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Re: Sweet!- some answers

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In Reply to: Sweet! posted by Paul Joppa on June 21, 2004 at 10:48:35:

Paul,
Actually the PGP8 original was 5@2A and then on the 8.1 these where upped in wire size (or down in gauge however you want to say it) so actually it is higher than 2A. That is why the 2.5V's are always 2.75Vac @ 2.5A's.
Also remember much of a power transformers specifications fall on the total power rating. In this configurations max on each winding is 6@2 (actually PG8 was 2.5A), 5@2 and HV at 125ma ac. So we are well below the design area and I think the 8.1 in this usage will be loafing.
I was always taught that ultalinear in SET was a poor choice. Since it is not balanced it can change the effectiviness of the output transformer because of the DC draw and the change in DC draw depending on output power. Triode wired pentodes to me is a total waste of time, might as well just use a triode.
I think this is a great starter amp for anyone looking for a great little project.
Thanks
Gordon
J. Gordon Rankin

Follow Ups:

Re: Sweet!- some answers - Paul Joppa 16:08:37 06/22/04 (0)
Thanks for the feedback.
Mike has posted on the PGP; looks like however you cut it there is plenty of filament power.
I agree about triode-wired pentodes, if you are determined to do a triode. Your info on the sonics of ultralinear vs pentode is a valuable addition, I have not heard that before. I guess I have a soft spot in my heart (or head?!) for an ultralinear EL-84 because it was the first amp I designed and built. I loved it, but it's been a long time - over forty years ago; I was probably 15 or 16. I certainly have not done any comparative listening since then, at least any controlled comparison. I may have to build this amp just to hear the differences.
Yeah - but SE UL really kicks ass in a guitar amp! (nt) - Jim Dowdy 22:11:06 06/21/04 (2)
nt
- In tests only 1 out of 10 ejoyed it - Gordon Rankin 09:59:06 06/22/04 (1)
  Jim,
  If you really like distorted then that may be true. But most people who tried it when I did it, didn't care for it.
  Gordon
  J. Gordon Rankin
  - Well, there were only two of us... - Jim Dowdy 14:23:14 06/22/04 (0)
    but, every time we flipped the switch from UL to triode mode, we just looked at each other and said, NAH!!!
    :-)
    Output tube was an Eimac 4-125A tetrode.
Re: Sweet!- some answers - GSI 16:09:10 06/21/04 (0)
What is output impedance (or damping factor) for this pentode output stage with 5k trans on 8 ohm tap?? With only 1-3dB NFB I think output impedance is very high. Internal impedance for EL84 pentode tube is about 38 kohm. This transformed (lowered) 625 times in output trans and we have about 60 ohms output impedance without NFB. So in the end we need at least 20dB of negative feedback to reach even 6 ohm output impedance. I'm calculating right or..?

Girius

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