Questions about replacing power tubes in HT Stage 100 (older model)

Discussion - HT Venue amplifiers. Inspiration from Studio to Stage.
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Posts: 2
Joined: Mon Mar 29, 2021 12:24 am

Thu Apr 08, 2021 10:30 pm

This is the first time I have ever owned a tube amp. I looked, and there's no amp techs anywhere near me (over 100 miles away) and my only vehicle is a motorcycle. Needless to say, I'm not hauling this thing on the back of the bike, taking 2 separate 200-mile round trips to drop off and pick up the head.

I do realize biasing and messing around in there with the amp live can be quite dangerous if you're not careful.

I liked the sound that it had, so I am replacing the tubes with the exact same kind: Ruby EL34BHT matched quad power tubes.

My first question is this:
Given that the tubes are the same brand and model, do I need to re-bias the power tubes, or should the existing bias adjustment be pretty close?

Assuming one or more power tubes is going bad, can I just measure the existing bias and set the bias to being the same when replacing them with new tubes, or will a bad or going bad tube throw off the mV readings? I considered measuring the bias as it exists now to ensure it is the same after so that I don't need to do measuring and math as per my third question.

I have read this post: ... 093#p24420 and I do understand this procedure, with the exception of measuring the plate voltage (no idea how to do that). Is this really necessary to go that far into it with new tubes if they're the exact same brand and model tubes?

Posts: 2
Joined: Mon Mar 29, 2021 12:24 am

Sat Apr 10, 2021 2:25 am

Well, I decided to go ahead and do it myself. Replaced the two ECC83 preamps, hooked it all up, gave it 5 minutes to warm up and measured the mV bias on the existing power tubes: 100.2 mV at the D27 resistor. Switched her off, and un-plugged, waited 30 min for her to cool down and dissipate any residual electricity (not sure if that's long enough, I'm no electrician or amp tech) before flipping it back over. Replaced the tubes, 5 minute warmup, re-checked the bias: 173.5 mV, yowza! Set bias back to around 100 mV.

So I guess that answered one question I had: when replacing tubes, even if they're the exact same brand and model, no, the bias will not necessarily be close.

And the mV when I measured remained pretty steady so, I am assuming even with the old tubes going bad, that the mV measured was still accurate. Mind you, the tubes hadn't totally gone bad, they just started relatively recently popping/crackling and having inconsistent tone/volume.

But, she's all done now, didn't blow anything up or hurt myself. Just waiting on it to cool back down and hopefully any residual electricity to dissipate (giving it 30 minutes) before I flip it back over and put it back in the box.

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