bias readings with JJE34Ls and KT77s from weber bias rite

[rabies]

The readings are for one pair of tubes at a time. What is the problem (if any) with "unbalanced" pairs as below?

Rivera S120 bias rite readings:

pentode, full power, E34Ls

plate voltage: 435-438
current draw: 41.7, 37.6

/*******************************/

pentode, low power, E34Ls

plate voltage: 218-220
current draw: 33.0, 30.6

/*******************************/

triode, full power, E34Ls

plate voltage:
current draw: 40.5, 36.8

/*******************************/

pentode, full power, KT77s

plate voltage: 434-437
current draw: 37.4, 33.5

 

[boogietone]

My Fryette 50/12 biased with a Weber Bias Rite:
Total average plate voltage 460 (Tubes were within a couple volts of each other)
25 divided by 460=54.3 (JJ EL34L is a 25 watt tube)
54.3 X .65 was 35.3
Hope this helps. My amp was at full power with SS rectification (switchable to tube rect).
30 min. warm up.
Was originally set at 83% when I first got the amp. Biasing it down to 65% made it sound
soooo much better!

 

[fishyfishfish]

None. Most "Matched" tubes are around 5 mA apart anyway, so everything looks good.
When they are to far apart I always got a real low hum, so as long as there is no noise from the power section you should be good.

 

[moondawg]

None. Most "Matched" tubes are around 5 mA apart anyway, so everything looks good.
When they are to far apart I always got a real low hum, so as long as there is no noise from the power section you should be good.

+1

 

[94Tremoverb]

I would not personally consider those tubes 'matched', because the difference is more than 10% on some of the settings (going from the low one to the high one anyway), and even 10% is a fairly liberal tolerance on matching. They should be better than that. But, there may be an inbuilt error in the readings because they are with a Bias Rite and therefore include the screen current, which can vary a little as well even for tubes with a consistent plate current, so that could be the source of at least some of the mismatch.

But in practical terms neither factor makes much difference. The worst that will happen is a little more hum. They may sound a little different from a matched pair too, but how different and whether that's a bad or a good thing is subjective. Just remember when biasing them to set it for the hotter of the two not the colder or the average - the reason for this is that in a normal Class AB amp there is a large amount of crossover between the two halves of the waveform, so the hotter tube will easily mask the cooler one in this zone and you won't get crossover distortion; but you don't want the hotter one failing when you crank it.

It's also better, if you can, to put the cooler tube in the position that gets the signal from the directly-driven (signal input) side of the phase inverter, not from the secondary grounded-grid side. On some amps - vintage Marshalls are a good example - the tube on this side takes quite a lot more stress than the other one once the amp goes into overdrive, because it's the phase inverter that clips first and this limits the drive to the second side, whereas the first side is still being driven by the full input signal; if this sounds too technical, look at the tubes when the amp is cranked, and you'll usually see the screen glowing a lot brighter in the tube on the first side, which is the one closest to the preamp tubes in Marshalls (although this may not be true for all amps, it depends on the internal layout).

 

[moondawg]

Also... one has to consider that the two circuits around the tube socket are not perfectly equal.
For example if you were to swap tubes in the sockets you may see them drift more apart or come closer together.