Hi Guys,
Sad git that I am, I worked out how the threshold control works and some implications which may interest some of you with curious minds, here we go.
If we start with Altec's published figures of 0dBm, +10dBm and +16dBm on the compression chart, we get the following voltages across a 600 ohm load, 0.775, 2.45 and 4.88 Volts.
Now thanks to CJ's "take apart" http://www.groupdiy.com/index.php?topic=35658.0 we know that the turns ratio on the output TX is 6.2:1, this means that our voltages are now 4.805, 15.19 and 30.26 on the primary.
These RMS voltages appear across both 270k resistors so we must divide by 2 and multiply by 1.414 to get the following peak voltages on the cathodes of the 6AL5; 3.40, 10.74 and 21.39 Volts. These are the voltages available by turning the threshold control through its range. By adjusting the threshold control, the 6AL5 only conducts when the peaks exceed the set voltage, it's a very elegant system.
Now for the interesting stuff on the other models; the 436A has a voltage divider on its HT to provide 10.6 volts, so this model is set at +10dBm with no adjustment (10.6 is within the spec of 10.74V above).
The 438A does not have the divider but connects the threshold to the cathodes of the 6CG7 at 2.3 Volts, so this model is fixed at 0dBm threshold. Please note that the HT supply to the 6CG7 is 145 volts on these models so the cathodes are at ~2.3 volts. The 436C has an HT of 150V which in fact pushes up the cathode voltage to 3.4 volts (I measured it) so again the lowest setting is dead on 0dBm threshold.
Two last points, The Sowter 9745 Transformer for the 436 is only 5:1 so this changes the threshold/gain slightly, but as long as you are aware of this and allow for it, it is no deal breaker and it works a treat.
PRR mentioned in an earlier post that he thought silicon diodes were an ok sub for the 6AL5 but I found this post, http://blogforumpost.blogspot.com/2009/04/theory-of-vacuum-tubes-5.html which states that the 6AL5 has no diode drop. This means that silicon diodes would have a voltage drop of say 1.4V which would alter the threshold voltages around the 0dBm mark by quite a lot and by a tad right through the range.
I hope you guys find this interesting, now what did my wife mean by "get a life" hmmm.
all the best
Dave P
Just turned 60 and getting younger by the day!
Sad git that I am, I worked out how the threshold control works and some implications which may interest some of you with curious minds, here we go.
If we start with Altec's published figures of 0dBm, +10dBm and +16dBm on the compression chart, we get the following voltages across a 600 ohm load, 0.775, 2.45 and 4.88 Volts.
Now thanks to CJ's "take apart" http://www.groupdiy.com/index.php?topic=35658.0 we know that the turns ratio on the output TX is 6.2:1, this means that our voltages are now 4.805, 15.19 and 30.26 on the primary.
These RMS voltages appear across both 270k resistors so we must divide by 2 and multiply by 1.414 to get the following peak voltages on the cathodes of the 6AL5; 3.40, 10.74 and 21.39 Volts. These are the voltages available by turning the threshold control through its range. By adjusting the threshold control, the 6AL5 only conducts when the peaks exceed the set voltage, it's a very elegant system.
Now for the interesting stuff on the other models; the 436A has a voltage divider on its HT to provide 10.6 volts, so this model is set at +10dBm with no adjustment (10.6 is within the spec of 10.74V above).
The 438A does not have the divider but connects the threshold to the cathodes of the 6CG7 at 2.3 Volts, so this model is fixed at 0dBm threshold. Please note that the HT supply to the 6CG7 is 145 volts on these models so the cathodes are at ~2.3 volts. The 436C has an HT of 150V which in fact pushes up the cathode voltage to 3.4 volts (I measured it) so again the lowest setting is dead on 0dBm threshold.
Two last points, The Sowter 9745 Transformer for the 436 is only 5:1 so this changes the threshold/gain slightly, but as long as you are aware of this and allow for it, it is no deal breaker and it works a treat.
PRR mentioned in an earlier post that he thought silicon diodes were an ok sub for the 6AL5 but I found this post, http://blogforumpost.blogspot.com/2009/04/theory-of-vacuum-tubes-5.html which states that the 6AL5 has no diode drop. This means that silicon diodes would have a voltage drop of say 1.4V which would alter the threshold voltages around the 0dBm mark by quite a lot and by a tad right through the range.
I hope you guys find this interesting, now what did my wife mean by "get a life" hmmm.
all the best
Dave P
Just turned 60 and getting younger by the day!