MARSHALL MINOR 2026

[Ritchie200]

Timbooooo!


Beautiful work as usual.  However, you made a mistake.  How are you gonna plug 4, 4x12 cabs into this thing with only one jack?  Make sure you post some soundclips so folks can hear how great this thing sounds.


Jim

[jojokeo]

Hi Timbo!
Rule #1: NEVER listen to the noise coming from St Louis! All squawk and talk will just lead you down a dead end street! If he only knew something about building amps rather than just clicking on the power switch all the time and actually played in the same room as the amp at that it might help to think he even knew what his settings were but i digress.


Remember when running two stages with one load or cathode resistor to get the same performance you halve them and they share the current to get the same performance and load line specs as when they are used separately.

For the AU7:
Lower the load resistors to 22k or even 15k, then the shared cathode to 750r. Don’t be afraid to lower the power dropping/isolating resistors fir higher voltages to the nodes. These changes will provide more headroom for you.


Jojo

[TIMBO]

Thanks guys, I had my stunt guitarist give it a good going over.
The results are kinda what I expected.
The clean channel (volume on 3, PPIMV on 8, everything else on 5) sounds AWESOME and increasing the volume gives the clean a bit more edge.
Engaging the second triode also gives the clean a bit more drive, VERY nice.
BUT, if the the two volumes are increased to get the BEAST to ROAR, well it didn't happen....
One word, CLEAN HEADROOM.
We cranked the two volumes and all we were getting was preamp distortion and the KT88's weren't responding.
There was no SUSTAIN or CHIME in the hard picking or power chords.
And it was FREAKIN loud.
Still very happy with the end result, could still do with some tweaking, sound clip on the way.

[SILVERGUN]

Timbo, first, you should be honored that the butt-hurt stable hand has made his way back to the forum to comment on your post.

Second, since the original Major was operating with 4 output tubes and a 16ohm feedback tap, have you compensated NFB for this? With the reduction in output voltage and loss of closed loop gain I would have to experiment (because I suck at math).

I would temporarily tack in a pot in place of your 56K and see if I could find a sweetspot, or even just temporarily disconnect it and see if there is any increase in roar-ability.  I would also jump out and eliminate your PPIMV in the process, so it's not clouding the results.

There was nothing special about the original Major anyway so don't be surprised if it just a loud speaker fryer. It always was.

Your work looks great!

[TIMBO]

Thanks guys, I do think there is a bit more room for improvement.
Both of you have touched on subjects that I lack tech knowledge.
So these are the areas I'll concentrate on...


NFB- Got room on the back of the chassis to mount pot for FB adjustment
Power valve resistors/caps - Increase resistor values to decrease headroom and volume and change bypass cap to suit
12AU7-Changes as per Jojo's post
Preamp- Resistors/caps??

[shooter]

We cranked the two volumes and all we were getting was preamp distortion and the KT88's weren't responding.

If you have a scope and signal generator;
your 88's are setting at ~29Vdc
you need at least that much Vac to get the 88's  bent
I like to shoot for 1.5X that amount, maybe 2X


try ~150mV AC in, set all tone knobs @ 5 all volume EXCEPT PMV to 10
scope the wiper of the PMV, it should be ~~~ 50V AC,  hopefully close to clean.
scope speaker + next and record V AC (and note what speaker impedance is)