JCM 800 Series 50 Watt Bass Amp

[W5FH]

I am considering this circuit for a bass amp build either 2 or 4 tubes in output stage. Have read a lot of good reviews about it. I like all of the specialized tone controls for bass. Wanted to ask for comments in regards to some of the circuitry in regards to bass amplification:
    1. Input stage V-1a is warm biased but has no cathode bypass capacitor.
    2. V-1B appears as a cold clipper.
    3. V-2a appears as an AC coupled cathode follower. After studying the operation of the DC coupled cathode follower will this AC coupled one give a similar smooth compression effect? In the DC coupled follower it is the "extra current" flowing in 100K plate load resistor (from the the grid current flowing in follower) that gives the smooth compression; just curious about AC coupled version.
    4. Is there a more clear version of this schematic, not sure of a few component values?
    5. Anyone have recomendations for better bass amp circuit?
Thanks all.
https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM800-Bass-50W-1986-Schematic.pdf
 

[thetragichero]

i made the 100w version few years back. VERY clean, lots of negative feedback

[pdf64]

Back when they were the latest thing, my bass player got one.
Big mistake, too much range in the tone controls, every sound except a good one.
A regular old JMP 1986 set things back on the straight and narrow.
Since then, for bass I’ve used 5F6A type or a regular Fender black panel normal channel with a beefy OT.
But the best sounding amp Ive heard and used for bass was a Vox AC50 MkIII that I’d repaired, and was giving it a final test with the band.
The OT in those is massive, much bigger than any regular 100W type.

[W5FH]

Thank you both for the replies. I am continuing my study on the JCM 800 bass amp, and starting to study the VOX AC-50 MK3. If you have a moment pdf64 I wanted to pick your brain about the AC-50 bass amps. Three things -
    1. Which do you prefer, the 12AU7 or 12AX7 input stage? One of the bass instruments is a stand-up bass with piezo pick-up; will need more gain.
    2. Regarding the output transformer - - - the one referred to as the "large stack" was said to be reverse phase wound in order to increase bass reproduction. I understand the advice to use a large output transformer to help bass reproduction. Would it be worth the trouble, and expense, to seek out an OT of this type? I wonder if the Mercury Magnetics VXO-50 OT is wound to the original large stack specs?
    3. If you were to do a clone build as I am contemplating would you change anything in the amp? I know I will not be using EL-34 tubes so mostly interested in preamp/PI sections.
   Thanks!

[pdf64]

I recall that the 12AU7 sounded better for bass. For guitar a 12AX7 perhaps worked a bit better, depending on the goal.
The owner uses it for bass, so a 12AU7 it is.

I’m not sure what’s meant by reverse phase wound? Maybe bifiliar. I’m not absolutely certain that the OT was where the magic lay, but who knows? I recall fixing another AC50 that had a Marshall 784-139, and that sounded fine too. Perhaps other transformer companies have repro designs too, try asking.

I suggest to use a simpler bias supply, the Vox design seems needlessly convoluted.