Project summary:
This tube amp is to become available through the forum tube amp pro's (once phase 3 is complete). My reason for DIYing this amp is that something like this is not available commercially (well, you can buy a Mesa Boogie Road King but that's a bit oversize and expensive for my taste). It is meant as the back bone of a rig-with-preamp and can switch between entirely different voicings. I am not going to claim you can nail a Fender Bassman, Vox AC15, Marshall 18W and Boogie Recto sound but that is the project objective anyway. I decided to build the project in three phases: basic amp with some switching, amp with full manual switching and amp with MIDI switching.

Have been playing through phase 1 for a year now, sounds fine but I'm dearly missing some switching possibilities. I also don't like its point to point chaos look. Here is the plan for Phase 2:

Full size image: http://img683.imageshack.us/img683/4121/gearheadstudiokingv2.jpg

Switching is as follows:
Output 1 switches from tube to solid state rectification, putting in more distortion and a lot less sag
Output 2 changes the plate resistor, puts more drive but also increases distortion and/or 'tube warmth'
Output 3 does something similar, but without adding 'tube warmth' to clean tone
Output 4 switches negative feedback on or off, which cleans up clean tone but also takes away some drive
Output 5 loads the phase splitter stage more or less for more or less distortion
Output 6 switches between EL84 and 6V6 amplification, drastically changing tone
Output 7 switches between ultralinear (hifi/acoustic sound) and straight up (rock)
Those in the know will notice that the circuit can get very close to the amp designs mentioned. There is no speaker switching (yet?).

Next step is to give all components a nice place on the turret board and build the second amp as per phase two. After that I will upgrade amp one. I still need to work out the amount of NFB by listening. I am building two to facilitate the A/B comparison thing and ultimately have a stereo rig.

Comments anyone?

Goodie gumdrops, this is going to be very cool!
Good luck Gearhead!

Gearhead wrote:
Switching is as follows:
Output 1 switches from tube to solid state rectification, putting in more distortion and a lot less sag
Output 2 changes the plate resistor, puts more drive but also increases distortion and/or 'tube warmth'
Output 3 does something similar, but without adding 'tube warmth' to clean tone
Output 4 switches negative feedback on or off, which cleans up clean tone but also takes away some drive
Output 5 loads the phase splitter stage more or less for more or less distortion
Output 6 switches between EL84 and 6V6 amplification, drastically changing tone
Output 7 switches between ultralinear (hifi/acoustic sound) and straight up (rock)

I presume that most of those are not "hot switchable". IE, you need to turn the amp off, reconfigure and then power it back on again.

Nooooo Bob, everything switches hot. In fact, there is not even a click or pop for the stuff I had in phase 1, and the switching that is going to be added is not very likely to be heard either.

I must actually post credit to the output tube switching - a patent by Randall Smith. Further credit to Karel Mars for suggesting the way some features work.

Gearhead wrote: Nooooo Bob, everything switches hot. In fact, there is not even a click or pop for the stuff I had in phase 1, and the switching that is going to be added is not very likely to be heard either.

Oh wow!

Sorry if the question seemed insulting or to cast aspersions about your design. I thought the question an interesting one (though it may turn out to be redundant) and I opted for the limitations of my own understanding and imagination ?

I am especially surprised at being able to hot switch the rectification stage, but maybe I just haven't been keeping up with world of things that you plug a guitar into.

X-rated Bob wrote: Sorry if the question seemed insulting or to cast aspersions about your design.

Don't worry man, I have grown up amongst Dutchmen so I don't insult easily ?

X-rated Bob wrote: I am especially surprised at being able to hot switch the rectification stage, but maybe I just haven't been keeping up with world of things that you plug a guitar into.

The funny thing about that is that the diodes churn out a voltage that is significantly lower than the inputs of the recto tube see, but higher than the tube puts out. Switching the diodes into circuit will reverse bias (thx Tom for the term) the recto tube and stop it conducting seamlessly. The "pi" filter with its resistance against current changes takes care of the rest.

About this filter - there's something wrong with it. The amp should switch on in tube rectification mode to save putting in a standby switch. The first filter cap is (at 80uF) too big for the 5Y3 double diode tube - the initial current inrush would cause arcs and over time burn the plates. I will go for 40uF/5H/40uF/10k/16uF/10k/8uF and see what happens. This is still a lot more capacitance than Leo used...

It's the Hammond 1608. It was smallish to me (opted for the 1650E, asked the guru about how much it would sag) but Karel explained the thing about guitars not putting through such low bass, so the rated hifi power is less than what you would use for guitar. I took his word for it, and have felt it not get very hot. It does churn out plenty bass...
One of the reasons for going for this OPT and not the 125E is that it has ultralinear taps, which I use. Otherwise, I think the 125E is a great trano for EL84. Maybe even take the D.

I did the whole parts list, only 320 parts for the head, including MIDI, including chassis excluding cab :-[
Since I have been asked for a price indication: the parts for the head, when bought commercially, I reckon would set you back about 9k.
Then, you need to go and buy an nice speaker, a TubeTamer (that you could switch between two settings through the MIDI interface) and a speaker. Then you'd need to build a cab.
You could of course buy two speakers and switch between those, but I'm sure that would take a little more doing. ?

Update: while studying Ampeg preamps (don't ask why) I ran across this fabulous tube: 12DW7. It basically is half 12AX7 and half 12AU7 and is made today by JJ who call it a ECC832. This would be perfect in the V1 position, with a value change of the two cathode resistors. It will make for more clean headroom and will enable the volume pot to become a master volume between gain stages and phase splitter.
The other news is that I have been rolling tubes for V2 as well and settled on a 5751. Good tip Alan ?

The downside of taking 'perfect' but poorly available tubes is that, well, people might have trouble finding them. I will make sure the amp still works with 2x12AU7 or even 2x12AX7 8)