Newb electronics questions

[brother rat]

1. How can I estimate the power output of a tube amplifier without a schematic? I know that it uses two 6V6 output tubes and 6SL7, 6SJ7, 6SC7 preamp/gain stage tubes.

2. How can I determine the load impedence that the same amplifier is expecting?

3. How can I make a 4 ohm speaker appear as an 8 ohm load to the amplifier?

Thanks for your help!

 

[cyrano]

1. How can I estimate the power output of a tube amplifier without a schematic? I know that it uses two 6V6 output tubes and 6SL7, 6SJ7, 6SC7 preamp/gain stage tubes.

The size of the output transformer only gives a very rough estimate. The 6V6 pair will probably have a max. output of approximately 12 Watts.

2. How can I determine the load impedence that the same amplifier is expecting?

You can't. Measuring the DC resistance of the speaker side of the output transformer will give you a guesstimation, tho.

3. How can I make a 4 ohm speaker appear as an 8 ohm load to the amplifier?

2 of those speakers in series, or a 3.9 Ohms, 5 Watt series resistor. The latter will waste half of the power output, tho. You could also use a transformer, but that will cost more and still change the sound a bit.

 

[brother rat]

thank you!

 

[ranjam]

1. How can I estimate the power output of a tube amplifier without a schematic?

You'd need a dummy load, a signal generator, and an oscilloscope. And then you'd need to know how to use them. It's very simple once you know how. In a nutshell, measure the AC voltage across your dummy load once you have the maximum 'clean' signal appearing across the dummy load (as seen on the oscilloscope). Now P = V x V / R.

2. How can I determine the load impedance that the same amplifier is expecting?

Tales From The Tone Lounge; Classic Articles-Indentifying Salvaged Transformers​

3. How can I make a 4 ohm speaker appear as an 8 ohm load to the amplifier?

You can't. It's splitting hairs, but the transformer doesn't really have an impedance. It's a ratio, and the reflected impedance will be based on the winding ratio. If you took say a 5K to 4-ohm transformer and hooked an 8-ohm speaker to it, huge rocks will not fall from the sky, nor will large cracks appear in the Earth's surface. But you will lose a few watts in the deal, and the primary will behave as if it were 10K Plate-to-Plate. Sometimes not a big deal at all, and other times when every watt is precious it is a big deal.