Variac differences and use

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

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.