[Staxnuts]

Hello!
 
I am building a vaccum tube amp, testing it with stax electrets as of now so no need for bias supply fot the moment. The headphones are connected directly to the plates, which are sitting close to 0V by using a negative supply for the final stage.
 
However i am a bit puzzled on how to connect a bias supply. My headphones are standard bias, so i used a simple voltage doubler fed directly from the AC line as in the srd-5 box. I have already made such a doubler for a DIY adapter box and it works fine.
 
But in the context of an amp with its own supply, how do you guys implement it?
 
I could probably just tap the B+. But how to connect the 0V point of a separate floating bias (maybe for a pro bias at 580V)? To the amp's ground? Wouldn't that create a short to the AC line on the negative phase (see pic)?
 

 
Thank you for any enlightment.
Joris

 

[wakibaki]

Connect 0V together, connect bias to diaphragm.
 
w

 

[jcx]

don't use the mains for direct connection to anything you handle, put on your head - it can be done safely but there are several critical topology and component rating issues to get right for safety
 
and the shown circuit doesn't have them - yes may have been done in the past, but standards, acceptable risk evolve
 
use the secondary isolated Vac to drive the doubler and then divide or regulate - and the series R may need to be multiple series devices to add up to safe margin for blocking the V due to the flashover V of the resistor body

 

[kevin gilmore]

The reason the stax transformer boxes are "safe" is because the power plug
is polarized so neutral is always ground, the voltage multiplier is different
from the one you show, and the output of the multiplier feeds a very
high resistance. But in general doing this is a bad idea.

 

[Speedskater]

To repeat:
The above drawing is extremely unsafe!

 

[Staxnuts]

Hello and thank you for all the replies.
 
The circuit is verbatim from the srd-5 box:
 

 
No fuse, and even if they wanted to connect the 0v to neutral, they came with an unpolarized plug.
You may have seen the circuit from a latter srd-7:

This one uses a cockcroft ladder that multiplies voltage to any ratio instead of just doubling like in the old srd-5; this allows both pro and normal bias. They added a fuse, but it's still directly connected to mains.
 
In my case, i used an isolation transformer, but in the absence of significant load, it overheats and i can't leave it on for long periods. Or maybe it is a special isolation transformer, since it was salvaged from a "razor only" outlet?
 
In any case, to be on the safe side I will folow advice and tap the B+ to get the bias. I will even allow a little adjustment pot, as it seems you can adjust the sound somewhat by varying the bias level.
 
Thank you very much!