Marshall bass spec (sort of) build schematic review.

[theRagman]

I have the transformers from a Marshall-esque amp that I'm hoping to reappropriate into a sort of simplified 50-watt Marshall bass spec. I've only built from a kit before, so any guidance you folks can provide me would be much appreciated.

It's kind of a weird deal, since the original amp I got the transformers from was a 50-watt amp with a 100-watt power transformer. I've tried to diagram that power supply as it was in the original amp and stared at it six ways from Sunday, but I would not be at all surprised if I've done something stupid here.

Schematic attached.

Thanks!

[sluckey]

Schematic looks good to me.

[theRagman]

Schematic looks good to me.

Thank you!

[Latole]

Wouldn't the bais adjustment circuit be better like this?

[sluckey]

Wouldn't the bais adjustment circuit be better like this?

His Marshall bias circuit has been working very well for Marshall for over 50 years.

[Esquirefreak]

I'd use either 2*100μF or 2*200μF in series as first filter stage/reservoir caps in the power supply.

/Max

[Latole]

I'd use either 2*100μF or 2*200μF in series as first filter stage/reservoir caps in the power supply.

/Max

I agree , the Ragman's schematic show 2 X 50 mfd in series = 25 mfds

Marshall JCM800 Lead schematic show 2 X 50 mfd in parallel = 100 mfds

[theRagman]

I'd use either 2*100μF or 2*200μF in series as first filter stage/reservoir caps in the power supply.

/Max

Thanks! I wouldn't mind adding some more filtering, so that makes sense.

[theRagman]

Is there a general rule of thumb regarding safety margin on filtering caps? For example, B+ for this transformer is 460V, and the electrolytics that were in there were rated for 500V. I assumed they were put in series because the normal tolerances on those caps could easily result in one at a true voltage rating less than B+. Are there other considerations?

[Latole]

Is there a general rule of thumb regarding safety margin on filtering caps? For example, B+ for this transformer is 460V, and the electrolytics that were in there were rated for 500V. I assumed they were put in series because the normal tolerances on those caps could easily result in one at a true voltage rating less than B+. Are there other considerations?

With Power Tube load, B+ may drop a bit.

On some Fender amps with 420 B+ I see 450 volts filter caps.
Deluxe Reverb AB763

[shooter]

here's one of my "stiff" PS's
the in-rush, before tubes warm up exceeds the 500vdc rating, but "stacking" (in series) 2 350vdc caps = 700vdc at 1/2 the capacitance.


a good bleeder resistor comes in handy if you don't want to wait 15 minutes after power-OFF to NOT get zapped.  large capacitance PS have a tendency to hold a charge long after power is off.

[pdf64]

Is the amp intended for bass or guitar?
Marshalls require a lot of power supply sag to achieve their characteristic overdriven response.
Additionally, the sag is required to protect the output valves from excessive dissipation.
A super stiff 100W capable power supply will sag very little when supplying a 50W amp.

Hence for overdriven guitar use, it won't respond like a Marshall, and the output valves may get overstressed.

Hence it may be better to sell that transformer and buy one that's more appropriate?

[theRagman]

Is the amp intended for bass or guitar?
Marshalls require a lot of power supply sag to achieve their characteristic overdriven response.
Additionally, the sag is required to protect the output valves from excessive dissipation.
A super stiff 100W capable power supply will sag very little when supplying a 50W amp.

Hence for overdriven guitar use, it won't respond like a Marshall, and the output valves may get overstressed.

Hence it may be better to sell that transformer and buy one that's more appropriate?

Hmm, that's interesting, and it may explain why I didn't really care for the amp I cannibalized it from. I was planning on using this mostly as a clean amp, but I think you've got a good point. If I could find an appropriate 50-watt transformer instead, that might be the way to go.

[theRagman]

I finished wiring the amp up last night, and I'm happy with it. Thanks to everybody who weighed in on the schematic and kept me from going awry before I even got started. You folks are great!