HT fuse location in circuit

[alerich]

I am doing a JCM 800 scratch build. The JCM 800 uses the square DPDT rocker style power and standby switches. I am using SPST bat style switches common to Fender amps. I'll put the power switch in the hot line just after the inlet and before the mains fuse just like a Fender amp. Marshall uses the DPDT standby to open the PT secondary on each side before the diodes. Can't do that with a SPST so I will be placing my standby switch just before the choke as Fender has done successfully in jillions of amps (despite the Valve Wizard site's advice to the contrary). The more research I do online about building amps the more confused I become.

Marshall uses a 500ma HT fuse after the first main filter cap reservoir and before the choke. Fender does not use a HT fuse at all. I want to use one. I am planning to put the HT fuse immediately before the standby switch which will then lead to the choke. Will this work? Should it be after the standby switch (between the switch and the choke)? Does it really matter?

[catnine]

There are many ways to add a HT fuse. Valve Wizard has a fuse on both HT secondary leads and one on each 6.3 volt and one in the 5 volt heater. Plus if you use a rect tube I have seen diodes in series with the HT to the rect tube.

 I put mine after the standby switch . Thing is if a cap shorts my thinking is all the caps should be after the fuse.

 My builds don't have a coil . Another thing , yes the main fuse is to protect from a fire . Yet when you think about it on my build for example the 6.3 heaters are rated at 4.5amp and the 5 volt is rated at 2 amp . My main fuse is 1 amp . I could be wrong but I would think that the 1 amp main would blow if there was a short in the heaters. Now if the rect tube shorted then there is nothing to protect the HT secondaries.

 I have never had an amp have a rect tube short out and many very old fenders I had had the origninal rect tube and one 5Y3 in a SF champ I once owned kept blowing the main I asked this was over 20 years ago at a guitar and amp shop and they sold me a new rect tube and proper fuse and that fixed it. So it depends on how the rect shorts out.

 To play it safe it may be best to add the two Ht secondary fuses one of each lead between the HT and rect at the very least and then one before all the filter cap either side of the standby shouldn't matter. If I had the room I would do the three at least.

[Merlin]

The best place for a fuse is always in the AC part of the circuit. This is not just because fuses are usually designed for AC, but because one of the most common failure modes in a power supply is rectifier diodes failing short, which can short out the whole transformer winding.

[PRR]

> Fender does not use a HT fuse at all.

Good plan.

If you try to cover ALL failure modes, your protection becomes more complex than the amp, adds new failure modes (loose fuse clips? Tin-foil??).

If it doesn't catch fire, you can fix it. Fuse the primary.

While fuses can be good on DC (it does make a difference) note that most compact fuses are rated 250V! Using them in a 400V B+, they can "blow" by leaving molten metal on the inside of the glass, which at the higher voltage may arc-over and continue the fault current. 600V fuses are available but rare and large. Marshall's experience says the 250V jobs are not a disaster.... but then how many Marshalls come in totally crisped, fuse didn't save them?

[eleventeen]

What's the consensus on placing a fuse in the HVCT to ground?

[The_Gaz]

> Fender does not use a HT fuse at all.

Good plan.

If you try to cover ALL failure modes, your protection becomes more complex than the amp, adds new failure modes (loose fuse clips? Tin-foil??).

If it doesn't catch fire, you can fix it. Fuse the primary.

While fuses can be good on DC (it does make a difference) note that most compact fuses are rated 250V! Using them in a 400V B+, they can "blow" by leaving molten metal on the inside of the glass, which at the higher voltage may arc-over and continue the fault current. 600V fuses are available but rare and large. Marshall's experience says the 250V jobs are not a disaster.... but then how many Marshalls come in totally crisped, fuse didn't save them?

Not a good plan unless you copy something else Fender did - low wattage screen resistors. If you use 5W screen resistors and a screen melts down causing an internal short (fairly common tube failure mode when cranking vintage amps with modern tubes), then don't be suprised if your PT 'fuses' before your screen resistors do. Honestly, I'm all for simplicity, but I don't like the idea of a failure on the secondary side having to pull current through the PT just to blow the primary side.

[JPK]

Another thing , yes the main fuse is to protect from a fire . Yet when you think about it on my build for example the 6.3 heaters are rated at 4.5amp and the 5 volt is rated at 2 amp . My main fuse is 1 amp . I could be wrong but I would think that the 1 amp main would blow if there was a short in the heaters. Now if the rect tube shorted then there is nothing to protect the HT secondaries.

You're talking about a fuse that's located on the Primary of the PT, 120VAC. The heaters are on the secondary. The 5 amps and 2 amps are at 6.3VAC and 5VAC respectively. So you're not drawing 2 and 5 on the primary. It's much much less than that. 6.3/120 x 5 =  0.2625 amps for the 6.3VAC if it's drawing 5 amps. 5/120 x 2 = 0.0833 amps for the 5VAC. That's not taxing that 1 amp fuse much on the primary.

PS: I may have misread your post. You're talking about a short on the heater circuits. Not sure that would blow the 1 amp on the primary before destroying the PT heater windings. It'd probably be best to fuse each heater circuit to protect from a short if you don't want to damage the PT, HT circuit as well.

[alerich]

There are many ways to add a HT fuse. Valve Wizard has a fuse on both HT secondary leads and one on each 6.3 volt and one in the 5 volt heater. Plus if you use a rect tube I have seen diodes in series with the HT to the rect tube.

I put mine after the standby switch . Thing is if a cap shorts my thinking is all the caps should be after the fuse.


To play it safe it may be best to add the two Ht secondary fuses one of each lead between the HT and rectifier(s) at the very least and then one before all the filter cap either side of the standby shouldn't matter. If I had the room I would do the three at least.

I am leaning towards dual HT fuses in the PT secondary before the diodes. I should have room for two fuse clips inside the chassis and this amp will be easy to open and replace them if needed. In that case I see no benefit to adding another HT fuse downstream. If you use dual HT fuses in each side of the PT secondary in this fashion would they still be the same ampere rating as a single fuse? Many amplifiers (Marshall included) use a single 500mA HT fuse. Would dual HT fuses in the PT secondary both be 500mA or some other rating?

I am also rethinking the switches and considering going with the Marshall style rocker switch. Despite the hassle that it would be to cut those square mounting holes I'm really beginning to appreciate the whole double pole power/standby switch model.

Thanks to all for the comments and input. It's all been very helpful and I appreciate it.

 

[John]

I'm really beginning to appreciate the whole double pole power/standby switch model.

Just in case you didn't see it, Doug just added a Carling power/standby switch to his catalog.

[catnine]

> Fender does not use a HT fuse at all.

Good plan.

If you try to cover ALL failure modes, your protection becomes more complex than the amp, adds new failure modes (loose fuse clips? Tin-foil??).

If it doesn't catch fire, you can fix it. Fuse the primary.

While fuses can be good on DC (it does make a difference) note that most compact fuses are rated 250V! Using them in a 400V B+, they can "blow" by leaving molten metal on the inside of the glass, which at the higher voltage may arc-over and continue the fault current. 600V fuses are available but rare and large. Marshall's experience says the 250V jobs are not a disaster.... but then how many Marshalls come in totally crisped, fuse didn't save them?

 That's one reason I didn't add the fuses on the DC  secondaries. Now (if) one could find the type of fuses used in cars whicYet h are dc rated the type that don't have the glass then they would work and you can get small inline rubber holders that have a rubber cap to cover the fuse. Trouble is the lowest I've seen is perhaps .5 amp and they are rated at 12VDC and they are fast blow. Doubt they would handle 400VDC . Yet aren't we talking current draw here rather than voltage that blows the fuse?

[catnine]

The best place for a fuse is always in the AC part of the circuit. This is not just because fuses are usually designed for AC, but because one of the most common failure modes in a power supply is rectifier diodes failing short, which can short out the whole transformer winding.

 That would require all the fuses to be right near the PT secondary side . Most older fender amps don't have the room to place these fuse holders beside locating fuses with the proper rating . At best I would need 150mA for the ht secondaries and who knows what for the heaters . It seems to me if the heaters like the 6.3 volt are rated @ 4.5 amp then we would need a max of 4.5amp fuse . I say this because the heaters are on the secondaries and you cannot exceed 4.5amp draw or risk overheating the 6.3 AC heaters say like placing to many tubes to exceed that rating. Same for the 5 volt heaters for the rect . mine is rated @ 2 amp so if I plugged in a rect tube that draws 3 amp this can burn the 5 volt out so in this case I would need no more than a 2 amp fuse . Since we have low voltage at these points 6.3 VAC and 5 VAC are we just talking current here or do we need to include the voltage rating of the fuse as well? The glass fuses used for the 120 VAC line are rated @250 VAC and seem to work , yet I can't say how they would hold up at 400 VDC or even 400 VAC for that matter.

 To take this a step further take DVOM's for example one I have states 1000 VDC and 750 VAC . All it states as a warning against fire use only 0.5 amp 250 VAC fuse. It seems that I can run 750 VAC max through this meter  and to allow this voltage without it bursting into flames in my hand a 0.5 amp 250 VAC fuse will live . I have a analog V , ohm and amp meter says the same thing yet can go to higher AC and DC  but neither have fused amp of mA . Another DVOM I have has the 10 amp fused max 10 amp and mA fused max 0.25 amp , VDC 750 and VAC 600 saved byeems to be one of the 2 fuses since they seem to go to the neg lead 0.63 amp @500 volts and a 12.5 amp @500 volts.

[eleventeen]

To restate and re-ask my earlier query: What about a fuse in the HVCT to ground? If the HV fuse (in that position) blew and the AC line side DID NOT (quite probably the most common "blow" mode)  then we would have big HVAC still delivered to the plates of the rectifier tube but no return for the circuit. We would still have big AC volts on those plates (and only on those plates) but residual filtered DC would, I take it, slowly drain off the electrolytics (even better if there were two series caps with the 220K balancing resistors across them a la Fender)...and the still-illuminated tubes would also serve to conduct and thus drain the charged up electrolytics.

Opinions?

[John]

You might find these helpful.

[sluckey]

To restate and re-ask my earlier query: What about a fuse in the HVCT to ground?

That works for a lot of amps. But there is a hidden danger for other amps. The same danger applies for putting a standby switch in the HVCT for those amps.

The amps that are at danger are the ones that have a bias tap that shares that HVCT. When the fuse blows, or the standby switch is opened, the B+ will still find a sneak path to ground through the bias supply. The bias filter caps will be subjected to a large reverse polarity voltage that will likely damage or outright destroy the bias caps. Now you have even more problems.

There are a lot of amps that fit this category, including most of the Fender fixed bias amps.

[eleventeen]

Good point, Steve. Thank you!

[alerich]

If you use dual HT fuses in each side of the PT secondary in this fashion would they still be the same ampere rating as a single fuse? Many amplifiers (Marshall included) use a single 500mA HT fuse. Would dual HT fuses in the PT secondary both be 500mA or some other rating?

Let me toss this out again since it appears to have been lost in the shuffle. The Marshall 2204 has a single 500ma HT fuse located after the first capacitor bank and before the choke. I would like to instead use two HT fuses - one in each side of the PT secondary to protect the PT secondary in the event of shorted diodes or shorted first filter caps (not wishing to wait on the mains fuse to blow). In this scenario would I still use the same fuse rating for each fuse (500ma)? I can't find any specific info online about proper fuse ratings for this approach.

[sluckey]

I would build the amp with the two fuse holders. Put temporary jumpers across the fuse holders. Once you have the amp up and running, remove one jumper and place your amp meter across that fuseholder and measure the actual idle current. (Remember to set your meter to measure AC current.) Replace with a fuse twice the rating of the measured current. Repeat for the other fuse holder. Hopefully you'll end up with two same size fuses. For example, if you measure .2A, use a .4A fuse. Probably have to use a .5A fuse.

Carry spare fuses as well as some larger fuses in case you have some nuisance fuse blowing and need to 'tweak' the fuse rating.

[alerich]

That's a clever idea. That should work. Thank you, Steve.