Rocker 30 end tube choice & idle current question

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RichardR
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Joined: Fri Oct 05, 2018 3:17 pm

Rocker 30 end tube choice & idle current question

Post by RichardR » Fri Oct 05, 2018 3:56 pm

Hello,
Someone asked me to look at his Orange Rocker 30, which had poor performance and one of the end tubes (6CA7) running very hot, and sound dropping out after a minute or so.

I replaced those tubes with EL34's, but even the ones with the lowest bias rating that I have here still run at a high idle current of 85 mA per tube. Yes, I know the amp runs in class A, so I guess the idle current should indeed be quite high. But as the whole thing still runs quite hot, I wonder if the other tube (6CA7) would be a better choice (even though EL34 listed as 100% equivalent, but perhaps that only goes for normal class AB-biased amps). Any ideas on this? I don't feel like buying a rather expensive set of 6CA7, especially since class A means that the end tubes have a relatively short life.

And oh, there's another rather strange thing: I get a low but audible 100 Hz power supply hum from the speaker, even with no end tubes present. Could this mean that there's something wrong with the output transformer? I don't have a schematic, but I don't think that there should be any signal coming through the transformer without the end tubes, and I don't see anything else hooked up to the primary side that could cause this (e.g. diodes).

Anyway, thanks in advance for any ideas here.

Jondog
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Joined: Thu Aug 21, 2008 12:16 pm
Location: Ontario, Canada

Re: Rocker 30 end tube choice & idle current question

Post by Jondog » Sat Oct 06, 2018 8:59 pm

6CA7’s came stock in some Rocker 30’s. Sound great. EL34’s should work the same. I’d check the values on the screen and bias resistors. Hum without the power tubes installed seems odd.
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