Power Control circuit

[Aurora]

I've been into electronic instrumentation for quite number of years, and in the HiFI asylum just about as long. I've been working with tube circuits since the early 70s, repaired quite a few guitar amps, but never built a complete guitar amp from scratch. My main instruments are woodwinds, - not guitars, but my son is heavily into the "fiddles".

I've been zapping around the net for quite some time to find a good project, and more or less decided on a Dumble, - of sorts.
The power control have pussled me for some time, until I finally found this excellent site!
On the 124 schematic, though, the 50/100W power switching is accomplished by switching 10k resistors betwen cathodes and ground, with a LED in series ?????
This cannot be correct?

[Aurora]

DOH

Sorry folks.
It's past midnight here, - my head pro'lly went to sleep a few hours ago!
I just realized.......................

OTOH - what sonic effect does this have, - except for the apparent volume change?

[Bob-I]

Welcome.

I put a 1/2 power switch on an amp I recently built for a friend, at the request of my friend. It has almost no affect at all in volume and the overall tone is just a tad darker. The difference in volume is non-existant below about 3:00 on the master, and at that level it's too loud in either 50 or 100 watt mode to be useful.

Bottom line, this switch is a waste IMHO

[ic-racer]

Here is mine in action. The LED really does work as described by the schematic. In terms of the effect, it is as described above but, there also is an increase in the impedance mismatch with the 50 mode. Audible
Another way to describe the effect is in relation to a power attenuator. Mine have 10 or 12 clicks at about 3db each increment on the knob. So the 50/100 switch is like ONE click of the knob.

[tjauernig]

You have to remember that 100 watts down to 50 is not 1/2 the volume. It's only a 3db change. 50 watts is 97% as loud as 100. There's a chart on this somewhere on this site. I think it had a 1.5 watt amp being somewhere in the realm of 46% as loud as 100 watts. What the power switch can and does change is headroom, and the touchy feely characteristics of the amp.

[ic-racer]

You have to remember that 100 watts down to 50 is not 1/2 the volume. It's only a 3db change.

3db decrease is 1/2 volume. (Or 1/1.9953 to be exact)

[Aurora]

3 dB is 3 db, anyway, halving the power, no matter how you calculate.
OTOH, effectively switching one output pair out of the circuit, alters the load line for the remaining pair, as load impedance is unchanged.
I suspect the effective otput is still quite more than 50W, unless someone has a measurement to confirm this.
So far, I think I'll settle for a fixed 50 W version and one output pair.


( BTW- by occupation, I'm an EE, BSc in radio transmission.
I could have kicked myself last night when the truth about the power switching really hit me. I really felt completely stupid!)

- Move on..don't look back!

[Bob-I]

3 dB is 3 db, anyway, halving the power, no matter how you calculate.

Thank you... now let's stop this stupid debate and agree you can't effectively measure volume. It's like saying something is twice as red

OTOH, effectively switching one output pair out of the circuit, alters the load line for the remaining pair, as load impedance is unchanged.
I suspect the effective otput is still quite more than 50W, unless someone has a measurement to confirm this.

I did measure the 2/4 power tube switch on the amp I did. The 4 power tube output was over 100 watts, I think about 110 but my little pea sized brain forgets things. The 2 power tube output was close to 65 watts. It's hard to exactly determine because I can't measure distortion, I just turn up until the sine wave starts to flatten out.

[Tavda3172]

Hi guys,
Tjauernig is correct that half the power only yields a 3db decrease in sound, which is NOT half the volume. If you want to have half the volume you need a 10db decrease, so go from 100w to 10w.
-Kevin