Power Tube Matching
Can someone translate the two #’s on the tube boxes?
What does the first two digit # signify? Plate current I think. And I assume millivolts, milliamps? Or what?
And the 2nd four digit # means transconductance I gather. What is transconductance?
How far apart can these #’s be before they are considered unmatched? And if you explain with percentage, I will need to know how high the numbers go, so maybe stick to actual number increases. My point being, on the internet people just say "5% difference." This means nothing if the plate current numbers can go up to 35 million, or only 100, or maybe 50, or 120.
So, if I Bob has 3 tubes that all have the first number of 14.3.....what numbers on a forth tube would be considered matched? 17.1, 18.4, 19.0, 25.8, or even 12.3?
What happens really if Bob used an unmatched pair of tubes in an amp. And it would be nice to hear about direct experience.
Thank you for your time,
TD
@clio09 Thanks, From that article, it looks like he is saying that the top, 2 digit, number on a tube box will be between 32 to 42. Well, I know Ive seen tubes as low as 14 and as high as 34, so.....maybe even 44. It is quite mysterious. Were tubes always matched, say in 1950's guitar amps? If no, when did matching become a thing? And did this correspond to a change in amp design? And if yes, why the change?
T
|
@tonydennison No that is not what he is saying. If you read the very last paragraph Roger wrote Bias is matched within 4% and Gm (transconductance) within 8%.
The answer is 13.7 or 14.9 using our method of testing. We actually offer even tighter testing upon request. I personally would not accept anything below 14 or above 14.6. So none of the other values in your example offer a good match to the tube labeled 14.3. |
Thank you @clio09 for clearifying, Im no engineer. What tube shop do you run? Also, not that Im going to try it, but what would happen using mismatched power tubes? Would there be SQ issues? amp damage? And what type of damage?
Thanks |
No don't worry so much about the Ip value , as long as it is the same on all four. Some places will put a sticker on the base of the tube. Apex tube matching puts a label on the base of the tube with the value as measured on their equipment. I have had pretty good luck with the JJ tubes I have had that were Apex matched. There are other places that fully test and grade their tubes in a similar way On your amps if they have an LED for bias will be around 50mA when it starts to glow brightly. I use this when I retube a 6L6 or EL34 based amp. I use this https://www.vhtamp.com/avtttt2 It is a great tool for adjusting bias and safe because you are not using meter probes on test pads Quicksilver has all of the bias values except the Mid Mono so I asked Mike and he said 53 mA for best performance. That VHT tool is more accurate than the LED , although it gets you close if set to where it just starts to get bright. |
