HT fusing

[daveyajd]

Hello,
I guess I will fuse my HT legs in my current build. I’ve never done this on previous build but maybe I should have.   This amp has bias derived from tapping the HT and I’ve read that is not a good time to fuse the CT.


I will be putting an in-line fuse on each wire before the rectifier tube. My question is, if the original amp uses a 1A fuse on their CT what should my ratings be for each leg? 500mA? 1A?


Aaron

[shooter]

probably looking at a tinkers choice, start small, work up, keep the fuse makers in business 

[DummyLoad]

1A.


--pete

[trobbins]

ss diodes?
Have you measured or have a target DC load current that you want to allow up to as normal max operation?

[daveyajd]

This build is using a tube rectifier. I don’t have any target current that I am going for. I am just wondering how to adapt a CT fuse for fuses on each leg prior to the rect. I will follow Pete’s suggestion of 1A on each.


Aaron

[trobbins]

Typically, the rms current through a full-wave diode arm is about 0.72 x the CT rms current, so something like a 0.8A fuse may be the closest, given a 1A CT fuse.


There doesn't seem to be a starting point for choosing a fuse (ie. no info provided on the secondary winding current capability, or transformer VA rating, or ...).  Apart from measuring the delivered current, if you were to get technical then there are articles that help to identify an appropriate fuse value based on design. 


Imho, if you are using a valve rectifier, and are concerned enough to use a secondary side fuse, then also including ss diodes in series with the valve rectifier anodes is equally if not more appropriate.

[vampwizzard]

There doesn't seem to be a starting point for choosing a fuse (ie. no info provided on the secondary winding current capability, or transformer VA rating, or ...).  Apart from measuring the delivered current, if you were to get technical then there are articles that help to identify an appropriate fuse value based on design. 

Get the mA rating of your secondary. Multiply it by 2 to be conservative, 3 to be a bit more aggressive, and go with the nearest value. 1A would be good for up to 2x protection on a BF/SF Twin and would be 3x or so for smaller.

[daveyajd]

There doesn't seem to be a starting point for choosing a fuse (ie. no info provided on the secondary winding current capability, or transformer VA rating, or ...).  Apart from measuring the delivered current, if you were to get technical then there are articles that help to identify an appropriate fuse value based on design. 


Imho, if you are using a valve rectifier, and are concerned enough to use a secondary side fuse, then also including ss diodes in series with the valve rectifier anodes is equally if not more appropriate.

This is for a Friedman Dirty Shirley type build (JTM 45-ish) so it uses a 345-0-345 150mA PT. In the gut shots I have seen (no schematics available) it shows a CT fuse of 1A. I have never been inclined to use HT fusing previously. I guess because those builds never lead me that direction.


Do you all use fusing on your secondaries? 


What SS diodes would I use going into the rectifier? I have seen those in Ceriatone layouts but never had it come up in any of my build education.

[vampwizzard]

There doesn't seem to be a starting point for choosing a fuse (ie. no info provided on the secondary winding current capability, or transformer VA rating, or ...).  Apart from measuring the delivered current, if you were to get technical then there are articles that help to identify an appropriate fuse value based on design. 


Imho, if you are using a valve rectifier, and are concerned enough to use a secondary side fuse, then also including ss diodes in series with the valve rectifier anodes is equally if not more appropriate.

This is for a Friedman Dirty Shirley type build (JTM 45-ish) so it uses a 345-0-345 150mA PT. In the gut shots I have seen (no schematics available) it shows a CT fuse of 1A. I have never been inclined to use HT fusing previously. I guess because those builds never lead me that direction.


Do you all use fusing on your secondaries? 


What SS diodes would I use going into the rectifier? I have seen those in Ceriatone layouts but never had it come up in any of my build education.

Youve at least made the better decision to fuse the B+ rails vs the center tap.. all the center tap fuse does is lose your reference to ground and thats akin to blowing the tension belt on an engine.. wild voltage swings are never a good failure mode.

Fusing the PT secondary is common enough. This article walks you through the whole thing. Id trust some of the more experienced builders on whats necessary/best https://www.premierguitar.com/articles/the-immortal-amplifier-mod-1

[sluckey]

I've never used any HT fusing in my stuff although I have mixed feeling about HT fuses, especially with big amps.

It seems reasonable to me that if an amp uses a 1A fuse in the CT that you should use the same 1A fuse in the HT lead. The two fuses are in series (even though they are not connected to each other) so the same current flows frough each fuse. I say this because only half of the HT winding is being used at a time, ie, one half cycle current flows through the top HT wire, through the diode, thru the load, and returns to the CT. The bottom HT lead is switched off while the top HT lead is conducting. So you have the same current flowing through the CT as flowing through the top HT.

During the next half cycle this all reverses. The top HT is switched off while the bottom HT is supplying current.

1N4007s would be fine for "protection" diodes.

[trobbins]

The 0.72 factor comes in from the 'rms' calculation of the current through a phase arm fuse, compared to a CT fuse - ie. the phase arm fuse experiences a lower rms current value than a CT fuse, because it is subjected to only half the current pulses over time.  Fuses heat up and open due to the rms current level (well at least for circuits which have some impedance so that the instantaneous current doesn't go stratospheric in the first few milliseconds), so often a fuse is used to protect the rms heating survivability of the power transformer secondary winding.

You may not know for certain how much rms current and time can be endured by your PT secondary, so another approach is to realise how much fuse rms current may be drawn by the amplifier acting as a 'load', and then using a fuse value that tries its hardest to open for any higher current level.  Fuses have tolerances, and faults come in all guises, so no simple guarantee that adding in a fuse is a going to 'reap rewards'.  And it is very difficult to 'design in' a fuse that could give you some guarantee of confidence (like a car air bag possibly could for some kinds of crash).  Often the topic revolves more around how expensive or unobtainable a power transformer is to replace if....

[vampwizzard]

A lot cheaper to replace a fuse vs a transformer in general.. but I will say most of the amps ive seen with HV secondary fuses were either significant mods on vintage amps or boutique amps with custom transformers.

Youve nailed it there Trobbins.

[tubeswell]

1A for 4 x 6L6 or EL34
0.5A for 2 x 6L6 or EL34
0.25A for 2 x 6V6 or EL84


all T-fuses

[daveyajd]

Thank you all for your knowledge! I am still on the fence about whether to even worry about it. I’d hate to have to suffer some sort of failure but in a non vintage amp that I have built I’m not to worried about having to do the work and $ if it went bad. If I do put the fuses in would I use slo-blo type like the mains or fast-blo?


Aaron

[shooter]

slo-blo

the initial "kick" will take out a fasty