Are Hiwatts tough on power tubes?

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Are Hiwatts tough on power tubes?

Postby Rockin_Dachshund on Thu Jun 08, 2017 1:57 pm

I've noticed that I've needed to change power tubes more often with my CP-504 head than with other amps I own. The plate voltage in this amp is about 500V. I assume this contributes to the great clean headroom. Does it also mean that the power tubes will not live as long?

I've been using JJ power tubes in the CP-504 because my understanding is that they can handle high plate voltages. Currently I have a set of JJ KT77 tubes in there. I have also used EL34 and 6CA7 tubes in the past.
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Re: Are Hiwatts tough on power tubes?

Postby Gooch on Thu Jun 08, 2017 3:01 pm

500V is fairly high, but not as bad as several other amps. Make sure you have at least a 1K resistor on the power tube screens, that will help.

Also, check if you heater voltage is much higher then 6.3V. If it is, you'll need to take measures to bring down the mains voltage that the amp sees.
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Re: Are Hiwatts tough on power tubes?

Postby pdf64 on Thu Jun 08, 2017 4:49 pm

The hotter their plate dissipation at idle, the quicker they are likely to wear out / fail.
When installing them, and periodically thereafter, are you checking their cathode current at idle?
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Re: Are Hiwatts tough on power tubes?

Postby Rockin_Dachshund on Thu Jun 08, 2017 5:07 pm

Thanks -- definitely has the 1K resistors...

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Re: Are Hiwatts tough on power tubes?

Postby Rockin_Dachshund on Thu Jun 08, 2017 5:13 pm

pdf64 wrote:The hotter their plate dissipation at idle, the quicker they are likely to wear out / fail.
When installing them, and periodically thereafter, are you checking their cathode current at idle?


I bias the tubes at 70% of the maximum plate dissipation, but I don't check to see if it drifts over time. Two much of a pain to keep removing the chassis!
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Re: Are Hiwatts tough on power tubes?

Postby pdf64 on Thu Jun 08, 2017 10:42 pm

The HT on Hiwatts tends to be pretty stiff, at 500V I think the idle dissipation may need to be cooler than that for reasonable tube life.
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Re: Are Hiwatts tough on power tubes?

Postby OldSchoolDave on Fri Jun 09, 2017 2:01 pm

It's been discussed in the past here that, when HIWATTs were originally produced, AC power at the wall was in the 110-115/220-230V range and the Mullard EL34's grabbed off the shelf were both consistent and stout. Those things have changed.

What's the voltage at your outlet? On my more sensitive vintage amps, I run a power conditioner to limit the mains to 110VAC. Barring conditioning, the options are either to modify the circuit to lower voltages or accept a shorter power tube lifespan.

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Re: Are Hiwatts tough on power tubes?

Postby pdf64 on Fri Jun 09, 2017 3:47 pm

The heater voltage may be used to indicate whether the mains voltage is appropriate.

The UK mains voltage has kept fairly consistent at ~240V over the decades, even though the nominal has dropped to 230V since EU harmonisation.

The issue with EL34 based amps I was alluding to is that Marshall, Hiwatt et al tended to use a p-p load suggested for a 400V HT, but wacked the HT up to ~500V.
With Marshall this was mitigated somewhat by their saggy HT (thereby reducing the effective, loaded HT) but Hiwatt HTs tended to be stiffer.
Hence plate dissipations under signal can go way over their limit.
See Aiken http://www.aikenamps.com/index.php/idle-current-biasing-why-70-percent
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