tube amp

[brian t]

have just started useing a valve amp after many years of solid state amps, two questions - am i right in saying you should not switch on a valve amp without the loudspeakers connected if so why ? also what happens if you short the loudspeaker terminals . cheers

 

[Sherman]

have just started useing a valve amp after many years of solid state amps, two questions - am i right in saying you should not switch on a valve amp without the loudspeakers connected if so why ? also what happens if you short the loudspeaker terminals . cheers

In general yes, though if you are not feeding an input signal into the amp and the volume control is fully attenuated you probably will do no harm switching on without speakers connected, at least for a short time. Why you would want to is another question. If you want to take readings you can use a dummy load resistor across the speaker terminals.

 

[Geek]

Re: Re: tube amp

In general yes, though if you are not feeding an input signal into the amp and the volume control is fully attenuated you probably will do no harm switching on without speakers connected, at least for a short time.

Unless it's a really bad (unstable) design - they tend to oscillate without speakers (or at least a Zobel attached).

 

[Sch3mat1c]

Yeah, two things.

The amp may be unstable with no load attached. Pentode amps with NFB may oscillate; most triode and all ZNFB amps will be stable. If it does oscillate, the chances are it's oscillating to its full capacity, which means high voltages and odd currents in the circuit, which may be dangerous to it.

If the amp is stable with no load, then about applying a signal with no load: with triodes, the output is always well controlled, so dangerous voltages are unlikely. However, if the signal is overdriven, causing the amplifier to cut out momentarily, this can put unusual voltages on the OPT (due to flyback effect). A good circuit will control this (snubbers, zeners, etc.). Most tubes are safe for 1 or 2kV peak as well.

In general... tubes are robust. 'Nuff said.

TIm

 

[ray_moth]

A good safety precaution against the high primary EMF that can be caused by the unintentional disconnection of the speaker is to solder a 2.2k resistor permanently across the OPT secondary winding.

 

[Sch3mat1c]

Er, the hysteresis losses in the iron are giving the flyback pulse more loss than 2.2k on the secondary would...

2.2k on the primary would certainly have a more than noticeble effect.

Something under 100 down to maybe 22 ohms would be best between ensuring stability and wasted power.

Tim

 

[Yvesm]

Most tubes are safe for 1 or 2kV peak as well.

Not all OPT are

 

[richwalters]

Originally posted by ray_moth
the unintentional disconnection of the speaker is to solder a 2.2k resistor permanently across the OPT secondary winding.

Hi there....I have logic problems with this one.....although I seen it done on some amp o/p's.... but... against the o/p tranny step up ratio of a standard p-p tranny....the 2k2 across a sec seems darned small tickling load to of any use across a 4 ohms sec winding. Anyone come up with a theory to convince me it ?

rich