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1/4 Power - "The Twin"

4/28/1998 11:31 AM
1/4 Power - "The Twin"
Fender "The Twin" amp has a switch to select 1/4 output power. It is said in the manual that this can be coupled to the old trick of pulling out a pair of tubes to get 100W (full power), 60W (2 tubes), 25W (1/4 switch) or 15W (2 tubes + 1/4 switch) from the same amp. Most people will be surprised on how loud it is even in 15W mode.  
The 1/4 power switch works by connecting the supply for the power tubes to half voltage. The bias voltage gets a shift too so that current remains almost the same. The phase splitter, reverb and preamp tubes still get full voltage.  
My questions are:  
Since the pre and driver are at full voltage and have full headroom, won't they drive the power tubes too hard? Maybe even make some grid current flow (specially since the bias voltage is changed)? Will this decrease the power tubes life? If there's grid current, won't this also wear out phase splitter tube (it will have to supply this current)? Will all this make tubes last shorter even at lower plate voltage? Is it better (in sound and reliability) than to use an external power attenuator?  
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