Modifying a stock Alctron HST-11B DYX PSU

[roger.unc]

Hi - I’m looking to modify a stock Alctron HST-11B DYX PSU (B+ 140v unloaded) to work with an Avantone CV-12 tube mic (B+ 153v unloaded). The PSUs are very similar and have the same 7-pin XLR pinout. Both have R1, R2 = 20k, R3, R4, R5 = 6k5 and R6 = 1.5M. The alctron xformer is labeled 200v/9.5v, which I believe the Avantone PSU is as well. Measurements below.

Zener voltages (unloaded):
Avantn 153v - D5 - 76.6 - D6 - 0v
Alctron 140v - D6 - 69.8 - D5 - 0v

Resistor voltages (unloaded):
Avantn 275v - R1 - 234 - R2 - 194 - R3 - 180 - R4 - 166 - R5 - 153v
Alctron 298v - R1 - 245 - R2 - 194 - R3 - 176 - R4 - 159 - R5 - 141v

1) Assuming I don’t change the zeners, can this be done solely with resistors? (e.g., R5 = 2k2)?
2) If I replaced the zeners, what voltage and wattage would I need on the zeners?
3) Which resistor would I potentially replace to trim B+? R5? (Or will the zeners be the only limiting factor here?)

Sorry for the basic questions.

Thanks,
Roger

 

[kingkorg]

They are not similar, it is the same PSU. Avantone is just rebranded Apex 460, Alctron HST11a....

The difference you measure is irrelevant, especially unloaded. Probably due to part tolerance, you can interchange psus between the two. 13v of difference doesn't change anything.

 

[roger.unc]

Great info! Thanks. I was concerned I'd get a noticeable difference running one capsule 10v lower than the other. I see that may not be a problem when actually loaded up.

Just for the sake of argument, any thoughts on my questions above if the PSUs were in fact different?

Best,
Roger

 

[kingkorg]

Those 13v of difference get split in two by voltage divider in the mic. Capsule sees half of the B+ voltage. So the real difference in pol. voltage is 6.5v. We are talking unloaded.

If you were to lower capsule pol.vol. by half, (ca. 35v from originally used 70v) you would get -6db in level. You can easily do the math how much difference 6.5v makes.

I don't have the schematic so i can't answer your question.

 

[roger.unc]

Thanks again!