Variac differences and use

A unit like this(10A) would suit your purpose nicely: (http://cgi.ebay.com/PowerStat-3PN116B-Variac-Variable-Transformer_W0QQitemZ220451775080QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3353f32268&_trksid=p3286.c0.m14&_trkparms=65%3A12%7C66%3A2%7C39%3A1%7C72%3A1205%7C293%3A1%7C294%3A50) I've always powered equipment up, and monitored power supply voltages without the tubes in the circuits(whether new construction or old gear), to be certain everything was working as intended. If a cap is ready to blow at operational voltage; it will(whether you take three days to bring it up, or three minutes).

It may be overkill for your purposes, especially at $539, but I use an earlier version of this B&K Precision variable ac power supply.

Note though that its output current capability is 3A continuous, 4A intermittent, which makes it unsuitable for some power amps.

I like the fact that it provides metering for current as well as voltage, and it also provides isolation, which is important for safety if you are doing troubleshooting while the equipment is powered up.

I disagree with the suggestion to run without a speaker connected. With many tube designs that can raise internal voltages to greater than normal values, possibly overstressing capacitors and other components. I use a $10 Radio Shack speaker driver as a load when I'm checking out or bringing up the ac voltage on equipment that is in unknown condition.

Re powering up without tubes inserted, that is a good idea to do initially on new construction or old gear in unknown condition, but keep in mind that if the design utilizes a tube rectifier you won't be reforming the electrolytics (or verifying a lot of other things) until the rectifier tube is installed.

Regards,
-- Al

thanks for your observations Al I wouldn't want to post bad info. of course. Not sure why a transformer coupled device would behave that way though?

I suppose a better choice of words would be "do not connect any speakers that you care about being blown up" because that certainly could happen if something goes wrong. A cheap pair of car speakers, yes.

FYI: I once energized a tube receiver without one of the tubes inserted & an electrolytic blew it's top. It was working decently up until that point but had a hum I was attempting to diagnose. Bad day.

Hi Bob,

There is a pretty good explanation of why it is dangerous to operate a tube amplifier unloaded in item 15 of this faq, and I've quoted the relevant paragraphs below:

The crux of the problem lies in the inductive nature of the output tranny. Inductive loads are pretty special things, since they STORE energy in a magnetic field. A property of this effect, as has been pointed out, is that the voltage can soar to levels above the supply voltage in the amplifier-- sometimes WAY above. You can't do that with any other kind of load other than inductive.

Now the transformer doesn't have an impedance of its own; it only reflects an impedance from one winding to another in proportion to the turns (or voltage-- they are the same) ratio squared.

So imagine that you've got an open secondary. This impedance is for all intents and purposes infinite. Thus, regardless of the turns ratio, the primary impedance is infinite as well. (leakage inductance and parasitic capacitance-- two unavoidable nasties of real-world trannies-- will limit this to some finite number less than infinity, but suffice it to say its really high.) This means that the primary will act like a constant current source, attempting to keep changes in currents through its windings to a minimum. This will be an important point later.

Operating into such a humungous load impedance will cause the plate to swing HUGE voltages, according to V=IR. Especially with tetrodes/pentodes, which are much better at cranking out current, the delta Ip will stay the same regardless of the load. Consider what happens when the R goes sky high.

Now, if the load were NOT inductive, the maximum possible voltage generated would be equal to the rail voltage. No problem. This is how it is in SS amps. But with tube amps, that's not the case.

The primary danger here is in the development of these extraordinarily high voltages, which can punch through winding insulation, arc over tube sockets, even arc inside the tubes themselves. Once an arc has struck you can be pretty sure it will happen again. And again.

This is not good. Probably the worst scenario is that the output transformer primary arcs to the core, which is grounded, and that will cause mega current to flow. The output transformer is toast, and the power supply will be too unless something stops that current in a big hurry.

Obviously the risk is greatly lessened if there is no signal going through the amp, but even with no input connected any number of things can be envisioned that could cause transient "signals" to be processed through the amp, including emi from nearby fluorescent light fixtures being turned on; sudden changes in leakage currents through capacitors, as the voltage applied to them is increased when the variac setting is raised; or perhaps even just a sudden increase in the ac voltage itself, as the variac setting is raised.

Re your experience with the electrolytic, now that I think of it, it probably is not a good idea to run an amp with the rectifier tube in place but other tube(s) not in place (especially the power tubes), because of the increase in B+ that would likely result.

Best regards,
-- Al

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