45 SET

[Nick]

Must resist...

[andrew Ivimey]

Futile....

[Ali Tait]

No right one Simon.

That was Naked Gun. Just saying.

Well, you know what my memory is like.

[RhythMick]

No right one Simon.

That was Naked Gun. Just saying.

That clip still makes me spit my tea...

[simon]

Even such a simple amp has lots of bits. It's a Bugle - AC filaments and heaters, it's a long time since I've used AC filaments, hopefully it'll be quiet enough...

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But first I need to debug it...

[simon]

Right. With AC disconnected from the 45 filament trafos and the GZ33 removed the 5751 heaters came up fine.

I added the GZ33 and the 5751 heaters and the HT came up fine.

Then I connected AC to the 45 filament trafos but with no 45s inserted and there was a proper flash, bang and a trace of magic smoke from the fuse holder in the IEC socket.

I've not seen this before and it looks to have damaged the fuse contacts in the socket. Quite impressive really.

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This was a 2A glass bodied slow blow.

Continuity of the 45 filament trafos looks okay - but whilst writing this post I've realised I've wired the primaries and secondaries the wrong way round... which would do it.

So, a break first I think to clear my head, then find another IEC inlet and reconnect the filament trafos.

[andrew Ivimey]

Couldn't happen to me oh no - not even the slightest chance of something so careless and sloppy....nah, utterly impossible but ....wait a minute....what's that!?


Only kidding. Such tomfoolery just doesn't couldnt never ever ever happen in Bedford Towers - no sir eeeeee bob

[Nick]

I've wired the primaries and secondaries the wrong way round

I too have done that once.

[Mike H]

[simon]

There is a lesson here, something about not doing stuff whilst under the weather...

But thanks for your support chaps.

I'm wondering now if I might have taken the cathode resistors out, and the two resistors between the filament pins with the top of the cathode resistor at the middle to give a centre tap. Their voltage rating would no doubt have been exceeded, but it would only have been instantaneous. And hopefully I haven't shagged not the 45s...

[Nick]

Its unlikely, don't know the values of the R's but assuming 45R, then maybe a voltage rating of 300v, that would need 6.6A, to get that current in the secondary would need 600A in the primary (assuming a 2.5:230 ratio) so I think the fuse would have done its job. And by the same reason, those R's would have prevented the secondary voltages getting to the point where the transformer would be damaged. I am optimistic.

[simon]

I think you're referring to the two resistors across the filaments? If so, I was thinking that 240V up the 2.5V secondary might give 2400V across two half W resistors. I can't remember what the values are now, but something smallish like you say. I prefer your optimism

[Nick]

Yep, but that's assuming that:

1. The voltage on the primary got to peak voltage before the fuse blew, its more likely that the inrush current took the fuse out before it got that high.

2. The power rating is based on actual dissipation of power, so there is a time element as well. The voltage would not be there for long enough for the resistor to start to heat up which is why I worried about the voltage rating of the resistor instead of the power rating.

The only thing I would worry about is I have seen metal film power resistors sometimes be killed by inrush into capacitors when between capacitors, they go open circuit, whereas an actual wire wound one is fine in the same situation. But that's simple to check with a ohm meter. But again, I would doubt the current would be high enough as it will be stepped down by the same ratio as the voltage is stepped up and the internal resistance of the secondary is likely to be the limiting factor there. BTW, I know there are pulse rating on MF resistors for that reason.

It may be optimism, but its not blind optimism.

[simon]

Yes, I'm hoping that it was all instantaneous and the fuse blew before much could happen. I will have a poke around tomorrow with fresher eyes. I'm just back from the hospital so probably not a great idea now.

They're Takman carbon films, and the cathode resistor is a Mills wirewound BTW.

[simon]

I couldn't help myself, the resistors all check out - 42R for the centre tap resistors and 1k5 for the cathode resistors. Both 45 filaments have a little resistance too. So far so good.