Tube DI Box

[Sorr]

Whats the resistance of that 150H choke?

 

[Gus]

OK, so you mean it would be mains powered?

Cheers

Ian

You could use a listed 12VAC wall wart to power the box with the other transformer in the box that MIGHT be good enough for code you would need to check.
I could not find the thomas holly? Phantom powered tube DI thread I think it was posted around 2004 does anyone remember it?

 

[Tubetec]

The Max 1771 switcher module seems to have a few advantages ,
it can opperate to 300khz , its high effeciency and small size means it should fit in a tube mic ,
A small screening can might help reduce the noise .

 

[Rob Flinn]

That's OK but I have to ask, what would you like? Surely not a mains cable either so are you asking for it to be battery powered?

Cheers

Ian

I would much rather have an iec mains lead plug into the box, even if the box were considerably bigger as a result. I did exactly that when I made a sanasamp bass driver copy.

 

[ruffrecords]

You could use a listed 12VAC wall wart to power the box with the other transformer in the box that MIGHT be good enough for code you would need to check.

I did mention using an AC wall wart but I think someone said they are not readily available any more.

Cheers

IAn

 

[ruffrecords]

Whats the resistance of that 150H choke?

Which 150H choke do you mean?

Cheers

Ian

 

[merlin]

I did mention using an AC wall wart but I think someone said they are not readily available any more.

Broskie is still using them, I find it hard to believe they've gone the way of the Dodo yet?
https://tubecad.com/2020/06/blog0505.htm

 

[ruffrecords]

I quite like the idea of a battery powered unit. Possibly use a DL92 (1.4V @100mA heater) along with SMPS HT supply of say 48V? With just 3mA at 48V the switcher would require another 100mA at 1.5V. A C size cell at 100mA discharge has a 6000mA/hr capacity so it would last 30 hours. Is that long enough? The DL92 can be used with its heaters in series at half the current. This would also simplify the SMPS and halve it current draw giving something like 60 hours life.

Cheers

Ian

 

[Perm0]

I did mention using an AC wall wart but I think someone said they are not readily available any more.

Cheers

IAn

Just did a quick check and..... damn, what happened ?

These are indeed very hard to find now.

 

[ruffrecords]

Broskie is still using them, I find it hard to believe they've gone the way of the Dodo yet?
https://tubecad.com/2020/06/blog0505.htm

They are still available from Farnell and on Amazon and the Broskie power supply is exactly what I had in mind.

Cheers

Ian

 

[Perm0]

I quite like the idea of a battery powered unit. Possibly use a DL92 (1.4V @100mA heater) along with SMPS HT supply of say 48V? With just 3mA at 48V the switcher would require another 100mA at 1.5V. A C size cell at 100mA discharge has a 6000mA/hr capacity so it would last 30 hours. Is that long enough? The DL92 can be used with its heaters in series at half the current. This would also simplify the SMPS and halve it current draw giving something like 60 hours life.

Cheers

Ian

Something with a charge port would be cool indeed, plug it in to charge, unplug when in use.

I like what the hifi guys do these days, run a DAC from supercapacitors, when playing music the PSU is disconnected from the system, when no music is playing it charges the supercaps.

With current battery technology, power is not really a problem, they will take quite a bit of space though.

"Do not drill holes in this DI"

 

[BIASKING]

Jameco still has a lot of older "clearance" type stuff. I've found many hard to find wall-warts there.
https://www.jameco.com/c/Power-Supplies-Wall-Adapters.html

 

[chrispbass]

I quite like the idea of a battery powered unit. Possibly use a DL92 (1.4V @100mA heater) along with SMPS HT supply of say 48V? With just 3mA at 48V the switcher would require another 100mA at 1.5V. A C size cell at 100mA discharge has a 6000mA/hr capacity so it would last 30 hours. Is that long enough? The DL92 can be used with its heaters in series at half the current. This would also simplify the SMPS and halve it current draw giving something like 60 hours life.

Cheers

Ian

This could be the way forward Ian as I suppose there are plenty of other mains powered valve DI’s one could make, so having something a bit different would make it a more unique project.

 

[ruffrecords]

Just did a quick check and..... damn, what happened ?

These are indeed very hard to find now.

For some reason 12VAC outputs are much more common than 9VAC.

Cheers

Ian

 

[Tubetec]

Theres a few things about battery the battery powered tubes here , mainly destined for radio ,
https://www.radiomuseum.org/forum/a_simple_battery_shortwave_superheterodyne_receiver.html
Im not sure how suitable the DL92 tube would be as an audio amplifier , they seem to reccomend others in the series for that purpose, the small signal tubes in the range opperate with only a few hundred uA's of anode current .

Seems a shame to just utilise one side of an ECC88 and leave the other section unused ,
Could a pair of cathode followers with their anodes connected , form a kind of upside down long tailed pair , either balanced input from grid to grid , or unbalanced with one grid grounded or impedence balanced line ?

The other thing about self biased cathode followers is they present a load many times the grid resistance value , so it works out something like the actual grid resistor value is multiplied by the gain of the tube , due to the way the feedback is applied .
That property is useful for tube mics , as we can realise a much higher input impedence than the resistor itself . Take an EF86 , they have a maximum reccomended grid resistor of 40Mohms but it achieves very high input impedence with the back biased CF .
the other handy thing with a CF tube mic is the voltage at the grid of the tube can provide capsule bias , grid connected directly to backplate with outer membrane grounded .

 

[rock soderstrom]

Here are some crude real world examples that give you an idea of how loud the switcher PSUs I mentioned are. In this test it sounds quite ok (ignore the overdrive and the bad singer), you can hear some PSU noise, but that's not surprising. I'm asking for trouble here, as member Tubetec has correctly recognized. No shielding, the PSU board is practically on top of the reverb spring. Nevertheless, the background noise is kind of ok, and can certainly be optimized.

Edit: Video deleted

 

[ruffrecords]

This could be the way forward Ian as I suppose there are plenty of other mains powered valve DI’s one could make, so having something a bit different would make it a more unique project.

I would not be too hard to make it rechargeable (from a regular stomp box 9VDC supply). C size Nickel Metal Hydride batter C cell at 6000mAHries are about £6. A pair would give a raw dc supply of 2.4V. THis might be a little low for the DL92 which is specified for 2.8V (series heaters) but still worth investigating. Only 4500mAHr compared to an alkaline primary at 6000mAHr so life would be shorter but it should still easily last 24 hours.

Cheers

Ian

 

[rock soderstrom]

However, it seems that the PSU gets louder with increasing duration and also sporadically produces other noises.

Listen to it yourself.

Edit: Video deleted

 

[Perm0]

So the board you are moving around is a PSU filter ? some caps and chokes ?

At the output of that board the hash is probably gone, filtered, but it is still at the input jack and the cable.
Move the filter to the source of the noise, at the other end of the cable.

Also the wood does not provide a lot of sheilding...

 

[rock soderstrom]

So the board you are moving around is a PSU filter ? some caps and chokes ?

No, its a complete switcher PSU (minus transformer) for B+ and heater voltage.

At the output of that board the hash is probably gone, filtered, but it is still at the input jack and the cable.

The output voltages are pretty clean, with little additional filtering absolutely useable. Ruud used this board for an U67!

The problem is the PSU radiates noise through the air, every inductor in proximity will catch up this without screening. For instance, a OPT without mu metal shield in close proximity will have a hard time. This could be the situation in a tube DI box.

The example is certainly a little exaggerated, but it shows what problems you can get into.

Also the wood does not provide a lot of sheilding...

Yes, that's clear. It's a test setup for now. If in doubt, I will shield the entire PSU. Distance is also your friend when setting up a amp like this...