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previous: pumpstein Well, last night I did a little tro... -- 9/1/2000 8:08 PM View Thread

Re: Help with motorboating?

9/2/2000 7:26 PM
Wild Bill
Re: Help with motorboating?
I wouldn't worry about those voltages - Traynor ran a lot higher than that! So did a Heathkit hifi amp I worked on once.  
Ditto the filament voltage. +/- 10% is the spec for tubes.  
Motorboating is often due to poor filter decoupling to a preamp stage. That's why amps have a string of filters with cascadingly lower voltage taps. It not only gives better filtering by the time you reach the 1st and 2nd preamps but it isolates the stages from each other. You'll notice if you compare various amp schematics how if they tap 4 triodes in the preamp they may use a larger filter value at that point than with a single channel amp with only two stages. Since bigger values cost more and in the days of tube rectifiers were harder on the rectifier with charging/surge currents designers tried to get away with as small as possible. I solved one motorboating problem just by adding one more dropping resistor and a 10 uf filter, so's I couold feed that 1st 12AX7 all on its own. I also run a separate leg to feed a reverb driver tube, if I'm running one.  
 
Hope this is of some help...  
 
--Wild Bill

 
Replies:
Peter S Pump, Wild Bill is on it! ... -- 9/5/2000 8:11 PM