Cathode-Biased 6V6 Plexi with FX Loop and LOTS of tonal options

[SnickSound]

A year ago I set out to build the "ultimate" 6V6 Plexi, that could do JTM45 to JCM800 types of tones (not sure we can call it a Plexi at this point, but you get the gist), at a very reasonable power level.

This week I've finally managed to install all the switching options I wanted to do at first, and got the FX loop working:

This is the schematic of the amp in the vid:
https://4.bp.blogspot.com/-YsJpmPZcc2I/Wk5mgZAgkvI/AAAAAAAACpI/H918HFw4CSYyKDyoNILjU7ZfOxOTMzfzQCLcBGAs/s1600/Ultimate%2B15W%2BPlexi%2Bby%2BSnickSound%2Bv4.2.gif

The various options would work whether you go cathode of fixed bias, and no matter what kind of power tube you're using. But the power level has an effect on which options are useful.
For example, at this power level (12-15W or so), cascade mod is can of useless at club volumes: power amp is not clean or tight enough. It's fun for noodling at home but that's it.
Also, while it can do some superb cleans, headroom is very limited.
Where it shines though, is in classic Plexi mode since it's easy to get the power amp going, but it saturates very quickly. Between the shared and split cathode modes, as well as the two NFB levels, I can do dirty blues or gnarly AC/DC.

It's a really fun little amp, although I might just do away with the cascade and cold bias options since in the past year I have NEVER used them on stage. But for a studio only amp, you can never have too many options. All these switches don't seem to add any liability, amp won't oscillate, and there's no odd artifacts unless I do something stupid (shared cathode in cascade mode for example)

I've got 30 videos so far about designing, building and tweaking this amp if you're curious:

[turtle441]

That is super cool.  Would you be willing to share the DIYLC file?  I might try that with 1-2 fewer switches.

[SnickSound]

That is super cool.  Would you be willing to share the DIYLC file?  I might try that with 1-2 fewer switches.

Thanks!

Problem is I don't have a DIYLC file that looks anything like the amp, so much tweaking and experimentation has gone on. When I get time I hope to properly document some of those things.

The only switch that requires an update to the board layout though is the Cold/Warm bias switch on the Normal channel. I basically have the normal cathode resistor and cap combo (except it's a 1k resistor instead of 820) go to a turret that does not connect to the ground buss, but I have an 8k2 resistor connect to ground (could also be a 10k, YMMV). The Warm mode simply connects the "lifted" resistor and cap to ground, giving you a 890 ohm resistor equivalent that is fully bypassed.

The Bright channel has the typical 2k7/0.68uF Rk network, all you have to do is connect both cathodes together via the Split/Shared switch to go into Shared mode.  You end up with a 670 ohm Rk equivalent (instead of 820), but compromises have to be made to allow all those modes, and 670 is good enough in shared mode.

Ah yes... the switched bright cap on the mix resistor does require an extra turret to be done cleanly, but it can be done with the cap on the switch.

Cascade mode has been document plenty, just switches the Normal channel's grid between the output of the Bright channel's volume (the wiper) and the Normal channel's input jack. Needs a DPDT to also stop sending the Bright channel's signal to the mix resistor in cascade mode. A bit more planning could allow that DPDT switch to bypass the mix resistors entirely, as well as the Normal volume, replace those with the typical 470k/470pF + 470k to ground voltage divider of the 2203/2204 circuit. SKy is the limit!