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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 Mark, I've built a couple of spitfire-type amps and I really enjoy them. Here's my $0.02 worth: 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

7/10/1998 9:08 AM

Mike B

Re: spitfire questions

Mark,
I've built a couple of spitfire-type amps
and I really enjoy them. Here's my $0.02
worth:

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

Replies:

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