Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on March 06, 2020, 01:16:46 pm

Title: Stress on power tubes question
Post by: punkykatt on March 06, 2020, 01:16:46 pm

example:  a pair of 7591A power tubes cathode biased Pv - Kv = 361vdc Pv x .054mA Ik = 19.5w. OPT turns ratio =  6k/8 ohm.  Amp owner wants to run a speaker combination load of 5.333 ohms.  Tube data sheet says for cathode bias load R plate to plate should be 9k @ 450vdc, For fix bias its only 6.6k for all other voltages listed.   Will the power tubes be stressed running a 5.333 load? Sometimes this stuff can get confusing.  Any help is much appreciated.
Thank you in advance:  Punky

Title: Re: Stress on power tubes question
Post by: sluckey on March 06, 2020, 01:27:45 pm

Well you certainly provided enough red herring info to confuse anyone.   :icon_biggrin:

Your 6K:8 OT has an impedance ratio of 750:1, so 4K:5.333. I'm OK with that.

Title: Re: Stress on power tubes question
Post by: pdf64 on March 06, 2020, 01:45:52 pm

I think that cathode bias will inherently tend to self limit plate dissipation in this scenario.

Title: Re: Stress on power tubes question
Post by: shooter on March 06, 2020, 03:02:52 pm

+1
your example math in the 1st sentence IS the dissipation.  when I use wonky impedance I re-do the Pdiss math with the mis-match load and verify it's <100% pdiss.

where you notice is sound, or tone which may be better or worse on a individual basis  :icon_biggrin:

Title: Re: Stress on power tubes question
Post by: punkykatt on March 06, 2020, 03:54:33 pm

Shooter,  "when I use wonky impedance I re-do the Pdiss math with the mis-match load and verify it's <100% pdiss."


What formula do you use to calculate the Pdiss for the mis-match load?   The OPT with the new load is 4k/5.333 as Sluckey pointed out.  Thanks

Title: Re: Stress on power tubes question
Post by: shooter on March 06, 2020, 04:43:51 pm

I don't worry about the "load" on the tube, it "shows up" in your Ik.  a mis-match will typically make the tube conduct more, so
Vk/Rk will change, use new I then Vp-Vk * Ik ~~~~ Pdiss

NOTE: I try staying +/- 0-50% of data sheet  :icon_biggrin:

Title: Re: Stress on power tubes question
Post by: punkykatt on March 06, 2020, 07:05:10 pm

Thanks shooter,  and all that replied. New power tubes on order, will pop those in and see if I need to play with Rk to cool down a bit.


Thanks again everyone.