Nice forum!! -- Question on Plate voltages / resistors

[tedm]

Great forum Guys, great job!!

I've got a question on cathode resistor and plate voltages. This weekend I went from 130 ohms to 220 ohms, and my plate voltages went from 370 VDC to about 390 VDC.

The sound is terrific, but I'm worring about tube life.

I remember a file on the Yahoo site where many folks were getting 350-360 VDC but I don't recall if resistor values were posted.

Anyways, just curious if anyone else has experimented around with 130/180/220 etc. cathode resistors. bypass cap is 100uF.

Thanks!

[Gabi]

tedm

I have a 220 ohm cathode rez (R20)... I experimented with:

125 - (with this one I was redplating),
180 - redplating was gone but my dissipation was still high
220 - dissipation within safe limits, sound - GREAT!

Note that I am using the Yugo EI tubes that don't take high cathode voltages as the JJs or other brands...

HTH,
Gabi.

[RossH]

Are you trying to bias the amp? In general terms you can just about brutalize the plate if you are nice to the screen. That being said 390V is high for a tube spec'd at 300V but I've seen a lot of amps run that hot. Some think 320V is a sweet spot although I REALLY like an amp I have running ~350V. In a general sense you can say that higher plate voltage increases amplifier headroom. But is isn't all about the plate voltage. It is also about total plate dissipation. Tell me what you are trying to do and maybe I can be of further assistance.

Ross

[tedm]

Thanks Gabi and Ross,
Yes, I've redplated a Sovtek with the 130R, but so far the JJs seem to take ~395 on the plates fine, only tried 2 prs. The tremolux EL84 that used the 6L6 iron also ran at 395VDC, different or cheaper tubes then...

Ross -- just foolin' with resistors. not trying anyting too radical here, am aware of the tone differences. I've got a SE EL84 AX84 amp with sweet distortion at 240VDC. Another stereo amp about 6 feet from me (Dyna SCA35) running a quad of EL84s that keep me nice and warm all night!!

[sws1]

By changing R20, aren't you only just changing cathode voltage? Or does that ultimately affect plate voltage as well?

[RossH]

The latter. When you go from 120 to 125 it isn't very noticable but if you go from 130 to 220 the plate voltage shift is more noticable.

Ross

[markh]

The main reason the plate voltage goes up is that the B+ is going up, due to a lighter load at a lower static bias point.

--mark