Could someone post how to do this, or a link?

Could someone post how to do this, or a link?

Here is the SRD-7 Mk2 schematic with the bias supplies on the bottom. It's pretty straight forward with a 150k resistor on the input and then the z1100 bidirectional zener but you can use two 81v units instead facing each other. The rest is easy and it doesn't take long to build on a perfboard.

The back to back zeners need to be 100v not 81v.

The back to back zeners need to be 100v not 81v.

Absolutely correct. I was looking at the minimum standoff voltage in the spec sheet.

Hi Birgir:

Resurrecting an old thread.

I have a SRD-7 I am thinking about converting to Pro bias to fill in while I build my KGSSHV. The link you posted in this thread is broken. Any chance you can link me to a schematic?

Thanks!

Edited February 28, 201511 yr by mwl168

One google search away....

 

I am a bit embarrassed. I did google it and came across this image but did not know it's what I was looking for )

 

Thanks Birgir!

 

Edit: For reference for others that may be interested in this topic, here is the normal-bias SRD-7 schematic.

Edited February 28, 201511 yr by mwl168

I did this using the schematic above, but kept one socket at normal bias, taking the voltage for it from halfway up the zener ladder.  One thing I did was to glue a nylon 4-40 screw into middle hole in the socket I converted to pro bias.  This prevents you from accidentally putting a normal bias phone into a pro bias circuit

There are two things very wrong with that schematic though, under no circumstance should that capacitor me after the ballast resistor and the ballast should be 4m7 and not 2m2. 

There are two things very wrong with that schematic though, under no circumstance should that capacitor me after the ballast resistor and the ballast should be 4m7 and not 2m2. 

 

To be clear, are you saying that the SRD-7 MK II schematic was wrong and that R104 should be 4.7M and C108 should be at a different position in the circuit.?

Yup.  Stax finally stopped doing this as it will damage the diaphragms.  King Sound still thinks this is a good idea though... 

Thanks Birgir.

Do you still have the custom PCB you made for this modification available? I checked your website but did not see it.

Nope, I haven't had any for a few years now. 

Thanks Birgir. I'll have to fabricate it on a perf board then. 

 

To confirm the right way to convert the SRD-7 to run on Pro bias (580V):

 

1. I disconnect the factory bias portion of the original circuit after the 150K resistor and neon bulb.

2. Construct the bias circuit of the SRD-7 Mk II in post #6 but skip C108 and change R104 to 4.7M.

 

Thanks again for your help!

It's fine to keep C108 but put it before R104. 

So essentially move R104 to between C108 and the bias pin of the output jack?

Yup

Thanks Birgir!

Excellent points by Birgir.  When I built my version I did as he suggests and had the 4.7M resistor after the last cap - been awhile and I forgot that- early senility sucks sometimes. 

S5277G and Z1100 diodes are both obsolete. Can someone recommend current production replacement? Preferably Mouser stocked parts - I am about to place a big order with Mouser for a KGSSHV build.

 

Thanks! 

So for Z101 and Z1100 you can use 100 volt zeners.  S5277G diodes are specified as 400 v PIV, so you can use 1N4007 diodes there.

Edited March 1, 201511 yr by JimL

It was the S5277G I was not sure about.

 

Thanks! 

100V zeners back to back and 1N4007's is what I used. 

Just wondering, what is the bias circuit used in the SRM-252S? It would be a lot safer to work with 12VDC than mains AC.

Voltage doubler off the built in switcher.