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5 Watt tube amp output into line level input
Old 7th March 2012
Gear Maniac

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5 Watt tube amp output into line level input

I built a little High Octane AX84 tube amp a few years ago. Now i want to use it as a distortion box. How close to line level is a 5 W output?
Old 7th March 2012
Lives for gear
2N1305's Avatar

about as close as San Francisco is to Montreal.

seriously, do not even think of plugging that output into a line input, if you value your line input... also, it will damage your output tube if you were to connect only the line out and no speaker load. If you built your ax84 you should know that no tube amp can function without a load...

I'd say, the proper thing to do would be to pad that output down with a resistive divider, somewhere along the lines of a 5 to 10k load. then send that to a single ended to balanced transformer so you can send it to a balanced input, thus reducing the noise pickup.

To answer mathematically to your qeustion, a 5W signal in an 8 ohm load is about 6.3 volts peak, or roughly 4.5 volts RMS. You would need a voltage division ratio of about 1:4. So four 1K ressitors in series and you take the output on the last one that's connected to ground.


Old 7th March 2012
Lives for gear

Just get a mic and mic the speaker. You'll probably like the results better anyways than direct.
Old 7th March 2012
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Piedpiper's Avatar
You could also replace the output transformer with a much lower ratio one. That would match the impedance better but you'd still need to pad the voltage down.

Really, you're better off building a tube distortion pedal from scratch...
Old 7th March 2012
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5 Watts into 8 Ohms corresponds to somewhere around +19dBu (+15VU) so is a touch 'hot' for a line input particularly if you want to play loud.
You should therefore 'pad' it down with an attenuator so the 4:1 as proposed above is probably about OK.
You MUST however have a load on the the output transformer either by keeping the speaker connected, or by fitting a resistor of 8 Ohms (10 Ohms would be fine) rated at say 10 Watts and mounted on a heatsink (a few square inches of metal is fine).
Matt S
Old 7th March 2012
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Whirlwind (and others, I'm sure...) make a speaker level direct box.
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