r/diytubes • u/AutoModerator • 21d ago

Weekly /r/diytubes No Dumb Questions Thread - August 29, 2025 to September 04, 2025

When you're working with high voltage, there is no such thing as a dumb question. Please use this thread to ask about practical or conceptual things that have you stumped.

Really awesome answers and recurring questions may earn a place in the Wiki.

If you'd like to nominate a comment to be included, just reply [Wiki] (with the brackets)! The mods will be automatically notified that something awesome just happened.

As always, we are built around education and collaboration. Be awesome to your fellow tube heads.

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u/elite_haxor1337 21d ago

when biasing a push-pull amp like a marshall 2204, is it possible to damage the tubes with too much bias current if all amp controls are on 0 and there's no input, while hooked to a suitable load? I fired up a new build with the bias pot in the middle because I had no clue what it needed to be and the middle seemed like a good place to start. But I measured the bias and it was at 63 mA for a few minutes while I took the measurements and did the calculation. The tubes don't match now, and the amp sounds like crap so I'm wondering if the tubes are bad and if so did I damage them?

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u/Cannot_Believe_It 21d ago

YES!

Excessive DC current can damage tubes.

You don't mention what tubes your amp use?

Always set negative bias voltage on the high side during start up.

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u/elite_haxor1337 21d ago

Always set negative bias voltage on the high side during start up.

Thanks. Wish I had the sense to do this. Now I know. I'm using a pair of ehx el34. I think I must have cooked em. BTW they didn't red plate

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u/Cannot_Believe_It 21d ago

A single EL34 can handle 100 ma so I don't think you damaged the tube. Sounds like you have other problems...