Splitting Cathodes of V1 questions

[lespaul3]

I'm pre-planning here... I'm definitely in the "lite" range (no tremolo), but I think I'd like to split the cathodes, and still try to stay as stock as possible otherwise (ie, not go so far as the cascade mod).

After much research on here (thanks to the forum), I've learned that in addition to splitting the cathodes, I need to split the plate supplies also. I know that I need to have 100K resistors for each half of V1 instead of one 100K feeding to both plates.

Looking at the stock and the Superlite schematics (I'm seemingly trying split the difference), I'm having difficulty rationalizing how I properly feed the signal to the stock tone stack if I split the plates. (sorry if my terminology is wrong, but I hope you get what I'm asking).

I would love to use the standard tone stack unless many of you strongly suggest a modified tone stack when splitting the cathodes, or unless it's absolutely necessary.

Could it be as simple as not splitting the plates but putting a 200K or 50K resistor in place of the 100K? (logically, I'm not sure which one would be correct)

Could I please ask someone to try to shed some light on how I would feed into a stock tone stack with separated plates?

Also - as a secondary question: Concerning the single and parallelled inputs as noted on the Lite IIb schematic, that sounds great... But, I can't for the life of me figure out if this is an actual mod to the original "hi and low" inputs or if the original were doing this all along and the Lite IIb just "renames" it. The best I can figure, it's doing the same thing as the originals.. ??

Any help appreciated.

Thank you!
bart

[zaphod_phil]

So you want to build a Lite IIb, and split V1's cathodes? And just to help me understand, what is it you're looking to gain by doing that? In any case, to answer your question, there's no need to split the anodes (plates) as well, since that's a convenient place for the signals from the two triodes to combine together. The only reason to split the anodes with separate resistors would be if you wanted to have individual volume and tone controls for each side.

To answer your other question, AFAIK, the Lite II's way of paralleling the triodes on the Hi input jack isn't a feature of the original Marshall 18W amps. The cool thing is that if you plug into the Lo input jack, it splits the inputs of the two triodes. You can then plug a guitar into each input jack and each effectively gets its own input channel, which is real handy for impromptu jamming.

[Raja]

To answer your other question, AFAIK, the Lite II's way of paralleling the triodes on the Hi input jack isn't a feature of the original Marshall 18W amps. The cool thing is that if you plug into the Lo input jack, it splits the inputs of the two triodes. You can then plug a guitar into each input jack and each effectively gets its own input channel, which is real handy for impromptu jamming.

Should someone be able to use an A/B box going to the inputs of a Lite and then switch between inputs? I've tried and all I get is low input.

[lespaul3]

Thank you Mr. Beeblebrox,

Judging by your posts, it seems you are an invaluable source around here.

The amp is being built out of an old mono hi-fi amp, and it has a 5 position rotary switch on the front. I was thinking that splitting the cathodes (and applying different values of course) would be a fun thing to put in for some tonal variations, but would still keep things fairly simple and still remain close to the wonderfully simple original circuit.
I hope that is correct thinking. (please let me know if it's not worth it!!)

Your answer makes it clear... if I don't split the anodes and keep the single 100K, then the tone circuit connection stays stock. sweet. That's easy.

Concerning the parallelled input, I do believe I have it figured out. I was looking at schematicheaven's 18W Rev5 schematic, thinking this was "original." IT is electrically identical to the inputs on the Lite IIb, though drawn slightly differently. I just searched on this site for originals, and the one that's listed is in fact different, and I can see how that would NOT be parallelled. Sorry for the corn-fusion.

RAJA: If it looks like if you plug something into both inputs, it effectively cuts the connection between the two sides (the parallel part) because the mod takes advantage of the self-grounding part of the jack. You shouldn't expect there to be any difference in the tones with a cable in both jacks (ie, an A/B box). If you switch ONE cabl between the two inputs, you *should* hear a difference in the inputs. If you really want to use an A/B box and hear a difference, I bet you'll just need an A/B/Y box (one that sends a signal to both A and B simultaneously - "Y"). That should effectively parallel the inputs outside the amp, since the option is lost internally when plugged into both inputs.

Thank you again,
Bart

[Raja]

Thanks for the info Bart.

[zaphod_phil]

I was looking at schematicheaven's 18W Rev5 schematic, thinking this was "original." IT is electrically identical to the inputs on the Lite IIb, though drawn slightly differently.

The PedalMonkey Rev 5 schematic on this site is from one of the very first production batch aka "first born" 18Ws. Without looking, I'm not quite sure what they've got on schematicheaven. As with many vintage amps, the idea of the dual or multiple inputs was that you could have more two or more players sharing the same amp.

If it looks like if you plug something into both inputs, it effectively cuts the connection between the two sides (the parallel part) because the mod takes advantage of the self-grounding part of the jack.

Actually plugging into the Lo (ie single triode jack) separates the two inputs. Plugging a guitar into the Hi input at the same time is optional.

I am told by those who've tried this trick, the fun part is if you bias one of the preamp triodes hot and one cold. When you parallel them, you get more shimmer in the amp's tone, while the two triodes kind of take turns handling different parts of the signal cycle. So for example you could have one triode with a 680 ohm cathode resistor and the other with a 2.7k, or maybe even 3.3k. After that, you could also do the bright/dark channel thing with the usual 50uF cap across the 680 ohm resistor and a .68uF cap across the 2.7k. This is one of those things I'm just going to have to try out myself sometime soon after I've finished moving house.

[zaphod_phil]

I just looked at the first stage of the two circuits (Pedalmonkey Rev 5 Normal Channel and Lite IIb) and they are the same. What Pedalmonkey calls Lo and Hi the Lite IIb calls Single and Parallel, respectively.

They still look different to me on the schematics. The Lite IIb schematic seems to show an extra contact on the jack that PedalMonkey's Classic 18W doesn't. Having said that, I've only ever done it the Lite II way, because that always seemed more useful to me, so I've never really seriously tried to work out exactly what the Classic 18W was doing in it's original form....