• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Tube Matching Question

Walter R

Walter R

Active Member
I just bought a tube socket adapter to measure the zero signal plate current in the two 6L6s in push-pull operation in each of my two HF-20 amplifiers. The venerable RCA tube manual says the typical zero signal plate current is 116mA for a Class AB1 amplifier in push-pull operation at 450v plate voltage (The schematic says 440v). What are the criteria for deciding that the two tubes are adequately matched? Not that I’m experiencing noticeable distortion, but I’m a curious guy. The tester is coming from China, so it will be a while before I test them.
 
Register to hide this ad
Two effects, first is asymmetry if the tubes conduct unequally (at power levels above continuous conduction) though this depends on matching transconductance more than matching current at idle. Second is saturation of the output transformer which occurs at a lower power level with some DC imbalance (bass notes only). Both will create even harmonic distortion, first will be greatly reduced by the amp's negative feedback. Second effect will affect max output at some bass frequency; feedback can't help. Matching of 10% may be enough, 5% is better (you have two pair, could possibly pair the closest ones).
 
Thanks for the information, I’ll pull out my TV-7 tube tester and compare the transconductances. But I understand that tube testers are generally limited to about 250v plate voltage, a different operating point than the 440v in the circuit. Hence I’ll compare the in-circuit plate currents too.
 
IF you had a 6L6 running at 116ma and 450v plate current it would melt down. That's over 50 watts static dissipation and the 6L6 is rated at 30w max.

38ma current at idle would be more like it. As for matching, I call them matched if the idle current between tubes is 5ma or less.
 
Walter R said:
I just bought a tube socket adapter to measure the zero signal plate current in the two 6L6s in push-pull operation in each of my two HF-20 amplifiers. The venerable RCA tube manual says the typical zero signal plate current is 116mA for a Class AB1 amplifier in push-pull operation at 450v plate voltage (The schematic says 440v). What are the criteria for deciding that the two tubes are adequately matched? Not that I’m experiencing noticeable distortion, but I’m a curious guy. The tester is coming from China, so it will be a while before I test them.
Click to expand...
That is for TWO 6l6gc tubes not a single tube. not all 6l6 tubes are 6l6gc.
 
Look in the RCA tube manual for the 6L6GC in Class AB operation. The zero signal plate current is 116ma at 450v. The maximum signal plate current is 210ma. The circuit plate voltage is 440v.
 
You must log in or register to reply here.
Back
Top Bottom