|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
98.71.145.94
In Reply to: RE: I am shocked! posted by Jim McShane on July 22, 2012 at 11:26:39
What about those who advocate using a 1/2 watt or even a 1/4 watt resistor? They contend that it would serve as a "fuse" and open if the tube began drawing excessive current. Is there a negative to using a lower wattage, other than the possibility of occasionally replacing a resistor?
Follow Ups:
Many years ago in my day job, we considered adding a 10 ohm, 1/4-watt AB carbon-composition resistor in the power feed to each of roughly 100 solid-state circuit boards running off a 15 V power supply to isolate any board that had failed shorted. Before we installed them, we did a simple test with a current-limited lab supply applying a few watts of power to the resistor. They smoked and stunk up the lab, but refused to open circuit. We used "picofuses" in roughly the same package size for the final array. Note that such fuses have a very high positive temperature co-efficient of resistance, to ensure reliable blowing on high current.
But just like selecting a real fuse you have to evaluate the circuit!
You have to consider the current capability of the tube. I use 2 watt resistors in the Cit II with the KT-88s - for EL34s I normally use 1 watt. For EL84s or lower wattage tubes you can often get by with 1/2 watt. There is no single hard and fast rule...
But in considering the cathode circuit you must consider all the possibilities - bias current is not the only thing the resistor sees. Put a scope probe on the cathode pin of the power tube when music is playing - you'll see other waveforms too.
Too small a resistor and the resistor will pop unnecessarily.
I would do just that, Jim, if only I had one.
In my particular case, I'm bringing a pair of Dynaco MkIII 's out of a twenty-year storage. The guy I bought them from way back then set them up with dual-bias pots, and used 8.2 Ω 5 watt resistors on pin 8 of each 6550, simply because he had a shopping bag full of them from a ham fest.
With 10 Ω, it will be a lot easier to remember the bias setting, but I'd read about using the low wattage resistors specifically in posts about the MkIII. Since I already bought four 10 Ω 1/2 W Dales for them, I'll use them, and then move to one or two watt versions if they give me grief.
Thanks.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: