Re: Amp - loss of power?

Ian Iveson wrote:
> "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.

Just wanted to confirm the obvious. If I want to put 40W through a
resistor, that resistor should be rated at least 40W?

Buying components requires mounting an expedition. We are not exactly
local to major retail.
Mail order means days...
I was just being hopeful that I could build a "dummy load" with what I
had on hand.


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

I was asking how to test an OPT.

> 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.

It was custom wound.
It has 374 vac (hand)written on it.
I connected a DMM across the input of the rectifier and it measured the
AC at between 369 and 374.

I suspect whoever wound it confused 374 and 347.

> 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.

This was back in 1989. It did ten+ years of constant work afterwards.

> And did whoever change the transformer also change other things?

No. I watched them put it in.

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

I don't disagree, but does history count?

> 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.


Could well be the case that the complexity of the problem exceeds my
position on the learning curve. Fair enough.
I don't think I can test the effects of an over voltage condition and
work out the variations in component values required to compensate.
Not without the indulgence of the group.
I believe the transformers for this amp are no longer available.
Mercury Magnetics do not list the IPT. Would take ages to get here
anyway.
I haven't been able to find much information (yet) on test procedures
for OPT's.

Amp techs are few and far between in our part of the world and the only
one I know of with any reputation is always busy. And several hours
away.
And if solving the problem really does require replacing both
transformers, it might not be worth it.

Building one from scratch using the existing IPT is still an option.

Thoughts?

.

Relevant Pages

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