corndog71 wrote:Jim McShane wrote:sKiZo wrote:Good info ... nice to see a "real world" review. Also impressed with the EXACT matching based on the box stickers ...
Just for everyone's info - those tubes will not match perfectly once installed as the components in the amp are not EXACTLY matched...
Once tubes are matched within a reasonable tolerance there is nothing else to be gained by getting a closer match in all but a VERY few cases. What is more important is how the matching was done:
1. Was the tube run-in for a significant length of time to minimize the amount of drift that occurs after the tube is placed in service?
2. Was the tube matched at voltages/currents like those the tube will see in real world service? And while it was good and hot?
3. Was the tube checked for excess screen current draw?
4. Since most gear requires measuring cathode current - not just screen current - to set bias, was the tube matched according to cathode current (not just plate current!)?
Those are FAR more important to the purchaser than "exact matching", especially if the exact matching was of the plate current only instead of cathode current.
Hey Jim, in my experience, a lot of tube sellers are not using the right equipment for testing and are essentially selling random tubes.
I see on your sight...
When we match tubes, we are most interested in how the tube "idles" or draws current.
You cannot effectively match power tubes on a typical tube tester. Tube testers never applied the amount of voltage necessary to get accurate measurements of power tubes. In fact, there are only a couple very rare models that will allow you to get close to the voltages needed and allow you to read the parameters correctly. We use custom-built testing equipment that is the best in the business. We burn in and test power tubes at real voltages, and we test them for shorts, grid leakage, and excessive current draw before and after burn in to help minimize the chances of using your amp as a tube tester.
I did buy a set of Tung Sol 6550s from you and they've been fantastic sounding.
Are you familiar with Roger Modjeski's RAM Labs tubes and his testing / selection techniques? Here's what he writes about the subject...
My experience gained from testing thousands of EL-34's, 6550's, KT-88's, and other tubes has produced a wide bell curve of the two most important parameters, Bias and Transconductance. The center of this curve is the "bogey" value that the manufacturer is trying to hit, and the ends show the cutoff points of what he is willing to allow out of the factory. The tubes at the ends are still perfectly functional, but may not work well in all amplifiers, and will certainly not work well if mated with another from the opposite end of the curve.
Some amplifiers do not have enough range on the bias pots to handle the range of tubes so that selection-must be made from a particular part of the curve to ever achieve bias.
In the "good old days," I'm told, the spread of values was much less, allowing reasonable performance with random selection. Today, random selection is strongly not recommended
Corndog, I sent two octets of randomly bought NOS original TS 6550s to tubes4hifi bought locally and off fleabay and audiogoner for testing plus a couple of used ones. He stuck them in a MK-III he was building and all came within ~5% of each other on bias @ 425 VDC, according to Roy. He tried one pair at 460V and bias remained steady. So yeah, you're right, the oldies are the goodies.