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Convert a powered signal to a line out?

J

jon36992002

Jun 7, 2012
20
0
#1
Can I convert the powered speaker out on an old amp to a line out? I have heard this can be done with a resistance load, but I am hesitant to try without guidance or feedback.

I found an all tube, military PA amplifier from the 50s. I tried running it for a bit, and it gave some very cool tone. With a preamp over-driving it, it delivered phenomenal low end fuzz. It definitely doesn't have the amount of power I need for my setup, and it lacks a lot of features I use. I am wondering if it is possible to use it as an effect in my effects loop without damaging it (or my amp) from the lack of resistance load. I have heard I can set up a resistor in parallel with the output to prevent damage to the amps, but how large of a resistor should I use?
 
#3
To use it as an effect it still needs a dummy load.

What tubes are in it? Someone with more smarts than me can tell you what sort of output you would have to soak. Need the correct impedance to keep its power transformer happy also.

Then you can use another transformer to get it back to instrument level. The speaker DI box idea might work, or not work if the output is intended for mic.
 
#7
The amp is completely unlabeled, but there is a stamp on the bottom that says 1949. It looks like it was designed for a PA system, as there are a Mic in and (at one point, but no longer) a phono in. It has a tone control (which is also the on/off) and a volume control. It has 2 bridged (I think, based on no resistance between them) speaker outs, which were something arcane but I converted one to TS. Based on its max volume, I would say it is about 15 watts (before tube distortion) or comparable to a 100 watt solid state amp.

Here is a picture I took a while ago, the military one is one top.
http://imgur.com/PZOvv

Thanks for bringing power attenuators to my attention. That seems to be what I am looking for.
 
#8
There are all sorts of devices on the market now that are meant to provide a load on the output of an amp to allow for things like DI outputs, or volume control while driving a specific load. It can definitely be done, if you want to do it. There are two major problems, from a tonal perspective, however. First, the load from a resistor has a very different impedance than that of a speaker. The real part of the impedance may be equal, but the reactive part differs. A speaker is essentially an inductor, whereas a resistor will only exhibit a significant inductance if it is of the wire-wound variety. Even then, it is not the same as a speaker. Second, much of the tone we are used to hearing from a cranked amp can be attributed to the specific frequency response and other acoustic properties of the speaker itself, unrelated to its electrical function. Even if you simulate the load of a speaker, the signal you take probably won't sound the same when it gets amplified elsewhere, as it does when the amp is driving a cab.

You mentioned the possibility of damage from the lack of a load at the output. Note that it is the other way around for tube-based power amplifiers. Unlike a solid-state amplifier, a tube-based amplifier drives an output transformer. (This is for two reasons. One is to lower the output impedance, since the impedance at the anode of most tubes is far too high to drive a speaker. You need to lower it about three orders of magnitude in most cases. The second is to provide inductive coupling to block the high-voltage B+ supply from reaching your speakers, since both the DC and signal are connected to the same part of the tube.) The output transformer will provide a 100% reactance when its secondary winding is shorted, so it is ok to drive a very low impedance load. You will need to provide a reasonable finite impedance load on the output, for your line out circuit, however.
 
#9
Most decent attenuators have a line out. They're a guitarist's best friend allowing to crank the master volume on a tube amp but get down to a reasonable sound level. You can also run it as a full dummy load w/no sound out of your cab. This may be overkill for you but hey, it'll do the job fantastic and you might find another use for it...

Here's a standard THD Hotplate. These run about $150-$200 used.

thd-hotplate4-110404-2.jpg
 
#10
You can certainly do it with any number of gadgets. But for all the reasons line6man brought up, if it were me, I would run it to a speaker and mic it. It will sound a lot better and probably be simpler to run. I would do this even for recording.

Best of luck either way.
 
#11
Hi.

What output tubes?

Very strange (to me anyway) that it'd drive a speaker impedance load instead of the more usual 70V/100V line, but anything's possible.

If You want to spend $100's instead of $10's for the soak, go ahead, but if the 15W is anywhere near the actual power output, a couple of wire wound "power" resistors will do the trick very nicely. Slap in a couple of capacitors onto the circuit if You want a more reactive load to mimic a speaker load.

The type of the (power) tubes is the most important thing to know ATM though.
Those babies will need replacing sooner or later and if a direct replacement can't be found for a reason or another, all You've done up to the failure point will be in vain. Literally.

Regards
Sam
 
#12
T-Bird said:
Slap in a couple of capacitors onto the circuit if You want a more reactive load to mimic a speaker load.
Click to expand...

A capacitor is the opposite of an inductor. The phase delay in an inductive circuit is such that current comes after voltage. Conversely, voltage comes after current in a capacitive circuit. Usually capacitors are added to inductive circuits in an attempt to bring the Power Factor closer to 1. As an example, if you have a purely capacitive circuit, the current is greatest when the rate of change in the electric potential is greatest, and that occurs at zero volts, and thus, a 90 degree phase shift. If a circuit is purely inductive, the phase shift is 90 degrees the other way around, so an equal capacitive reactance will "cancel out" the inductive reactance, allowing a circuit to behave in a purely resistive manner. Of course, most circuits are not purely inductive or purely capacitive, lying somewhere in-between, depending on frequency. In an LCR resonant circuit, for example, everything changes constantly as the frequency alters the balance of inductive and capacitive reactance.

Staying on topic, though, adding a capacitor will cause the circuit to behave opposite what the OP wants. A speaker is an inductor, and thus, its behavior needs to be simulated with an inductor rather than a capacitor.
 

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