Tube DI Box

[ruffrecords]

I know there have been and are a few tube DI boxes but I do not know of any DIY ones so I thought it might make a good groupDIY topic/project. Here is my thinking so far.

1. Keep it simple. Little more than a cathode follower - we will be feeding a mic input so huge levels are not expected.
2. Input pot and output pot. Instrument in and out ockets
3. Small output transformer.
4. Run it off a 9v wall wart. Regulate the heaters with a linear regulator and use a small SMPS to switch that up to 90V or so fro the cathode followers. Again, we do not need huge signal levels so we do not need huge HT voltages (but they need to be large enough for the tube to operate in a normal region.
5. ECC88/6DJ8/6922 would be my first choic6. Probably the obligatory ground lift switch (actually, thinking about it we might not need it as this thing is floating anyway)
6. All in a nice diecast box with a custom front panel.

Thoughts?

Cheers

Ian

 

[C.B. - Boudio]

Maybe use an E/PC92 (basically half an ECC81). Or even a D-series tube, since it'll be running on a low B+, a DC9x or something (those are 1,5V directly heated, but not impossible in this case, maybe even a C-cell battery) - I'm just imagining away now.
I don't have much else to say, only that it's really quite interesting!

 

[chrispbass]

I know there have been and are a few tube DI boxes but I do not know of any DIY ones so I thought it might make a good groupDIY topic/project. Here is my thinking so far.

1. Keep it simple. Little more than a cathode follower - we will be feeding a mic input so huge levels are not expected.
2. Input pot and output pot. Instrument in and out ockets
3. Small output transformer.
4. Run it off a 9v wall wart. Regulate the heaters with a linear regulator and use a small SMPS to switch that up to 90V or so fro the cathode followers. Again, we do not need huge signal levels so we do not need huge HT voltages (but they need to be large enough for the tube to operate in a normal region.
5. ECC88/6DJ8/6922 would be my first choic6. Probably the obligatory ground lift switch (actually, thinking about it we might not need it as this thing is floating anyway)
6. All in a nice diecast box with a custom front panel.

Thoughts?

Cheers

Ian

Liking the sound of this Ian…especially something that could be kept physically quite small for live work etc.

Is the smps a ready made module?

Cheers

 

[Gus]

I would investigate the Royer supply with no DC to DC converter

I would not use a 6dj8 type tube

Thomas Holly had a thread about a phantom power DI at this forum I think in the 2000's.

 

[chrispbass]

I would investigate the Royer supply with no DC to DC converter

I would not use a 6dj8 type tube

Thomas Holly had a thread about a phantom power DI at this forum I think in the 2000's.

How do you derive a heater supply from phantom power GUS scuse my ignorance?

 

[Gus]

it is in the old thread

 

[Okimy123]

I have a mastering labs tube DI, Tubes are naturally an unbalanced output so using a 1/4 in and out is simple and easy, You're going to need a 100k/1meg input load resistor since it's a high impedance source but this can change depending on the sound/volume etc. outside of normal tube design there are plenty of subminis that use low voltages, even 12au7 use 9v wall wart on the plate and heater for usage in some stompbox usages. You won't necessarily need an output transformer with it unless you want a balanced output. You can just take singal pre-transfo for 1/4 unbal out and then have an output post transformer for balanced xlr out. this will give you an unbalanced 1/4 out and a balanced xlr out both mic level.
My gear is more for studio work; however, i would imagine that converting bo hansens active DI box 1975 can be converted to tube fairly easily. considering it's already seemingly phantom powered.

check this out and pick one that you like https://frank.pocnet.net/sheets51.html

 

[Brian Roth]

Obviously, we can't use a conventional tube for a phantom powered DI box. 48V into two 6k8 resistors in parallel....and the resistors shorted at the "far end"....is around 0.667 Watts dissipated in the resistors and NO voltage at the "far end".

My beloved Grandma Roth had serious hearing problems. Decades ago, I recall she had a hearing aid pack that she kept in her bra...and two pairs of wires going to two ear pieces. It was battery powered. and I watched Grandad Roth open it to replace the batteries several times. He showed me a tiny vacuum tube inside.

So, hearing aid vacuum tube for a phantom DI??? Seems a bit obscure to me.

Bri

 

[Brian Roth]

@ruffrecords .... Ian, I guess you've had great luck "parking" a SMPS inside a vacuum tube device. I somehow haven't.

Years ago, a friend/client stuffed the PC board for a Drip REDD47 kit and I finished the chassis wiring for him. One addition was an "enclosed" Meanwell supply for phantom. It was a very "cute" unit....smaller than half the size of an Altoids tin.

I mounted it to the rack chassis as far as I could from the active stages. Just the existence of that Meanwell in the chassis (not using phantom power to a test mic) we had strange digi-noise. I tried loading the output with various load resistors, including low enough that I need a 5 Watt resistor. With the DC wiring disconnected from the Meanwell (but load resistor in place).....noise.

As an experiment, I extended the AC mains wiring (including a chassis ground wire) that fed into into the Meanwell, and detached it from the chassis so I could move it away from inside the chassis. Sure enough, once I was maybe 6 inches away, the noise subsided significantly

When I moved the Meanwell in my hand, I could use it as a variable level noise source as I neared the active circuitry. It was acting as a noise radiator. We tried several dynamic mics (located many feet away from the test setup)as well as a 150 Ohm resistor inside of a XLR connector.

Just an anecdote....and we all KNOW the Drip designs are at best "dicey" <g>

Bri

 

[Okimy123]

Obviously, we can't use a conventional tube for a phantom powered DI box. 48V into two 6k8 resistors in parallel....and the resistors shorted at the "far end"....is around 0.667 Watts dissipated in the resistors and NO voltage at the "far end".

My beloved Grandma Roth had serious hearing problems. Decades ago, I recall she had a hearing aid pack that she kept in her bra...and two pairs of wires going to two ear pieces. It was battery powered. and I watched Grandad Roth open it to replace the batteries several times. He showed me a tiny vacuum tube inside.

So, hearing aid vacuum tube for a phantom DI??? Seems a bit obscure to me.

Bri

yea, but i still think a 9v wallwart would work on a 6p30b or 5902 or something submini or even 12ax7. afterall the redd DI is a 12ax7 or something like that. just eliminiate phantom power and makes the circuit a lot easier no?

 

[Brian Roth]

I don't understand the math using a 12AX7 (or similar) that is somehow powered from a phantom supply. The heater alone requires 1.89 Watts.

Bri

 

[Okimy123]

I don't understand the math using a 12AX7 (or similar) that is somehow powered from a phantom supply. The heater alone requires 1.89 Watts.

Bri

I've just worked on a few stomp boxes that use these tubes off a wall wart. I didn't look into it to much other than figuring out why the client had a intermittent cut out issue; however, it's possible. there are other tubes that can work i'm sure too.

 

[ruffrecords]

I would investigate the Royer supply with no DC to DC converter

THanks, I will investigate this

I would not use a 6dj8 type tube

Why not? What would you use?

Thomas Holly had a thread about a phantom power DI at this forum I think in the 2000's.

Unfortunately phantom power is not sufficient to operate a tube heater.

Cheers

Ian

 

[ruffrecords]

Sounds like existing stomp boxes with tubes in could provide some useful insights into the design options.

If we used a 9V ac wall wart then we could replace the SMPS with a simple voltage multiplier for the HT.

Thanks for the inputs so far.

Cheers

Ian

 

[L´Andratté]

I always felt intigued but didn´t build yet, because of PS troubles, by this one that you, ruffrecords, supposedly participated in:

 

[Tubetec]

My recent experience with SMPS powered tube gear tells me you cant have an unscreened SMPS anywhere in the vicinity of low noise tube circuitry or you'll pick up switching hash .
Sure the RF voltages might be smaller in magintude than typical mains related PSU noise , but the subjective annoyance factor of even smaller amounts of RF ,seems to me to be far far worse .

A submini tube as cathode follower , built into a barrel jack that plugs directly into the instrument , is something Ive been meaning to try out . CF is quite capable of driving an unbalanced cable run ,
but then what ?

Theres plenty of manufacturers who used regular 12ax7's with 25v at the anode , with op amp supported circuitry before and after , its garbage , it doesnt do what real tube circuits do .

The phantom power idea is also not feasible , so lets rule that out also .

 

[rock soderstrom]

Thoughts?

Good idea!

I suggest the "Ruud" solution for the power supply. Very flexible, very small and with minimal additional filtering very clean for a switcher PSU. 9-12VDC or AC in, 150V-280VDC/15mA out plus 6.3VDC/1500mA. Top bang for the buck.

There are "better" tubes than E88CC and 12AX7 for a nice instrument sound. 6SN7/6CG7, 6n1p or some nice pentodes like 5654, EF800, 5840...

Edit: in very close proximity to inductors/transducer you will need screening. See my test-rig with the spring reverb. Otherwise you will hear your PSU sing.

 

[ruffrecords]

My recent experience with SMPS powered tube gear tells me you cant have an unscreened SMPS anywhere in the vicinity of low noise tube circuitry or you'll pick up switching hash .
Sure the RF voltages might be smaller in magintude than typical mains related PSU noise , but the subjective annoyance factor of even smaller amounts of RF ,seems to me to be far far worse .

I think screening goes without saying but if we can get an ac wall wart we could use a voltage multiplier instead. Old school but effective.

A submini tube as cathode follower , built into a barrel jack that plugs directly into the instrument , is something Ive been meaning to try out . CF is quite capable of driving an unbalanced cable run ,
but then what ?

Yes, the ancients used cathode followers to drive video sgnal down long cables.

Theres plenty of manufacturers who used regular 12ax7's with 25v at the anode , with op amp supported circuitry before and after , its garbage , it doesnt do what real tube circuits do .

The phantom power idea is also not feasible , so lets rule that out also .

25V HT is a not a proper HT IMHO. That;'s one reason I favour the 6DJ8 and friends because it is designed to operate at 90V HT.

Cheers

Ian

 

[ruffrecords]

Good idea!

I suggest the "Ruud" solution for the power supply. Very flexible, very small and with minimal additional filtering very clean for a switcher PSU. 9-12VDC or AC in, 150V-280VDC/15mA out plus 6.3VDC/1500mA. Top bang for the buck.

Can't seem to find that on the net. Do you have a link?

There are "better" tubes than E88CC and 12AX7 for a nice instrument sound. 6SN7/6CG7, 6n1p or some nice pentodes like 5654, EF800, 5840...

Tube choice is always bound to be contentious.

Cheers

Ian

 

[rock soderstrom]

Can't seem to find that on the net. Do you have a link?

https://www.ebay.de/itm/235284606232