HT Transformers for Vacuum Tube Preamps

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ruffrecords

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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
 

Matador

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Nice list!

I will second the Triad VPT recommendation.  For linear heater supplies, I've taken to using the Gyraf G7 scheme of buying two, VPT12's, and using the first to reduce 115VAC (parallel primaries) down to 12VAC, then connecting the second transformer to the secondary of the first "backwards" (series primaries) to go immediately back to 230VAC.

You can typically buy both for about $35, and the cost only goes up to about $55 if you need 50VA.  They are compact enough that they can be mounted to the side of a 2U project case and not even take up any real estate in the bottom of the case.

At least where I am, the Hammond transformers have steadily climbed in price to the point they aren't particularly cost effective.
 

ruffrecords

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Here are a coupe with 6.3V heaters as well:

Carnhill VTT2302 has dual 115V primaries, a 240VAC secondary at 30mA which is good for about 20mA HT current and alos has a 6.3VAC 1amp rated heater winding.

The next one is much bigger. It has dual 110V primaries plus a 20V primary winding so it to accommodate a wide range of mains voltages. It has a 275VAC HT secondary rated at 200mA so good for 125mA of HT current. Lastly is has a centred tapped 6.3VAC heater winding rated at 3A.

Cheers

Ian
 

ruffrecords

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Ampmaker.com have a nice tube power transformer for a very good price, it's possibly a bit overpowered compared to what has already been mentioned in this thread (it's 0-190-275V @ 160mA, rather than 240V @ <50mA) and it also includes 3A for heaters.

Pretty hard to beat for £36.

Oops I forgot to add the links for the last two transformers I mentioned, both of which are also from Ampmaker.com

Cheers

ian
 

wlinart

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Another option i tried: i asked some manufacturers on aliexpress for making a custom toroidal transformer. The one in wanted was special: 8 secundary windings, for a total of about 30W. I asked several companies, and a few just weren't able to make one, but after a bit of a search i found one. They asked for this transformer about €30, which is a very good price IMO. It arrived in about 10 days, and i measured it. (didn't have the chance to actually use it in circuit yet). The voltages were spot on. I also did an isolation measurement and that too was perfect. Here's the company i worked with: Custom toroidal transformer
But there are a few other companies like that. Just look around for a bit, and send them a message with what you want.
 

PermO

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If you are in EU.

These guys in Poland do high quality custom work at very reasonable prices;

 

xander

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Loads of kits (amplifier, preamp, etc) use the non-toroidal Hammonds. Most of the kits provide the Hammond 200 series (such as the 270CX or 270CAX). The odd thing about that series is the high voltage secondary's wires are pulled under the core and exit the case on the primary/mains side of the case (increased noise and other demons?). Where as Hammond's 300 series does not have that odd wiring run. Plus they have a Faraday Shield built into them. For not much more than the 200 series (all thing being relative, of course). Same basic block of stuff, with the 300's doing it better.
 

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GussyLoveridge

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Hi folks - I have been bugging Mr. Thompson-Bell about transformers for a while. Sorry sir! Thanks for all your help and thank you for pointing me here.

I am building a bench supply for prototyping some mic amps. Need 300-350 VDC HT, phantom and 12 vdc heaters. I'm using a PCB generously gifted by Mr. Thompson-Bell, his Lunch Box External PSU V2.

I'm finding it hard to source the correct transformer. I have found the following from Don Audio:


This gives me pretty much what I'm looking for; however, I'm in Canada and would prefer not to ship from overseas.

I'm not opposed to using separate transformers for the heaters, phantom and HT at all. But I am struggling to find cost effective options.

I'm looking at probably somewhere between $150-200 CAD by the time I tack on shipping and duty. Is it unreasonable to think I can do better than this? Does that seem reasonable? Certainly there is more than enough current capability for what my needs are now.
 

soapfoot

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Could this Edcor unit be a contender?

 

Tubetec

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A universal 230v bathroom shaver isolation transformer (20VA) makes a great easily available source for a HT transformer . Most electrical retailers do them from around 10 euros up here .
You can wire it for 110 or 230 v output .
Those horrendous 60's-70's Bates Motel sink splashback vanity units with the light and shaver socket , if you kick the crap out of them they have a useable transformer inside , either way its something you'll often see in electrical recycling facillities in one form or another .

Theres no doubt the low profile UI transformers have advantages over the usual single bobbin lump , encapsulated is probably all the better from the point of view of any kind of lam rattle or buzz in the windings occuring .
Also the fact that the transformer core is isolated from chassis (appart from the ground connection you make intentionally ) helps reduce unessesary noise currents flowing in the chassis .
I noted in another thread a while back Hammond do the low profile units down to a few Va, with 120 or 240 volt output , dual primary ,dual secondary windings allows great flexibility , both primaries in parralel for 110v ,in series for 230 and likewise at the output depending on what yout HT requirements are , you could just as well make a negative HT rail off one of the secondaries , if the circuit required it .

Ian,
you had been looking at a way of generating HT from the heater supply lately if I recall correctly , maybe you have abandoned the idea now in favour of the transformer for simplicity.
Id also been looking along the lines of LT battery derived HT , simple switching circuits opperating above the audio range in the 10's of khz , with horribly fuzzy wuzzy waveforms .

I have a Bruel and Kjaer SPL meter , it derives several supply rails including 200v capsule polarisation , 48v fet supply and +/-14 volts for op amps all from a nicad battery powered low distortion sine wave reference oscillator , this 1khz tone is stepped up via a transformer, rectified and filtered in the usual way to produce the various rails .
I know Whoops mentioned the back to back transformer like Jacob uses to get the HT , Ive tried it and it works ,
Could a sine wave oscillator in the KHZ range be applied to a low profile mains transformer so its stepped up ? example circuit below . I think it was shown before the Low profile UI mains transformers have plenty of clean bandwidth in the audio range , the basic sine wave invertor circuit can run at anything from 50 hz up to mhz .

A benchtop Arb gen with a with a few hundred ma drive into 50 ohms could be used to energise a small transformer in step up mode ,
Might make a handy variable HT supply where you only need smaller currents like in tube mics or preamps
I have a negative bias supply I made using a 3w 120v:6v transformer in reverse , if i can dig it and the arb gen out I might take a look . Very easy to bread board up in any case , just your transformer ,bridge rectifier , filter caps and dropper/bleed resistors , you dont even need noisy zeners , any voltage you want is easily dialed in via the ARB down to mV resolution if you want it . A small HT choke might be a worthwhile addition if you want more quietness and still remain with all passive filtering , any small low current junk box ht choke will do the job fine.
The arb gen itself is capable of around 20 volts peak at 250mA per channel max so whats the best transformer ratio/vA rating to go for ? In theory the arb gen with linked channels ,phase reversed on one side should give me 40 volts differential drive ,so maybe a 24v +24v :120v +120v transformer ?
A handy benefit of diferential drive is you no longer need any ground connection from the transformer to the generator , you have galvanic isolation to prevent ground loops and your free to establish ground in the most apprpriate way for the circuit your using
 

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