Re: Amp - loss of power?

"Ben Gray" <o2xtal@xxxxxxxxxxxxxxx> wrote

> [below, except...]
> What type of resistor?
> The largest resistors I have seen are 5 watt types.

Make an effort. Look in a catalogue. Or use lots in series and/or
parallel.

P = Vsquared / R. where V is the voltage across the resistor.

Don't get worried by where and how you post...Phil just wanted a way
out.

I don't quite follow why you need that resistor anyway, but don't
let me put you off...

Your problem could be nearly anything.

You have a non-standard power supply transformer that may or may not
be considerably over the correct voltage, but it's hard to say
because I don't know how you measured it.

>From what Phil says, Marshalls are pretty skimpy in the first place,
so running over voltage is likely to shorten the life of *many*
parts.

And did whoever change the transformer also change other things?

If there are several problems arising from over voltage, fixing them
is futile because they will blow again.

For example, your problems could be a result of heater/cathode short
in the 12ax7. That is possible because the correct heater/cathode
voltage for that valve is quite high anyway, so an over-voltage
supply could take it dangerously close to the limit of 100V. It is
also conceivable that this problem will clear as the HT drops after
you switch to standby. When you fixed the bias problem (which may
also have been caused by over the over-voltage) you increased the
current through the power valves, and so dropped the voltage to the
12ax7 a little. This may have relieved the problem but not cured
it...just slowed it down.

OTOH, the intermittent OPT failure theory could also be right for
the same reason. As could many other theories, including filter
caps. All of these have maximum voltages that may have been
exceeded. All will result in crackling, sizzling, or rustling
noises.

You need to get a power supply transformer of the right voltage and
current rating. Something sold for the purpose, by Marshall or a
reputable supplier.

Then you may need a replacement 12ax7, or output transformer, filter
caps...whatever is failing.

cheers, Ian

in message

news:1128236431.503244.5850@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> Chris Hornbeck wrote:
>
>> >> Do you have a sustitute rectifier?
>> >
>> > I have diodes marked IN4004
>> > The schematic specifies IN4007
>>
>> IIRC, this is just a difference in voltage rating.
>> And, in practice, maybe just a difference in pricing
>> or specing from the same production run. Whatever;
>> 2.5 amp 1000 PIV rectifiers are next to free
>> nowdays, so no biggie.
>
> There is the bridge rectifier that used to supply the preamp.
>
>
>> >> Do you have a substitute load?
>> >
>> > What do I need?
>>
>> If I'm reading the schematic correctly, 435 volts
>> at, let's say, 100 mA is a load of 4350 Ohms and
>> 43.5 Watts. A 4000 or 5000 Ohm resistor from
>> Fair Radio Sales or some other surplus supplier,
>> mounted in a safe (dangerous voltages; high temps!)
>> cage, should do fine. Don't allow children or
>> pets into the room, natch.
>
>
>
>
>> >> Also, can you measure line voltage?
>> >
>> > I have a multimeter and a strong will to live.
>>
>> Then you'll have already eliminated the Edison as a
>> potential problem. But I haven't read (have I missed it?)
>> that you've measured the B+ (the high voltage supply
>> at the center tap of the output transformer).
>
> You didn't miss it.
> Could really use some guidance on this one.
>
>
>> The obvious inference from your symptoms is that the
>> standby switch is failing, so a few DC measurements
>> should go a long way. And all this (above) is
>> most likely unnecessary.
>
> I am all ears.
>


.

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