contemplating Version 2...

The prototype McGrath Mini amp which I've built for myself, the one you see in all the photos, may just end up being the only one built with that configuration: three 12AX7s and an EL84.  I am now starting to sketch out what version 2 will look like, and it will probably become the first version that I manufacture and sell.

The main impetus for a new version is to improve the effects loop, by adding another 12AX7 tube.  This will allow the loop return amplifier to be non-inverting, and to easily have enough gain for "lossless" operation.  The send circuit will remain a passive resistor-divider, as it is now.

Given the heater current capacity of the power transformer that I'm now using (and there seem to be no other close alternatives: beyond this one, current ratings, size, weight, and price all seem to jump dramatically higher), there is not enough power to add a fourth 12AX7, unless current is reduced somewhere else.  Fortunately, there's a perfect solution!  The 6V6 draws enough less heater power than the EL84, that the fourth preamp tube becomes a possibility.  I've been interested in using the 6V6 anyway.  The trouble is, it has less input sensitivity than the EL84, requiring an extra triode stage as a driver where the EL84 can get away with none.  But we now have that extra triode stage available, from the fourth 12AX7.  This will also let me now run negative feedback (NFB) around the power amp -- with a switch to optionally disable it, naturally.

With four preamp tubes, the larger-diameter 6V6, and the rectifier tube as before, the tube collection will no longer fit in a nice single row along the back (I want to keep the case the same size); instead, the tubes will be in a staggered zig-zag pattern.  But I think everything will still fit.

The 6V6-with-NFB output stage is also what I'm planning to use (two of them) for the "big" amp I'm planning to build next: the McGrath Stereo amp.  So developing the circuit for the Mini will help me get a start on using and configuring this output stage, before moving to the more complex Stereo design.

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(later musings from Jan 1 2024)

Actually, I have had good experimental results with these clever little switching power supplies designed specifically for tube amps, coming from China via Amazon/Ebay.  Could be perfect for a small amp like this, and then I could have four preamp tubes plus the power tube, still in a single line of five on the back edge of the chassis as now: instead of the EZ81 regulator, I'd have another 12AX7.  This can be considered to be my new design as of now, though I haven't built one this way yet.

The extra 12AX7 stages will be used as follows: one for additional boost in the preamp, switchable, ahead of the "linear preamp".  The other stage will be used as part of a 2-section circuit on the effects loop recovery side.  This will make the effects loop non-inverting, and will allow enough gain to have significant NFB here, to make the return very linear.  Still no tubes on the "send" side, just a resistive divider.  (This layout will also improve the "logicality" of the tube-section assignments: one tube for the input preamp, one for the v-mid circuit (drive 1), one for the M-section, and one for the effects return.  As it stands in the first prototype, these functions were spread across tubes a little haphazardly.)

I have thoughts on how to implement the footswitch more usefully, as well: by adding some more relays and making most of the switches 3-position, either on, off, or controlled by a fixed choice of footswitch input.  Each relay-controlled function will be assigned to a given footswitch, 1 or 2.  Probably both gain stages would be mapped to footswitch 1, along with other gain-increasing functions, while the effects loop would be mapped to footswitch 2.  The two orange LEDs on the front panel will stay, still reflecting on/off status of the two switchable drive stages; however, both will (probably) be mapped to footswitch 1.  There will be a new blue LED reflecting effects loop status.  There probably won't be LEDs for all the other controllable functions, but... maybe?  (Actually, yes.)

The colour of the footswitch LEDs will no longer be orange; perhaps it'll be white?  Or maybe orange for 1, blue for 2.  (Yes.)

I've become interested in the 6AQ5 as a possible power tube in future designs -- both this, and especially larger amps such as the upcoming McGrath Stereo (4 power tubes).  This tube is apparently very similar to the 6V6 sonically and electrically, though it has a lower max power rating.  Both the 6V6 and the EL84 have become what I'd call "crazy expensive" of late, although maybe it's just because I'm out of touch, being from "the 1900s".  The 6AQ5 seems to be much cheaper; if it performs anything like a 6V6, that is of interest.  Unfortunately, there does not seem to be a similar viable substitute for the 12AX7 for less money, and these too have become crazy expensive.

However, to use the 6AQ5 (or 6V6) here, I'd probably need to allocate one 12AX7 stage to the driver, since power tube gain is lower than with the EL84.  So more re-arrangement of the preamp design might be needed, beyond what I described above.  Probably the first implementation of these ideas (i.e., my second McGrath Mini prototype) will still be based on the EL84, and I'll use the larger Stereo amp as the first test platform for 6AQ5 designs.

It might be ideal to just provide two sockets, one for a 6V6 and one for a 6AQ5, for each power amp tube.  Assuming the design can truly use the two interchangeably, in terms of voltages, impedances, etc..