DIY 2024 (4010) mismatch bias

[jasonvilla]

Hi there, I cant get my tubes to bias the same. I've tried two different sets of match tubes with the same results. Ive read in a few places that a difference of 5ma is generally acceptable, but what causes this and how do i "fix" it? Or is it fine as is?
On EL34 1 I get 447 plate voltage and 35.5ma. On EL34 2 i get 449 plate voltage and only 30ma
So one tube is running about 65% and other one is at around 55%!
Pin 5 on both tubes are both seeing -38vdc. Pins 3,4, and 6 are both seeing 449, 444, 449, respectively.
Thanks

[sluckey]

I'd call it good.

[jasonvilla]

I'd call it good.

Thank you!  But just curious what causes the mismatch? I cant seem to find any causes online. Expect for OT mismatch

[sluckey]

The most probable cause is the tubes themselves. You can install a bias balance circuit or a dual bias circuit if you want to bias them both exactly the same.

[jasonvilla]

Alrighty, thank you Sluckey.

[Willabe]

Every tube will have a different output. Just the nature of vacuum tubes.

The better the tube is made the less difference there will probably be, but still they will all have a different output.

Part of it is the alignment of the parts inside, heater, cathode, G1, G2, G3/beam plates.

Groove tubes used this as a sale point. You could buy tubes from them rated from 1 - 10. They said once you bought a set from them, say 6L6GC with a #6 rating, then after you set the amps bias with that new set, any set after, if they were a #6, you could just pop them in and not check/reset the bias. All they were doing was testing the tubes for output current and matching them up into sets of cooler -to- hotter running tubes. I'm not sure you could actually get sets that were the outside ex streams? Like a #1 or #2, or a #9 or #10.

I guess it would have depended on the run/batch of tubes they had just bought, how close the tubes were to each other in current output and what they deemed a matched set, within -X- mA? 

[jasonvilla]

Yea but Im having the same result with 3 different sets of matched tubes... which leads me to believe its not the tubes.

[sluckey]

Yea but Im having the same result with 3 different sets of matched tubes... which leads me to believe its not the tubes.

You're talking about a 5.5mA difference. How are you measuring the current?

[jasonvilla]

Im using a Eurotubes bias meter.  Ok I did some testing and I'm not sure what to think of it. This time with JJ6v6s - which is the whole reason I stumbled upon this weird bias thing.

Front socket, Tube A    470v /19.1ma - after 10 minutes 470v / 18.2ma
Back socket, Tube B    468v / 19.8ma - after 10 minutes 470v / 20.2ma

Front socket, Tube B   468v / 20.9ma  - after 10 minutes 470v/ 20.1ma
Back socket, Tube A   469v / 19.1ma  - after 10 minutes 471v /18.5ma

[Willabe]

Those numbers are great. Very tightly matched tubes.

(Well, other than the B+ is really high for 6V6's.)

[jasonvilla]

Those numbers are great. Very tightly matched tubes.

(Well, other than the B+ is really high for 6V6's.)

Thank you Willabe, It seems to be working well now. This is a DIY JCM800 that I wanted to use 6v6s with. At that time I had a 120R resistor from CT to ground but yesterday installed a 25w 150R resistor in its place. That brought things down another 10v. Im using JJ6v6s so hopefully it wont be a problem. The only other thing I did was change the 220k grid leak to 100k. Sounds pretty good and the resistor doesn't even get warm. My reverb doesnt sounds quite as nice since dropping the voltage, but it still works fine.