Bass Amp Build Circuit-Clone Suggestions

[W5FH]

I apologize for asking such a broad-scoped question here but am seeking advice for a build-
     Since I am not a player I have no experience with different amps. I need to build a bass amplifier that will also be used with other instruments such as a cello, violin and banjo. For this build I have an over-rated (150w, APD8072H) output transformer for 4 x 5881's and power transformer 640 VCT @ 500 mA. Plenty of room on a blank Fender SuperReverb steel chassis. Playing through a 2X12 cabinet. The user of amp does not play real loud, mostly at smaller gigs but occaisionally a larger venue.
   Have been studying the Marshall JCM 800 series 1986 with interest. It is curious to me, for a bass amp, the input stage has no cathode bypass capacitor however it is warm biased with 820 ohm cathode resistor. The next stage appears as a cold clipper with an unbypassed 10K cathode resistor. I am intrigued by the circuitry around V2-b.
   I attached a schematic for the 50 watt preamp, I believe it is identical to the 100 watt version. Does anyone have any clearer schematics of this amp, some component values are hard to make out.
Thank you.
     

[sluckey]

That SR chassis will need an extra octal socket. Maybe enlarge some noval socket holes?

The odd-looking tone stack on V2 is an active tonestack. Notice the feedback path through C8? I believe this is called a Baxandall tone circuit.

Why the jcm800 bass amp? You may consider the jmp bass 1986. It has the more common TMB tonestack and the inputs may be more friendly to other instruments. Worth a look...

     https://el34world.com/charts/Schematics/files/Marshall/Marshall_jmp_bass_50w_1986.pdf

[W5FH]

Thanks Sluckey-
    My chassis is a blank undrilled unpunched one, built to SR dimensions, to go into a matching head cabinet. I will look at some of the other ones that you mentioned. The JCM800 series 1986 had caught my eye as I read quite a few opinions touting it as a good bass amp. OK, will get back to researching circuits.

[thetragichero]

i've built the 100w jcm800 1992 bass amp in an old marshall dual super lead chassis, mainly because it's about the weirdest all-tube circuit marshall ever did. the 50w seems to have a lot less negative feedback which is one of the issues i had with the 100w

[tdvt]

As a bass player, I would encourage you to look at Ampeg designs. They were always ahead of the pack as far as bass amplification & remain there today.

The B-15 particularly (2-6L6) has a history of use with both electric & upright bass, a quad version would be an interesting take on it. There are several iterations of the B-15 with differing placement in the circuit of the tone stack among other tweaks (here's one,  https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b15n.pdf)

That said, I have played (electric bass) through Fender Showmans (4-5881), Bassmans & had one a rig that consisted of a Princeton Reverb with a pre-out into a SS power amp. All sounded good & would play nicely with instruments other than bass.

As far as speakers, I currently use Eminence Neo Kappalite 3012 12's which I am really happy with. Great sound, light-weight, high power handling.

[W5FH]

Thank you both, tragichero and tvdt, for the suggestions. I will be checking them out!

[tubeswell]

A simple 1ch Fender AB763 blackface preamp with i.e. input triode driving a TMB tonestack and volume control then into a 2nd triode (and no effects) then followed by a blackface amp 12AT7 long tail pair phase inverter and a quad of 6L6GC will work well for bass or pedal steel. Use a Fender Twin PT, Choke and OT. And make sure they have heavy duty (200W minimum) bass guitar speaker(s).

[W5FH]

Thanks Tubeswell-
     Good info there, thanks. Very interesting on 12AT7 phase inverter. I was studying a schematic I obtained on the Mojotone site for a British 100 watt bass amp kit they sell. It appears very much like the 5F6A Bassman preamp and phase inverter, driving a quad of EL34's. I have a desire to have that sound from the Bassman cathode follower and am thinking of using the HiWatt fixed bias phase inverter/presence circuit following the Bassman style preamp.

[W5FH]

Also meant to mention to you Tubeswell -
     I have set aside for the bass amp build a Heyboer made output transformer that has an oversized core, the APD-8072H. I believe it is 150 watt rated. I have read that it will produce a deep bass response due to the larger core; the Edcor 100 watt equivalent has a 1/2" less thick core. I believe both transformers are using same core materials. Plus, I believe you get a little help in the low frequency end response on these output transformers when using a lower impedance primary (such as 1.8K for 4 tubes). For power transformer I have the Heyboer made clone, I believe to be the equivalent to the Fender unit you mentioned, 640 VCT at 518 mA. And yes I always use a choke, I try to use a larger value if I can squeeze it onto the chassis. Actually I have the chassis mostly done on this bass amp build. I bought one of the blank Fender Super Reverb size ones. I have a hermetically sealed choke picked out for it from my mil-surplus stock, a 10 Hy at 150 mA. It has a smaller footprint than equivalent open frame type. Another thing is that most bass amp circuits I have studied use lots of filter capacitance, a fair amount more than conventional designs. I am planning on following that design. The amp will be going into a 2x12 Bassman style cabinet from Peter Mather, built just a little heavier. I will pass along the speaker recommendations to the band member who has the cabinet.

[tubeswell]

Yes, more OT iron = better ability to conserve AC at longer wavelengths without saturation of the core by magnetising current.

Re the impedance ratio, you will be looking for something around 500:1 if you’re wanting a quad of 6L6s with a 4R speaker (or 1000:1 if you’re using 2R, or 250:1 if 8R).