Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on February 04, 2021, 10:01:37 am
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A friend just got a Victory Copper amp which uses a combination of ‘Cathode’ and ‘Fixed’ biasing. In the description included in the manual it says, "... Many early EL84 design guitar amplifiers exceeded the recommended plate dissipation and this can lead to early valve failure and the output stages running extremely hot, even when they are not being played."
When I look at the data sheet for an EL84 it says the VaMax = 300V and Vg2Max=300V. My Vox AC15 has more than 330V on both of those. Is this what the manual is referring to?
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Look at current and plate dissipation.
Yes, everybody works EL84 hard. Hey, they are cheap.
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I never see a amp builder use the recommended voltage. They are all way over.
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The Tung-Sol 7189 (EL84 equivalent) reissue is rated at 400V max for the plates. I have no experience with them, but they're over $30 a piece new.
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... in the manual it says, "... Many early EL84 design guitar amplifiers exceeded the recommended [/font]plate dissipation ... My Vox AC15 has more than 330V on both of those. Is this what the manual is referring to?
Early AC30s had an 80Ω shared cathode resistor. By 1963, they dropped that to a shared 50Ω, which in practice was often a shared 47Ω resistor.
Plate current & idle dissipation went up, generally over the ratings. The the output tubes were easier to drive and some folks really prefer the sound of the EL84s being worked hard.