I get a lot of correspondence about simple tube pre amps for use as mic pres or gain make up in passive EQs. Probably the most common question I am asked is which mains transformer to use for the HT (and possibly also for the heaters). And of course, the answer depends a lot on where the enquirer is located and hence what is likely to be available as well as the mains voltage required.

I have had to revisit this topic myself recently as part of the search for low cost power supply components for the poor man's tube mixer which itself has quite modest HT requirements so I thought it would be useful to share my results and to solicit other candidates.

Before I begin, let's first dispense with the question of heater supplies. I no longer use linear supplies for heaters. All my designs are based on 12V heaters and there are plenty of SMPS around that can provide any amount of heater current in a compact, cost effective product. The only thing to remember is to rate the SMPS for at least twice the nominal heater current so it can cope with the cold heater inrush current. I find the SMPS with hiccup mode made by MeanWell are excellent for this purpose and generally do not need any additional filtering for heater duty.

In terms of specification the first requirement is for a universal mains input which usually means twin 115V primaries. Secondly, we are unlikely to need more than 50mA of HT current for simple class A preamps and often a lot less. Allowing the standard factor of 1.6 to convert dc current to transformer secondary rating, we need this parameter to be about 80mA. Assuming a nominal 230V or 240V secondary for a final dc voltage around 250V to 300V this means the VA rating of the transformer should be in the region of 240V x 80mA = ~ 20VA.

And the results? So far I have found the following useful transformers:

TRIAD VPT230-110. This has 2 x115V primaries and 2 x 115V secondaries and is rated at 25VA so this is capable of a secondary current of 109mA which implies a maximum dc current load of about 68mA. It is available from Mouser and Farnell. It is a toroid type.

Hammond 182B240. This has dual 117V primaries and a 240V secondary rated at 63mA so it is good for 40mA of HT current. I t is available form Digikey, Mouser and Newark. It is a toroid type

TRIAD FP-230. This has dual 115V secondaries and dual 115V secondaries rated at 50mA (230V) so is good for 30mA of HT current. It is available from Digikey, Mouser and Newark. This is a low profile UI type (sometimes called semi-toroid)

Bell Signal IF-18-230. This has dual 115V primaries and dual 115V secondaries rated at 80mA so good for 50mA of HT current. It is available from Mouser and Digikey. This one is a little unusual in that it is fully encapsulated and also PCB mounting. They also have 14VA and 10VA versions of the same transformer which share the same PCB footprint and providing 38mA and 27mA of HT current respectively.

If you know of any others please let me know.

Cheers

Ian