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Sorry guys, didn't mean to sound inclusive, (I thought Z was close by).

Anyway, in a Lightning PI circuit, what is the function of the 100K resistor between the .01 coupling cap and the ground end of the tail resistor?

When this one is lifted, everything still appears to function normally, just a lot louder. Closer to what I would expect from this amp.

Thanks for any input.

Steve

I think it is just an attenuator....could be wrong though
Stew

Yes, you're right. Like I said in the other thread, the PI input is being fed via a 220k/100k resistive divider, so that the PI is only getting around 1/3rd of the signal coming out of the tone stack. And the tone stack itself is also heavily attenuating the signal. Removing the 100k to ground effectively disables the divider, since the PI input is at a very high impedance. So the signal only gets attenuated by the tone stack.

In schematics like this one, www.schematicheaven.com/newamps/matchless_lightning.pdf , it's drawn kind of strange, so it's not immediately obvious what's going on.

BTW with Vox/Matchless type amps, at some gain levels, the cold-biased PI can clip in a fizzy way. So I like to reduce the PI's cathode resistor from its typical 1.2k to a lower value, say 820 ohms, although I've known some folks go to around 700 ohms. I also like to increase the 1200pF coupling cap after the first gain stage, to 5nF, to thicken up the tone a bit. I know this may seem like sacrilage to some folks...

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Great tones come in small glass jars!

Zaphod, I've noticed that "fizziness" with some settings as well - though I love the amp. I think I'll try what you suggest. But to clarify, do you mean by the "1200 pF coupling cap after the first gain stage" C1? (The schematic I have shows 0.0015uF there = 1500pF. And by "the PI's cathode resistor" do you mean R15 on the schematic, at V3? Pardon my obvious ignorance!

Thanks!
Jim

I agree with Z about 5nf C2 it, it beefs up the tightens up the bottom end.
I also took a 1meg resistor to ground off of this cap.

I prefered a 100K plate resistor for V1, it's less thrash.
I installed a switch on the V2 cathode bypass with a 40uf in that position. I did pop for Sozo vintage caps throughout.

I currently am running at 820 ohms on the center PI resistor and 68k for tail with .022 coupling caps (Mallory 150's) and no 100k attenuator or divider. Odd but I went with what sounded good

I changed my MV to a cut control with a .009 El cap in series and lowered the volume pot cap to 47pf silver mica.

My cathode bypass for the EL 84's is 40uf with a 120 ohm resistor.
I found that if I pulled the B+ for my output transformer (MM 8K)from the middle 30uf cap (after the 125ma 10H choke so that it's in a Pi configuration), using the Westlake power transformer, I got 295 between pins 3 & 7 on the EL84s.
I calculate out at 12 watts dissappation and 40ma.

I followed Z's advise and am running V1 as a single or parallel triode using the 18 watt lite IIb input topography. Clearer sounding than the Ceri@tone input topography.

I used a 4way rotary switch with the following values 12k,33k,68k,open in parallel with 80k at the bass pot. This gives me around 10k,24k,37k,80k as a mid cap even if the switch fails.

Even with all of the above changes I still had to run 200k single and 220k parallel V1 grid stoppers to tame the high end. I know my values are from outer space, and I have triple, triple checked my layout and wiring.

This circuit has so much gain and high end, it's just delightful to tweak.
The amp just rips and still sounds "pretty" chimmy and shimmering at a normal gain level. I'm running a 12" 16ohm Warehouse Black and Blue Alnico in parallel with a 16ohm 12" Cannibis Rex on the 8ohm tap.
I have a oversized Lopo convertible cabinet coming next week. It will be nice to get the drivers in a cabinet and off the coat hangers on my shop shelf. I can go from the Beatles to Tom Petty to the Faces to Rory Gallagher to a Marshall in one stop. Just amazing. A+ for Mr. Sampson.

Thanks again guys, I will post pictures next week when I get the cabinet.
Steve

I was working off the Matchless Lightning schematic on the schematicheaven web site. I'm not sure which Lightning schematic you're referring to. So the cap in question is definitely 1200pF on that schematic, but might well be 1500pF on yours. Anyway, I think it's pretty easy to see which one it is. There's only one resistor on the phase invertor cathodes that's 1.2k, so I think that one's not too hard to identify either.


I prefer 56k or 47k so that I can drive the power tubes harder, for more of that sweet power tube distortion.

I doubt that he would recognise much of your amp.

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Great tones come in small glass jars!

Interesting, I was looking at that 68k and wondering if I should A / B it since I went to the .022 couplers.

You are right on the money Z, it was just a tad bit on the weak side, better at 56K, just right for my old ears.

Just for my own edification, where does all the "briteness" come from in this amplifier circuit?

I was referring to the TC-15 circuit/schematic, which I realize has been modified from the original Matchless/Vox models used as basis. Perhaps there are other mods in the TC-15 that compensate, such that the changes you're talking about would not improve but degrade the TC-15?

Thanks.

The EL84 power tubes are naturally quite bright. So in my experience most EL84 amps have a tendency to be somewhat bright. So Vox amps usually have the Cut control to compensate. Marshall 18W amps have 8.2k grid blocking resistors on the power tubes, which also helps somewhat.

It seems that the TC-15 does indeed have some extra magic over the regular Lightning. Certainly the EF86 channel is a huge extra. The TC-15 also seems to yield some great tones without the choke, which the Lightning has, and you have the additional flexibility due to the 5k/8k OT.

However, at some point it starts to become more a matter of personal taste. For example, I happen to like the thicker tone you get with C1=5nF, but that doesn't mean the next person would. It all starts to get quite subjective.

_________________
Great tones come in small glass jars!