Tube testing

[bsco]

Mesa sold what they called SPAX7's, that's the description. 25-30 years ago they were supposed to be pick of the crop, that is low noise, low hum & low microphonics selects, then they put on the heat shrink which further reduces microphonics. I have some from late 80's - early 90's that are Shuguang. For a while they were selling EI. I wouldn't be surprised if they're using JJ these days.

That was the ones I got from the store today and I did find them alot quieter....at least when compared to the Gt's.....

[Mick Bailey]

Any tube can be quiet or noisy, regardless of brand. I have a pile of NOS Mullards and Brimars that are too microphonic to use in a high-gain amp, and conversely some Shuguang 12AX7s that are as quiet as you'd ever get. It just comes down to selection criteria. My guess is a lot of NOS tubes now on sale were originally 'B' grades, and the Shuggies I have are ones I've picked from a whole bunch to get a decent bench set.

My tap test is to install in V1 position and press down slightly on the top pip (to keep the tube seated) and flick the tube with the back of my fingernail with the amp turned up on a clean setting. Then try this with higher gain channel. Often a reshuffling of tubes will get better microphonics/noise characteristics. I think maybe it was R.G. who suggested using one of those phone/pager vibration motors as a possible steady reference for a more scientific comparison.

At the end of the day, I don't need to look at scope traces to see noise because I'm repairing amps and not calibrating tubes for resale. I use the amp I'm working on at the time and my ears.

[bsco]

Thank you for the reply Mick.....when I have to test tubes, I do use the amp itself and have speakers connected as you have to be able to hear what is going on.....but lately I was also using a scope to monitor it as well......just to see what was happening as well as listening to it.....I just put this thread out there to find out what other techniques people use......and maybe get some valuable information that would benefit me......I have swapped noisy tubes around in the past as you suggest....sometimes it worked out great...other times not so much....Thanks for the reply......It is much appreciated.....
Cheers

[Mick Bailey]

Here's a handy piece of equipment I thought I may build one day. A simple Nixie SMPS would easily power most preamp tubes and so the unit could be self-contained and built to run off a 12v wall-wart. A scope output could also be included;

https://www.youtube.com/watch?v=205__gxWJc8

Nice just to be able to A-B the tube and listen to it.

[bsco]

Here's a handy piece of equipment I thought I may build one day. A simple Nixie SMPS would easily power most preamp tubes and so the unit could be self-contained and built to run off a 12v wall-wart. A scope output could also be included;

https://www.youtube.com/watch?v=205__gxWJc8

Nice just to be able to A-B the tube and listen to it.

I seen that video while I was scouring youtube for something else awhile back...that is where I got the idea of scoping the output but they recommend 40mV as the max acceptable noise level....If I choose that for a reference I can pretty much throw all the tubes out....so I figured .1mV and the video I saw was with a 12AX7 .........I will do some more testing over the next couple of weeks as time permits and finally come to a volts /div level that is reasonable...maybe .1 is also too sensitive as well......somebody already mentioned that here to me.....trial and error and playing around with it and I will have a decent reference to go by.....
Cheers