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Old 9th January 2009, 10:48 AM   #1
Skorpio is offline Skorpio  Denmark
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Default Protection of tubes/amp...

I have searhed around and found that the best way to protect the amplifier/tubes is to mount small 10R/0.25W cathode resistors instead of the larger 3 or 5W types...

Will 0.25W/10R be ok for a KT88 UL 2x60W amplifier? The standard mounting is 10R/3W...will burning one of these be good for output transformer (disconnected coil?)

If replaced with 10R70.25 0.1% one can ensure very precice bias setting...

I am a little concerned about defective tubes damaging the amp. But with all the amps made, on the other hand it doen't seems likely that major damage comes form defective tubes?

How does normal tubes wear-out? Short? Disconnection? Other?

When is the best time to shange tubes?
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Old 9th January 2009, 10:59 PM   #2
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There's a guy doing this in his guitar amp with no ill effects so far.

Depends on the dissipation of the resistor in normal operation (too hot and they drift) and if there's a bypass cap (which explodes if the resistor opens up).

Cheers!
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Old 9th January 2009, 11:23 PM   #3
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Don't fiddle with the amplifier, it's dangerous. If you want to fiddle, get a solid-state one to break.

Amplifiers blow up. Then you get a new one. We call this 'the cost of doing business'. Just like cars, don't buy an amplifier if you can't afford to have it blow up.

w
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Old 10th January 2009, 03:37 AM   #4
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I use small "fusible" resistors in my Citation II and it saved an output transformer and KT88 once. This is only appropriate for fixed bias amplifiers without cathode bypass caps, as Geek says, where the cathode resistor is used for measuring current and not for providing bias. You'd need to post a schematic of the amp in question, then carefully choose a size that will blow under abnormal conditions and not during clipping, etc. 10R .25W is in the right ballpark, depending on tube operating point. Using a 3W 10R cathode resistor just to check bias current is nutty, IMHO, unless you'd rather replace a $30 tube and $100+ output transformer than a $0.25 resistor.

I do not recommend changing anything unless you feel confident in your abilities - otherwise, it is better to have a tech who is familiar with the particular amp do the work for you.
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Old 10th January 2009, 04:16 PM   #5
Skorpio is offline Skorpio  Denmark
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I know what I am doing, I would just like to hear some experiences with tube faults....

Why use 10R/3-5W resistors on fixed bias circuits to save resistors, when more expencive components fry?
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Old 11th January 2009, 12:42 AM   #6
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Default Re: Protection of tubes/amp...

Quote:
Originally posted by Skorpio
Will 0.25W/10R be ok for a KT88 UL 2x60W amplifier?
No.

Quote:
Originally posted by Skorpio
If replaced with 10R70.25 0.1% one can ensure very precice bias setting...
...the accuracy of which will be destroyed by overheating if the resistor survives at all. Morgan Jones in Valve Amplifiers recommends running 0.1% resistors at no more than 1/8 their rated power if accuracy is to be preserved.

The choice of resistor is based on a number of factors. You haven't given us enough information to work with, therefore you don't know what you're talking about, plus you are overconfident, therefore you should not **** around with valve amplifiers.

w
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Old 11th January 2009, 02:24 AM   #7
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Default Re: Re: Protection of tubes/amp...

Quote:
Originally posted by wakibaki
The choice of resistor is based on a number of factors. You haven't given us enough information to work with, therefore you don't know what you're talking about, plus you are overconfident, therefore you should not **** around with valve amplifiers.
In the words of the dearly departed Bob Marley: Simmer down.

As Jon said, .25W will be fine. Even if the KT88 is drawing 100mA, the resistor will only be dissipating .1W of heat, which is well within spec for a .25W part.
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Old 11th January 2009, 09:51 AM   #8
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I protect my amplifiers only with a fuse in series with high voltage line that feeds the anodes and the G2's for short circuiting of the speaker lines, and an overvoltage protector form anode to anode of the output transformer for open speaker lines. The overvoltage protector I use are Siemens glass sparking divices. They are available in differend voltages.
Doing so I still never have problems with burned output transformers or wornout powertubes.
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