Toroidal power transformer shielding

[beatnik]

Hi everybody,

I am building a portable tube based mic preamp based on a RCA BA-2 circuit and I would like to get a custom made toroidal power transformer for this project.

The manufacturers I contacted offer various options in terms of shielding and I am trying to understand what would be best for this application.

From what I understand external shielding minimises the magnetic interference of the transformer with the signal circuitry, so it's probably a good option to have in this case.

They can also put internal shields between primaries and secondaries and between the various secondaries and I am unsure wether this is necessary.

I should point out that the filament and high voltage secondaries are somehow "shared" by using a tube rectifier. So maybe this is something that needs to be considered ?

My goal is getting a transformer with the best noise performance possible, but obviously I want to avoid spending extra money on unnecessary features.

 

[johnheath]

You could try to see what don-audio has to offer... I use his toroidal transformer a lot. He has a lot ov various transformers wit different secondaries


Bets regards

/John

 

[beatnik]

I did have a look at the don audio stock but unfortunately there isn't a transformer with 460V center tapped secondary, that is required for the BA-2 circuit. There are some that could probably be adapted but are way overkill for this circuit

 

[johnheath]

460V for each leader? That sounds like a...lot? Don-audio has a lot of transformers with 250VAC secondary which will lead to roughly 350VDC after rectification.


Best regards

/John

 

[johnheath]

I have the schematic but it does not say anything about the voltages, but 350V should be enough for plate voltage.



Best regards

/John

 

[beatnik]

230V for each half of the secondary.

I have added the schematic I'm referring to in the first post.

I understand other transformers can be used, but I would like to use the original power supply circuit.

It's actually quite affordable to get a custom made power transformer, I just wanted to understand what I need in terms of shielding.

 

[My3gger]

Do you need tube rectifier in new portable or rack build? Mine has solid state diodes because preamp is in class a which doesn't give sag. Properly dimensioned toroid in big enough chasis normally doesn't need shielding, just turn it around until it gives lowest interference. They mostly come from where wires come out.
I use shielding for secondaries only when i want isolation for tube heaters which is rare. V76 and others where i want really low noise at high gain benefit from such shields sometimes, some compressors too.
BA-2 is very easy build when you have correct OT (Cinemag has cheap and good 50Ni/50Fe), power trafo might need outside shielding only in a very small space.

 

[johnheath]

230V for each half of the secondary.

I have added the schematic I'm referring to in the first post.

I understand other transformers can be used, but I would like to use the original power supply circuit.

It's actually quite affordable to get a custom made power transformer, I just wanted to understand what I need in terms of shielding.

If you want to use the original circuit with the rectifying tube I guess this transformer would do just well...
https://www.don-audio.com/TB750S-Audio-Power-Toroidal-Transformer-Sec-750VCT-63VCT-5V-Shield


Best regards

/John

 

[johnheath]

Or this one might be even better due to the lower voltage for HT


best regards

/John

 

[beatnik]

Do you need tube rectifier in new portable or rack build? Mine has solid state diodes because preamp is in class a which doesn't give sag. Properly dimensioned toroid in big enough chasis normally doesn't need shielding, just turn it around until it gives lowest interference. They mostly come from where wires come out.
I use shielding for secondaries only when i want isolation for tube heaters which is rare. V76 and others where i want really low noise at high gain benefit from such shields sometimes, some compressors too.
BA-2 is very easy build when you have correct OT (Cinemag has cheap and good 50Ni/50Fe), power trafo might need outside shielding only in a very small space.

I have a bunch of 6X5 laying around so I just decided to stick with the original power supply.

I would like to make the chassis as small as possible, so it's probably good to have an external shield.

Regarding internal shields, from what I understand the shield between the primary and secondaries serves to stop rf interference from mains to power supply lines. Is this actually necessary with tube circuits ?

 

[beatnik]

If you want to use the original circuit with the rectifying tube I guess this transformer would do just well...
https://www.don-audio.com/TB750S-Audio-Power-Toroidal-Transformer-Sec-750VCT-63VCT-5V-Shield


Best regards

/John

That is way overkill for a single preamp channel, and too expensive. But thanks for pointing out

 

[buildafriend]

I built a BA-2. I thought it was thick sounding and that it didn't have a have a lot of gain. It never went on an analyzer though since it was before I had a decent one. I probably could have gotten more gain out of it had I used different transformers. It's a cool circuit and the simplicity is nice. The valve grid caps might be a little annoying for travel..

You will sometimes see that the inside of rack cases are divided by a piece of metal which helps. There are also covers available specifically for toroidal transformers. If you really want to eliminate any potential issues then use an external supply, but that might not meet your needs for a portable unit.

Put a well insulated rubber glove on and with one hand move the transformer around while the amp is at max gain, where ever any hum is audible is where the transformer should not be.  Use all the tricks. Fake center tap on heaters or DC heaters, large smoothing caps, shielding. Listen with a compressor slamming the unit to really hear what the transformer placement is doing. Ian told me to use a 150 ohm R to terminate the input from pins 2 a 3 to help observe the self noise of the amp. I found it useful.

Best of luck,

 

[beatnik]

Thanks for the suggestions.

Regarding the ht current rating,

6J7 datasheet shows 6.5mA plate current for triode connection,

For two tubes 15mA would be OK or it would be better to leave some more headroom ?

 

[My3gger]

Both tubes take ~5,6mA at 248V after RC filters. Here is chart for calculating power transformers:
http://www.sowter.co.uk/rectifier-transformer-calculation.php

I aim for Iac x 2-2,5 because custom toroids are very cheap here and they radiate less if far from saturation. I got situation when winder didn't follow my instructions and tx didn't reach targeted voltage when loaded, that's why higher power.

 

[PRR]

> 6J7 datasheet shows 6.5mA

Yeahbut.... V1 here has 120K in plate. If the tube were dead-short it would only be 2mA. Dead-short is not likely; next-guess is the tube fair-fights 120K and sits halfway up the 240V rail. 120V on 120K is 1mA, not 6mA.

V2 is a little trickier. But the 2,270 ohms cathode bias and 1620's Mu near 20 suggests the tube "acts like" say 45K. This leads to 5mA-6mA.

You can't buy a transformer as small as 7mA; the wire would be too fine to manage.

Distance is better than magnetic shielding.

Inter-winding electric shield reduce crap from bad power; elevator motor pops etc.

 

[My3gger]

When start ordering other material you could make cathode caps bigger and get smallest possible OT gap for lowest -3dB. I got problems with low end's -3dB at 55Hz when using iron core gapped for 10mA. The same model from Cinemag with 50Ni and 7mA sounds better and has -3dB at 20Hz, iirc. I'm sure you will find this important.
You mentioned grid caps; EF86 should sound the same as 6J7 or 1620 in this circuit, it doesn't need redesign.
DaveP has good examples and explanations of shielding in Lorenz V-241 thread: https://groupdiy.com/index.php?topic=67386.0

 

[beatnik]

thanks for the good information.

since the total current draw of the tubes should be 7-8mA as PRR pointed out, a transformer rated at 15mA should work fine ?

for output I am planning to use an UTC A-25 as I have read good reports on the forum.

 

[Tubetec]

My guess is that if you want to run a tube rectifier you will benefit from internal sheilds in the transformer ,often a pulse from the ht rectifier can bee seen on the other rails . You'll also want a 6.3 volts ac line dedicated to the 6x5 ,this winding see's the full ht volts ,so you dont want to have to share it with other valves . There are several Chinese R core transformers with suitable voltages and currents for your project , although aiming for a transformer delivering 15ma is a bit mean , 50-100ma is more likely what you''ll find ,and although its much more than you need ,it may mean your transformer doesnt spew out as much radiation too.

 

[buildafriend]

50-100ma is more likely what you''ll find

This matches my experience and has always worked fine for preamps with just a few tubes. Also, expect the unloaded secondary voltage to be above what the data sheet usually says and for it to sag a little bit when loaded

 

[My3gger]

Transformer delivering 15ma is a bit mean , 50-100ma is more likely what you''ll find ,and although its much more than you need ,it may mean your transformer doesnt spew out as much radiation too.

This matches my experience and has always worked fine for preamps with just a few tubes. Also, expect the unloaded secondary voltage to be above what the data sheet usually says and for it to sag a little bit when loaded

50mA is AC current i use for BA-2 too, one of reasons was easier calculation for winder and no difference in price.
When loaded voltage changes around 1V or so during functioning, measured by cheap DVM. Both builds have almost unmesurable noise by Tek 2247A. So comparable to good fixed gain preamps, despite BA-2 having a bit more complicated shield grounding of level control between stages.

Does anyone really hear difference between SS and tube rectification with preamps runing in class a? Could be difference because of less stiff psu from old schematics.