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previous: Mark Buckingham After I got a great deal on parts, ... -- 7/9/1998 9:16 PM View Thread

Re: spitfire questions

7/10/1998 9:08 AM
Mike B
Re: spitfire questions
I've built a couple of spitfire-type amps  
and I really enjoy them. Here's my $0.02  
1. That power tranny you have puts out way  
too much voltage to safely run the EL84's  
in class A. Modern EL84's (Sovtek, etc.)  
don't like more than about 375Vdc and that's  
pushing it. Keep in mind that the original  
spec for these tubes showed Vamax at 300V.  
2. Using a solid state rectifier will compound  
the problem due to the greater efficiency of  
the silicon diodes. A tube rectifier would help bring the voltage down a bit. Does your  
transformer have a 5V rectifier winding?  
If not, you could use a 6CA4/EZ81 rectifier  
tube which will run from the 6V filament  
supply. This will probably give you about  
400V which is still too high, IMHO.  
3. I installed a pentode/triode switch on  
my amp using the standard 100 ohm screen  
resistors. I was not particularly happy  
with the sound in this mode - loss of high  
end, kind of muddy. There was a definite  
decrease in volume but I didn't care for  
the tone so I ripped it out.  
4. I personally like the master volume  
circuit used in this amp. It's simple and  
it does the trick. It's usefulness is limited,  
however, to small reductions in overall  
volume. This amp does not have enough preamp  
gain to shred at bedroom levels - it relys  
on power amp distortion to get it's signature  
sound. That being said, on my second spitfire  
style amp, I deleted this function.  
5. Yes, you will get power tube distortion  
sooner, but you may not like the tone. It's  
worth adding the switch to see if you like  
the triode option. Foe lower volumes I ended  
up building a single-ended EL84 amp that I  
could crank and not fry my ears (as much!).  
This is a great circuit for experimentation.  
My next project is to replace the parallel triode preamp with an EF86 pentode. I'll  
post with the results.  
Mike B

Mark Buckingham Thanks, Mike.I'm trying to ... -- 7/10/1998 9:41 PM
Jack Orman Mike,I've been doing some w... -- 7/13/1998 8:29 AM