"Manufacturer bypassed" fuses - WTF?

[Robert M. Martinelli]

It's true every day brings something new to be seen!

Just opened an old PV from the glorious 80s, it's an ARTIST VT Hybrid ( SS/op-amps pre, plus a 6L6 P/P output stage ) belonging to a friend which is a guitar teacher and studio artist.

He complained about the amp stopping working and smelling considerably ( veeery baaad sign ).

After opening the amp, much to my surprise, I found all the PCB fuse holders on the PS board have been bypassed with a jumper - and it looks like the amp's been built that way - the jumpers are located on the components' side UNDER THE FUSE HOLDERS, all the solder joints appear to be untouched/original, and the jumpers are ink-stamped ( in white ) on the PCB as well!

To cut a long story short, the ONLY fuse left to protect the amp is an external ( mains ) fuse, which didn't blow immediately ( being a 3,15 slo-blo, matching the type and rating recommended by PV ) leaving time for the power amp PCB to literally burn ( I found arcing signs on the PCB in the output tube area ).

Needless to say, I' ll fix the amp, remove the f***ing jumpers and measure the currents across the fuse holders to properly size the fuses to throw in on each power supply rail....

I was just curious, and wondering if someone's stumbled upon something crazy like this in the past, especially someone with considerable experience on PV amps, like Enzo, who is an authorized PV repair center, if memory serves me well, and has probably repaired thousands of PV amps.

Am I going nuts, or did some design engineer at PV have his/her brain disconnected when developing this amp?

I'm all ears!

Cheers

Bob

[Enzo]

You will find many products like that. Every place on earth has its own idea what the electrical codes should be. I see things imported from Japan that have wires where fuse should have been . SOme places require secondary fuses on everything. Not the USA.

If you read the schematic for your amp, the fuse chart clearly shows that the internal fuses are not used in domestic versions of the amp. The export version should have them. But if the amp were going to an export destination where secondary fuses were not required, they might well have deleted them. I wonder if the amp you have started out as a US amp.

They install the jumper wires and the clips. To convert to export, just clip the wires.

No one beefs that there are not all those internal fuses in a Fender Twin. Only one fuse on that.

What was the last amp you saw with a fuse in the bias supply?

Remember, fuses don;t protect the amp, they protect your house from catching fire.

But however the amp got there, if you local codes demand fusing, clip the jumpers and slap fuses in there. The values are in the schematic, no need to calculate them. They are not cheap ones either - 1/4A slow and 3/4A slow.

[Robert M. Martinelli]

Hi Enzo, and thanks for the lightning-fast reply!

I agree with you, every place has its rules and regulations, ( more or less stringent ), but I think putting fuses in the right places is ( or should I say "should be" ) a matter of common sense.....I would have protected at least the +B rail, and I really think that the amp came very close to catch fire.

I'd like to highlight that the arcing could have occurred in a "domestic" amp of the same type operated within the US as well, and this indicates the design is not "safe and sound" IMHO.

Maybe I'm just "biased" by my background and I worry too much ( or maybe I'm just turning into an old fart complaining about everything ).

Back to the issue, at this point, I think this amp is a result of a mistake made at the factory, 'cause it' s labeled "240VAC" ( and this should qualify it as an "export" model ) but the jumpers are still there ( like in a standard "domestic" model ), so I think someone at the factory simply forgot to remove the jumpers and throw the fuses in.

Thanks again

Best regards

Bob

edit : In this amp there's no 6,3VAC ( heaters' supply ) fuse either, the PT sec wires are directly soldered on the power amp PCB.

[Steve Conner]

If I remember right, it's Canada that requires a fuse in every PT winding. The rest of the world is less paranoid.

I usually fuse the HT and the bias winding, nothing else. The bias winding can be protected with a fusible resistor, it doesn't really need a fuse. My thinking behind this is:

Fuses in heater lines drop voltage. Also they may get loose in their sockets, dropping even more voltage. And tube heaters have a huge turn-on surge that would make fuse selection difficult. There's only so much room in an amp for fuses anyway. If I had to fuse a heater line, I'd probably use those blade fuses for cars. (If your heater winding has a grounded center tap you need 2 fuses: one more reason to use the 100 ohm resistors instead.)

The HT winding is made of fairly thin wire, especially in amps that had tube rectifiers, so a short may not blow the mains fuse before the winding burns. Especially if the short happens after a tube rectifier. Having a fuse in the HT gives more of a chance of saving the transformer. Though as Enzo always says this isn't in any way guaranteed.

The bias winding is usually made of the same gauge wire as the HT, but has a lower voltage. So if it gets shorted, it will definitely burn itself out instead of blowing the mains fuse.

[Robert M. Martinelli]

Hi Steve,
thanks for chiming in!

Well, apart from this particular specimen, which clearly is the result of a mistake of some sort at the factory, I'd say I prefer to have fuses on each power supply rail anyway; even if I am aware they're not an all-curing medicine, they surely add some peace of mind to my paranoid brain, and can help in saving huge amounts of $$$$.....oops, €€€€ ( ££££ in your case ) should something go wrong.

( I've got a paranoid feeling someone is reading/will read this post - are they spying on me? Leave me alone! )

Cheers

Bob