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STC version 807 5B25x LOCTAL

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This is how some of them look.
I have been really bothered and annoyed by constantly being sold "NOS" items, which never match properly, or fail suddenly for no good reason.
It's become impossible to do quality work on that basis, dso I make my own solution.

Testing them PROPERLY, so they are properly matched is not possible in a valve tester, you have to FIT them in a proper working amplifier at genuine working conditions.

There are large variations in production between one sample and another, just like the original 807.
Finding a pair of 807s which match for current and bias voltage at 600-800 Va and don't red plate in one corner of the anode, is quite an art.

They have to pass my 80W square wave test with the anode colour evenly ALL over, without any hotspots, and no sign whatsoever of glowing electrodes or uneven waveform from the matched pair.

Nobody else goes to this sort of trouble.
I have had to adopt exactly the same strategy for the KT8C ( a nightmare of its own), and the 5B25x series which has a smaller top cap.

The 1949-50 Bogen AB2 amplifier shown here has been extensively modified to be able to adjust bias, stabilise the screen to exactly 300V with no changes whatsoever under full load, and a silicon rectifier package BEFORE the paralleled diode valve rectifier in order to remove the original (choke input filter) original HT line sag under full load.

It's the perfect test amplifier for testing NOS "807" style valves, revealing the slightest manufacturing defect instantly.
I have the same sort of equipment for testing several other high gain valves:- 8417, 6G-B8, 7591, A2134, which are all easily damaged, and need to be checked both for quiecent current and gain match through proper waveform read-outs.

I have at least 20, "good second hand" 8417 which failed exactly that test.
They are all defective, but are exactly what people sell on EBAY.
Unmatched pairs, create serious cross over distortion, which may not show up at the standard 1W at 1khz test.


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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.