DIY DI BOX or LINE OUT for 60W tube head?

Tube or solid-state, this section goes to eleven!

DIY DI BOX or LINE OUT for 60W tube head?

Postby tedsorvino » 22 Nov 2017, 18:34

Is there a way to build a good DI BOX like (THE JUNCTION by PALMER or the RADIAL one) for tube amps? Any schematic or layout available?
I need it in order to avoid the cabinet for easy home recording of my 60w tube amp head, without the cabinet at line out levels.
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby FiveseveN » 22 Nov 2017, 22:16

You will need a dummy load first, and you may pick a simple resistive one or a reactive load. Then use a resistive divider to bring the signal down to line level. Add a balanced driver if you feel fancy. You will also need a cabinet simulator, but I suggest adding one in software, with plugins like LeCab and the vast variety of impulse resposes available online. I presume you'll be recording in the box and not on tape :D
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby ppluis0 » 23 Nov 2017, 01:59

tedsorvino wrote:I need it in order to avoid the cabinet for easy home recording of my 60w tube amp head, without the cabinet at line out levels.


Hi Ted,

Be aware that tube amplifiers MUST remain connected to a loudspeaker or at least a dummy load.

If you run this kind of amps without load a catastrophic failure at the output transformer is usual to happen. :!:

Cheers,
Jose
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby tedsorvino » 23 Nov 2017, 02:09

Thanks for your replies both of you.
Of course I'm aware of the fact ppluis0.
Fiveseven could you be more specific please. If you could share a schematic-layout or even a picture of something similar. It could help.
I mean..ok dummy load (resistive or reactive) is pretty simple, but what value for 8ohm/60watt and what Resistive divider? And what
is balanced driver (are you talking about an xlr input or something)? Yes the cab simulator is not something I care since I want just to record in the box.
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby phatt » 23 Nov 2017, 12:17

SPEAKER2LINE.JPG
Here is a simple Speaker to Line load box I built some time back and is still working today. It sits under a little 10 Watt Valve Amp I built.
Just make the Load resistor twice the Amp wattage and it should get Hot but not overheat.
I used wire wound jug elements to test the idea out and they are rated at 1,000Watts but out of water you have to de-rate that number a lot but they handle a 50 Watt Amp fine.
If you don;t know about Balanced line out then don't bother with it as the only advantage to those is if you want to run very long cable.
Have fun,, Phil.
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby tedsorvino » 23 Nov 2017, 15:35

Thank you very much Phil. That's really brilliant.
Just a couple of questions
Does the cap go before the pot to ground or from the pot lug to ground?
Is 330r/ 10w adequate for 60w?
According to your advice it must be.
Thanks again for the excellent advice and schematic.

If you have a picture of the box available it would be really great.
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby phatt » 24 Nov 2017, 13:45

Hi again,,
It's written on the diagram but to be clear;
for ~50 Watt you need TEN 330 Ohm Wire wound 10 Watt resistors.
These are wired in Parallel which gives you a 33 Ohm 100Watt resistive soak.
So a cranked 60 Watt Amp should not overheat the bank of resistors but they will still get pretty Hot so leave air space between the 10 load resistors.

I don't have any photo's of the unit I built and it's all packed away till 2018. I'm sure a bit of googlefu will find pictures of resistive loads,,This link came up on top of a search; https://paulinthelab.blogspot.com.au/20 ... m-for.html

(another case of 101 uses for dead computer PSU's, :applause: )

This chap uses a fixed voltage divider where as mine uses a pot. His divider network uses lower values but you can use any resistor combination just shoot for a 20/1 division and it will be close. So a VDiv of 500k/25k is going to give similar results as 50k/2k. No matter what type of soak you implement the line out V Div will work the same. If the line out is too hot and clips the input of your recorder then just change the divider network.
On my circuit the the output was a bit too hot so I just added a 39k resistor across the 50k pot to back it off rather that change the whole thing.

The link above shows those fancy alloy finned resistors but the plain ceramic units are a lot cheaper and work just fine. :secret:

Regards the Cap option; just wire it across the pot. :thumbsup
Phil.
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby tedsorvino » 24 Nov 2017, 16:13

Thanks a lot Phil. Fantastic info. I will try it soon
Best
Ted
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby tedsorvino » 24 Nov 2017, 23:19

Sorry Phil but by just reviewing the info I realised the impedance value your calculation shows. 33 Ohm???? My amp is 8ohm...So I guess I need 10 x 80ohm/10 watts resistors in parallel. Am I right?
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby phatt » 25 Nov 2017, 13:34

OK don't panic,,,
This is one of those tecky conundrums that gets debated by the experts and confuses the rest of us who can't read hyroghlific maths [smilie=a_makeitstop.gif]
If you use 8 Ohm load it will work but the outcome can be improved by using ~30 Ohm load.

When they say; 8 Ohm speaker they are referring to it's DC resistance.
But the Z (impedance) is vastly different, Z is resistance of AC and if you look at a graph of the Z of a speaker you will find it's all over the place like a dogs dinner.

30 Ohms is the *Average* Resistance over the whole bandwidth. which opens up the Valve amp output a bit better. OK still not perfect but better than an 8 Ohm load box which will make your $$$ Valve rig sound like a tube screamer into a cheap Amp.

If it worries you then keep in mind that a famous Attenuator named "The Ultimate Attenuator" uses a 30 Ohm load resistor and IIRC works for 4,8 and 16 Ohm outputs.

There was an indepth build of the UA on another forum, a chap built one from the ground up but all the pics are no longer there due to Photo-phuket. :block:
The chap could not believe how well they sounded so he built one. :horsey: :horsey:
There was much debate (by highly competent technicians) one agreed it obviously works and it's safe.

Note;
Like all attenuation of Valve Amps I say; Make DAMN sure your Amp it capable of running flat out into an 8 Ohm load BEFORE you BLAME Attenuators when it blows up. A lot of later Amps run Valves way past there limitations and Power Valves burn out within 6~12 months.

I've run my little 10 watt rig flat out for hours and had no issues. I also built one for a mate who had a Fender Pro Junior and absolutely no problems.
Phil.
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby tedsorvino » 25 Nov 2017, 18:52

Thanks Phil for the in depth info. I know the ultimate attenuator. Actually I'm using a simple L pad I've made myself but I didn't try to build a line out. I guess my rheostat -10ohm/150w- is a load, but I don't want to try it as a line out since there is no such provision on the schematic I used (very basic thing)
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby lead2203 » 25 Nov 2017, 21:11

phatt wrote:OK don't panic,,,
This is one of those tecky conundrums that gets debated by the experts and confuses the rest of us who can't read hyroghlific maths [smilie=a_makeitstop.gif]
If you use 8 Ohm load it will work but the outcome can be improved by using ~30 Ohm load.

When they say; 8 Ohm speaker they are referring to it's DC resistance.
But the Z (impedance) is vastly different, Z is resistance of AC and if you look at a graph of the Z of a speaker you will find it's all over the place like a dogs dinner.

30 Ohms is the *Average* Resistance over the whole bandwidth. which opens up the Valve amp output a bit better. OK still not perfect but better than an 8 Ohm load box which will make your $$$ Valve rig sound like a tube screamer into a cheap Amp.

If it worries you then keep in mind that a famous Attenuator named "The Ultimate Attenuator" uses a 30 Ohm load resistor and IIRC works for 4,8 and 16 Ohm outputs.

There was an indepth build of the UA on another forum, a chap built one from the ground up but all the pics are no longer there due to Photo-phuket. :block:
The chap could not believe how well they sounded so he built one. :horsey: :horsey:
There was much debate (by highly competent technicians) one agreed it obviously works and it's safe.

Note;
Like all attenuation of Valve Amps I say; Make DAMN sure your Amp it capable of running flat out into an 8 Ohm load BEFORE you BLAME Attenuators when it blows up. A lot of later Amps run Valves way past there limitations and Power Valves burn out within 6~12 months.

I've run my little 10 watt rig flat out for hours and had no issues. I also built one for a mate who had a Fender Pro Junior and absolutely no problems.
Phil.

Also to add to this .... The UA is basically a load resistor and a power amp ...kind of ....it's an anode follower. So I wouldn't put this in the traditional attenuator category. It's very cool ....it does change the top end a little ....but it sounds very good. There used to be a schematic online ...but I can't find it .....
Also when an amp blows when using an attenuator ....Most likely the tubes were going to blow anyway....just running the amp hard just made them blow sooner....so ...if you are going to use an attenuator ....make sure the amp is running with good tubes and the amp is in top shape.....and you really shouldn't have any problems.....
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Re: DIY DI BOX or LINE OUT for 60W tube head?

Postby tedsorvino » 26 Nov 2017, 04:00

Thanks lead2203...
Just to add my club etc. stage experience with my DIY L PAD attenuators...

A fender twin with a DIY L pad at around 50% attenuation sounds clean at about 3.5 volume pot (of course without 4 6l6 - by just having 2 of them,...or better 2 kt66. Cut bass....add treble..average mid
And a JTM 45 sounds excellent with the same settings , with the same diy attenuator..and 2 KT66 tubes. The tone stack has to be cut treble...full middle...average bass. And of course the volume can go up to 6 ..because...We love jtm 45 crunch.

In the studio...who cares about attenuators and cheap sound proof solutions and reduction thingies ????????
I mean the equipment goes to its limits ...without annoying anyone...and damages may happen.
But that's the price you pay if you don't play it safe. I mean...if you ..want to ...experiment..to rock.

But what I'm asking and what Phil advices and explains is something totally different, I think.
Thanks anyways
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