About the fear - omnipresent in this forum - of reaching 1mA unbalance in the primary of the output transformer:
(I didn't see any fear for tubes going bad in a SE amp...)
Suppose we have a decent transformer with good specifications, max unbalance 1mA.
Also having a good tube for V2, a tube with reasonably well-balanced triodes.
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In my preamp:
B+ 248V
Tamura SMG-1000: Rdc of the windings used as primary: 679 and 677 Ohms, total 1356 Ohms.
ECC82: both cathodes are connected to a common resistor of 15K.
114V over this resistor.
Total current drawn by both tubes: 114/15= 7.6 mA, or 3.8 mA each.
(only 3.8 mA in the primary...while 1mA unbalance is permitted!)
1/2 ECC82 - one triode - is limited to max 2.75VA anode dissipation.
in my amp: 248-114=134Vx3.8mA=0.509VA for each triode.
The tubes will have a long life!
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I measured the unbalance in the primary of all my Philips tubes (8) in this circuit, nos and used. Connected without the trimpot of 1K.
Max unbalance between the anodes was 0.1V, most tubes measuring less. (Ok, good tubes, these Philips.)
0.1/1356=0.000073746A or unbalance of 0.0737mA. Far below the max allowed 1mA.
After trimming the 1K pot I measured <0.01V unbalance or <0.00737mA
Don't be afraid to see little DC fluctuations caused by changing temperatures and in my case also changing B+ .
(High tension is not stabilised).
Don't be tempted to do the trimming over and over. Conditions will never be twice the same. DC unbalance will fluctuate but is low enough after the first trim. Of course, with new tubes it is good to do a new adjustment after a run-in of about 10 à 20 hours.
This is the starting point.
As said, tube wear is low. We have a wide margin.
There is no necessity for frequent adjustment.
If you are in a hurry, adjustment will take 3 minutes and 7 seconds.
3 minutes for the tubes to warm up. 7 seconds for adjustment.
If you are relaxed, with plenty of time, it is good to use 10 à 15 minutes and 7 seconds.
10 à 15 minutes for a cup of coffee and maximum stabilization of the tube, 7 seconds for adjustment.
When after a long time tubes have reached the point where too much correction is needed for proper balancing, then at least one of the two halves is bad. It's time to replace the tube.
You can pull it out and ... use it in your SE-amp???
Paul
PS
By choosing 1K for the trimpot (instead of 220 Ohm) amplification of DC is reduced through local NFB.
Perfectionists can do even better by adding a resistor of 180 Ohm to both sides of the trimmer.
Then there is NFB across both cathodes caused by a resistance of approximately 500+180= 680 Ohms. Equal to the Rdc charge on the anodes. So, no more amplification of fluctuations in the tube...
Enough about all this. Time to take a few photos.
No...oh no...
Let's take a break first.
Time for my coffee!
October 24, 2004, 09:13:56 PM