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Re: Help!Weird prob with "Super" DR clone!


 
6/16/2000 3:57 PM
Wild Bill
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Re: Help!Weird prob with "Super" DR clone!
Well Mr. "P", the schematics are already posted. I simply "stole" most of the Fender Deluxe Reverb, which you can find at lots of sites. I used only the reverb channel. I used a 6AQ5 instead of a 12AT7 for the reverb driver, still pretty standard. Instead of two 6V6's I used 6, 3 on each side. Each has a 400 ohm screen resistor and a 1K6 grid stopper.  
Which leads me to my present conundrum - (I originally had intended to cathode bias the tubes. Of course, with just over 400 volts on the plates this meant that by the time I had the right value of cathode resistor to keep the plates from glowing red at idle I was no longer Class A but rather AB1.  
As you probably already know cathode bias is self-adjusting. As the current flow goes up the cathode resistor drops a higher voltage, raising the bias even more to hold the plate current steady.  
This means that the amp was essentially "compressing" itself. Higher signals meant higher bias, lowering the gain of the output tubes. The amp was putting out little more than 10 watts.  
This wasn't all to the problem - those 10 watts sounded GREAT!  
The amp was punchy as hell, with lotsa mids! I did convert to fixed bias and the amp now screams like a 50 watt Marshall! But it sounds different - good, but not the same.  
This is an obvious beginner's mistake to some of the old pros on this board but having done it and listened to both bias methods I can't help but thnk that I liked that compressed sound better! This must account for the sound of some of those AC-50 Vox amps - they used a combination of fixed and cathode bias. I had wondered why they used a more complicated approach and now it's easier to understand having heard the results.  
After I rest up from so much soldering and making metal shavings maybe I'll climb back in and see if I can come up with a compromise...  
Even with such a bonehead mistake the amp still sounded way better than something with silicootie transistors!  
 
Wild Bill
 
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