OT preservation

knuckleberryfinn

This might be a stupid question, who knows.  I want to test the bassman I'm building without having to connect it up to the cab - it's a pain in the arse.  Can I use a single low wattage speaker (I think it's 15W) simply to take the load of the transformer?  There won't be any power valves in there yet, so I assume this will be okay.  Wattage only matters when the speaker is being thumped with a signal right?

ICBM

If there are no power valves you don't have to connect anything to it at all.

Once there are power valves in it's a good idea to have some load on it even without a signal going through it - there are a very few amps which are unstable with no load and will self-oscillate, which is effectively the same as cranking the amp up with no load. It's not common though, and assuming you've connected the NFB loop up correctly the Bassman isn't one of them. Any load of roughly the right impedance is fine, it doesn't have to be accurate or capable of taking the full power of the amp.

knuckleberryfinn

Ah okay, thank you.  I did not know that - it must be such common sense that nobody even bothers saying it anymore.  Still learning... :-S

Misterg

ICBM said:

.... assuming you've connected the NFB loop up correctly {...}. Any load of roughly the right impedance is fine, it doesn't have to be accurate or capable of taking the full power of the amp.

Trouble is, the first sign that the NFB isn't correct (50% chance?) is wild, full power oscillation the first time you switch the amp on...

So I would venture that you would be safer with something (speaker or dummy load) that *can* take the full power of the amp, at least for a short time.

ICBM

True, although I would always tend to turn the standby on with a firm grip on the switch to flip it off again *immediately* in the event of any problem - which need not be self-oscillation.

Cirrus

ICBM said:

True, although I would always tend to turn the standby on with a firm grip on the switch to flip it off again *immediately* in the event of any problem - which need not be self-oscillation.

It might be 350 volts connected to a floating standby switch
)

ICBM

Cirrus said:

It might be 350 volts connected to a floating standby switch
)

I know you're joking, but… no.

The switch body will always be connected to the chassis so it's safe. Unless you're a complete idiot and you really do mean a switch that's literally floating, fitted temporarily to test an amp which will not finally have a standby switch. Not that I've ever done that of course.

ecc83

You should always run an "experimental" up with a load and voltmeter and or scope on the output.

A good substitute is a 22W car lamp which has a "burning" resistance of ~6.5 R close enough to 4 or 8 for jazz and if the amp IS instable it will glow quite well.

In the PA game with 100V line amps we used a main neon. These "strike" at about 90V but if there was any ultrasonic instability they would glow like a toc H lamp!

Dave.