No speaker load and the output transformer

Pete Farrington

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None that I’m aware of, though 2W may be a bit underrated on a cranked JTM45 16ohm output, and definitely would be on most 50 watters.
Bear in mind that almost all black and silver panel Fenders already have it, by virtue of their negative feedback network (eg 820+100ohms). Though as they’re on lower impedance outputs, the 920ohm (or 867 on the DR) may be a bit less effective.
 

SwedishWings

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None that I’m aware of, though 2W may be a bit underrated on a cranked JTM45 16ohm output, and definitely would be on most 50 watters.
Bear in mind that almost all black and silver panel Fenders already have it, by virtue of their negative feedback network (eg 820+100ohms). Though as they’re on lower impedance outputs, the 920ohm (or 867 on the DR) may be a bit less effective.
Thanks. I've been thinking of adding this to my home brew amps but was unsure if there are any downsides, and also if it actually works. Guess I'll go ahead now.
 

Pete Farrington

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It is on the switching jack so not in circuit when lead plugged in.
Thanks, I’d not noticed that.

It’s tricky to know how effective they are, but the rationale behind them seems good to me.
As per the AC50 (470ohm 3W across the 16ohm winding), I use about a safety resistor across the OT secondary with a value about 30x the intended load impedance, rating about 0.1 of the clean power output.
If hardwired to the secondary, all eventualities are catered for.
30x the rated impedance so that the speaker’s bass resonance doesn’t get damped by it.
 

PelliX

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It is on the switching jack so not in circuit when lead plugged in.

As I believe @Pete Farrington indicated earlier, putting it on a switched jack is actually not the best way of doing it in my view. Suppose you have a lead plugged in, but no cab/load on the other end. The jack is switched, the output has infinite impedence. Go with the OT secondary (not switched) and the values outlined above and you actually have a sort of failsafe.
 

Pete Farrington

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The value of a safety resistor on the secondary winding was brought home to me when I was tinkering on the test bench with my JTM45 build, which has an RS DeLuxe OT.
Even with a safety resistor on the speaker jack, I noticed that with the amp operational but idling, if the output load impedance switch was flipped, there was a spark between the primary terminals - yikes!
Due to the secondary momentarily being open circuit. Equivalent to a bad connection in the secondary circuit vv
With the safety resistor moved to be hardwired across the main A-F half winding, there was no spark when flipping the impedance switch.
 

XTRXTR

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Interesting topic.

The idea is that the OT output and the reflected load to the tubes have something rather than nothing, without a speaker. And, you're adding 470R or 1kR in parallel non switched when the speaker is plugged in. So the actual load with speaker plugged in will be near enough what the reflected load wants to see, not affecting the tube plate currents much and not lose much of the secondary current to the speaker out. The higher value of a safety resistor the better as it decreases the plate currents but also better matches the speaker when plugged in.

On the other hand if you used a dummy load sized resistor, eg 16 Ohms at 50w, its great if no speaker but with 16 Ohm speaker you reflect 1/2 the impedance to the tubes and increase 2x the Tube plate current and the output current, worse if its an 8 (less than 1/3 while 3x plate current) or 4 (less than 1/4 while 5x plate current) Ohm speaker.

So the non-switched side tip of the output jack seems like the best place for the safety load with a value of at least 1kR 3 to 5 watt rating as you want to dissipate something across a load to ground but not so high of a resistor that it appears open. With respect to a 2204, 2203, or similar amp.

Am I understanding this correctly?
 


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