Re: Best ST70 idea


pfjw@xxxxxxx wrote:
> OK.... The ST-70... I have done a few of these in my time, including
> one in the inventory right now that gets pretty steady use. Some things
> about them:
>
> True Things:
>
> The ST-70 was designed against a nominal 115V wallplate current....
> back in 1960, or so. Depending on the region and provider in the US
> today, that may or may not be a problem. In Philadelphia, the place of
> their birth, the nominal tariff voltage these days is 120V. At our
> wallplate pretty near to the local step-down and not far from the
> substation, it is 117V. The tariff permits +/-3%, so that makes sense.
> Solutions/cautions to a perceived overload of the power transformer
> include, from the easiest to the most complicated:
>
> A simple dropping resistor in series with the hot lead. That can be
> calculated to a fare-thee-well. This does not add any noise, if it is
> done in an outboard box (fused, of course), it can be made to control
> several items, and big, honking power-resistors capable of 60 watts of
> dissipation or more (which will NOT be needed) are cheaper than a
> transformer, certainly.
>
> Thyristors and triacs can (mostly will) add noise. But such devices
> will clamp voltage at whatever setting is desired. Similarly CV
> transformers, which tend to add a LOT of noise. Both of these can be
> controlled for noise, but both add complexity. A CV transformer, even
> as part of a power-conditioner/regulator will be relatively expensive.
>
>
> One can replace the 5AR4 with diodes and reduce the load on the
> power-transformer. A better solution is a solid-state 5AR4 with the
> same slow-start characteristics, still cheaper than an NOS US-made
> 5AR4.
>
> http://www.revolutionguitar.com/andmore/andmoreN.htm
> http://www.webervst.com/ccap.html
>
> Are just two of many. Or, a 5U4 can be installed, somewhat derating the
> amp and somewhat defeating the slow-start of the 5AR4.
>

t's just easier to mount some diodes and dropping resistors on an
octal plug so the heat generated is above the chassis and does not
much heat the amp, if that's your desire.

Voltage dropping can be done with a boost/buck isoformer-largish ones
being common ex-teevee-shop items at hamfests, or one can
series-connect a paralleled light bulb array. I have used one side of a
discarded sealed beam for this in fact.

What should NOT be used except for test purposes, is a Variac.

But the external PS idea saves a lot of work and also gives the best
possible performance. As long as the cable is properly made it is safe.
Remember a lot of commercial equipment was built this way and the DIYer
has to use his own judgment, rather than that of unelected,
unaccountable bureaucrats. Hammond did this with their organs too. The
cable fed B+ from the tone cab to the preamp as a method of
"Leslieproofing" the B-3. (We see how well that worked!)

.

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