additional fuses for Fender style circuits

[mikepukmel]

I noticed some Marshals have a fuse (size dep on ckt) between the HV center tap to ground. Reading up on some fender amp repairs in the repairs section of the blog, various smoked resistors, etc., is this something that we should add on new builds? Any other places that it would be good to fuse on the old AB763 era circuits?

[pdf64]

Marshalls are Fender type amps too!
I think that mod Marshall made to the 5F6A was definitely a good idea; if there's a tube rectifier then a suitable silicon diode in series with each of its anodes should be fitted whatever (or a fuse on each HT output, as per Merlin The Valve Wizard ).
I seem to remember that RG Keen advises all PT secondaries should be appropriately fused; automotive type blade fuses being perhaps the best choice for the high current, low voltage heater circuits.

[Justin Thomas]

Do a search for "immortal amp mods." Goes into all the protection stuff. The main reason for most of it is because the mains fuse s to protect your house from your amp, not your amp from itself. I only fused everything in one build; I don't necessarily add them to vintage amps unless they're already hacked. You know, no hole drilling.

The main reason we (should) fuse/diode everything is to protect the expensive parts of the amp - the trannies and speakers. The mains fuse MIGHT save your PT, but that'd be lucky.

Justin

[alexradium]

HV to ground is not good.
Better fuse both phases to rectifier.
Also fuse heaters,both rectifier and power tubes,bias optional.
Yes,check out RG Keen articles for PG,Immortal mods, and Valvewizard site for similar things.

[mikepukmel]

Thanks, all, for suggestions and links to Immortal Mods. Pretty good. Looking into sources for fuse clips I can mount in the chassis.

[cjenrick]

guitar players are known to prefer an onstage melt down over sudden dead silence from a blown fuse, well not really

but some people put in a bigger value fuse than he amp is meant for in order to have the show go on.

so you can add fuses to compensate for this, even placing some of them inside the chassis so they can not be tampered with.

also, tubes are not as good as they used to be, and amp makers are running them at higher and higher voltages these days, so more expensive components like transformers can be protected with additional fuses.
Vintage Ampeg amps often have two fuses in series, one on the outside and one on the inside just in case. I have seen this in Garnet amps also.

I find that the small fuses used for B+ lines and heater circuits do not seem to hold up as well as the standard AGC types, maybe it is the vibration, maybe it is because they are being run at 500 VDC instead of 120/240 AC like they were designed for, or maybe they do not like capacitor surge and peaks like you see in the charging cycle in a power supply, this is good for the repair business but bad for the customer. So I would say if you are going to add fuses, use the AGC types and use a fuse holder that will withstand high voltage.

[trobbins]

Adding a fuse to the secondary side is well worth it imho. However, like adding any part to a circuit, there is design required (unless you are following an existing competent design).

I use the linked techniques for designing up a fuse for a valve amp. I wouldn't call the design process simple, as it requires effort and appropriate awareness.
https://www.dalmura.com.au/static/Va...p%20fusing.pdf

[mikepukmel]

guitar players are known to prefer an onstage melt down over sudden dead silence from a blown fuse, well not really

but some people put in a bigger value fuse than he amp is meant for in order to have the show go on.

so you can add fuses to compensate for this, even placing some of them inside the chassis so they can not be tampered with.

also, tubes are not as good as they used to be, and amp makers are running them at higher and higher voltages these days, so more expensive components like transformers can be protected with additional fuses.
Vintage Ampeg amps often have two fuses in series, one on the outside and one on the inside just in case. I have seen this in Garnet amps also.

I find that the small fuses used for B+ lines and heater circuits do not seem to hold up as well as the standard AGC types, maybe it is the vibration, maybe it is because they are being run at 500 VDC instead of 120/240 AC like they were designed for, or maybe they do not like capacitor surge and peaks like you see in the charging cycle in a power supply, this is good for the repair business but bad for the customer. So I would say if you are going to add fuses, use the AGC types and use a fuse holder that will withstand high voltage.

Thanks for the tips. Oh yeah, back in the day, I remember finding a paper clip wrapped in aluminum foil in one fuse holder. That amp wasn't producing any sound at that time. Thanks for the tips on fuse holders and fuses. Not very easy to find the right fuse holders, for me anyway. Sifting through hundreds of items on the big electronics sites (Mouser, Digikey, etc). Most seem to be rated 120/240 vac or similar. A few I found by accident 300 range. But didn't see one 350. stil looking. Also, reading up on agc. ...

[mikepukmel]

Adding a fuse to the secondary side is well worth it imho. However, like adding any part to a circuit, there is design required (unless you are following an existing competent design).

I use the linked techniques for designing up a fuse for a valve amp. I wouldn't call the design process simple, as it requires effort and appropriate awareness.
https://www.dalmura.com.au/static/Va...p%20fusing.pdf

Thanks, that's a worry for me, since Im weak in electronics. The HV lines from teh PT are short and go right to the rectifier tube. No place to put an inline unless I re-route those wires, and I can't tell what kind of problems that might cause. THinking about taking the PT back out, mounting two fuse holders on the side of the chassis, then running the 350vac lines to the fuses there, then from teh holders to the rectifier socket. The other side of the PT is too crowded with other various signal wires, and that big bundle of wires from the OT and choke comes in there too. I don't want to send termites into that house!

Thanks for the link, reading . . .

[mikepukmel]

Great article, thanks again.

[nsubulysses]

how about these ? http://www.mouser.com/Search/Product...y576-05200004Z

[mikepukmel]

how about these ? http://www.mouser.com/Search/Product...y576-05200004Z

Thanks!!! Oh man I searched through so many parts, didn't come even close to that, Exactly what the Dr Ordered.

[Leo_Gnardo]

how about these ? http://www.mouser.com/Search/Product...y576-05200004Z

Nice find. 600V rating I expect is DC. I've used this sort of fuseholder for the typical 1/4" x 1 1/4" fuses, probably has same voltage rating but takes up a good deal more space.

[nsubulysses]

I'll have to read up on that fusing article posted by TRobbins.

I can't decide if these are the same two fuse holders. I guess not because the datasheet is different.

The mouser one says 600V on the mouser specs but when you click the datasheet it says 300V? also says dialectric strength is 1500V.
http://www.mouser.com/ds/2/240/520_M...LOCK-16036.pdf
http://www.mouser.com/Search/Product...y576-05200004Z

CE one says 300V and says dialectric strength 1500V
https://www.cedist.com/products/fuse...pe-full-molded

Understanding these specs exactly is a little beyond me. Anyone care to weigh in?

Or maybe Leo already did, "600V ratings I expect is DC."

[trobbins]

I looked at all the datasheets connected to those links - imho they would all be fine for your application.

A fuse holder is rated for a range of fuses - the datasheets identify standards compliance for parts up to 6.3A or 10A parts, where the fuse usually dissipates more power as the current rating goes up, so the capability of the clip end and the max temp are most stressful for 6.3-10A fuses - as you would be using a fuse <1A then the design margin is well in your favour.

The linked datasheets show particular compliance references for 250Vac and 300Vac ratings - the Mouser webpage reference to 600V is either a typo (as the datasheets make no such reference) or related to some other application code or compliance that is not identified. The reference to 1500V in the datasheet is the creepage/clearance capability of being able to apply a test voltage of 1500V min between the fuse and the chassis/earth (as is sometimes required when doing insulation resistance testing on a product using test equipment such as a 'megger').

As indicated in the fuse article (with a Littlefuse reference), for amp applications where the fuse is on the secondary side of a transformer, a 250Vac rated fuse can be fine for a much higher working voltage (such as 360Vac), especially as the fuse current rating will be low. Similarly, a 250Vac rated fuse holder would also be fine for that application.