HT Transformers for Vacuum Tube Preamps

[ruffrecords]

I like he shaving transformer idea. Thay also include a thermal fuse to it is not going to burst into flames. 20VA gives you 83mA max secondary current which translates about 50mA dc HT output. Useful for a lot of projects.

I did do some research on generating HT from 12V. I was hoping to avoid using a transformer but it turns out to get 240V out of 12 V you need a very high duty cycle (over 95%) and of course you need 20 times the current at 12V so for 50mA HT you will be drawing 1amp pulses from the 12V supply. You can reduce the duty cycle to a more usable 50% using a transformer but you then end up with2A pulses. If you have to have a transformer then you might as well go the whole hog and make a mains input 250V dc output switcher. At least then the switching currents are a more reasonable. However, nobody seems to make this or supply transformers for it so it went no further.

Cheers

Ian

 

[abbey road d enfer]

These things just don't seem to exist outside the UK (and probably Australia and a couple of other countries). They are available from Ali express and Banggodd, though.

 

[moamps]

I am building a bench supply for prototyping some mic amps. Need 300-350 VDC HT, phantom and 12 vdc heaters......
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...

On ebay.com (from China, if acceptable) you can find cute cheap R-core power transformers for tube preamps or testing like these:

https://www.ebay.com/itm/162245232731?hash=item25c691c85b:g:8BgAAOSw-URiFEYmhttps://www.ebay.com/itm/165358140983?hash=item26801cfe37:g:7~cAAOSw6iNiGvwBhttps://www.ebay.com/itm/353169960397?hash=item523a91edcd:g:35gAAOSwwFZfNhK1

 

[Tubetec]

Thanks for the info Ian ,
So I see if you want larger currents from a switched HT you end up running into trouble with the current pulses and probably extra noise in the LT if thats where the Ht is derived from .

My idea to make a HT supply from a battery and switcher arrangement was for a tube mic so only very small HT currents involved , likewise driving a step up transformer from the arb gen ,its only a few mA I need with variable voltage , basically a test rig to check how tubes opperate under the conditions found in tube mics .

Of course the arb can put out all kinds of wave shapes and frequencies, its easy to store a waveform ,its frequency and amplitude to a preset allowing instant recall of any HT voltage you want , no need to mess with soldering series resistors or zeners . Either ferite or iron core transformers depending on frequency .

Theres lots of choice as far as bobbin and cores with the ferrite ,
Looks easy enough to wind even with basic equipment
How would I go about winding a small ferrite step up transformer thats capable of using the Arb 50 ohm output to generate HT ?
Found this to explain the basics ,
https://microcontrollerslab.com/ferrite-transformer-turns-calculation/

 

[gyraf]

You are aware that there are off-the-shelf solutions out there? Specially for the "nixie-tube" type of projects, there's a whole lot of up-converters to 250'ish volts - some of them also down-convert to 6.3V (usually from 12V input)

https://www.aliexpress.com/item/1005002998520867.html
https://www.aliexpress.com/item/32926216972.html
https://www.aliexpress.com/item/32986857367.html
/Jakob E.

 

[Tubetec]

Ive seen the R cores alright , so far the 20w cores only seem to be dual LV secondary ,
I saw none with a HT winding . The companies that make these seem to invite orders for custom secondary voltages , although I havent inquired , they might be willing to supply samples in smaller numbers . The little 20w core would be prefect for a small tube pre or mic head amp .
What voltage/current would people interested in ?
By my calculation 9.5vac @1A and 175vac at 50mA comes out at 18va , so do-able on a 20w core with a little margin to spare .

 

[ruffrecords]

Not a transformer but a small SMPS that can provide up to about 30mA at 250V from a 12V dc supply:

Power Supply - Page 1 - Taylor Electronics

Looks similar to the type I have seen in some 500 series modyules with tubes in them but could be generally useful for preamps.

Cheers

Ian

 

[dmp]

I'm trying to find a toroid power transformer for a 1U tube preamp build.
The Carnhill VTT2302 would be perfect but I can't find it available anywhere. It is discontinued at AmpMaker. Anything comparable or another source for this Carnhill?

It is a 2 tube preamp so I only need 6.3vAC, 0.7mA heater current, B+ ~250V with 10mA. The height less than ~40mm is the difficulty.

If I can't find a single toroid, I could do two toroids for the 12vAC heater and B+, but I'd rather find the Carnhill or something like it.

 

[abbey road d enfer]

https://fr.aliexpress.com/item/1005...00026895734995!sea&curPageLogUid=JYLVtHmrR8Xuor
https://fr.aliexpress.com/item/3280...1f!64317005697!sea&curPageLogUid=xRiXTJPVGePhmaybe this one
https://fr.aliexpress.com/item/1005...00015642809545!sea&curPageLogUid=rbDZHetfJC9T
You need to check the size with the seller.

 

[dmp]

Thanks - those are good possibilities. I think EI and R-core will have higher stray magnetic field leakage though. A toroid would be the best for a single 1U build for minimum magnetic coupling.
Found this at 36mm height: Upgraded Copper Audio Power Transformer 0-260V 0-6.3V For EAR834 Tube Phono Amp | eBay
I emailed to ask if it can be used for 115v, description says it can, but I don't see wires for dual primaries.

 

[beatnik]

tube-town has a good variety of toroidal power transformers for tube projects at reasonable prices

this one should fit in a 1U rack and has two additional taps that could be used for relays , phantom power, or even powering V1 with regulated DC

https://www.tube-town.net/ttstore/toroidal-27va-preamp-engl-520.html

 

[gyraf]

What about just a simple switchmode supply like this:

https://www.aliexpress.com/item/1005004463285408.html
FF 6.3V
HT 200 or 260V

/Jakob E.

 

[scott2000]

I emailed to ask if it can be used for 115v, description says it can, but I don't see wires for dual primaries

Maybe they mean you can use it for 115 but won't get the same results? . Says 0-6.3V, 0-260V...

 

[Tubetec]

These guys have the R core 30w with 9-0-9v, 0-9v ,0-6.3 v and 240v
https://www.audiophonics.fr/en/transformers-accessories-c-6356.htmlTransformer mounting brackets and shields might be put to good use , one of the toroid cans might fit an R core inside for extra screening ,
Even the 30W R is to high for 1U enclosures .

Seems to me they prewire the toroid for either 115 or 230 at the factory ,

Specifications:

Output Power: 30VA

Input: 0-115V 0-115V (we connect as 115V or 230V input)

Output:
0-AC260V @60Ma
0-AC6.3V @2.28A

https://utmindustry.com/utm9050
UTM9050
Primary: 115+115V or 230V AC
Secondary: [email protected], 6.3V@4A, 9V@0,2A
Wattage: 50W
Core Size: EI76, Z11 Grain Oriented lamination steel.
Bell-style package.
Weight: 0.8kg

IGS mentioned they were going to be offering another version with a different winding config soon,
The 9v winding has to little current to be of much use for a DC heater supply ,
Maybe a little less current @ 6.3v and around 1A @ 9V would be nice .

 

[scott2000]

Seems to me they prewire the toroid for either 115 or 230 at the factory ,

That actually makes sense....

(we connect as 115V or 230V input)

 

[dmp]

tube-town has a good variety of toroidal power transformers for tube projects at reasonable prices

Those look good for 230v but they don't seem to have 115v, dual primaries. Good for EU though

What about just a simple switchmode supply like this:

That looks pretty good. Very affordable. Not the way I want to go for this tube preamp, but definitely something to consider in general.
I'm going to order one to try. Also available on ebay. The description does not list the mA for B+ and H+, but for a two tube preamp it should have the capacity.
This similar one has more than enough B+ and H+ mA, at 100mA and 3.5A, respectively

https://www.aliexpress.us/item/3256...ku_id":"12000018653274062","sceneId":"30050"}

Seems to me they prewire the toroid for either 115 or 230 at the factory ,

Yes, that is correct. They made a custom buy it now for me to get them wired for 0-115v. I am going to try this

 

[thor.zmt]

Probably the most common question I am asked is which mains transformer to use for the HT (and possibly also for the heaters).

Excellent Post, Ian. But I would like to add to it a little.

There is a simple trick that can be used with "OTS" low voltage transformers.

Say we want 12V 2A DC for heaters and 300V/50mA for HT.

Let's buy two 9V+9V AC transformers, one 50VA and one 25VA.

We connected the 25VA transformer "backwards" to one of the low voltage windings of the 50VA one.

Voila, we have 230V @ 0.1A AC and 9V @ 2.777A AC to power our project.

Need different power levels, voltages etc? Adjust the recipe.

How about using two 25VA/9V+9V Transformers with both sets of windings in series and 12V AC input from a dual 6V winding on a 100VA transformer?

We get 310V @ 0.16A AC and 12V @ 4A AC.

Need more voltages? Most toroidal transformers have a 3 Turns per Volt setup. Want 5V/2A for a tube recitifier?

Get high voltage insulated stranded wire, wind around 15 Turns (Check voltage under load with multimeter, voila.

Heck, I once even wound a 180V AC (540 Turns) onto a relatively small toroid.

Need a center tap? No you don't, use a hybrid bridge rectifier with BYV96E in the negative legs and your tube rectifier for the positive legs.

So there are many ways of doing this easily and safely with generic off the shelf low voltage transformers as well.

Thor

 

[thor.zmt]

I did do some research on generating HT from 12V. I was hoping to avoid using a transformer but it turns out to get 240V out of 12 V you need a very high duty cycle (over 95%) and of course you need 20 times the current at 12V so for 50mA HT you will be drawing 1amp pulses from the 12V supply. You can reduce the duty cycle to a more usable 50% using a transformer but you then end up with2A pulses.

Herr Geinacher should be your friend.

Use a 12V : 48V capable switcher chip (or even the old MC33064 with 40V) and use a voltage multiplier cascade to get whatever HT you need.

T

 

[ruffrecords]

Herr Geinacher should be your friend.

Use a 12V : 48V capable switcher chip (or even the old MC33064 with 40V) and use a voltage multiplier cascade to get whatever HT you need.

T

Hello Thor, and welcome to the group. We have discussed using SMPS and dc:dc convertors for tube projects at length in several threads over the years. SMPS have improved so much that I use them all the time for heater supplies and even for phantom power. Meanwell is my go to SMPS manufacturer. Using a 12V to 48V switcher chip plus a voltage multiplier still has the same problem. Since 240V of HT is 20 times the 12 input it means the 12V current will be 20 times the the 240V current. If you want say 50mA HT current that becomes 1A at 12V and the current spikes will be a lot higher.

Cheers

ian

 

[thor.zmt]

Since 240V of HT is 20 times the 12 input it means the 12V current will be 20 times the the 240V current. If you want say 50mA HT current that becomes 1A at 12V and the current spikes will be a lot higher.

Step-up IC's these days handle several ampere easily.

My "go to" from Richtek has 5A max. switching current and operates at 1.2MHz. The result is a very compact layout.

With correct design, parts and layout it is quiet enough that neither 300MHz/1mV/Div Oscilloscope or 5GHz spectrum analyser plus LISN shows much of switching frequency breakthrough.

I am actually using something like described for the Tube HT in commercial hybrid products.

It does imply a lot of very small SMD parts, but there are now suitable PCB/PCBA houses that can do the work needed and deliver pre-assembled boards.

So I still think doing this is a highly valid option. Especially if you already use SMPS for heaters, why not design a suitable Module for HT?

Thor

FWIW, I also designed a direct off-mains SMPS with both heater and HT supply on a single transformer, using Fairchild/On-Semi FSCQ1565RT, this was used for a stereo EL84 PP Amplifier and also ended supremely quiet even at full load which dropped the switching frequency to around 24kHz.

SMPS / Switched DC-DC conversion has rather come of age.