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As I was sorting through my tubes and tryng to get the best matched set for my new sIII, I hit on something that confuses me.

I have a BBQ bias rite which I decided to use to try to match EL84s in my new amp. I had already tested them for transconductance.

I tried a set of "matched" JJs and found the current to be about 10 ma apart. I had 6 other RCA's and I tried to match them up. Most pairs I measured would give me a reading about 10 ma apart. If I took the lowest reading tube of two given pairs and put them in the amp together, I still got about 10 ma separation, even though both of them had read 35 ma when matched with the other tubes.

I have one pair that almost exactly matched but I couldn't get any of the others to get closer than about 10 ma.

What's up with that? I just doesn't seem logical to me. Thanks.

Does the Bias Rite test them in place in the amp? I've never used that tool.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Yes, it has two 9 pin sockets that are switchable and it reads plate voltage and current.

https://weberspeakerscom.secure.powweb. ... r_page.htm

Need to think about this a bit..

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Yeah. I thought about it until my head hur and still couldn't come up with an answer. I kept asking myself, "what's wrong with this picture?"

Maybe BBQ has an answer?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I think I'll post it on 18 watt just to see if I can get some ideas.

I should have known better.

All I got was emotional pontifications about whether tube matching was needed or appropriate for guitar amps.

I guess I was kidding myself to think I might get a well considered technical answer to my real question.

LOL,

So, since the tubes share a common cathode and screen resistor, the other resistors in the chain must be causing them to draw different current. These are the two 8K2 (R31 & R31) to grid each EL84 grid and the two that join them and go to ground - 470K (R18 & R19). They ar eall +/- 5% tolerances. But if one is hi 5% and another is low 5% , that's a10% spread. That would cause some current differences.

You can check them if you want.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I checked the 8200s but didn't check the 470s. I'll check both of them more closely when I get in the mood. Right now I'm enjoying the amp and it's working well. I'm just trying to understand what the deal was.

Thanks. I'll let you know if I figure it out.