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questions about Spitfire

8/9/1999 1:11 PM
questions about Spitfire
I finished my spitfire clone amp. When I powered it up I got 350 volts on the plates; 337 volts across the tube. With the 120ohm cathode resistor on the power tubes, though, I got a tube current of about 45mA; too high for a tube that can only dissapate 12 watts. Having a 270 ohm 5W resistor laying around, I tried that instead and got a much lower tube current reading of 24 mA. Is this just a hot set of tubes, or am I missing something? It seems odd that I had to use a cathode resistor more than twice the specified value to get the current down low enough.  
8/9/1999 1:40 PM
Steve K
Your first readings seem proper to the original.  
The el84's can handle more than they are  
advertised to... I've been running mine at over 14  
watts dissapation at 50ma with no problems...  
The tubes last me many months which seems normal  
to me. My 15 watt Vox/Matchless clone runs at 360v  
on plates with 120Rk which dissipates over 14  
watts per el84 and I get nearly 6 months on a set  
of the Sovtek el84M tubes. Less on the NOS ones  
for sure but if you want that swwet tone, you've  
got to push it...  
Steve K
8/9/1999 2:35 PM

OK, good. I'll try putting the 120 ohm res. back in there. I'm not as experienced with cathode biased amps. Is this a class-A design? Are there different rules or guidelines for tube bias and tube idle current?  
8/9/1999 3:28 PM
Benjamin Fargen

I think your setup is on the right track. Your biggest problem with a cathode biased amp like this is heat. 12-15watts dissipation in a small chassis starts to take its toll on the overall ambient temperature of the whole amp(chassis,internal parts,PT,OT,ect.)  
I would up the cathode resistor to a 130-150/5watt resistor to bring the dissipation closer to 12Watts. In my experience there is a fine line between running the tubes to hard and just right. IMHO......(opinions here...not discounting what Steve K said at all!) running the tubes in the 13-15 watt range produces a cold brittle sound and not enough of that sweet EL84 top end breakup that we all know and love. I would try to shoot for 9-12watt dissipation Experiment with it and find out what you think sounds best.  
8/9/1999 4:08 PM
Steve K
Yes, I use a fan in all my class A (or close) amps  
which seems to help tremendously. But I disagree  
that running the tubes lower sounds brittle. In fact  
to my ears its just the opposite. The older NOS  
el84's (don't remember the brand now) really  
deliver that classic el84 tone. The SOvtek's are  
harder sounding (especialy the el84M's) but really  
deliver a great tone when fed a humbucking LP...  
Everyone who has heard it loves that tone so I  
know I'm not alone...  
But beauty is in the ear of the beholder of course..  
Steve K
8/9/1999 4:38 PM
Mike B

The Vox AC15, which the Spitfire circuit is based on, used a 130 ohm Rk with a B+ around 330V. So your circuit is not far off. I have built several cathode-biased amps using EL84's and have had no reliability problems running the tubes in the 14W area. I typically use a 130 ohm Rk and run the B+ around 345V.  
As far as tone is concerned, I have found that increasing the value of Rk thins the sound out. Increasing the bias current, within reason, seems to beef up the tone, at least to my ears. I think Matchless came to a similar conclusion and that is why they use 120 ohms Rk. They tend to run all of their amplifiers very hot. I was concerned with reliability, so I eventually settled on 130 ohms for my DIY amps. Something to keep in mind - these amps run class A without a feedback loop, so if they are too lightly biased crossover distortion will occur. Running a high bias current is a necessary evil in order to keep the tubes conducting throughout 360 degrees of input signal.  
Mike b
8/10/1999 2:01 AM

OK, a 150 ohm Rk gives me 14 watts diss. @ 350 volts. I think I'll try this out for a while. Thanks.  

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