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sardonx 23rd August 2005 09:43 PM

How do I turn an high current signal into a line level signal??
I wasn't sure which forum category to ask this in... but since it involves my tube amps... here it is.

I've been wanting to try this for a very long time. I want to take the output of my tube amps (15 watts/ch or so) and 'condense' the current to a line level signal which will be able to go into my pre-amplifier...

how do I do this??

sardonx 24th August 2005 03:58 AM

nobody knows?

xplod1236 24th August 2005 04:01 AM

Enzo 24th August 2005 04:05 AM

That tube amp MUST have a load, either a speaker or a resistor.

To send a line out from the speaker terminal then, just make a voltage divider across it. I would start with about a 10 to 1. In other words from the speaker hot, run a 10k resistor, and from cold run a 1k resistor, join the free ends and take your line signal from that point. Replace the 1k resistor with a pot and the wiper becomes the line out and is now adjustable.

I suppose a 50k resistor and 5k pot would work too.

PRR 24th August 2005 04:25 AM

> take the output of my tube amps (15 watts/ch or so) and 'condense' the current to a line level signal which will be able to go into my pre-amplifier...

Define "line level".

Preamp input never needs more than 2V. Levels under 1V are often fine.

15 watts in 8 ohms is 12V max sine-wave. A guitar amp can play "louder" (not much more volts but a lot more loudness) and much softer, but will usually run fairly near full power.

Tube amps MUST have a load unless known otherwise. Hi-Fi amps can use an 8Ω or 10Ω power resistor. Guitar amps are strongly affected by speaker impedance and won't sound the same with a resistor; however a good fake speaker load is hard to dummy-up, so try a resistor. Radio Shack used to have 8Ω 20W as a standard part. If not, they had 10Ω 10W in 2-packs, wire four in series-parallel for 10Ω ~40W. (Note that a "15W" guitar amp can throw 25 watts of distortion and resistor-heat when cranked past "11". If you play hard, and the resistor smells hot, get a bigger resistor.)

So you have the amp happy but silent at 3 to 12 volts output. You want 2V max at the preamp input. Use a resistor divider. The total resistance must be larger than the amp's load. The lower resistor must be lower-Ω than the preamp input, or the reactance of any cable. This is easy: 1K in the bottom resistor will feed any home preamp or cable, and the total resistance will be far-far higher than the amp's load. To turn 12V into 2V you need a 6:1 divider. This could be 4K7 and 1K. It could just as well be 470Ω and 100Ω, and 47Ω+10Ω would be OK too, especially if you use a 10Ω dummy load on an "8Ω" output.

I hope one terminal of the guitar amp is "grounded". This is true of almost every post-1935 tube amp I've seen, and nearly all sand-state amps until you get past a couple hundred watts.

sardonx 24th August 2005 05:19 AM

Thanks for the replies everyone... that should give me enough to experiment with.

The amps are monoblocks, Pilotone from the 50's i think.

Could I use a light bulb to drain power too?

PRR 26th August 2005 01:20 AM

> Could I use a light bulb to drain power too?

You can use a bucket of salt water. (This was actually used for dimming stage lights before autotransformers and other tricks evolved.) But it is messy, high maintenance, and a pain to set to the right ohms.

Problem with an incandescent lamp is the resistance changes a LOT from cold to hot. In fact I tried it once with an amplifier very like yours. About 15 watts from a pair of 6V6. I used an ohm-meter and found that a "120V 60W" lamp was about 15Ω. I put it on the 16Ω tap as my dummy-load. Did some power tests, and got HUGE voltage, much more than the expected 15 volts. And when I killed the room lamps, I could see my test lamp glow dimly. I did some more measurements and learned that my "15Ω" dummy load was rising to 50-100Ω when warm. (It is obviously 240Ω when fully white-hot: 120V at 0.5A is 60W and 240Ω.) The under-loaded amplifier was able to make a lot more voltage that way.

Another old trick is to put a car tail-light in series with a tweeter. When cold, it is under 1Ω. Hot it rises over 10Ω. Long-term average voltage into a high-crossover tweeter is just a few volts, which mostly lands in the tweeter, and it works right. But sustained high tweeter voltage warms the lamp, which increases in resistance, so it takes voltage away from the tweeter, which warms the lamp more: the tweeter ends up padded-down 6 or 10dB. It is hard to cook the tweeter that way, it does get weak but still tweets, and comes back to full voice quickly when power is reduced.

Use a proper resistor. A 20W resistor is not expensive.

sardonx 26th August 2005 03:45 AM

Yeah that's why I asked about using a light bulb... because i read somewhere of someone using it with a tweeter like you did.

I tapped a line level signal from the speaker outs of my chip amp last night... and the resulting sound? Going into my OTL pre-amp and listening to the headphone out... the sound has absolutely no resolution... sounded like i was listening to a discman.

As soon as i get new rectifiers for my tube amps, i'd like to try tapping the signal off their outputs... I hope that is sounds better but probably won't be anything great...

Burnedfingers 27th August 2005 11:16 AM

Try a 70/25volt speaker transformer. Hook up the 8 ohm side of the transformer to the amplifier that is supplying the signal. Use the tap of your choice to provide your line level signal. I usually use the 5k or 1 watt tap from the transformer.

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