18Watt.com

I built a Mojotone 18W clone about 15 years ago. The PI and both preamps (3 x 12AX7) are all fed by the same 16uF filter cap. I’m considering using the existing 16uF for the PI and add another dropping resistor and 16uF for the preamp tubes. Have any of you tried this, and do you have recommendations on what value dropping resistor to use? My initial thought is to reduce the 10K to about 8K and add another 2K before the new 16uF cap to keep the preamp voltages essentially the same. Thanks in advance.

Although your reasoning is correct about separating the filter caps between the preamp and the PI I still wonder why bother after 15 years. Is there a specific reason for doing that other than good practice? Are there indications this is a necessary step? Hum? Noise?

Thanks for the reply. I made a mistake in my first post. The schematic I attached was pulled from Mojo’s current documentation. I looked at mine (attached here) and it has the power screens and PI fed from the second node 32uF. I’m not sure when or why Mojo made that change.

The reason I was considering it was to see if it changed the level of intermodulated distortion or ghost notes. It’s something you can hear when playing alone but not in a band setting. I changed my power resistor to 180R and screens to 470R a while back and I think that may have reduced ghosting a little bit. I know ghosting is a pretty common thing with these vintage Marshalls.

It looks as though they removed an RC filter in your kit. Probably cutting cost and corners. I’m not sure how this affects the tone. As long as you have space on your board you can add an additional RC filter. Change your current 10K to a 2K2 and put an additional 8K2/16uF filter to feed the PI. The screens and PI would then have isolated nodes. I would also beef up your screen resistors as per what you see in most of the contemporary 18 watt schematics.

Could you post some pics of the build? Let’s see where the best spot for the R/C is in your amp.