...they put 3 in series diodes instead of only one (and I've no idea for the reason) ...
Me neither
On the face of it, that allows up to 3V across the 1ohm 1W resistor under fault conditions, ie 9W! So the diodes seem kinda pointless
PDF are you following all the posts to this thread?...
I thought so; could you point out if you think I've missed something?
...The way I look at it is that if something catastrophic happens in a tube something else is going to give before that 1 watt 1 ohm resistor blows...
Indeed, but then output valves do occasionally fail catastrophically.
My view is that it's good to attempt to limit the collaterol damage arising from likely incidents. So the user just replaces the problem valve, blown fuse, checks bias and the amp is back in use, no need for time on the bench.
PDF yes we could run the diode in parallel with the 1 ohm resistor. But lets look at Fender amps from the 50's 60's 70's 80's how many times have you seen one of these amps have a failure as you describe?
I can tell you my experience. Never have I seen that kind of failure... how many amps have you seen or experience that have had the failure you describe? ...
But those amps didn't generally have 1ohm cathode current sensing resistors
I suppose we all have to be guided by our personal experience. I've not repaired many amps, valve amp tinkering /fixing etc is just a hobby, so a few dozen maybe over the years. And only a couple of 60s Fenders, being as they're fairly rare here.
But when I'm given something to fix, it plays on my mind how the equipment's design might be amended to reduce the likelihood / consequence of recurrance. Of course valves are always going to short, but it's feasible to make simple changes to the reduce collaterol damage.
I've described a few failure types in this thread, but I'll assume you're referring to the heater - cathode insulation damage to other valves in the amp when an output valve shorts and the heater circuit is not solidly DC referenced to 0V common(?)
I think that's not unusual, at least in my limited experience. Bear in mind that unless the breakdon is total, it might not cause problems in many applications, eg trem oscillator, later stages, earlier stages whose cathodes are fully bypassed. But if used in earlier stages with partially or unbypassed cathodes, the damaged valves cause a buzz to contaminate the signal path.
If we built cars for every possible catastrophe that may occur the cars would weigh tons more with 17 air bags per square inch and multiple bumpers ... along with throttle resirictions.. and we would have to wear helmets and bullet proof vests..
There comes a point when we have to look at the costs versus the reality ..
The fitting of a diode for each cathode current sensing resistor, and a suitably rated, fast acting HT fuse, is all I've suggested; which doesn't seem an inordinate degree of effort to go to?
...
So I do whatever I can to practically bullet-proof an amp, whether that is to suppress stress causes for starters, or include failure protection such as poor-mans fusing, and that includes adding parts to avoid further collateral damage if a cathode fuse/resistor open-circuits. I am in the camp where I'd prefer to use a 10R cathode resistor of 0.25-0.5W rating, and hope that it opened or told me that something was wrong, rather than alternately designing a 1R resistor to survive by upping its power rating or adding a protective part.
It seems fairly common for output valve shorts to result in a heater circuit humdinger or balancing resistors blowing. So unless the heater winding has a CT that's connected to 0V common, on balance I think it's best to try and avoid an output valve short resulting in the cathode losing its connection to 0V common.
Here is a tidbit of history...
Thanks, that reminded me of the mixed bias arrangement used in the later Vox AC50, expect that 47ohm, rather than 12ohm, resistors were used. But then the goal was 47mA idle cathode current, rather than 130mA!
Topic: 1 ohm cathode resistor (Read 8752 times)