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Doesn't a Big Resistor do the Same Thing?

6/26/2005 8:52 AM
dave b
Doesn't a Big Resistor do the Same Thing?
If you've got a 5 to 10W, 250 to 350 ohm resistor across the OT secondary you don't need spike protection diodes right?  
The resistor acts as a high impedance load if the speaker load blows or is disconnected, preventing the OT from frying until you realise something is wrong and can power down; yet is "invisible" electronically speaking.  
One part, Easy, Cheap  
I haven't read all of the posts here so sorry if this has already been mentioned.  
6/26/2005 12:00 PM
Ray Ivers

It does something, but not the same thing.  
The AB2 circuit I mentioned earlier shows no-load full power OT-primary spikes of 9kV (all voltages peak-to-peak). I know of no audio OT using wire insulation that can withstand this kind of voltage, although RF modulator transformers are out there that can. The primary swing with an 8 ohm load is @ 1.2kV.  
My dual-Zener w/resistor circuit tops out at 2.25kV, which is higher than I'd like but much less than 9kV. A 250R resistor shows a swing of 2.7kV; a 2.5kV-rated OT might withstand this for a while, but would eventually fail.  
I found that to get adequate protection with a single resistor I needed to go much lower in value than I was comfortable with, at which point the resistor was dissipating significant power and could no longer really be called 'invisible' sonically - but YMMV, there's no question that it will work, and how audible it is would have to be determined by the IUT (Individual Under Test ;)).  
6/26/2005 3:00 PM
dave b

Thanks for the clarification Ray, it also makes sense to have protection on both sides of the OT.  
I used to to both the resistor and the 3x1n4007 circuit for OT protection, then I read a post here from someone who seemed to be pretty sharp electronically speaking who was explaining only the resistor was needed so I stopped adding the diode sting to the primary side. Maybe I need to go back to it again.  
So, what exactly is your dual zener/resistor circuit?  
6/26/2005 4:19 PM
Ray Ivers

My thing is just two 43V 5W Zeners in series, cathode-to-cathode, with a 24 ohm 3W resistor across the speaker output; nothing fancy. With the speaker load removed, lowering the resistor value increases waveform-peak loading at the expense of added power dissipation in all 3 devices, and raising or lowering the Zener voltage matches the circuit to various power levels and/or output impedances.  
I didn't realize that a 1N4007 could withstand even a single avalanche event; I know I've replaced more than a few of them, along with the special HV and fast-response diodes, from the OT primary to ground - Peaveys, Music Mans, newer SVT's, etc. - so I'm not really a big fan of them from a reliability standpoint. I know that later on the amp's owner will be really happy that the problem was just a $.20 diode instead of an OT, but to be suddenly playing air guitar in front of a club-full of people makes for a lingering memory, I'm sure. ;)  
It might be interesting to try the resistor-only idea out with a sacrificial OT, but the results would probably only be applicable to that OT, at that B+.  
7/15/2005 11:54 AM
Conclusion: Doide + Resistor...
My conclusion, after some research, is to use a Diode (or a stack of them) in each side of the primary. But not a the usual 1N4007, but for example the two BYV96 in each side, these diodes seem to work well in this kind of application see datasheet:  
To help them a 470R resistor on the secundary will be very usefull. It's a "big" resistor so I expect that will not change too much the tone.  
MOV's could be better, but they are too much "obscure" devices to trust.  
Conclusion: two BYV96 on each side of the primary, and one 470R 5Watt resistor on the secondary.  

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