Running a Tube Amp w/o Speaker?

[RayG]

I'm thinking about using a Tube Amp Head as a pre-amp into a Quilter Steelaire solid state amp. I would go from the record out of the Tube Amp Head into the main input of the Quilter and know that would be safe. However, I realize that you should never turn on a tube amplifier without a speaker plugged into it... or risk some serious damage.

The Head in question needs to see an 8 ohm load so I'm wondering if it would be safe to rig up the correct load resistor on a 1/4 inch jack and plug it into the speaker input. My thought is that the Amp Head would see the 8ohm load and all would be well... at least I think/hope that's the case.

If I'm wrong would anyone know another way to fool the Amp into seeing the correct speaker load?

Thanks!

[Leo_Gnardo]

The Head in question needs to see an 8 ohm load so I'm wondering if it would be safe to rig up the correct load resistor on a 1/4 inch jack and plug it into the speaker input. My thought is that the Amp Head would see the 8ohm load and all would be well... at least I think/hope that's the case.

Sounds good to me. Some cautions: make sure the power dissipation of your load resistor exceeds that of your amp, twice as much is generally good. Why? If you drive that amp into hard clipping it will be putting out well over its rated power, and you don't want your resistor(s) going up in smoke & stinking up the place. FWIW there are good affordable resistors available at Mouser, Parts Express, and I'm sure a couple other outfits, aluminum cased 50W units that can be bolted to a heat sink.

While we're on heat, be prepared to have those resistors get super hot in use. Extra heat sinking would be a good idea. I put a 50W resistor into a 2 x 4 x 5 inch box, ran an amps output thru it, then when I turned my back on it for a coffee break, found it had burned a rectangle into the benchtop carpet, a permanent reminder "don't do that". Even though the box had rubber feet on it, those melted and the singed carpet stunk up the room for a couple days. SO - don't do that, if anything overdo the heat sinking, and use metal feet on the resistor box. Good luck with your rig!

[Chuck H]

Well... A little more circuit clarity might be helpful. Since the "record out" on most amps is in the circuit prior to the power tubes it may be possible to simply pull the power tubes from the tube amp in question. And that's it. But...

The sound of that tube amp from it's power tubes IS NOT necessarily the same (not likely in fact) as the sound from the record out jack. So we also need to know what aspect of the tube amps tone you wish to capture.

If you wanted the tone of the whole amp, power tubes clipping, etc. you'll want to build a dummy load with a "line out" jack to allow for whole amp tone with silent operation. If you like the sound from the record out jack, and it is prior to the power amp in the circuit, you should just unplug the power tubes and use the record out jack, saving you the added heat in the load and added wear on expensive tubes that aren't even being used.

More details please.

JM2C

[RayG]

Well... A little more circuit clarity might be helpful. Since the "record out" on most amps is in the circuit prior to the power tubes it may be possible to simply pull the power tubes from the tube amp in question. And that's it. But...

The sound of that tube amp from it's power tubes IS NOT necessarily the same (not likely in fact) as the sound from the record out jack. So we also need to know what aspect of the tube amps tone you wish to capture.

If you wanted the tone of the whole amp, power tubes clipping, etc. you'll want to build a dummy load with a "line out" jack to allow for whole amp tone with silent operation. If you like the sound from the record out jack, and it is prior to the power amp in the circuit, you should just unplug the power tubes and use the record out jack, saving you the added heat in the load and added wear on expensive tubes that aren't even being used.

More details please.

JM2C

Thanks, nickb!

Thanks, Chuck! I was not aware that their is/may be a difference in sound from the record out. I'm not sure if the record out encompasses the power section or not as I have not yet tried the setup I described. Actually I would definitely want the sound of the entire amp.

[g1]

Further to what Chuck said about disabling the tube power amp, I'm always a little leery about pulling power tubes unless I've verified that the filter caps voltage rating can handle it. This is because the voltage on the filter caps rises when the power tubes are removed.
A couple options, if you kill the signal going to the tube power amp, you can get away with a small, low power load resistor.
2 possible methods of muting the signal to the power amp would be
a) insert a shorting plug into FX return jack (if amp has switching jack on return)
b) removing the PI tube

[J M Fahey]

Since the "record out" on most amps is in the circuit prior to the power tubes it may be possible to simply pull the power tubes from the tube amp in question. And that's it. But...

Not so sure about that.
It´s true on full-of-ICs ones, (say JCM900 and similar) where sound is fully cooked on the preamp, but not on classic ones.

[Richard]

There are some amps that take the recording output from the output transformer.

[Chuck H]

Hey Juan. That's Mesa's "slave out". Not the same as their "recording out".

[Robert Technology]

If you wanted the tone of the whole amp, power tubes clipping, etc. you'll want to build a dummy load with a "line out" jack to allow for whole amp tone with silent operation.

JM2C

What is the preferred method for adding a "line out" to a dummy load?

I've built a couple of solid dummy load boxes for testing amps, but have been curious about this for a while... if I wanted to have a small speaker to give a representative sound for testing, or a more direct "line out" style output to plug into a recording interface what would be the best way to accomplish this?

Thanks!

[The Dude]

Before you get all crazy with any of this and FYI, some modern tube amps will provide output at the record out jack while the amp is in standby. No loads or tube pulling necessary. Try it first. It may be as simple as leaving the amp in standby.

[Jazz P Bass]

There is not a 'best way' to get a good sounding line out signal.

For years, Marshall used a simple resistive divider to knock down the signal amplitude for their DI jack.

[Chuck H]

What is the preferred method for adding a "line out" to a dummy load?

I've built a couple of solid dummy load boxes for testing amps, but have been curious about this for a while... if I wanted to have a small speaker to give a representative sound for testing, or a more direct "line out" style output to plug into a recording interface what would be the best way to accomplish this?

Thanks!

The simple line out that I use a lot (well... I don't personally use it because I don't play out or record much anymore, but...) is very much like the one Jazz linked in the Marshall schematic. I always adjust the output for the amp it's going in and/or make the output adjustable. That is, the values shown are not generic and to provide a line level output at max volume the values need to be adjusted for the amp and circumstances it's being used with.

Some take it so far as to use an iso transformer for a balanced line out and/or include a filter with inductors and caps for speaker simulation. IMHE it's good to have the plain line out for use with stage PA's and such where your speakers are still operating and affecting the impedance at frequency dynamically. Once you go to a dummy load all bets are off, but I would think that plugging the normal slave out/line out into a speaker simulator DI like a Red Box (or some cheaper unit, though the Red Box has a good rep) would be the best option because 1) you haven't colored the sound of the amps line out for normal use at gigs, and 2) the Red Box is real handy to have for several amps and uses if you do any recording. So...

A dummy load is just a resistive load in place of the speaker. Observe wattage and heat requirements as noted by leo. Wire up a slave out/line out to the dummy load the same as you would for a speaker and run that either to a SS amp driving a guitar speaker cabinet or through a speaker simulator/DI box and into a PA. Not exactly eezy peezy, but not too bad either.

[The Dude]

+1 on the Red Box.

[Robert Technology]

Thanks for the info and schematic for reference!

I didn't realize I could simply drop the power with a resistor divider but that seems obvious now.

I've got 300W worth of power resistors (3x100W) in a box I use for diagnosing amp problems, but there are some things that I can still diagnose faster by sound than on the scope.