VARIAC: Procedure??? 2 old 30 wpc tube amps, unused for 4 years


Two Fisher 80Z, made 1958, overhauled 6 years ago, not used for 4 years

https://www.hifiengine.com/manual_library/fisher/80-az.shtml

Borrowed my friend’s VARIAC.

How many volts ___ to start?

Progress in ___ Steps?

How much time _____ each step?
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I found this, but always seek and welcome advice here

https://www.circuitspecialists.com/blog/using-a-variac-to-power-up-an-old-radio-or-amplifier/

says sniff/look for smoke while:

slowly dial up to 50V; 30 mins (pre-heat tubes; reforms electrolytic capacitors)

slowly                75V; 30 mins (power transformer/windings/drives out moisture)

slowly .............. 90V; 30 mins (if hum replace electrolytic capacitors) (old capacitors: replace em anyway).

slowly up to 110/120V
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I’m gonna drop my Cayin off to have it’s internal bias adjusted by Steve at VAS

so, a chance to use these beauties (beautiful sound, actually hard lookers) with my McIntosh mx110z for the 1st time.

They will be driving the Electro-Voice speakers they were paired with in the 1958 Fisher President II I inherited. Always gives me good memories of my Uncle Johnny.

He had a fabulous apartment in NYC with one of a matching pair of Steinway Concert Grand Piano’s from the 1934 Chicago Worlds Fair that my Grandfather bought. My Uncle Bill had the other one. Trust Funds for the brothers, but not for my mother or her sister, they were expected to marry well. My aunt married into the Vanderbilt family, not my mother. Hence I’m the DIY bargain hunter.


elliottbnewcombjr

Showing 2 responses by lewm

I’m still perseverating on your basement with “zero” humidity. One important  bit of advice that I and some others failed to convey is to always use a load resistance on the amplifier outputs. If you don’t have a suitable power resistor, you can use an old speaker or a single driver from an old speaker. But it sounds like you’re already up and running without any mishaps.
In some tube amplifiers Zenor diodes are used to provide voltages to key elements of the circuit, for example the bias voltage on the grids of the output tubes. Diodes don’t conduct until a certain threshold voltage has been reached.Other kinds of regulatory tubes also are often employed. In such a circuit, it can actually be momentarily dangerous to the circuit when it is not supplied enough AC input voltage in order to activate these regulatory elements.Therefore, without seeing the schematic of your amplifiers, it is hard to give a yes or no kind of answer to your very specific question about the algorithm you outline. I personally would spend less time at the very low voltage settings. I would bring it up to 80 or 90 V within a few minutes, for example. Then go slow up to 120 V but that’s completely off the top of my head. If it was really overhauled within the last several years, you’re probably going to be OK anyway.