'71 Marshall 50W HT fuse question?

[Neon]

I have a '71 Marshall 50W. I usually play this amp at moderate volumes with no problems -- although, it doesn't get played too often. Today, I was testing a new OD pedal, with the amp quite a bit louder than usual, and the high-voltage (HT) fuse blew after a few minutes of playing. I replaced the fuse, and it blew again (same scenario).

The fuse label specifies a 500mA fuse, but says nothing about the type (i.e. fast or slow blow). The amp had a fast-blow fuse installed, and I replaced it with the same. Is this the correct type of fuse for this amp?

The only other time this amp blew the HT fuse is when a power tube shorted. I wanted to make sure the fuse is correct before troubleshooting further.


Thanks,

[John Phillips]

It shouldn't make that much difference - a fast-blow is usually fine in the HT, although not in the mains, due to the big transformer surge on power-up.

If the amp is drawing enough HT current to blow a 500mA, even a fast-blow, I'd suspect the power tubes, especially if you're driving it very hard at the time.

Does this amp have screen-grid resistors fitted? Stock 50W Marshalls from around this period don't, but they are essential for reliability if the amp is being driven really hard - without them, they have a tendency to overheat the screens and cause tube failure, especially with new-production tubes. Even if the tubes don't fail completely they'll likely run far too hot and draw excess current.

Look inside for a large resistor on each power tube socket - the stock ones if fitted are usually a dark green/grey color about 3/4" long. Aftermarket ones may be these or white/tan ceramic box types. If you need to fit them, the stock type are the best, but any 1K-ohm 5W wirewound type will work OK.

Check the bias too - do NOT aim for 70% of max. dissipation, that's too hot in these amps; they're hard on tubes in general and were intended to be set more conservatively - Marshall did it by the crossover notch method which usually works out at around 50%.

[Neon]

Hi John,

Thank you very much for the explanation.

I'm guessing the bias may be running a bit hot. I usually use the cross-over notch method. However, my scope and generator are in storage, and I did not check the bias when last replacing the power tubes. The previous power tubes were in the amp for about 18 years, and I bet the new-production tubes are spec'd differently. I'll definitely check the bias.

As you suggested, I had a quick look inside the amp, and it looks like there are no screen grid resisitors. This sounds like a good maintenance item to add.


Cheers,