Dumb questions: a list parading my ignorance^H^H^H^opportunity to learn

Snags

Following on from the Squeal, phut, hummm thread, the process of reviving my Tweaker 112 combo continues, and I need to plug additional gaps in my knowledge...

Fuses: sourcing

Best place to buy them? Presumably any electrical place that sells the right size (physical and rating)? Can't be anything special as none of the sites that sell valves seem to sell them in any meaningful way.

This is because the fuse is now a nice empty glass tube, without a filament going across it.

Fuses: operation

Which bit of the circuit does the internal fuse protect (not the one in the plug )? I'm guessing it fuses something between pre-amp and power-amp stages because at present I have no noise when plugged into Input or FX Return, but if I stick a tuner on the FX send that works. So signal is definitely getting in, through the pre-amp and out to the loop, but not coming through the power amp to the speaker.

Does that make sense, or does it indicate that something else might have gone bang?

Power Valves: which bits are critical, which are optional?

Do any of the suffixes on valve designations matter in terms of actually working safely/correctly? I get there might be differences in tone/break-up etc. but if it's a 6V6 can any other 6V6 be used?

Reason for asking is that the spec sheet says 6V6GTA.  I've bought a pair of 6V6GT as no-one listed GTA (and @ICBM said it would be OK, so it must be ). The valves I've taken out are, it transpires, 6V6-S.

I'm starting to assume that really I only need to care about it saying 6V6 - is that fair?

Valves: is there a visual way to check if they're blown?

The old valves don't look damaged in any way. One of them is possibly missing a tiny internal wire but it's not obvious.

Valves: should they rattle?

I would instinctively assume that this is bad, but one of the new valves, which came incredibly well packaged in a good-sized box wrapped in about 4" of bubble wrap in every dimension, does rattle slightly when (very gently!) shaken.

That hot burning smell ...

Before pulling the chassis apart to swap the valves and get at the fuse I did the diagnostics with input, fx send etc. and at one point got the horrible squeals briefly. On getting close to the back of the amp to dismantle it (more or less immediately afterwards) there was that familiar unwelcome hot electrical smell lingering, I think from the "Tweaker PT-15 power transformer". Hard to be 100% sure because it took a bit of time to get the back off and be able to sniff components individually (oo-er).

Is it likely (I know there's no way to tell for definite from a forum post) that the transformer might be futzed? If it is that's obviously well above my paygrade and it will need to go to a grown-up, but I don't want to shell out if I just need to whack a new fuse in. I also don't want to fuck up two new valves

Since re-assembly and testing with the new valves there has been no hot smell, and no squealing (no noise at all, in fact) but given the echoing void of a fuse that's probably not too surprising.

Old hands, feel free to mock the dumb-ass, but please do so in an educational mocking style

McToot

Winny_Pooh

One dumbarse to another until ICBM comes along:
Fuses, you need "slow blow", in the right rating and that's about it.

Fuses generally protect the power amp section/OT/valves in a well designed amp.

Generally you can swap all types of 6L6s etc however certain variants can take more voltage on the plates or bias a bit differently. Unless you have a special needs amp that runs very high voltage (like some ac30/matchless or musicman) on the power tubes you won't see too much difference although mesa for one "grade" their tubes so replacements are consistent.

Valves should ideally not rattle as that can be amplified by vibration in the chassis, less of an issue with power valves.

Hot smell could be a resistor or other part burning up, which is never great....

ICBM

Fuses: Ebay is actually as good as anywhere, dozens of sellers sell fuses in small quantities. (5 is plenty, if you blow more than that in testing there’s something much more serious wrong.)

It’s likely the blown internal fuse is an ‘HT’ - high “tension”, an archaic term for voltage - fuse, which protects the supply to the active part of the valve circuit. (Not the heaters, or filaments, which will still glow and are low voltage.) What’s strange is that normally that includes the preamp valves, but they appear to be working if you’re getting signal out of the FX send. I don’t have a schematic for the Tweaker but it’s possible it has an slightly unconventional fusing arrangement.

Valves: blown valves may or may not look visibly damaged. They can rattle slightly even when perfectly fine, but it is also often a sign that they might rattle or be otherwise microphonic when in the amp. This may or may not cause an audible problem.

Transformer: the Tweaker did have a known issue with transformer failures in early versions so that unfortunately can’t be ruled out. However, it would normally blow the amp’s internal main fuse, so it wouldn’t power up at all. But it isn’t impossible that a different fault has occurred - if it blows a new HT fuse when new power valves are installed this becomes more likely.

Try a new fuse first - they’re only pennies. To start, remove both power valves (leave the preamp valves in) and power up with the new fuse. If there’s a fault with the amp it will blow again, and then you definitely need professional assistance. If it doesn’t blow, fit both new power valves and try again. If it then blows it’s most likely there’s still a fault, although a damaged valve can’t be completely ruled out... but you still probably need someone who can test them.

Does that help?

Snags

Thanks @Winny_Pooh

I'd come to that conclusion on the fuses, although as the holder says "Fast Blow" I'll be putting a fast blow one in mine. Reading comprehension I'm generally OK with

When the fuses comes I'll power it up with the new valves in place. If it goes bang or the smoke gets out I'll take it to a grown up. Changing fuses, basic diagnosis, and swapping valves (armed with enough detail info for type-matching) I'm fine with. Changing transformers is above my pay-grade.

Snags

@ICBM thank you, that's great.

It would never have occured to me that it's OK to power up without the power valves in place!

It's currently got the new ones in, awaiting the arrival of a pack of spare fuses. I'll run it through that test cycle when they arrive, and if it does anything at all suspicious I'll take the whole lot (amp, old and new valves) to a local tech.