Marshall 3520 Bass Amp -- Capacitor Question


2005-03-10 11:57 pm

I am currently restoring a Marshall 3520 Bass Amplifier.

1. The input capacitor seems a little on the low value at 0.22uF. Would anything be gained by increasing the value to , say, 10uF ??

2. When checking the HT voltages, I measured 64.7v. The reservoir caps are rated at 10000uF 63v. Is there any tolerance on the voltage rating ? Is this overvoltage common in Marshall amps ?



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Was the amp biased cold? Hotter bias will draw down the rails a little... maybe.

That 63v on the cap is a working voltage rating. The cap also has a surge rating which will be somewhat higher.

Exceeding the cap voltage is not like stepping one step too far at the edge of a cliff. There is no sudden snap into failure at X volts over the spec.

What is common is for ANY amp to have voltages higher than the schematic, because mains voltages have steadily crept up over the years. Your 1.7v over is less than 3% beyond specs. MAybe marginal, but I am unaware of any Marshall model being a cap eater.

If you are replacing them, you could go up to 80v or whatever the next increment is in the lines you are looking at. But in my mind, making sure to use 105 degree caps instead of the 85 degree will have a lot more impact on reliability and life.

22nf doesn;t seem low to me, certainly 10uf would allow low notes even whales couldn;t hear through, but to what end. Making it 47nf would drop the bottom end considerably from whatever it is stock,
1) well, no, even .22uF there is already large .
Capacitors by themselves tell you nothing, you must see what else are they connected to ... in this case the 47k input resistor.
The capacitor connected as shown will cut some bass ... determine at what frequency and how much.
In this case: .22uF and 47k will cut frequencies below 15.4Hz .

A useful online calculator, add it to your favorites:
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Since lowest Bass frequency is around 42/43Hz, you are cutting somewhat more than a full octave below that, so .22uF is a well chosen value.

Personally I always cut at lowest frequency so would use .1uF and 47k , but that's a personal choice, to alleviate Bass speakers suffering ;)

Hi Fi guys sometimes like to design cutoff frequency some 10X below lowest expected, so capacitor will have no influence in practice.

To each his own,I respect that, but live MI amplification is a grueling task so it's often good to try to protect poor speakers ... which besides never reach that low anyway ;)

I measured 64.7v. The reservoir caps are rated at 10000uF 63v.

Not ouch they will explode but ouch, they will dry up faster than usual (which might still take years).

Mains voltages have creeped up steadily in the last years, old 220V European standard is nominally 230V everywhere and higher in many places, old 240V UK is sometimes as high as 257V as stated here by Forum users, old USA 115/117V officially creeped up to 120V but MANY places show hair rising 127V .

Of course, you rise mains voltage, you rise all internal unregulated voltages by the same % .

From personal experience, those caps will not blow catastrophically by any means, but may dry up, lose capacitance and increase internal resistance so eventually amp will hum and show "ghost notes" (actually mains frequencies mixed with what you are playing) or capacitors will eventually bulge and even leak salty capacitor blood through cover vents.

It would be good to replace those caps with modern higher rated ones.

Since Technology advances, *maybe* new 10000uFx100V ones are same size than old 10000x63 ones and can be measily mounted in the same place.

What you must really pamper are those Audio Mosfets: expensive/unobtanium (pick one or both) .

Yes, there are modern replacements if you need them, but just keep the originals alive if possible :D
If you have no 100HZ hum, leave them alone. They last for donkeys years and when they start to go it will be obvious. Leave the 0.22uF where it is. I would, personally, replace the fb capacitor C2 with a 220uF and leave the rest alone.
There is no bias adjustment because when FETs warm up the gain goes down so self stabilizing as long as they start somewhere near nominal. Like a valve; when the anode gets too hot the emission goes down, so the current reduces and the anode cools. It is called compression.