Tube DI Box

[Wavelength]

Generally.....PCB-fabrication shops can only hold their PCB board-outline dimensions to two (2) places (i.e., 3.93") UNLESS you want to pay extra for a "3-place held tolerance" on your board-outline and -- WHY -- would you even want to do that anyway???

/

That was a problem like 10 years ago. Now with 4mil traces and separation being the norm you can pretty much specify anything you want at no extra charge.

 

[Wavelength]

Ok since my AverTech is having issues with the new macOS, I ran some tests on the thing with the Jensen output on the Prism dScope III. I am going to order up a bunch of outputs including some UTM, Carnhill and others. Suggestions?
So here is the THD + noise, frequency response is above in a post. This looks pretty good and quiet considering it's strung out on my bench without a tube shield. Termination on the output was 600, Vrms output was ~1Vac, input as you can see was 0.22Vac rms gives it a gain of about 13dB.

 

[ruffrecords]

That's a lot of higher order harmonics at only 1vac output. That will definitely have a "sound".

Cheers

Ian

 

[MidnightArrakis]

That was a problem like 10 years ago. Now with 4mil traces and separation being the norm you can pretty much specify anything you want at no extra charge.

[you can pretty much specify anything you want at no extra charge] -- Maybe, but.....it actually boils down to which PCB-fabricator that you decide to use. I spent a couple of hours earlier this afternoon calling PCB-fabricators all over the U.S. here and their answers were all over the place when it came to how well they can hold their tolerances!!! Some stated that they typically route boards +/- 0.010" and +/- 0.005" will cost you extra, while others stated that they can typically route a board at 0.005" at no extra cost at all!!!

However, and this is similar for sheet-metal fabrication (although I am fairly certain your experiences are different), the PCB fabricators stated that they can hold a +/- 0.010" tolerance from the board-edge to the center-line of a mounting-hole and then a +/- 0.005" tolerance from that first mounting-hole to any other hole on the board. This is also how I dimension both my mechanical sheet-metal and PCB-design fabrication drawings.

/

 

[Wavelength]

That's a lot of higher order harmonics at only 1vac output. That will definitely have a "sound".

Cheers

Ian

Ian probably the highish output Z of the SRPP (~2300) is the reason for that into the 10K load. Still low thd but the drive isn't really as good. I have a folded cascode discrete DI board I was thinking of making up for a show I was doing in May in Florida (all acoustic) and never got a chance to finish it. I am going to make it up tomorrow so I can compare them.
Thanks,
Gordon

 

[Wavelength]

All finished up the board today by adding all the pads for the output transformer. I will order some via JLPCB soon.
I talked to David over at CineMag and he came up with a real nice 10K:600 PCB version. I ordered 11 of them in 50/50% M6/High Nickel.
I am going to do some front and back panels on a Hammond 1455 type chassis which is 4x4 and available in 6" and 8" long extrusions on Front Panel.
As above the new version has input Z at 1M, 500K and 100K and has an input TS and through TS with a male XLR. There is also a switch for termination at either 620 or 3K3 (of course you can change this to optimize to other outputs). There is also a switch for XLR PIN 1 to ground or open.
Thanks,
Gordon

 

[Wavelength]

Oh I almost forgot... I have been designing a number of tube DI for pedal boards as well. Maybe too many of them. I talked to a bass player friend this morning told him I am on probably design 164. Then talked to Jim Hagerman and had another idea. Just don't want the damn thing to draw a bunch of current. I have a 6418 clean boost with a HV supply and the whole thing only draws 35ma. Maybe something more robust than that one.

 

[Wavelength]

Guys,
It's been a while and I received the proto CineMag output transformers and they were really good. But they were 8:1 instead of 4:1.
Dave is making new ones this week for me and I should have some testing data and the new PCB's because I have a little free time next week.
BTW I got to thinking about a pedal version of this running on 9V. I tried the 5703 (wired pencil type) tube and it worked well with a 140H loading choke I had on hand. The whole thing drew a bit more current than I wanted at 340ma. I have a bunch of 6C4WA tubes that should work about the same and that would bring the current down to about 290ma.
Thanks,
Gordon

 

[merlin]

That's a lot of higher order harmonics at only 1vac output. That will definitely have a "sound".

Quite normal behaviour for an SRPP-alike.

 

[ruffrecords]

Quite normal behaviour for an SRPP-alike.

Yes, you are right. I somehow misread the fundamental as the 2nd harmonic!

Cheers

Ian

 

[Wavelength]

Also I was thinking of this pedal version again last night and there was other drawing sitting there I did a week ago. Maybe a dissimilar folded cascode would work and be less expensive. Use a 6C4 as the input and a PNP transistor on a 80V supply running each one around 2.5ma with the PNP collector to ground at 12.1K to ground would give good drive low output Z and not cost too much. I figure that completely isolated power would pull about 240ma from 9V.

 

[ruffrecords]

It is nearly three months since I was hospitalised with Covid and pneumonia and I am pleased to say I am now pretty much recovered. I tried some soldering last week with no effect on my lungs (although I took care to have a fume extraction fan turned on). So I hope soon to be back working on the Tube DI project.

For starters, I think it is a good idea to take a leaf out of the Midnight Arrakis book and sort out the enclosure. The basic requirements of it I can think of are:

1. Readily available throughout the world
2. Big enough - there is likely to be a big Carnhill transformer in the as well as the tube, a pot and a couple of connectors
3. Tough enough to withstand stage use
4. Easily customisable - we want the end result to look good so a separate panel that can house the connectors and pot(s) plus a professional looking legend is a must
5. Easy to work. Toughness means we need a metal enclosure so we are talking steel or aluminium. Steel is a PITA to work so my preference would be aluminium.

Suggested candidates?

Cheers

Ian

 

[JeffKall]

It is nearly three months since I was hospitalised with Covid and pneumonia and I am pleased to say I am now pretty much recovered. I tried some soldering last week with no effect on my lungs (although I took care to have a fume extraction fan turned on). So I hope soon to be back working on the Tube DI project.

For starters, I think it is a good idea to take a leaf out of the Midnight Arrakis book and sort out the enclosure. The basic requirements of it I can think of are:

1. Readily available throughout the world
2. Big enough - there is likely to be a big Carnhill transformer in the as well as the tube, a pot and a couple of connectors
3. Tough enough to withstand stage use
4. Easily customisable - we want the end result to look good so a separate panel that can house the connectors and pot(s) plus a professional looking legend is a must
5. Easy to work. Toughness means we need a metal enclosure so we are talking steel or aluminium. Steel is a PITA to work so my preference would be aluminium.

Suggested candidates?

Cheers

Ian

Glad to hear you are finding your way back, @ruffrecords !

We forget COVID is still out there and still morphing and infecting loved ones, friends and people we respect on GroupDIY !

I am glad to have you back among the living doing what you love … and selfishly pleased to know you are still available for your excellent comments and advice so many of us appreciate here !

Be (stay) well !!!

 

[Tubetec]

When we think of a DI , we think a compact enclosure that sits on the ground ,

This sounds more like a bench top enclosure would suit the job ,

Steel is probably a good thing if its got a mains transformer as it might serve to contain the transformers radiated energy better .
Effectively screening mains tx from output tx isnt easy , In top class lab grade gear like H/P or Bruel & Kjaer you often see chassis with a stepped shelf inside , in such a way that not only can transformers be placed at 90 degrees to each other but with two layers of steel between one and the other , the outer case forming two distinct areas within the chassis .

There really isnt a whole lot on offer in terms of mains transformers for tube gear at reasonable money ,
A way over specifed Carnhill sounds expensive , and by the time it lands in Ireland and clears customs its likely to have around 30% in added charges on top , probably the same anywhere else in Europe .

Last time I looked at IGS/UTM mains transformers were fairly limited , mainly larger components ,
there certainly seems to be a gap in the market for a small cheap tube power transformer capable of running one or two preamp tubes , like in a DI box or tube mic or simple single mic pre .

A group buy on a customised 10W R-core might be another cost effective option .

 

[ruffrecords]

If we want to go mains transformer in the box then an enclosure similar to the one used by the RED DI would seem to be appropriate. However, my preferred route is an external universal SMPS and internal dc:dc convertors for HT and in some cases heaters.

Steel can be a blessing and a curse. I have had instances where a steel case successfully coupled stray EM field from a mains transformer at one end to a microphone transformer at the other. Aluminium does not do this. By all means add a steel screen between the two but and entire steel enclosure can be a recipe for hum problems.

Anyone know exactly what enclosure the RED DI uses?

Cheers

Ian

 

[Wavelength]

All,
I made the above PCB fit in the Hammond 1455NS1601 for a 4x4" 6" length and there is an 8" as well.
https://www.hammfg.com/electronics/small-case/extruded/1455
I did FrontPanel Front has the XLR, input, thru, switches etc... has room for any off board transformers. Back panel has IEC and power transformer.
Hammond makes one like the RED DI and you can get it modified by them or others.
I will get the panel info out tomorrow when I get to the office.
Thanks,
Gordon

 

[ruffrecords]

The Hammond 1455NS1601 looks ideal, made in aluminium and not crazily expensive either. Thanks for the tip.

I think Phaedrus uses a smaller version for their tube DI:


Notice they use a 12VAC power input for safety reasons and then leave the tube exposed???

Cheers

Ian

 

[Tubetec]

The trouble is there you have a glass tube envelope seperating the outside world from a high voltage source
the code dictates a metal grill , at least thats how massmarket manufacturers of tube equipment have interpreted the guidelines .

When was the last time you saw a Marshall Fender or Vox without a caged tube sheild ?

Id say its well over 20 years ago since I did on a modern production amp

 

[Wavelength]

Ian,
A lot of these pedal types use the 12V supply and the filaments across that as it just makes it less of a design thing. Since they are all drawing pretty high current to get the HV. Except for these guys running the plates at 2 or 3x 9V.
You can get some real nice tube shields from GoodComponents.com Or even these cages. I have been thinking of options for the pedal one for the last week or so. Going vertical like that on a tube unit seems like a bad idea to me. I have seen a bit too many beers fly onto the stage for various reasons and that makes a great place for a liquid to enter.
Anyway for the powered one here are the two panel ideas I came up with and I think are here. I have 2-3 more hours on this crazy SPDIF thing I am custom making then I am kind of free for the rest of the week.
Thanks,
Gordon

 

[ruffrecords]

I was not putting the Phaedrus product forward as a good idea - having the tube sticking out seems more like a gimmick than anything else.

Whilst cogitating about enclosures last night I realised the extruded Hammond case could have access issues and also limited front panel space. It then occurred to me we could use the same enclosures as were used in the Classic Solo design (one version of which was a DI). If memory serves there were two main options. One was a 2U Galaxy enclosure made by Modushop in Italy. I also use them a lot for mixer power supply enclosures. The one I use most is the Galaxy Maggiorato GX288 230 x 280 mm which is only 35 Euros.

https://modushop.biz/site/index.php...arch=Galaxy+Maggiorato+GX288&description=true

The other Classic Solo enclosure was the Hammond 1402FV which is somewhat more expensive at £60.

https://uk.farnell.com/hammond/1402fv/enclosure-instrument-aluminium/dp/2912217

Both these enclosures have the advantage of removable tops which greatly simplifies building, and trouble shooting. They both also have built in ventilation.

Edit: CamdenBoss also make something similar for around £40: https://www.farnell.com/datasheets/2625403.pdf?_gl=1*1sdhegu*_gcl_au*NDEzNDY1MDg0LjE3MjAwMzUyMjk.

We might even get away with a really cheap box like the £20 Hammond 1411P but this lakes removable front and rear panels.

Other suggestions are welcome.

Cheers

Ian