RCA BC-2B restoration and power supply

[mrtnasty]

Hi All,

I recently restored 2 dual channel RCA BC-2B preamps. I replaced all the caps (signal and electrolytic), wired up the inputs and outputs and supplied it power originally with a heath kit tube regulated power supply. All channels worked, but there was a pretty substantial buzz. I replaced the electrolytics in the Heathkit. The hum was still there. So just for kicks I spent $25 on amazon on one of these Chinese 280V and 6.3V power supplies:

https://www.amazon.com/Preamp-Switc...2050&sprefix=280v+power+supply,aps,101&sr=8-3

I first measured the Chinese power supply, and I saw 280V DC on the first 2 terminals, and 6.3V DC (Not AC) on the second two terminals (The 12ay7 data sheet says AC or DC is ok for the filament). After firing up the BC-2B with the Chinese power supply, it works awesome and is dead quiet!

Someone please let me know if I am missing something and why I shouldn't be using such an inexpensive power supply with such nice preamps, will the sound quality of the preamps suffer by using this cheap power supply? Otherwise I think I'll use it like this, it's super convenient to be able to mount it right on the rack. Thanks!

 

[emrr]

Interesting option. I don’t see any current specs in that ad.

 

[mrtnasty]

Interesting option. I don’t see any current specs in that ad.

I first posted about this power supply here: https://groupdiy.com/threads/cheap-tube-mic-preamp-power-supply.90042/

and mjrippe pointed out the link I shared there says the specs are 5A @ 6.3V and 200mA @ 280V. I just bought it from amazon so I could return it easy if it didn't work. The 12ay7 data sheet says one 12ay7 takes 300mA @ 6.3V and 3mA @300V so it seems to me that this power supply should work for 4 12ay7s.

 

[rock soderstrom]

After firing up the BC-2B with the Chinese power supply, it works awesome and is dead quiet!

Someone please let me know if I am missing something and why I shouldn't be using such an inexpensive power supply with such nice preamps, will the sound quality of the preamps suffer by using this cheap power supply?

Thanks for your input, that's an interesting topic, I'm currently also testing such combined PSU solutions for tube amps.

The switching frequency of these modules is relatively low, you will probably see these artifacts in your signal, which won't necessarily bother you at first. However, it can become a real problem if e.g. other switching artifacts interfere with this and beat frequencies are formed in the audible range. Sometimes I have also had slight problems with oscillating amplifiers, but these can all be solved.

Keep the distance between the SMPS and the signal transformers as large as possible, use additional shielding if necessary.

My general suggestion is to clean the B+ voltage coming out of the module with an LCRC filter, that works pretty well. Take a look at B+ before and after the filter on an oscilloscope, then you'll know what we're talking about.

Overall, my experience with switching power supplies has been pretty good in this application, even with the rather inexpensive, simple boards!

 

[mrtnasty]

rock soderstrom thank you for this insight!