Hello,
how do you protect your OPT against open loop secondary ?
Do you put a resistor across the secondary - what would be the minimum current / max. value ?
Or do you use Zener ? Assuming an amplifier with 25W / 8 Ohm (10 Vrms) do you use i.e. 2x max. voltage = 28 V Zener in anti parallel configuration or other value ?
Thank You
how do you protect your OPT against open loop secondary ?
Do you put a resistor across the secondary - what would be the minimum current / max. value ?
Or do you use Zener ? Assuming an amplifier with 25W / 8 Ohm (10 Vrms) do you use i.e. 2x max. voltage = 28 V Zener in anti parallel configuration or other value ?
Thank You
Do not use a Zener. Parallel on the secondary a 100R 5W, or up to 1k 1W,
depending on the situation and how much abuse there is.
depending on the situation and how much abuse there is.
Last edited:
I usually recommend a 100 ohm resistor soldered across the 8 ohm outlet.
For a 50w amp 5w resistor is good enough.
For a 50w amp 5w resistor is good enough.
Reverse biased freewheeling diodes in parallel with the output tubes will protect against ALL forms of overvoltage stress caused by improper loading. The voltage swing on the primary cannot exceed 2X B+. There are, however, times when their effect can be audible. Particularly with guitar amps. RC snubbering (basically a zobel network on the primary) will keep peak voltages down under heavy clipping at resonance. A small load (100 ohms) on the secondary will help in most ‘ normal’ circumstances. All methods are.equally applicable SE or PP, triode or pentode.
True enough, if it’s not clipping. Anode resistance is enough to keep the voltages in check. But if the tube does cut off abruptly you can get a voltage spike.
I have a 47 ohm 1W resistor in series with a 0.047uF cap across the secondary. These snubbers were often called 'tweeter savers'. Best way to prevent an open secondary condition from damaging the amplifier is to get an amplifier that will tolerate that. Mine do. and I've had no trouble when I accidently ran them with no speaker attached.
Or you could always install a finely adjusted spark gap across the primary. 😀
Reminds me of some beginner days, where I installed an unloaded secondary interstage transformer for the first time. Sometimes I would unplug the input of the amp in max gain mode, then plug in again, causing momentary ground floating and thus amplifier clipping. Some sparks inside the power tube between its pins could be observed, more specifically between g1 and cathode.
The problem went away after loading the secondary with a 47k. Makes sense.
The main point here is, the input stage was a triode. So wg_ski's comment is valid. Trouble happens when the stage cuts off.
Reminds me of some beginner days, where I installed an unloaded secondary interstage transformer for the first time. Sometimes I would unplug the input of the amp in max gain mode, then plug in again, causing momentary ground floating and thus amplifier clipping. Some sparks inside the power tube between its pins could be observed, more specifically between g1 and cathode.
The problem went away after loading the secondary with a 47k. Makes sense.
The main point here is, the input stage was a triode. So wg_ski's comment is valid. Trouble happens when the stage cuts off.
Last edited:
Not a problem for ordinary Class A triode or triode connected pentode stage whether SE or PP. The circuit is heavily damped by the low rp of the tube.
But a real problem for pentode power stages. And any PP Class B, high mu triode stage. These both have very little inherent damping.
There are several cures circulating, some better than others.🙂
It would be interesting to know which are the better ones and why....😀
Why not a Zener ?
Last edited:
Zeners are semiconductor junctions and relatively fragile; I would use a varistor rated for 2X the highest peak voltage available so it never ever triggers under normal use, even extreme use, but limits car ignition type kilovolt peaks.
They roughly work like crude Zeners, but are way more abuse resistant.
There´s a reason we use varistors against lightning protection and similar mains peaks, instead of Zeners.
As of the spark gap, I have somewhere the pictues of a tube PA amp, not sure it was an old time Geloso or some German brand (Dynacord?) but it had an adjustable spark gap across primary.
They roughly work like crude Zeners, but are way more abuse resistant.
There´s a reason we use varistors against lightning protection and similar mains peaks, instead of Zeners.
As of the spark gap, I have somewhere the pictues of a tube PA amp, not sure it was an old time Geloso or some German brand (Dynacord?) but it had an adjustable spark gap across primary.