result in lower power throughput. With some the distortion varies.
This is exactly what I get. Many consider this as hard driven.The quies current could be reduced, but with some o/p trannies the thd will rise at the higher audio frequencies.
With the identical setup. and going further down with a low B+ to 260V;each tube biassed at 95mA quies; 110mA full load with same 2K A-A o/p tranny one gets 25W with Vg approx -15V. I consider this AB nearish class A performance. The rest depends on the power supply to deliver the current swings.This is a long life setup for modest power.
Interpreting the graphs is an approximate business: doing ones own charts will display considerable fluctuations.
Well known in the business,The JJ KT88 has a different bias point compared to standard 88's and 6550's.
In general, using an output tranny with a higher A-A impedance for a specific set of tubes will result in lower power throughput. With some the distortion varies.
Hello Rich Walters,
I have approx. 510V on the plates of a pair KT88 EH (Electro Harmonix) . The tubes just received together with a TVA-1 amplifier. Can I replace the KT88 with Tungsol 6550 reissue ( or TAD 6550C) , say with 520V on the plates and 50mA? - or is 500V the limit for 6550? The old "real" Tungsol 6550a was not designed for 500V plus??
Best regards
Kim
Rich,
I remember your preference for lower B+ and higher Iq for the same Watt rating from an earlier post, could you explain what is it that makes the tube last longer with such kind of biasing?
Cheers Michael
I remember your preference for lower B+ and higher Iq for the same Watt rating from an earlier post, could you explain what is it that makes the tube last longer with such kind of biasing?
Cheers Michael
Thanks for your detailed response.
The OPT options i have are limiting me. Ideally id like as close to 50 Watts Class A, i'm not too concerned about much more AB watts than that. My OPT options are: 2K1, 2K8, 3K3, 8K4. I may have incorrectly worked on the assumption of more class A from higher Rlaa.
The PSU voltages i was being lazy and trying to fit into an existing design.
Tom i have found that helps thanks. I'm familar with the site. I'm trying to build Patricks 8585 design with his available OPT OP1 or OP2.
for-sale-2-output-transformers
The OPT options i have are limiting me. Ideally id like as close to 50 Watts Class A, i'm not too concerned about much more AB watts than that. My OPT options are: 2K1, 2K8, 3K3, 8K4. I may have incorrectly worked on the assumption of more class A from higher Rlaa.
The PSU voltages i was being lazy and trying to fit into an existing design.
Tom i have found that helps thanks. I'm familar with the site. I'm trying to build Patricks 8585 design with his available OPT OP1 or OP2.
for-sale-2-output-transformers
Attachments
- Status
- Not open for further replies.