Running a tube head without a cab - Dummy Loads

[Dante]

Man, I hate posting in the technical section...I know not of what I speak - lol. I have a tube head that has a built in DI, but it clearly states to NOT run the amp without a cab connected. So, I started looking into dummy loads, DIY dummy load boxes, etc.

Attenuators keep coming up. I guess they'd work, if I just dialed them down to zero, but do they really provide a speaker load?

Can't I just use a stompbox sized unit like the RedBox for a speaker load? Am I missing something?

[rnolan]

Hey Dante, I imagine MJMP will give you a better answer, attenuators do provide a load, they also soak up some of the output (that's the attenuate bit).  A simple dummy load would just be a resistor of the right value (current/voltage and impedance) e.g. an 8 ohm resistor with high enough voltage tolerance.  Some circuits (seems yours is) need to have a load so the output stage doesn't run away and crap itself (like putting too low a load e.g. 2 ohm when it's not designed to cope, sucks the guts out of the output stage).  Don't know about the RedBox thingy  .

[MarshallJMP]

I don't think a redbox has a load. You can use a high power resistor like Van Halen did or something like the  2 notes captor? I use both sometimes.
I attached a pic for a resistive load, it's easy to make, I used 4 arcol 50W 16 ohm resistors to make a 100W load. Put 2 X 2 in parallel and these 2 in series to get 16 ohm.

[Iperfungus]

I just sold a Laney IRT Studio 15W rack head that has a good number of very interesting features (but I didn't like the tones... ).
One of those features is a built-in dummy load that allows you to disconnect the head from the cabinet without any damage to power section as soon as you want to use the head for direct recording through its integrated USB audio interface or go direct through the integrated DI (with selectable speaker emulation...I told you, there are a lot of features there!).

This is something that modern amps should have by default, today.

Otherwise, you've to add a dummy load: no DI or attenuator can do the job, 'cause they aren't loads.

[Dante]

MJMP, That sounds a lot like the video I found:

The good stuff starts at 0:50

So, if I use a couple 8ohm/25w resistors, I can achieve a 16ohm 50w load...if I wire it up right. Do I understand that correctly? If so, that would be plenty of room for my little 20w head (which runs 4/8/16 ohms). I found these (a single 16ohm/50w resistor) on Mouser....why wire up two, when I could just use one?

[MarshallJMP]

Well if you have a look at the datasheet you will see that without a heatsink (like in the YT vid) a 50W (HS50) can only dissipate 14W @ 25°C. That's why I use 4 of these and a heatsink to use it as a 100W load. And it get's pretty hot.

[Dante]

I really need to remind myself sometimes to RTFM!!

From the Peavey Classic 20mh user's manual:

SPEAKER ENABLE/DEFEAT SWITCH

This feature effectively disconnects any speaker/cabinet connected to the SPEAKER OUTPUT and redirects the output to an internal dummy load. This enables the user to monitor their playing using the signal from either the MDSI™ output, USB output or headphone output, without producing any real volume. Therefore, they can play or record at much lower volume levels.

So, I can run my little Peavey head with no cabinet without killing it (because it has an internal dummy load, like Max's amp)

Pretty cool

[MarshallJMP]

RTFM!!  ;D

[Iperfungus]

So, I can run my little Peavey head with no cabinet without killing it (because it has an internal dummy load, like Max's amp)

Sounds f**king good! 

[Iperfungus]

RTFM!!  ;D

What???