Quote:
Originally Posted by adn6244 /img/forum/go_quote.gif
Hi Tomb,
thanks for your reply. I use a tiny screw driver to retension the socket and now the tubes are firmly in and i can see the centre part lighted up in orange. I adjusted the PS voltage to 27V.
interesting findings :
PS Voltage: V+ with GND = 27V
Now both the 2 tube bias: TA2L/R with GND = around 24.1V to 24.2V
Buffers: Left/Right Channel = 24.1V to 24.2V (The last time I checked was 44mv and I did not adjust the trimmer after that)
Did I just fry something? sigh.....
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Be sure you're referencing the correct points. The Buffers
are not referenced to Ground. You must measure TA2L/R to TB2L/R (or TA2L/R to TB1L/R). Also - at 44mV, the output transistors are just barely switched on (if they are at all). You need to get them up to 90-110mV.
As for the tubes, remember that you may have to turn 15 turns or more on a 20 turn trimmer to see an effect, depending on where the bias shakes out. This is because the trimmers are sized to be able to bias three totally different tubes and their gains.
When it comes to the tubes - if you have power, and the cathodes are lit, these are the only explanations:
1. You haven't turned the trimmers enough.
2. The trimmers are bad.
3. You've got bad connections in the socket.
#3 is highly unlikely if the tubes are lit.
#2 can be easily checked - measure the resistance on the trimmer leads under the board. Turn them about 5 turns, then measure again. If you get different values, the trimmers are working.
#1 is the easiest, but be careful if you don't have clutched trimmers such as the Murata's. Bourns will snap if you turn them beyond the click warning. I believe Vishay's have a clutch that will allow them to over-turn, but I'm not sure.
EDIT: I should add that bad tubes are always a possibility, but again - if the cathodes are lit, this is fairly unlikely also. Two tubes being bad are very unlikely, so switching them is an easy way to confirm.