[Amps] Can I cool too much the tubes?

[Gary Schafer]

Yes, a typical amp runs at say 66% efficient at full power out. Now reduce
the drive so that you have 1/4 the power out (as is done for AM linear
operation) and you have roughly 1/2 the efficiency or around 33%.

At 1000 watts input the output would be 660 watts (66%) with a plate
dissipation of 340 watts (1000-660).

At 1/4 power output the input power is going to be around 500 watts. At 33%
efficiency that gives 165 watts out with a plate dissipation of 335 watts
(500-165).

As a general rule the efficiency drops 1/2 as much as power output does.
This holds pretty true for class B but is modified as you get into AB and
gets worse as you approach class A.

So if using the amp on CW or RTTY at reduced power you are much better off
retuning at the lower drive level for maximum power out which returns the
efficiency and reduces plate dissipation and power supply dissipation.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Steve Thompson
> Sent: Tuesday, August 24, 2010 2:15 AM
> To: amps at contesting.com
> Subject: Re: [Amps] Can I cool too much the tubes?
> 
> I've observed this across a wide range of class AB linear
> amplifiers - device dissipation is very roughly constant between
> 10 and 100% output.
> 
> With a tube amp you can adjust the tuning to get higher efficiency
> at lower output power, which will reduce the tube dissipation as
> well as giving the PSU an easier time.
> 
> Steve
> 
> > Gentlemen,
> >
> > You should take a look at the plate dissipation at the reduced power
> levels.
> >
> > In my amp, as the power output drops back, the efficiency drops at a
> faster
> > rate on some bands.  Which means that running the amp at half power, for
> > example, as is the current rule of thumb for RTTY operation, may not
> > actually reduce the effective plate dissipation very much.
> >
> > Of course, one point does not make a trend.  But it's easy to make the
> > calculation and then a guy can KNOW rather than just guess.
> >
> > And if we were perhaps talking about an SB220, that transformer is
> pretty
> > light weight to begin with, and a guy would definitely want to check the
> > efficiency there.
> >
> > Hope to see you in the SARTG.
> >
> > 73/jeff/ac0c
> > www.ac0c.com
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