Post by bananimal on Feb 8, 2007 21:52:59 GMT -7
I'm trying to figure out whether my testing equipment is mis-calibrated or my tubes are mis-matched. Or neither or both. Hopefully somebody here can help.
This situation arose this evening when measuring the bias of a recently acquired used KT-45 using a recently acquired used Weber Bias Rite. Measurement was ~27mA / ~32mA. Off by quite a bit and at each edge of the suggested range. I planned on changing these tubes anyway, so out they came.
I had on hand a matched (according to the seller) quad of SED EL34, the ones with the =C= on them. I put maybe 2 hours on them back when I briefly owned a 100W Marshall over 2 years ago. I figured I could put any 2 of these tubes together and get a good match...but that's not what my meter indicates.
I'll refer to these 4 tubes as A, B, C, and D. I went ahead and took measurements with different combinations without adjusting the bias trim pot. I started with A / B and C / D:
A=29.0 / B=31.6
C=29.2 / D=30.8
At this point I thought, "Maybe I bought 2 matched duets and not a matched quad...it WAS a long time ago." So I tried A / C since they were very close, figuring they were a matched duet:
A=28.4 / C=30.2
This was perplexing because I expected to get 29.0/29.2 which made me think, "Maybe my meter is not calibrated properly."
Realizing just how little I knew about electricity, I shrugged, tried B / D, saw that they were very close and decided to go with that combination. I dialed the trim pot until the meter read 31.0/31.3 and called it a night.
So my questions are:
1) Assuming the first 2 measurements are accurate, is that 3rd measurement even possible? In other words, do the properties of the left tube effect the properties of the right tube?
2) Could the 2 tube adapters on my Bias Rite be mis-calibrated? How could I measure this with common household items?
3) How close in terms of % difference is an acceptable match? Or put differently, how close are vendor-matched sets, typically?
Any insight is appreciated.
This situation arose this evening when measuring the bias of a recently acquired used KT-45 using a recently acquired used Weber Bias Rite. Measurement was ~27mA / ~32mA. Off by quite a bit and at each edge of the suggested range. I planned on changing these tubes anyway, so out they came.
I had on hand a matched (according to the seller) quad of SED EL34, the ones with the =C= on them. I put maybe 2 hours on them back when I briefly owned a 100W Marshall over 2 years ago. I figured I could put any 2 of these tubes together and get a good match...but that's not what my meter indicates.
I'll refer to these 4 tubes as A, B, C, and D. I went ahead and took measurements with different combinations without adjusting the bias trim pot. I started with A / B and C / D:
A=29.0 / B=31.6
C=29.2 / D=30.8
At this point I thought, "Maybe I bought 2 matched duets and not a matched quad...it WAS a long time ago." So I tried A / C since they were very close, figuring they were a matched duet:
A=28.4 / C=30.2
This was perplexing because I expected to get 29.0/29.2 which made me think, "Maybe my meter is not calibrated properly."
Realizing just how little I knew about electricity, I shrugged, tried B / D, saw that they were very close and decided to go with that combination. I dialed the trim pot until the meter read 31.0/31.3 and called it a night.
So my questions are:
1) Assuming the first 2 measurements are accurate, is that 3rd measurement even possible? In other words, do the properties of the left tube effect the properties of the right tube?
2) Could the 2 tube adapters on my Bias Rite be mis-calibrated? How could I measure this with common household items?
3) How close in terms of % difference is an acceptable match? Or put differently, how close are vendor-matched sets, typically?
Any insight is appreciated.