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I recently bought one of those gizmos to bias the tubes in my amps. I was able to bias one of my amps, but on my JCM 900, I had a little problem.

The reading was at 16mA, so I turned the pot to get a 35mA on my multimeter, but the pot reached the maximum turning point, yet I was only at 20mA. Is there anything I'm missing?

Amp specs:
JCM 900 MKIII 50 watts
EL34 tubes

Thanks!

Sounds like maybe a "lazy" tube with low emission.

When you swap tubes, turn that bias control all the way down before firing her up

Cheers!

I didn't know anything about low emission tubes. The original tubes I had on there was a Tesla EL34, but i replaced it with a matched pair of Groove Tubes 6CA7, which the amp tech said is compatible with the EL34s. Could that be a problem?

Yes, 6CA7 == EL34

Groove Tubes is a relabler of Shuguang (mostly) and aren't looked highly upon by too many in the musician communities I hang out at. Haven't tried them myself. TAD's I have and never will again.

I recommend JJ - never had a problem with biasing them.

Cheers!

Could be the Groove Tubes.

First off, Shuguangs QC is piss poor and GT isn't set up well to ferret out bad tubes these days.

Second, GT has their "hardness rating" which really just reflects how cool or hot the tube runs at a specific B+ voltage in a specific fixed bias circuit. This means trying to get 60% or 70% dissipation from a tube with a high or low "hardness number" may require significant compensation on a bias pot.

A real 6CA7 was a North American, power-beam equivalent to the EL34 by Sylvania and GE, it's like a 6550 with bigger-bodied tone. The GT 6CA7 was the first repro of this tube (Sovtek now markets another under the Electro Harmonix name). Everyone else just cross lists their normal EL34 as a 6CA7.

So it turned out those Groove Tubes were the problem. I put in a different set of tubes (Svetlanas), and it biased in and still had headroom. Gonna return those tubes back. Thanks for all you guys' help!

Marshall put several different values of resistors in series with the bias trim pot. A 56k in R28 is common and will not allow you the bias range needed for an EL34 but is fine for a 6L6. Some schematics show R28 to be 47k. I just had a JCM 900 with EL34's in the shop with this exact problem. With a 56k resistor bias range was 12 to 28 ma. When changed to 47 k range was is 22 to 48 ma (p5 -39 to -50vdc) allowing you to set bias at desired point of approx 35 ma. If you hunt around on the web you can find references to this part change.