When interest was rekindled in the 1980’s Marshall mods that I developed when I worked at SIR LA, I was able to find the diagram of Stock #39 that I drew when I first built it. On later mods, I had continued to develop that idea further, until I moved to the Midwest and got completely involved in analog synthesizers. Of course, I continued to play guitar and collect & modify amps for my own amusement (at last count I have over 30 tube amps).

Looking back at the 80’s mods with a fresh perspective, I wanted to correct some of the shortcomings they had, as well as incorporate some of my newer ideas and some features that people have become accustomed to in the last 20 years. At the same time, I realized that it was absolutely necessary to exactly duplicate the sound of Stock #39. After considerable soldering and tweaking, I believe I have accomplished those goals.The biggest problem with the 80’s mods was that there was no clean sound available, and with the extreme amount of gain, it didn't clean up very well by backing off the guitar’s volume control. Also, the amp had to be set for solo tone and volume, and couldn't be easily switched for other tones.

I have attacked these problems by adding a separate Clean volume control. A footswitch (supplied) selects between this and The Gain channel. Once in the Gain channel, you have the choice of a MK II Master circuit (good for crunch rhythm) or the Stock #39 extra tube stage (the center footsw).

Both of these circuits have gain boost toggles on the front panel, as explained on the Block Diagram page. There may, under certain conditions, be some popping when switching channels. I have employed relays, rather than the opto-isolators used in most channel switching amps, because relays don't affect the tone, and optos do. The penalty is some occasional popping. If no notes are held when switching between Clean and Gain and back, no popping will occur.

Most modern amps have effects loops, so this is included. Great effort was made to match the tone of the Loop to the natural tone of the amp, but there is a slight difference, due to the fact that the signal must be buffered and reduced in level to match rack mount effects, and then amplified back up to internal tube amp level again. This should not be a problem, since any effect in the Loop will affect the tone much more, and all the circuitry of the Loop is out of the amp’s signal path when the Loop is off. To use the loop, connect an effect and set the input level on the effect after you have established your playing volume. (Changing the Clean and/or Master level will change the level going to the effect.) Then, adjust the effect’s Output control while switching the Loop on and off, to match the volume, or add some boost if you like. The Loop will affect both the Clean and Gain channels.

Another problem with high gain amps is that the gain amplifies any noise inside the amp right along with the guitar signal. This has been reduced by running the critical 1st 2 stages’ tube with direct current (DC) on its filament, rather than the stock AC.

Lastly, I have added a Mid Boost, which is activated by pulling the Treble pot.

Try it, maybe you'll like it. One problem with channel switching amps with only one set of tone controls is that different settings are usually required for the clean and distortion sounds. The Stock #39 mod has a very bright sound, and I usually roll some Presence, Treble, and Middle off. Unfortunately, this is not compatible with a bright, shimmery clean sound. I toyed with the idea of reducing the hi end of the Stock #39 stage for a closer tonal balance with the clean sound, but decided it was more important to leave that the same as the original Stock #39. Of course, many players prefer a much brighter sound than I do, but if not, rolling off the guitar’s tone control while in the Stock #39 channel can help. On some amps, this problem is solved by multiple sets of tone controls, but I don’t really like amps that look like mixing consoles. On the Soldano/Caswell X99 preamp, we solved this problem by motorizing the controls, so switching sounds could instantly change the tone settings. Unfortunately, adding the motorized controls to an existing amp would be rather ugly (not to mention expensive); food for thought, tho.

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Block Diagram
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