Variac differences and use

Some additional explanation which I think is lacking in the writeup I quoted above:

The voltage across an inductance, and the current flowing through it, are related by the equation

V = L x (dI/dt)

where V is voltage, L is inductance, I is current, t is time, and (dI/dt) represents change (delta) in current per unit time, in other words the rate of change of current.

So if a steady-state current flowing through the transformer primary (which is essentially acting as an inductor when the secondary is unconnected) is abruptly reduced (due to an abrupt transient being applied to the grids of the power tubes), the resulting dI/dt can produce enormous values of V. That is sometimes referred to as "inductive kickback".

Best regards,
-- Al

I guess I've been extremely lucky all these years. The First Tube amp I built was a Dynakit ST-70. Their instructions(upon assembly completion)were to first test the voltages out of the power transformer, then insert the rectifier and test the voltages at the multi-section cap. Insert the tubes, connect input and output and play the sucker. I've followed those directions ever since(with variations according to circuit design), when assembling amps or upgrading their power supplies(quite a number). The most recent being the addition of 4 strings of IXYS FREDs in my Cary monoblocks, a few months back. Never have had a voltage issue of any kind. Maybe I AM living right after all?(NAH- That CAN'T be it) =8^)

Hi Rodman -- Those sound to me like good instructions for a new build, using new parts. And I note that you listed "connect ... output" prior to "play the sucker," which avoids the possibility of inductive kickback problems in the output xfmr, which as described above can potentially cause much more serious problems than the other issue which was discussed (operating without all of the tubes present).

As far as that second issue is concerned (B+ elevation due to some tubes not being present), I would think that generally would be fine with new electrolytics, assuming their voltage rating is conservatively specified in the design, but with old ones that may be partially dried out and/or need reforming, it would be more risky.

Best regards,
-- Al

You're right, and I've always considered the caps that were manufactured long enough ago to present a problem, inferior to current ones(whatever the year I was working on unit). I always replaced/upgraded the capacitance when doing a PS, or recommended doing so(if an older unit was opened for any reason). It was nice when the large Rubycon Black Gate electrolytics were readily available, and at reasonable prices(fond memories). I wish Sanyo would offer their OS-CONs in higher voltages and capacitances.

In case you plan to buy a Variac to reform caps on old valve equipment, an alternate can be had for almost nothing....

A series lamp with the mains input.

Depending on the product and the anticipated load, ( eg Pre & Power amps will draw Hugely different Mains current.. ) start with a Low wattage bulb (incandescent lamp).

Could be a 25 Watt bulb / lamp for a power amp. On switch on, the lamp will take most of the voltage across it. In case of a short circuit ibn the amp, the Lamp will limit the current in the circuit automatically.

Leave the 25 Watt lamp in place for as long as you want to form the caps at a low voltage and then move to a 50 Watt lamp, and later a 100 Watt ( or even multiple 100 Watters in parallel).

As the capacitors form, the lamp will glow Less bright ( could take overnight or longer. ) This is a good visual indicator too.

I live in a country with 240 VAC Mains.

I have also found the series lamp excellent as a Short Circuit Protection... current fold-back, while experimenting.

NOTE: This will work only with an Incandescent lamp and not with CFLs. I am told that incandescents are banned in Australia...