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"Marshall tone mods"

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7/1/1999 0:51 AM

"Marshall tone mods"

Just finished converting an old PA amp inot a LOUD guitar amp! Basically it's mostly a Marshall clone. I used the preamp circuit from this site, the one with the master volume. The output is a pair of EL 34's running 800v plates and 400v screens, at about 25 ma idle.

The amp sounds great through a 12" Celestion G12M-70 but I do have some small tone issues for which I'd appreciate some advice.

I'd like to add some "bottom" warmth. The bass and mid controls don't do a heck of a lot. The amp is very trebly (like a Marshall - of course!) to the point of being "clicky" when you pick a note.

I thought of adding cathode bypass caps to the 12AX7's but I have tons of gain already and I'm afraid the caps wuld bring the highs up along with the mids and bottoms. How about increasing a coupling cap or two from .022 to .05 or so? All of them? Or just certain stages?

Maybe I'm trying to get TOO much tone range! After all, I could have copied a Fender. Is it possible to get a good compromise?

Any thoughts would be greatly appreciated...

7/1/1999 1:02 AM

Skott;

Did you follow the JCM800 schematic to the "T" as they say? If not maybe you could draw up a quick schematic of what you have and email to me?

If you don't have bypass caps on the cathodes of the 12AX7's I would start there. It's a great place to shape tone. .68uF cap across the first stage is always a nice "marshall" value. For more bass you could go for 250uF or 330uF. Also try replacing the 33K slope resistor with a 47K or even a 56K.

Again, I'm not sure exactly what to suggest because I'm not sure if your amp is the same as the schematic.

Let us know!

Trace

7/1/1999 4:08 AM
Bruce	I'd say also to find a way to drop that 800vdc B+. Get those plates down to around 550vdc-525vdc or less! Then set the bias for more idle current, around 32ma-35ma range. Around 18 watts at idle. Now you are a little hot. What are the preamp plate voltages? Bruce Mission Amps

7/1/1999 3:24 PM

First off, I did copy the preamp exactly. The only bypass cap at the moment is that .68 uf. It's the schematic from this site that has two inputs with the lead channel fed through the input jack switch of the following "clean" channel.

As for the B+, I'm not too worried about these voltages. I've researched the tube data and with the lower screen voltage and the screen dropping resistors I've installed the tubes should be ok. I've been in contact with one of Svetlana's "gurus" and he thinks their tubes should be ok, as long as the screen current is no more than 20 ma. per tube. In effect, the lower screen voltage compensates for the higher B+. This is why amps running lower voltages have to run the screen voltage so close to the plate.

If I lowered the voltage ( a lot of work and money in itself!) I would have drastically changed the plate load impedance, which would mean a new output transfmr. That's why I did all the research and talked with the tube mfg. I would have felt safer being more traditional in design but I would have lost all the cost savings in modding an existing amp.

Of course, I'm kinda locked into Svetlana EL 34's now instead of cheaper versions but that's not really a hardship!

Anyhow, I do have to be careful about too much gain in the preamp. Cathode bypass with a low reactance value for the lows will of course also bring up the highs along with them.

That's why I'm wondering about the coupling caps. If I do go to cathode bypassing perhaps I can drag my scope out and see if I can cut down the overall gain to compensate. It would be a lot easier with the coupling caps.

I should say that even with the Svetlana blessing I felt the need to install a fan. Can't hurt...

7/1/1999 3:29 PM

Nearly forgot! Bruce, the preamp has its own LV supply, which also feeds the screens. It's the standard consecutive votage frops between cap sections. The first triode plate sees about 200v and you end up with about 225v on the platees of the phase inverter. The screens come from the first section with their own dropping resistors to prevent sagging the preamp plate voltages as the screen current swings.

7/1/1999 7:31 PM
dave	If you haven't done this yet, you might lower the volume bypass cap from 0.005 uf to 500 pf. If the first coupling cap on the bright channel is 0.0022, change it to 0.022. There's been tons of discussion on this BBS in the past about this very subject. Too bad you can't search the achives anymore. dave

7/1/1999 8:32 PM
Craig	See KG's 'Tone ruminations' thread below Ken has some really interesting thoughts on tone/gain.

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