Silvertone 1484/1485 Bias Voltage Doubler

[sonoflee]

Hello,

I'm trying to teach myself more about electronics while fixing up an old Silvertone 1485 (schematic: http://elektrotanya.com/PREVIEWS/63463243/23432455/silvertone/silvertone_1485_sch.pdf_1.png) and I was wondering if anyone can explain the differences between design in two different schematics to add a bias voltage doubler (below) - the first from VintageAmps (ignore the cap/diode in the upper right); and the second from Gerald Weber's All About Vacuum Tube Guitar Amps.





I'm seeing some small resistors in series with the diodes on Weber's (equalizing resistors?), as well as a different configuration at the 50k pot. 

Are they functionally the same, or is one better practice/safer than the other? 

Thanks a lot!

[Willabe]

This in the red square looks wrong to me.


              Brad     

[sonoflee]

Thanks for the response - yes, those were the original bias circuit that was accidentally left in the drawing, they should be gone.


I'm just wondering if the two different circuits are functionally the same, or if one of them be a better choice than the other (and why)? 

[eleventeen]

They are pretty much the same thing.


Transformer output side A goes to the junction of two series-connected diodes.
Transformer side B goes to the junction of two series-connected caps.


One phase of the transformer, through the forward-biased diode (D1 in the top dwg) charges the + end of the cap stack.
The other phase of the transformer, through the reverse-biased diode charges the negative end of the cap stack.


Thus, one phase adds charge to the "+" end, the other phase subtracts from the "-" end. That's the down and dirty explanation for how a  doubler works.


The series resistors in the second dwg don't do much. They will reduce the overall voltage output a miniscule amount. Change them into short circuits and you'll see the ckts are about the same thing. Bias circuits use "zero" current for all practical purposes...they just throw a voltage, a charge, upon the G1s of the output tubes. Since the bias rectifier uses "no" current, there is almost no voltage drop through those 100 ohm resistors to speak of. 


You say to delete the cap and diode in the the upper right of the top dwg. The ground connection, however, should remain.

[sonoflee]

They are pretty much the same thing.


Transformer output side A goes to the junction of two series-connected diodes.
Transformer side B goes to the junction of two series-connected caps.


One phase of the transformer, through the forward-biased diode (D1 in the top dwg) charges the + end of the cap stack.
The other phase of the transformer, through the reverse-biased diode charges the negative end of the cap stack.


Thus, one phase adds charge to the "+" end, the other phase subtracts from the "-" end. That's the down and dirty explanation for how a  doubler works.


The series resistors in the second dwg don't do much. They will reduce the overall voltage output a miniscule amount. Change them into short circuits and you'll see the ckts are about the same thing. Bias circuits use "zero" current for all practical purposes...they just throw a voltage, a charge, upon the G1s of the output tubes. Since the bias rectifier uses "no" current, there is almost no voltage drop through those 100 ohm resistors to speak of. 


You say to delete the cap and diode in the the upper right of the top dwg. The ground connection, however, should remain.

Thanks for the information, I appreciate it.  The ground would definitely be there in the top one, just the cap/diode are gone.


Is the 50k resistor --> ground on the bottom leg of the 50k pot in the top image a good idea for safety (i.e., if the pot is turned all the way down, you'd still have some measure of safety)?