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A Reponse from Jim Hagerman

Posted by anevsky (A) on October 20, 2003 at 00:34:51

In Reply to: Re: SF/Bay Area tube testing Collective - READ! posted by Jim McShane on October 19, 2003 at 12:34:53:

Here is a reponse I got from Jim after I sent him your critique.

I've also posted a few pictures which he sent me. They are curve tests of a 300B

http://www.trailtopia.com/akimball/300b.jpg
http://www.trailtopia.com/akimball/easyscope.gif
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Depends on what tubes you are using. The VacuTrace won't be doing a 845 or anything of that nature. Really, you don't always need super high volts to measure a tube at THE BIAS POINT of interest. You can get a pretty good feel for a power tube by looking at the lower voltage characteristics too. Check out the pic, it's a 300B overlayed onto a data sheet. You really don't miss that much.

Granted, the VacuTrace is better for lower power tubes, especially triodes. The real benefit is getting curves for the actual tube you use, or for comparison matching. In fact, I rarely bother with gm or other single point measurements. The curves tell it all. And a tube that matches at low volts tends to match at high volts.

Second pic is from a USB-based digital scope that works really well. It's $189 (www.saelig.com) and lets you get jpgs. Else, a very cheap analog scope will do fine (& digital camera). You can get plenty of these for under $100 on eBay. Heck, if you're doing serious audio work, a scope is super handy.

The VacuTrace will do a pair of 6550s at the same time, showing curves from both. But in pentode mode you get above the 100mA limit in a hurry with high screen voltages. So as was pointed out, you cannot run these tubes anywhere near full blast. For that you will need to make your own tester. Maybe a Tek 570 will do it?

If all you want to do is match 6550's, then this may not be the right machine. Better to make your own. The VacuTrace was designed to be very flexible and operate with a wide range of tubes.

jh

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Jim Hagerman
Hagerman Technology LLC
P.O. Box 26437
Honolulu, HI 96825
Cell: 808-383-2704
Fax: 808-394-6076
www.hagtech.com

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