Re: Marshall 4140 Amp question


"n cook" <diverse8@xxxxxxxxx> wrote in message
news:dqnl0n$6g0$1@xxxxxxxxxxxxxxxxxx
> Arfa Daily <arfa.daily@xxxxxxxxxxxx> wrote in message
> news:weCzf.4907$Kt5.855@xxxxxxxxxxxxxxxxxxxxxxx
>>
>> "Asimov" <Asimov@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
>> message
>> news:MSGID_1=3a167=2f133.0_43cecdb9@xxxxxxxxxxxxxx
>> > "n cook" bravely wrote to "All" (18 Jan 06 12:52:44)
>> > --- on the heady topic of "Marshall 4140 Amp question"
>> > ... "'Blues,' for all you people who paid to get in." - Pete Townshend
>> >
>>
>> I agree with Asimov regarding leaky grid coupling caps, but I am not a
>> fan
>> of running tubed / valved group amps without the output bottles in place,
> no
>> matter how briefly ( but if you must, take note that Asimov does say
>> briefly - IMO 40 seconds max ). Without the output stage drawing current
>> from the HT line, this can rise easily by 20% or more, from its nominal
>> running value, which can stress the power supply caps, and any other
>> decouplers along the way on the HT line.
>>
>> This is not quite such a bad thing if the amp employs semiconductor HT
>> rectifiers, as the HT rail will be high of its nominal running value
>> until
>> the outputs warm up anyway, and this factor is designed in when the
> voltage
>> ratings for the caps are chosen by the designer.
>>
>> However, if the amp employs a thermionic rectifier, its output will come
> up
>> slowly, as will the load imposed by the outputs as they warm up. This
>> results in an HT rail that comes up to a value, and stays there. If you
> now
>> run the amp up with the outputs removed, the output of the reccy will
>> come
>> up with no load on it, which is an unexpected condition for the designer,
>> and the resulting unloaded voltage may well come very close to, or even
>> exceed the voltage ratings of some caps on the HT line.
>>
>> You should be able to check the bias voltage with the outputs in place.
>> Marshall generally tend to use a negative supply of around 50v for the
> bias
>> source, potted down and fed to the outputs' control grids via a few
>> resistors.
>>
>> If a seperate bias supply were missing or low, this would more than
>> likely
>> result in excess output stage current demand, with consequent lowering of
>> the HT rail. Glowing anodes are the order of the day here, but I would
>> rather have that, as the outputs will stand this for a considerable time
>> without damage, rather than have exploding or shorting electrolytics,
>> brought on by voltage stress.
>>
>> Arfa
>>
>>
>
> Amp is 1975 , from electrolytics, and uses Si rectifiers.
> Amp was in a damp shed for maybe 15 years. Doesn't look too bad for that ,
> after blowing out the cocoony bug nests etc (1KW Martingdale ).
> Charred/burnt 1.5K grid resistors and blown HT fuses.
> All valves ( all marked Marshall) checked out good on Avo CT160 - I'd
> forgotten how problematic , with high current valves, to get the initial
> zero on the meter before rotating the SET mA/V.
> I always power up kit left idle for a long term with a variac + current
> meter + thermal trip.
> Is it still perceived wisdom in such circumstances to power up valve amps
> with full speaker load on output with all valves in place and to power up
> transistor amps intitially without load ?
>
>
That's my usual recommendation.

Arfa


.

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