S-5 Electronics K-12M (11MS8) 8 wpc PP Tube Amp Kit ($139)

[BuzzerBro]

I recently finished casing up my S-5 amplifier. I was reading the paper work that came with the amp and it says “Do not operate without speakers connected.” Does anyone know why this should not be done and what are the consequences of doing so?

Also, does the input impedance change if the potentiometer and input caps are removed?

I posted some pics of the amp in the Tube photo gallery:
http://www.diyaudio.com/forums/showt...008#post920008

[Miles Prower]

Originally posted by BuzzerBro
I recently finished casing up my S-5 amplifier. I was reading the paper work that came with the amp and it says “Do not operate without speakers connected.” Does anyone know why this should not be done and what are the consequences of doing so?

Even though you didn't include a schematic, since it's a VT amp, it's safe to assume that the speeks are coupled to the finals with an OPT. Since VTs operate as output transconductance amps (current output, not voltage output) the OPT operates as a current step-up xfmr. As such, it will try to output the same current regardless of the secondary load. According to Ohm's Law: V= I * infinity= INFINITY!.

Of course, the voltage won't get there because something will poof first, either the insulation of the OPT and/or the finals. This will probably take something else along with it: the power diodes and/or the power xfmr.

Operating with no load frequently renders the VT amp FUBAR.

Also, does the input impedance change if the potentiometer and input caps are removed?

Yes