Q's about tube Rectification: GZ34

JlMMY

Member
Messages
228
I have a few GZ34 tubes. The amp has 2 x 6L6GC power tubes (Vibrolux). I monitor the power tubes with a Weber Bias Rite and notice that, when using one particular rectifier (Japan made RCA), that my output -ma fluctuates, i.e., when the amp is idling the power tube measures -42ma but when I hit a strong chord the output goes up to as much as -46ma. When I tried a used Mullard GZ34 the output tube doesn't move much from it's idle current during attack. Both tubes test very strong. I am just wondering if there is anyone who can explain which scenario is better or if either scenario are ok? The fluctuation is not very much, maybe 2-3ma with one tube and 1 ma with the other. I am guessing the tube that moves the least is not being taxed and would be the better tube? Also, I run 2 x RCA 6L6GC tubes. Is there a general "best" setting for bias? Right now I am running 2-3ma apart. One tube is 42ma and the other is 44.7ma. thanks
 

pdf64

Member
Messages
7,926
When you hit a class AB output stage with a signal, the current in the tubes goes up - that's the signal increasing the average current level. Hit it with a big enough signal and you'll maybe measure over 200mA.
You have just been measuring that your amp is working, ie it draws more current from the B+ when it's putting out a signal. The difference between the 2 rectifiers is likely due to different test conditions, ie you didn't pick the string as hard with the Mullard.
Those idle currents are on the high side.
It's a balance between tone and tube life - the cathodes will degrade with every ampere/hour under their belt. But can you honestly tell the difference in tone between 35mA and 45mA? If not, turn it down and enjoy those v expensive tubes for longer.
 

TweeDLX

Member
Messages
3,756
You want your bias at idle somewhere in the 60-70% range. With a 30 watt 6L6GC tube you want between 18-21 watts at idle.
Calculate: 19 watts/plate voltage = bias setting. Really, go with what sounds good to you. Tube life is a factor for higher bias settings.

Mike
 

donnyjaguar

Member
Messages
4,194
On first guess it sounds like the Mullard's output is sagging under load. Your measurements could however be in error.
 

JlMMY

Member
Messages
228
You want your bias at idle somewhere in the 60-70% range. With a 30 watt 6L6GC tube you want between 18-21 watts at idle.
Calculate: 19 watts/plate voltage = bias setting. Really, go with what sounds good to you. Tube life is a factor for higher bias settings.

Mike
I guess I could lower the bias a bit. Using the formula above 19/435 = 43ma

I ordered a couple more GZ34 tubes. I will test things again when the new tubes arrive.
 






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