Allowable drop in gain of a band amp over an hour

[N Cook]

What would be normal and what acceptable for a solid state or valve/tube
stage amp. ?
Well you would not want the gain/ output audio to go up over an hour of use
with a constant input.
Testing a solid state amp that had a problem that certainly affected the FET
gain killer protection circuit so (falsely) cutting the output over an hour
to close to zero. But checking to see if there was any other gain drop
problem but then realised I don't know what the norm is.
Amp rated at 200W (music power) which I've taken as 100W (continuous RMS if
you could). Running a constant source of 1Kz through amp to give a
continuous 1/5, 20W in a dummy load , which with heating itself is not
necessarily constant. Result of testing over 40 minutes was a 9 percent drop
in V rms into the load so about 20 percent drop in equivalent audio watts.
No fans on this amp and the heatsink settled at 68 degrees C after about 30
minutes. The output was still dropping after 30 minutes but very much slower
and decided to cancel after 40 minutes

 

[Ron(UK)]

What would be normal and what acceptable for a solid state or valve/tube
stage amp. ?
Well you would not want the gain/ output audio to go up over an hour of use
with a constant input.
Testing a solid state amp that had a problem that certainly affected the FET
gain killer protection circuit so (falsely) cutting the output over an hour
to close to zero. But checking to see if there was any other gain drop
problem but then realised I don't know what the norm is.
Amp rated at 200W (music power) which I've taken as 100W (continuous RMS if
you could). Running a constant source of 1Kz through amp to give a
continuous 1/5, 20W in a dummy load , which with heating itself is not
necessarily constant. Result of testing over 40 minutes was a 9 percent drop
in V rms into the load so about 20 percent drop in equivalent audio watts.
No fans on this amp and the heatsink settled at 68 degrees C after about 30
minutes. The output was still dropping after 30 minutes but very much slower
and decided to cancel after 40 minutes

In an ideal world I wouldn`t want a pro amp to drop any level over any
amount of time. You should easily be able to locate just where the
signal is dropping with a scope. There may be a temperature sensitive
device in contact with the heatsink.

A constant sine wave isn't a very good way to test an amplifiers long
term performance, I`d use a signal more indicative of real life conditions.

IMO

Ron(UK)

 

[Eeyore]

Result of testing over 40 minutes was a 9 percent drop
in V rms into the load so about 20 percent drop in equivalent audio watts.

Are you sure the drop in power isn't simply due to the transformer heating up
and the consequent rise in winding resistance ?

Graham

 

[Meat Plow]

In an ideal world I wouldn`t want a pro amp to drop any level over any
amount of time. You should easily be able to locate just where the
signal is dropping with a scope. There may be a temperature sensitive
device in contact with the heatsink.

A constant sine wave isn't a very good way to test an amplifiers long
term performance, I`d use a signal more indicative of real life conditions.

I made a audio track on CD of a guitar track from a multi track studio
recording on CD to test amps with and just repeated the track over and
over. I would take a 12" 150 watt speaker and lay it on it's face on
carpet then wrap a blanket around the outside of the basket to get an
inductive load rather than run the amp into a 300 watt resistor. I don't
expect any power drop in a guitar amp over a period of time but pumping a
1KHZ sine wave into a non-inductive load may produce such results where
real life playing wouldn't.