Cheap tube mic preamp power supply

[mrtnasty]

Is there any reason why using one of these cheap power supplies to power a tube mic preamp like an RCA Ba1a is a bad idea? It says it produces 280V DC and 6.3V for the filament:

https://www.aliexpress.us/item/3256...srl2EXR8glhoCeVIQAvD_BwE&gatewayAdapt=glo2usa

Thanks! David

 

[mjrippe]

With 5A @ 6.3V and 200mA @ 280V you could power several BA1A. Extra filtering on the 280V rail could also drop it a few volts if needed. I have not used one of these switchers for a tube project yet, but for the price I might try one.

 

[pucho812]

The only reason to avoid it is that it is a switch mode. Will probably need filtering after the psu to eliminate switch noise

 

[mrtnasty]

I bought the power supply and it seems to work great and is dead quiet! I just posted about using it in my RCA BC-2B preamps here:
https://groupdiy.com/threads/rca-bc-2b-restoration-and-power-supply.90135/

 

[pucho812]

Oh really? cool,

 

[emrr]

I have questions about that B+ current rating, a little terrifying and is that really continuous. There are very few historical supplies with such a rating outside of bass amps

 

[mrtnasty]

I have questions about that B+ current rating, a little terrifying and is that really continuous. There are very few historical supplies with such a rating outside of bass amps

I agree, it seems a little too good to be true, unfortunately there is very poor documentation and absolutely no customer support for these power supplies. But it seems to be working very well to power 4 12ay7s, I guess the worst thing that can happen is the power supply blows up one day and I go back to a more conventional power supply?

 

[emrr]

I’m sure it has no problem at all with your usage, but on paper you might power an entire tube console, and that seems questionable. I don’t know enough about the behaviors of switching supplies at full load. Many require a minimum load to get into a lower noise/ripple region I think; i could be wrong, not enough coffee yet.

 

[rock soderstrom]

But it seems to be working very well to power 4 12ay7s, I guess the worst thing that can happen is the power supply blows up one day and I go back to a more conventional power supply?

The 4x 12AY7s are no challenge for that SMPS.

I have seen a similar SMPS module in a hi-fi power amplifier with fat power tubes. I guess 100mA or even more on the B+ rail is no problem. Above a certain load, I guess you have to ensure good ventilation. I've ordered this slightly larger module, I'm curious to see what it can do.


Edit: a comment on Ali

"The power supply works as described. It can drive 2 * e235L at approx. 100mA. It not break down if ther is a peek. It have to be cooled. If if if had a negative grid bias it were perfect."

 

[pucho812]

I am trying to find one that would power 16 x 12**7 tubes.

 

[mrtnasty]

I am trying to find one that would power 16 x 12**7 tubes.

Haha you could be the guinea pig and see if the provided specs are true. Or couple a few together?

 

[pucho812]

Haha you could be the guinea pig and see if the provided specs are true. Or couple a few together?

Well that’s just it. I am looking into that. But published specs for a 12**7 tube average 300ma heater current at 6v. To do what I need if the math is correct I need a minimum 4.8 amps for heaters. So it would be pushing it but I could probably swing it.

 

[ruffrecords]

I’m sure it has no problem at all with your usage, but on paper you might power an entire tube console, and that seems questionable. I don’t know enough about the behaviors of switching supplies at full load. Many require a minimum load to get into a lower noise/ripple region I think; i could be wrong, not enough coffee yet.

I bought a very similar one a while back. After tracing out parts of the circuit it became clear that the regulated voltage is the heaters. The HT is a separate winding that is unregulated. Fortunately class A preamp tube circuits are very tolerant of HT voltage so this is not really a problem. It is easy enough to to add a simple RC filter to the supply such that the R drops the HT to the required voltage.

My main concern would be safety. These supplies do not meet any European or US standards for safety or EMC.

Cheers

Ian

 

[emrr]

My main concern would be safety. These supplies do not meet any European or US standards for safety or EMC.

Ah, that’s a no-go for me with anything going out into the world.

 

[Cqwet Dbdfte]

Some cheap SMPS' have poor isolation between primary and secondary in their forward transformer. One could measure the leakage line frequency current with a DMM AC voltage range between its output and protective ground on the low voltage output.
I went thru a pile of these until I found one with low enough leakage. Of course regular 60 Hz iron suffers the same problem but the better ones have electrostatic shields between primary and secondaries. In these HV will leak to low voltage windings to the secondary windings, which may be a problem, or not.
Split bobbin transfomers are an attempt to increase safety and reduce leakage. Medical grade power supplies pay close attention to this. Even if some microamp current may not be lethal it would be at least annoying and would not have any positive effects.

 

[mrtnasty]

Can someone please suggest a simple quiet safe preamp power supply circuit I could build instead? Or one I could buy?

 

[rock soderstrom]

Can someone please suggest a simple quiet safe preamp power supply circuit I could build instead?

This design is for ordinary tube PSU transformers with 6.3V AC heater winding. Thanks to the voltage doubler, you could operate your 12AY7s with 12V DC.

https://groupdiy.com/threads/diy-pr...ps-like-preamps-buffers-or-microphones.84324/

 

[emrr]

My comment doesn’t mean it’s not safe, it means it’s unsuitable for work being done for paying clients with regards to liability. You can’t state that it’s safe. Given the explosion of people running boutique audio gear companies selling electrical products with no certification, does it matter? It will if there’s a problem and you’re sued by an insurance company. That was at one time a big topic around here.

 

[JohnRoberts]

Selling SKUs using power supplies that lack safety agency certification could expose you to legal liability.
===
One could perhaps, incorporate a GFCI outlet into the back chassis, and take the units mains power from that GFCI protected feed. In theory this could protect users from mains related shock hazards.

This is just a little outside the box thinking.... I don't think I have even seen or heard about this being done, so caveat lector.

JR

PS: I have multiple GFCI outlets installed around my house to protect me from old, 2 circuit (no safety ground) house wiring.