Best product for internal fuse?

[Mark]

Hi

I’m wishing to put an internal 3AG fuse from the GZ-34 to the filter caps in an AC-30 clone that I’m building.

The idea being it is simple protection against a bad cap or a valve going south. The value of the fuse will be 500mA.

I had intended to use something like this fuse holder.

https://www.amplifiedparts.com/products ... ow-profile

https://au.rs-online.com/web/p/base-mou ... s/7874316/

[pdf64]

That should be fine. I use 20mm fuses for such purposes - they’re smaller!
As there’s no standby switch and hence no current surge, F type / quick blow is fine.
I put them in the return to chassis 0V of the HT winding CT, then they don’t have voltage on them, until they blow.
In conjunction with beefy series protection diodes on the rectifier anodes.

[trobbins]

How did you determine 500mA fuse rating?

If the amp is default AC-30, and you hot switch the power then there will be a current surge through the fuse - whether that blows a Time delay or Fast fuse or not depends on a bit more assessment, along with the fuse current rating.

The standard AC-30 secondary voltage is 280-0-280V, and the load current not substantial, so a 1N4007 should do fine for protection of each GZ34 anode.

[Mark]

Vox use 500mA as a HT fuse in some AC-30 amps.

[trobbins]

Clone's can be as different as original versions are. Ie. 500mA may be too high, or low, as it depends a lot on the power transformer windings resistances, and the capacitance value, and whether you are running the same output stage, and if you are using the same type of fuse as Vox were (the commonly available fuses are IEC rated and UL rated for USA, and they have quite different performance).

RS has a good range of IEC fuses in 5x20 size. There can also be a lot of difference between a 315mA, 400mA, 500mA and 630mA fuse when it comes to saving parts in an amp if a bad fault occurs.

My son's AC30 CC2 has a 295-0-295V transformer and uses a time delay 500mA. I measured the PT winding resistances as primary DCR = 23Ω, secondary DCR = 51+53 = 104Ω. That amp uses up to 44uF. Is that close to your transformer and cap?

[stephenl]

Just an opinion, but you could lose the tube rectifier...unless you want it for aesthetics. The GZ34 doesn't really contribute much sag, Ra = 50ohms. Add a resistor to simulate this with a SS rectifier.

If you use SiC, Schottky diodes, you won't introduce the diode switching noise from reverse recovery.

[sll]

https://robrobinette.com/Generic_Tube_A ... ction_Mods

A good source of information.

[SoulFetish]

How did you determine 500mA fuse rating?

If the amp is default AC-30, and you hot switch the power then there will be a current surge through the fuse - whether that blows a Time delay or Fast fuse or not depends on a bit more assessment, along with the fuse current rating.

The standard AC-30 secondary voltage is 280-0-280V, and the load current not substantial, so a 1N4007 should do fine for protection of each GZ34 anode.

Tim, I was hoping you were going to chime in . Perhaps you can post a link to your power supply issues/fusing article?

[Mark]

I’d link to see Tim’s article too.

[trobbins]

The article below uses PSUD2 as a tool to help indicate what fuse may be appropriate for a given amplifier configuration. The topic can get involved as details need to be known or measured, and interpretation and assessment is often needed on what could happen.
https://www.dalmura.com.au/static/Valve ... fusing.pdf

Ciao, Tim