Vietnamese Marshall

Phil Allison · 2011-04-28T12:39:52+00:00

** A rather new looking Marshall " MA100H " valve head landed on my bench today - but it looked a bit different. On the back it says " Made in Vietnam " - so this is the first non UK made ( ie assembled) Marshall, at least that I have ever seen.The amp was silent, valves all lit up but no output whatsoever and no extra current draw when the Standby switch was closed. OK - so where is the damn HT fuse ? Nothing on the back so it must be inside - another first for a Marshall valve amp. On the PCB is a 630mA delay fuse - very blown - fitting a new one got the amp running.Bit of poking about finds that both 1/4 inch QCs on the PCB used for output valve grid drive are very loose - easy fixed. But was this the cause of fuse blowing? Nope.That 630 mA HT fuse should be 1 amp at least, since it is fitted in the secondary of the PT. On test, the fuse had to pass just over 1 amp rms when the amp is over-driven. The primary AC current draw is 2 amps rms at the same time ( ie 480VA) .On sine wave test, there is significant crossover distortion on the CRO screen at 1/2 output and above. The bias setting seemed OK - there are two trims inside and a pair of 1 ohm resistors in each pair of output valve cathodes.On checking the screen B+ supply, the cause is simple. There is no filter choke of course ( what are they ?) and instead a 470 ohm ,7W resistor. So the screen B+ supply falls like a rock soon as the amp is driven. Guess this helps the crappy Chinese EL34s to survive overdriving.Besides the above bad points, there are lots more.The AC tranny is undersized - it looks like a 250VA type and needs to be 400VA to withstand hard use in this amp. Even sitting on the bench with a 605mA idle current from the AC supply, the internal temp rise is almost 40 degrees C. A couple of hours of heavy metal on a warm night will see it off.The same * ridiculously fragile* plastic shaft pots as used on other recent Marshalls have been fitted - one light bump on the knob = nasty...

Apr 28, 2011 23 Replies

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Phil Allison 15 years ago

"Arfa Daily"

** See the "Overview" on page 2 of the pdf.

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Also note the pic of the back panel and the specs on page 4 of the pdf.

.... Phil

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Arfa Daily 15 years ago

'Tis beamed ... Let me know if any probs

Arfa

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boardjunkie1 15 years ago

mp.

ut.

ment

ies

Right ahn...got it. Thanks bud...

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Arfa Daily 15 years ago

Welcome ...

Arfa

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