I have been re-reading this old post about the UA 175 transformer calculation and I think there is a mistake. This is an important vari-mu reference post so it's important to get it right.
First of all lets clear up some doubts about the schematic. I have attached a clear copy of a UA 175B as a reference point. There is no doubt that the voltage divider is 82k plus 47k, so the threshold voltage is 36.4% of the voltage on the OB2 which is nominally 100V. If the 45V were correct then the OB2 would be running at 123V which is clearly wrong. It shows up as 105 or 108V on the schematics I've seen and my own OB2 ran at 105V, so I think the real threshold voltage is much nearer to 38V, which is also PRR's estimate.
Given that the stated threshold is 13dBm then this works out at 3.46Vrms on the 600 ohm output. 3.46rms is 9.786Vpeak to peak. We now come to where I think the error is:-
The 6AL5 is fed from a centre tap arrangement so each side goes through 38V as the primary alternates, so the peak to peak voltage across the primary at threshold is 2x38V which is 76V.
We can now work out the transformer voltage ratio: 76/9.786=7.76:1
This means the Impedance ratio is 60:1 so 60x600 ohms = 36K:600 a much more reasonable figure.
The error on jhaible's original calculation was to use a peak voltage on the secondary and a peak to peak voltage on the primary which doubled the ratio.
It might be possible that 10% resistors on the threshold voltage divider could have made an error but you would have assumed that on long production runs that would have averaged out in their QC.
best
DaveP
First of all lets clear up some doubts about the schematic. I have attached a clear copy of a UA 175B as a reference point. There is no doubt that the voltage divider is 82k plus 47k, so the threshold voltage is 36.4% of the voltage on the OB2 which is nominally 100V. If the 45V were correct then the OB2 would be running at 123V which is clearly wrong. It shows up as 105 or 108V on the schematics I've seen and my own OB2 ran at 105V, so I think the real threshold voltage is much nearer to 38V, which is also PRR's estimate.
Given that the stated threshold is 13dBm then this works out at 3.46Vrms on the 600 ohm output. 3.46rms is 9.786Vpeak to peak. We now come to where I think the error is:-
The 6AL5 is fed from a centre tap arrangement so each side goes through 38V as the primary alternates, so the peak to peak voltage across the primary at threshold is 2x38V which is 76V.
We can now work out the transformer voltage ratio: 76/9.786=7.76:1
This means the Impedance ratio is 60:1 so 60x600 ohms = 36K:600 a much more reasonable figure.
The error on jhaible's original calculation was to use a peak voltage on the secondary and a peak to peak voltage on the primary which doubled the ratio.
It might be possible that 10% resistors on the threshold voltage divider could have made an error but you would have assumed that on long production runs that would have averaged out in their QC.
best
DaveP
