Testing EML tubes
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After all the explanations, this is what is all about. Just open the set up file, and run a test.
What to say about test results?
- If Grid voltage is the same on the tube box with limits of say 5...10%, the tube is new. Such small differences can result from the tolerance of the tube tester, and also tubes that have been stored long, may need a short burn in, to get full 100% values again. So anything below 10% difference is nothing to look at further. Perhaps do the noise test 10 minutes instead of 3 minutes, and most probably things will be good now.
- If the tube is used, first the grid voltage will become lower. As a second change, Transconductance will become lower, and values of 70% can be tolerated always. Often, values below 60% can be tolerated, and we have seen amplifiers work fine even on 40% tubes. So a a lower transconductance for used tubes is normal, and no sign of a "bad" tube. As a third change, plate impedance will go up. As a forth change, gain will go up, together with transconductance going down. Somewhere at or after the 3rd change, the tube will stop functioning.
- Burn in can only be done in the FIXED BIAS mode
- The plate curve. In the set up menu, you can also check plate curve. There is only one in the default menu, but that is fine. If the plate curve function gives funny looking triangle results, check here. If the plate curve at the high end has a tendency to bend over to the right, this is a sign of low emission.
- Burn in the tube. This will improve emission, if the tube was over stored, (15 years or more) or if the tube was used at too low grid voltage. In either case, emission will rapidly increase by burn in. Open the set up menu (for the 300B-XLS for instance). Reduce the plate current to 50%. Save this as 300B-XLS-burn-in. Or another file name of your choice. Like this you can call the burn setting any time again. To start the burn in, select only "noise test" and "auto bias". Start the noise test, verify if plate current is 50% indeed, and start the test. You need to observe this continuously, and if you are successful, plate current will slowly increase. If more than 10% increased, stop and restart. This way, you can burn in a tube for a few hours. Do not leave it unattended, as burn in may sometimes not take place right away, and then accelerate unexpected.
For all other tubes it works the same, so you should make a small directory with test set ups, and with that you can work quick and convenient. Note the right heater voltage. You can change that with the set menu. For instance if you make an adapter for the AD1 tube, which you want to plug into the UX4 socket, you can begin with the 2A3, modify that at 4V and save it as "AD1".
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