Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on February 10, 2012, 03:22:13 pm

Title: FUSE SIZE??
Post by: TIMBO on February 10, 2012, 03:22:13 pm

I guys, This may be common knowledge but is there a way to properly calculate (in easy to understand math for us untech foke) to make sure we have the right size use for the job.
My new build has three fuses and i have no idea what size they should be  :dontknow:

Main Fuse
HT Fuse
Heater/12v supply Transformer Fuse

Thanks

Title: Re: FUSE SIZE??
Post by: navdave on February 10, 2012, 04:16:11 pm

Most 50 watt amps use a 2amp mains and a .5amp b+,
100watt 5amp and 1amp b+. For the heaters add up all the tubes
in use heater current draw if your gonna draw 6amps total
put a 3amp fuse on each ac line. Anyways that's what I would do
hope it helps.

Title: Re: FUSE SIZE??
Post by: darryl on February 10, 2012, 05:18:45 pm

Quote from: navdave on February 10, 2012, 04:16:11 pm

Most 50 watt amps use a 2amp mains . . . 100watt 5amp . . .

This assumes a 120VAC power source. In 240VAC countries, halve the mains fuse current rating.

Title: Re: FUSE SIZE??
Post by: DummyLoad on February 11, 2012, 12:01:17 am

fusing rule of thumb: for AC or DC not-purely-inductive loads - 1.5x to 2x average draw.

for filaments; series or parallel or combination? for parallel add up the specified current draw of each tube at the specified voltage, then 2x. for series, add up current draw of each of the series strings that are in parallel, then 2x

for B+; the max current draw, then 2x that - use a fast acting fuse.

for mains; use the VA rating of PT / the mains voltage to be used. to get VA rating add up VA of each of the secondaries. VA rating is Vrms * Irms. some mfg. give the PT VA rating, if they don't, calculate it.
 
i.e. fender twin -

640V x .5A = 320VA
6.3V x 5.5A = 35VA

total VA = 320VA + 35VA = 355VA

now calculate needed fuse size - 355VA / 120V mains = 2.95A - closest standard value is a 3A fuse. use a slo-blo type.

-or- take easy st. - copy someone else's plan - if it's custom - find something similar in tube count and output power.

--DL

Title: Re: FUSE SIZE??
Post by: TIMBO on February 11, 2012, 12:19:50 am

Thanks guys, the build i am doing is similar to Tubenit's SoLow Watt with the ECL84 power tubes. The PT i am using does not have the heater winding so i am using a separate transformer for the heaters, but this transformer also powers the SS circuit i have on board and also a 9v power supply for pedals.Thanks