Re: Best ST70 idea

in article 1134699323.392617.272810@xxxxxxxxxxxxxxxxxxxxxxxxxxxx,
pfjw@xxxxxxx at pfjw@xxxxxxx wrote on 12/15/05 9:15 PM:

>> E.g. to 'drop' 120V a.c. to 117V requires a diode bridge and a power 3V
>> zener diode.
>
> Which will add (or potentially add) noise. And I have seen quite a
> number of diodes fail-to-straight-wire. The resistor is definitely
> old-tech, and does waste some small amount of energy (calculates to
> about as much as a filament-type night-light for an ST-70), but
> resistors typically fail open. Power-resistors can also be nicely
> adjustable.
>
> http://minresistors.com/Adjustable.htm
>
> shows a pretty good range. Mouser or Newark will sell you
> onesies-twosies. But you can vary to your line voltage from a generic
> resistor. Neat. You can also get two voltages from the same unit, also
> neat. So, your older pair of monoblock amps designed at 110V can serve
> right along side of your 115V semi-modern amp. Just break the
> copper/brass colored tab on the duplex receptacle, feed the two
> take-offs one to either hot (preferably through separate fuses) and you
> are there. The neat thing about this situation is that all you need to
> do is switch the fuse to match the item. D/E fuses come from 1/16A
> through 30A and may be seen here:
>
> http://www.cooperet.com/library/products/MDQ_Specs.pdf
>
> Also from Mouser, Digikey or Newark.
> And here is also an excellent use for a properly calibrated & metered
> Iso-Variac. You can determine the peak load of any given item, install
> a fuse to match that peak load fairly precisely, and still have the
> ability for it to accept the cold-start surge. With this in mind, you
> actually can fuse an item to protect it, not just the real-estate
> around it.
>
> Peter Wieck
> Wyncote, PA
>




Another method I've tried to lower B+ is to connect a low value,
high-wattage resistor to the B+ center tap and ground. But it does
dissipate considerable heat beneath the chassis.

I'm going to remove the one I stuck in my Dyna and use a 5U4G instead, to
get the add'l voltage drop (that is, after I swap in the Eico tranny).

The warm-up time of the indirectly heated 5AR4 is a great feature, but lots
of amps used other rectifiers without apparent (I choose that word
carefully) cathode stripping. And if you use SS diodes you've got more
voltage to drop plus instantaneous turn-on.

So the different rectifier tube seems like the lesser of the evils, since it
is plug-and-play and requires the least effort, and is the easiest to undo.

Jon

.

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