Electrolytic Caps in my Plexi Build--HELP!

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Electrolytic Caps in my Plexi Build--HELP!

Post by ESR » Fri Dec 07, 2018 6:13 pm

I built a Super Lead clone from the Mojotone amp kit. I believe it's supposed to be based on a '68. Might be wrong about that. I've had a decent amount of ghosting going on with it from day one. I've read in various places that it's just the nature of the beast, and I've read in other places that there are ways to eliminate it.

A trusted friend told me that ghosting is a result of insufficient filtering or "too little capacitance in the B+"... In the kit that I used to build my Plexi replica, all six of the can capacitors were JJ 50+50 500v cans. All six of them. My friend suggested trying some 100+100 cans--at least for the two connected to my center tap.

So I've been doing some reading. I've discovered in some older Metro Forum threads that one COMMON way to go is:

1. the two cans connected in series to the center tap are both 50+50/500v (I'm assuming that's the same as a "100uF", since 50 + 50 = 100...yes?) -- this is what my friend suggested bumping up to 100+100/500v... good idea in order to eliminate ghosting?

2. then the two cans connected to the screens are 32+32 (500v)... (again--mine here are 50+50 instead)... why might I want to keep my 50+50's or switch to 32+32's?

3. I've also noticed that the two axial electrolytics near my rectifier section are both 8uF/150v instead of 10uF/150v--what is that yielding in my circuit?

4. Instead of 32+32/500V axial cap on my board going to the 10k 2W resistors on the preamp side, mine is a 50+50/500v can mounted to the chassis behind the board... should I consider switching it to a 32+32? why/why not?

5. The lone can after my power tubes is a 50+50/500v, which seems to line up with what i'm seeing everywhere else.

6. Lastly, I'm confused about the electrolytic(s) at far right/input end of my board--my amp has a single 250uF/25v (with an 820ohm resistor in parallel) coming off pin 3 of V1 and then grounding off on the other end... But I've read about others running "two 330uF/25v caps" instead... why am i seeing them run two of them when i'm only running one?

I know I listed a bunch of questions here. I would really appreciate any help/explanation I can get about how these electrolytics are affecting my circuit, and why I might want to choose different values than what I've currently got in my amp.

Thanks in advance!

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Re: Electrolytic Caps in my Plexi Build--HELP!

Post by alexradium » Tue Dec 18, 2018 10:58 am

the 6 caps 50+50 are normal in all 100w modern built Marshalls,going for smaller values could give a looser,less tight feeling.
About ghost notes,i guess that is due most probably to bad wiring layout,including grounds which in these amps are all over the place and contribute the most to noise.
you should post some pics of your amps to get the idea.
don't worry about the 8 uF,they are ok.
same for the 250u at first cathode,anything over 22u is the same.

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Re: Electrolytic Caps in my Plexi Build--HELP!

Post by danman » Tue Dec 18, 2018 7:34 pm

The smaller 8uf caps are for the bias circuit. 8 or 10uf won't make a huge difference though I usually like 10. The 220 or 330uf cap at the end of the board is a bypass cap for the first stage cathode resistor. Both values are very large and make for a lot of bass in the preamp. Fender used 25uf in a lot of their amps and they are also bass heavy. Some folks will lower the value down to 1uf-5uf which helps to tighten the amp's bass response by removing a bit of the low end flub. Marshall ended up splitting the cathodes on later circuits by using a large 220/330uf cap on one side of v1 and a much smaller .68uf cap on the other half. This gives a much tighter and brighter sound to the lead channel versus the bass heavy sound of the normal channel. There are other components in the lead circuit that help with the brightness (treble peaking caps) but using the small .68uf cathode really helps that channel stay tight.

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