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Vintage threads from the first ten years

No bias current ?

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4/25/2000 4:17 AM
JCS	No bias current ? I have a SFTR I just blackfaced and recaped. I had everything done but the 2 res in the power supply(was waiting on parts)It sounded and tested out fine. Then today I replaced the res with 1k and 4.7k 2 watt. When I fired it back up, no bais current? All voltages inclunding negitive voltage are reading normal,checked bias cap, rechecked all connections. Any Ideas? Thanks JCS

4/25/2000 1:23 PM
kg	do you have continuity from output cathodes to ground? ken

4/25/2000 7:24 PM

JCS,

If "all voltages are normal", then the only thing it could be are the output tube cathodes lifting, and even then I doubt that ALL voltages would be normal. Every single voltage on each pin of all the output tubes are normal? If you haven't checked them all, I would go back and check them, especially the screen grids (pin 4) - it sounds to me like during the replacement of the first (1K) resistor the screen grid supply voltage was lost, maybe due to a wire popping out as soon as the solder melted. The choke, first filter cap, the resistor, and the screen grids all are connected to this point - a lot of potential for soldering problems.

If the tubes' cathodes are grounded and every voltage on the tube sockets reads normal (heaters too!) and you still have no current flow through the tube, and the measuring instrument is OK and connected correctly - then I honestly have no idea what's wrong with your amp; it sounds like 'one for the record books' (unless all your output tubes decided to quietly fail at once).

Ray Ivers

4/26/2000 2:25 AM
JCS	Thanks for the replies guys. OK here a rookie mistake, The fuse in my meter blew, so I could check voltages but not current. DOOOOGH! It was late, after I slept on it, O well.

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