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I'm new to this forum, so be gentle! I'm an electronics and computer systems engineering student, and I always thoroughly enjoy audio circuits, especially examining older valve based designs (I play guitar, who would've guessed!). My degree is very much not an electrical degree, we don't look at anything above around 12VDC. That being said, I have a cursory knowledge on transformers, as it was briefly covered during a module in first year, though I will add that valves were not even mentioned due to their niche use-case in today's world. Anyway, onto the question:

I understand that having an open circuit across the secondary of a transformer-coupled valve amp is dangerous, but I would like to know why. My (again, incredibly base-level) knowledge of transformers would lead me to believe its due to the infinite impedance load being reflected in the primary, but again I'm uncertain as this was very hastily covered. If this is somewhere along the right lines, what does this actually do to the valves that causes damage?

I'm aware this is probably very painful to read, as I know some terms, but am probably using them wrong, but any answers would be very appreciated!

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    \$\begingroup\$ Where does it say that it is dangerous or can damage something? Any source for this claim? Is that a generic claim or specific to some specific equipment? \$\endgroup\$
    – Justme
    Commented Jun 4, 2022 at 11:46
  • \$\begingroup\$ This is true for many (class C) radio power amplifiers and because this was one of the last resorts of vacuum tubes, this may have beed extended to all valve driven amplifiers as a myth. Audio amplifiers include some negative feedback, which avoids open load problems and they are not tuned to a specific frequency. Open load does not reduce the transformer impedance as it can in detuned RF power stages. \$\endgroup\$
    – Jens
    Commented Jun 4, 2022 at 12:56
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    \$\begingroup\$ Valve audio amplifier stability may be dependent on the correct (approximate) output load, and LF instability (aka "motor boating") was said to be a problem : possibly from negative feedback becoming positive when the main load became the primary inductance. ( I don't recommend using "motor boating" as a search term in the modern era, unless qualified with "valve amplifier") \$\endgroup\$
    – user16324
    Commented Jun 4, 2022 at 13:27
  • \$\begingroup\$ Very strange. If the resistive voltage drop of the load is required and will fail otherwise, it’s a very bad design. Flyback without feedback and regulation (practically unheard of) has this problem. \$\endgroup\$
    – winny
    Commented Jun 4, 2022 at 15:38
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    \$\begingroup\$ @winny of course you realize that about a century worth of electronics knowhow has been generated since the inception of these amps. And whenever someone tried to modernize some design decisions, there were others who denied the development because "it destroyed the tone". As a result, many of the flawed design decisions have carried over into tube amp "canon". \$\endgroup\$
    – tobalt
    Commented Jun 4, 2022 at 18:02

4 Answers 4

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For those unaware, tube amps require the use of transformers (except for certain extreme examples), because the plate voltage is quite high (~300V) and current quite low (~100mA), essentially useless into a low impedance speaker by itself. Tubes are also only "N type" as it were, so push-pull amplifiers can only be done by totem-pole arrangements (awkward to drive), or using a transformer for phase inversion (which, with the transformer being more-or-less mandatory anyway, is fine).

We can refer to these examples, and draw similar conclusions, noting the differences :

https://www.tutorialspoint.com/amplifiers/transformer_coupled_class_a_power_amplifier.htm

https://www.tutorialspoint.com/amplifiers/push_pull_class_a_power_amplifier.htm

Basic operation with a load is, the transformer's magnetizing inductance is charged up to quiescent current, and as the device throttles down, that (inductive) current flows into the load, pushing the plate voltage up. This happens in a controlled manner because the load impedance sets the voltage due to a given current. (Tube amps with local negative feedback, or triodes (which can be considered to have internal NFB), can still control this voltage without a load connected.) Without a load, the voltage is limited only by the impedance of the transformer itself -- which has an LC characteristic, peak voltage given by Iq * sqrt(L/C). Which isn't necessarily going to be destructive or anything, but, as is usually the case -- it depends, and it could lead to arc-over, typically at the tube socket or something like that. Rarely internally, causing device damage. (For the transistor case, avalanche may occur, more likely to cause damage.)

In short, particularly when overdriven, the circuit resembles a boost converter with no load, and the flyback pulse can reach dangerous voltages.

For the push-pull case, note that one side turns on while the other turns off. With normally no quiescent current in the transformer (the currents in the two sides are balanced for zero net magnetization, and class B operation is possible so that peak signal current can be many times Iq), flyback isn't an issue, and for the transistor case at least, the opposite side acts to clamp that voltage.

We encounter a difference with tubes here: vacuum tubes are inherently [series] diodes (no current flow in reverse) -- the inverse of MOSFETs (inherent parallel diode, full current flow in reverse). (BJTs can handle some reverse current, but only when the base is forward-biased. They're kind of inbetween I guess you could say.) So we can have the situation that one side turns off and the other turns on, but the voltage dips too low, reverse-biasing the "on" tube. This doesn't result in dangerous voltages, so much, but with the high grid voltage (it's "on"), cathode current is demanded -- but there's no plate voltage to absorb it, and consequently a massive current flows into the screen grid instead (for tetrodes+; N/A for triodes, which are fine here). This can result in "toaster grid" and subsequent destruction.

There could also be instability where, due to the light load, feedback goes unstable and oscillation results; and with help of the above mechanics, a relatively large voltage could develop (i.e. more than twice B+ voltage). Which might go with toaster-grid operation too, so, all around not a good time. This does require that the circuit is poorly compensated, which as usual, depends.

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    \$\begingroup\$ Good content, but I think it is a bit needlessly verbose. \$\endgroup\$
    – tobalt
    Commented Jun 4, 2022 at 15:41
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    \$\begingroup\$ @tobalt Amazingly, that's the first I've been told here. I'm nothing if not verbose! \$\endgroup\$ Commented Jun 4, 2022 at 17:45
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    \$\begingroup\$ One reader’s “verbose” is another’s “thank you for the detailed explanation” \$\endgroup\$ Commented Jun 5, 2022 at 2:40
  • \$\begingroup\$ I don't see anything 'verbose' here, just a good and comprehensive answer. @tobalt \$\endgroup\$
    – user207421
    Commented Jun 6, 2022 at 0:38
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    \$\begingroup\$ And indeed, this is for me, a thank you for the detailed explanation, this was immensely informative, and really really helpful! \$\endgroup\$
    – AL270
    Commented Jun 13, 2022 at 10:24
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A vacuum tube audio amplifier, driven without a speaker, would get damaged by arcing at the valve base terminals.

Arcing is the result of high voltages induced in the output transformer primary winding when a speaker is not connected across the secondary winding.

Arcing may be prevented by connecting a neon lamp across the primary winding.

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It makes the load for the output valve an almost perfect inductor. If the output is not DC-coupled (and DC current through a transformer is a bad idea), that means that this inductor combines with the output capacitor into a largely undampened resonant circuit that can develop humongous amplitudes across either part, making either of them candidates for damage.

An inductive load is not good news even for DC-coupled push-pull stages since the resulting phase shift means that (for example) the pull stage does not get to work between output voltage 0V and +Vb (where it delivers most current when the voltage across it is smallest) but between +Vb and -Vb. In the extreme, that can dump about 4 times the power in the output stage circuitry as waste heat than when driving a mostly ohmic load.

Current-limiting circuitry does not protect the output stage against that since it is the combination of current and off-phase voltage that causes the problem.

By the way: a good way to cause problems even for transformer-less amps (tube or solid-state) is to disconnect the bass speaker while leaving the frequency crossover connected. This tends to leave a huge purely reactive load that will in a similar manner overload the push-pull output stage.

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What comes to mind immediately is any resonance that developed between the transformer parasitic elements (self inductance and self capacitance) and the output stage would have little to damp it.

A circuit diagram and the text of the warning would be helpful.

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