[ADXA] Gu74b tube question

[Jay Bromley]

HI Jussi,
Not sure any of this is super simple?  In the past, I think several have used HiPot testers between the tube elements.  I am sure if there are dedicated testers for that tube, but we have to use what is available to us.  To match them up, you could take a look at the bias drawn one tube at a time, during key down with no RF input.  That should match things up fairly well.  However, on amps like yours and some of the Alphas, etc, they use Bias switching so you probably can't see the bias or that the tube is drawing current wise during idle conditions.  Maybe there is way to disable Bias Switching on that amp for a test?    Others don't think matching is necessary in that amp, as long as the tubes are A-OK ?  If the tube is putting out good power on the highest band, you are probably A-OK.

If you haven't used those tubes in a while, some suggest putting them in the amps, without B+ and let them cook for a while before putting B+ on them.  I have always done that, the few times I didn't or got in a hurry, that when things didn't end well for that tube.  One really never knows for sure what is cause of death in tubes.

Gud luck and 73 de w5jay/jay..
________________________________
From: adxa-bounces at mailman.qth.net <adxa-bounces at mailman.qth.net> on behalf of Jussi Eloranta via ADXA <adxa at mailman.qth.net>
Sent: Thursday, September 17, 2026 12:29 PM
To: 'ADXA' <adxa at mailman.qth.net>
Subject: [ADXA] Gu74b tube question

Hi,

Looks like the tubes in my acom2000a have died. I have a bunch of gu74b tubes but now I need to figure out the following when selecting tubes from my stash:

1) what is a good way to check that a given tube is good? I don't have any special testers or the like  but I imagine that I can check for shorts or disconnect between some of the pins?
2) once I have found good candidates then I need to find a matched pair. Is there a good (simple) way of doing this?

The actual tube replacement I know how to do.

Jussi (aa6kj)
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