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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 27, 2018 9:08 am
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Tim, a tube which is shorted, will still conduct electricity --- even after a SGR pops. It'll be more like in triode mode, than pentode. That's all. That's why a lot of amps with popped SGR --- have other parts of output stage ruined.

This is different than a grid stopper resistor popping (maybe, only the "fuse" in Fender amp output stage). Instability, like parasitic oscillations, applied to the signal grid can cause the tube to go ballistic. This leads to excessive current passed by the tube. However, if the grid stopper pops --- the oscillation signals applied to the signal grid stops. At least, that is my take on the thing.

:)


Last edited by BMW2002Ti on Wed Jun 27, 2018 10:01 am, edited 1 time in total.

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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 27, 2018 9:35 am
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Utter nonsense Tim. Leo's "designs" show advanced copying skills, as he had "borrowed" from existing designs (but of course, Leo didn't "design" the BJr either). BTW, Leo was not an electrical engineer, he was a machinist. No one with advanced engineering skills is going to design a circuit that counts on a resistor to fail, that can and does catch on fire, often taking out other components too, when it is extremely easy/cheap to use a fuse (check many modern amps with an HT fuse). A shorted output tube in a BJr can easily pull 7/10ths of an amp through the SGR, which would be over 200 watts, so even a 5 watt resistor could smoke and fail, as well as other components failing along with it.

Many Fender amps actually did use 5 watt or higher resistors for shared SGRs. And the BJr did add a HT fuse, in the version II.

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Post subject: Re: Blues Jr III Mods Gone Awry - R47 2.2K 2W Smokin' after
Posted: Wed Jun 27, 2018 10:39 am
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I think Leo was an accountant and horn player.
Yes, it was a common feature to use low wattage sgr resistors on most amps from the golden age of tubes. There is no distinguishing feature of a modern fusible resistor. They are simply 1/4 watt resistors rated to 1/2 watt. They also blow with smoke, but are now flameproof. They are commonly used in modern SS power amps on the grids or bases of power output transistors
Your math also shows exactly why Fender never uses HT fuses. They don't need it. When a power tube in a Fender shorts, it blows the main fuse. An HT fuse is simply another unreliable component. A power surge can blow them. They are subject to ageing and vibration failures and they dont save any components when they do blow. It just thoroughly shuts down the amp and the music. This is where foolish engineers thinks its more important to save the amp rather than save a performance. Personally, I want my amp to be a smoking slag heap before it quits making music.
You can't ignore the differences in design philosophy that Fenders seldom have more than one fuse, while others use lots of them. Reliability. Its the same reason they stay away from too many connectors. They copy what works .


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