|
Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
Register / Login
|
In Reply to: Re: Not for long! posted by Vinnie on May 1, 2007 at 14:06:53:
If the goal is to size a fuse for your amp then what matters is your amp's operating conditions. Who cares about an irrelevant operating point shown in a manual?I hope you have heard and understood what others are telling you about your rectifier tubes, filiments, and using a variac.
In general, a fuse offers little protection to something like a transformer. So with this in mind, try the following. Fire it up, full line voltage, with only the rectifier tube(s) in place. Use something like a 1 amp fuse. See if your B+ comes up to rated voltage and current draw is in line. You are using a choke input supply so you may indeed need to add a bleeder resistor to keep the voltage under the cap's rating. This in turn could require a larger fuse. Calculate this fuse value. You are looking for a quick and dirty go/no-go test. Keep a hand on the cord, watch the ammeter, volt meter, and look for smoke. No matter how many times you go through this, it is always the moment of truth and everyone is a bit nervous. If something is wrong you will know in a hurry. Tubes will survive serious overloads for short times but now is not the time to walk away and get a beer:) If the voltage ramps up normally and current draw is normal (next to nothing with no load), you are good to go.
Now add the tubes. Use a dummy load resistor in place of speakers. Get a few meters with good clip leads. You want to measure plate voltages at your different voltage levels. All of what I have said so far is the voltage levels that you see as soon as the rectifier(s) conduct and the other tubes are hot enough to draw current. So these readings will be made rather quickly....and no you can't have a beer yet:) You want to measure primary current draw and all relevant plate voltage levels (which can be done at filter cpas usually).
Take your primary current draw and pick a fuse that as around 125% of that. Put it in (your ammeter should have been fused and in series with the primary BTW), that is why you need a few meters. Hook up the speakers....go grab that beer now....and lets see if she makes music....and oh, if fixed bias set that before you do the speakers, beer, music, and measure primary current to calculate required fuse.
Calculations are great for getting in the ballpark and picking a fuse to start with, but if you want max protection it is better/easier to measure. Normal sizing practice is around 125% of actual load for a slo-blow fuse and 175% for a fast blow(general info and not amp related). But with a choke input filter that meets critical inductance I'd go 125% (or less) and fast blow.
Russ
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- Then you are making a mistake - Russ57 14:45:19 05/01/07 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
