Moderators: pompeiisneaks, Colossal
What cap value would you suggest in it's place then?M Fowler wrote:Jason,
I'm probably the only person that enjoys the Ken Watt's Spitfire stock no mods. I do agree the Fat Boost switch doesn't add much with the cap values used.
But the rest of the circuit I left stock.
Mark
So, you're using 4.7K instead of the 22K circled in black here? And there's a notable difference in harmonic richness?orrong65 wrote:It seems this exact same topic comes up every 5 years or so, when someone builds one of these remarkable amps which are still widely used in recording studios around the world.
I built a Lightning clone from the schematics, and have ironed out nearly all of the bugs. It is the best amp I own (out of 12). I have it in a 2 x 12" combo with a Celestion G12H anniversary and a Weber signature 12s alnico. Pure heaven.
There are two Lightning schematics readily available:
1/ a crude hand drawn version called "Matchless Lightning 15" with pre-and power-amp, and half the power supply (supposedly the original schematic), and
2/ a better hand drawn version of the complete circuit called "MATchless LightNiNg" and dated 2-9-9? and signed MS?
The schematics are very similar, but I wanted to build the original version as it is supposed to have much better harmonics. So I built the original true to the schematic, including as much of the original power supply that was shown, and then used the later power supply for the rest.
But something was not right. I found that there was quite a bit of noise despite adequate power filtering, and the amp struggled a bit above moderate volume with a big signal. It also lost a lot of its punch, either overdriven or clean.
Going back to the original schematic, I noticed the EL84 screen supply was connected to the OT B+ supply, and came off the power rail at the same point (which was not shown). This is unusual and not present in the later version, or in many other amp designs at all.
What I did was to leave B+ supply connected to the power rail before the choke (where I had put it) and moved the screen supply to the rail after the choke, and after a 22k step down resistor to drop the screen voltage and 25uf of filtering, recreating the later schematic.
This cleaned up all of the noise instantly, improved the tone clarity, and preserved the amp power right up to 10 on volume (if you ever need to do that!).
So after trial and error, I ended up with and original pre amp and power amp circuit, with the complete later power supply circuit, and this worked just fine. In my view the original circuit with the screen supply joined to the B+ supply must either be an error, or just an early version of the circuit.
Note that I kept the original design 4.7k step down resistor on V2 supply instead of the 22k shown in the later schematic (Y supply), running the V2a voltage amp and V2b cathode follower stages at higher plate voltages. I think this is a big part of the better amp harmonics.
The only other significant difference between the two Lightning circuits is the 100pf treble bleed cap across the master volume on the original (as someone else has mentioned). This certainly brightens up the tone, and is ideal for studio recording. Installing an on/off switch on this cap is a simple way to add a new tone to this great amp. Another option is to experiment with a ceramic cap (for a 'velvet' tone) and a silver mica cap (very clean) in this position, or switch all 3 - off, ceramic, silver mica.
An amp to cut through any mix.
Dave
