Old rf valve amplifier

[jazzy]

Hello
I am repairing and overhauling an old bremi valve amplifier, I was wondering why the fuse is on the negative side of the input to the transformer?
Could this be as a safety precaution? ie someone connecting up the plug the wrong way round, poss live chassis earth?
Thanks

[Skywave]

The only thing that I can think of is that this reduces the insulation requirements of the fuse & its holder, since the potential difference between mains supply "N" and earth is low, compared to mains supply "L" and earth.

Al. / Skywave.

[jazzy]

Thanks for getting back to me,
Would there be any benefits of me swapping these around or adding a fuse to both l and n ?

[Nickthedentist]

As Al says, this way means that the fuse holder should not give someone a shock if they were daft enough to try to change a fuse with the machine still plugged in and the mains really was the right way round.

A fuse in both poles is frowned on nowadays, as if the neutral fuse goes, the set would stop working but current could still flow from neutral to earth.

Perhaps the best thing would be to re-wire it. But if that spoils the unit's originality too much, you could always short out the existing fuse, and merely use a suitable fuse in the mains plug (1A is readily available).

There is no right or wrong here

Nick.

[jazzy]

Thanks Nick
I think i will swap them around (best thing) this will not spoil the unit's originality, What i an trying to do is to make this amp as safe as i can,as it has 5 el519 valve and a voltage doubling circuit.
What i want to do fit is a fuse on the ht side, i was thinking a 1 amp fuse ? and may be a choke on the antenna out put just in case the capacitor failed.
Thanks

[Nickthedentist]

Should have said...

Originally Posted by Nickthedentist

A fuse in both poles is frowned on nowadays, as if the neutral fuse goes, the set would stop working but current could still flow from live to earth.

[Skywave]

Just for what it's worth, my take on a case such as this is to assign safety over originality.

1. Input mains supply to a double-pole, double-throw switch.
2. One fuse in the mains a.c. input feed - in the "L" wiring.
3. Input mains wiring "E" to chassis.
4. If a bi-phase FW rectifier cct. is used, I fit a delay fuse in the center-tap of the transformer secondary to ground / chassis.
5. All mains wiring to the blue, brown, green/yellow standard colour scheme.

Which leads me on to one of my 'pet issues' on mains wiring in equipment.

There is a school of thought that claims 1 and 2 above should be the other way round - input mains to fuse first. That way, it is claimed, the mains switch is protected by the fuse.
My take on this approach is that even if the chassis-mount fuse holder is wired correctly (input mains to the rear of the fuse carrier), in the event of someone removing the fuse with the unit plugged in and the DPDT on/off switch thrown to OFF, the user could still get a shock from an unblown fuse. (Yes, this has happened to me! )
If, however, the DPDT switch comes first, the chances are that the user will remember to switch the unit OFF on this switch - even if he forgets (or is unable) to disconnect the a.c. mains source - thus making fuse removal a safe operation.

In the method of 'switch-before-fuse', a suitably chosen fuse in the 13-Amp mains plug should provide fuse protection for the DPDT on/off switch.

It's been my experience that this method is common in the U.S.A., whereas the latter seems to be preferred in the U.K.

Al. / Skywave.

[glowinganode]

I would tend to agree with skywave here. DPDT switch first, then panel fuse (fed at rear) in Live feed to primary. This allows the fuse to be changed whilst isolated. This will need to be antisurge.
On the secondary side, I would recommend fusing the transformer secondary, as this is the part you want to protect. Again this will need to be antisurge , especially in a voltage doubler. This will need to be well insulated, I wouldn't advise fitting it in the earthy end as secondary could break down with an o/c fuse. I've used a 5W 22 ohm wirewound resistor as a protective element in a similar circuit, in the event of a rectifier failure the overload quickly burns out the resistor. Needs to be mounted with plenty of clearance mind. Also reduces the strain on the rectifier diodes, reducing the chance of failure in the first place.
Definately fit 2.5mH choke across output in case of coupling cap failure.
Regards, Rob.

[AlanBeckett]

I usually agree with Al (Skywave) but not this time
A not unusual failure mode of (mechanical) switches during overload is that the contacts weld together. This can result in a situation wher you think you've switched the unit off whereas it's actually still connected. I'm afraid that the only safe mechanism is disconnection from the mains while changeing the fuse.
On the subject of secondary fusing I've never really understood the logic of a fuse in the centre tap of a transformer, although I've seen it used lots of times. If the fuse blows, either due to overload or old age, the full no-load voltage will appear across the secondary. While a military grade transformers may well stand that, and are often tested under those conditions, I have grave doubts whether the average 'domestic' grade transformer will.
Alan

[kalee20]

Originally Posted by AlanBeckett

A not unusual failure mode of (mechanical) switches during overload is that the contacts weld together. This can result in a situation wher you think you've switched the unit off whereas it's actually still connected.

And this is very likely to have happened if the fuse has just been zapped by a fault.

For me, definitely input lead:fuse:switch:equipment (so the fuse protects against breakdowns in the switch as well as the equipment).

HT secondary fuses are a bit of a compromise whatever - as Alan says, with fuse in centre-tap to chassis lead, the full secondary voltage appears anode-anode even when the fuse has blown. And, the centre-tap voltage will swing wildly below chassis. It's a question of weighing up whether you risk burning out the transformer and rectifier with an HT overload or reservoir capacitor short-circuit, or whether you protect the rectifier but risk the transformer failing due to insulation breakdown. But then if the transformer is that close to breaking down anyway, it's probably going to fail pretty soon.

[beamcurrent]

quick comment on fuseholders and accidental contact, current spec panel fuse holders are made so that the fuse contacts are protected until the fuse is clear of the contact and you should not be able to get a shock.

Having had a mains side fuse fail to protect a transformer ( it had to high a value to blow) choose the fuse rating with care, a slow blow fuse of just high enought value not to blow.

regards.

[Skywave]

Originally Posted by AlanBeckett

I usually agree with Al (Skywave) but not this time
A not unusual failure mode of (mechanical) switches during overload is that the contacts weld together. This can result in a situation where you think you've switched the unit off, whereas it's actually still connected. I'm afraid that the only safe mechanism is disconnection from the mains while changing the fuse.
Alan

Presumably, you mean disconnecting from the mains PRIOR to changing the fuse! (Highlight above by me; not in original Post)

Within the limitation of referring to valve-equipped radios only, I have indeed met this situation on a couple of occasions. However, in those cases, the mains switch was the conventional toggle DPDT with a metal dolly (you know the type ) and the dolly would not 'go up' to the off position on account of the contacts being welded inside the switch. Of course, you cannot rely on this 'fail-safe' condition always occuring, & I do accept Alan Beckett's (& Kalee20's) point.
If it is a piece of kit you are building / modifying / reparing for yourself, I suppose it's a question of what you feel comfortable with: either involves some risk.

Finally, there is always the very simple expedient - which I have seen on some USA-manufactured kit - of attaching a label near the fuse on the cabinet back that states "Caution! Fuse before switch" - or vice-versa.

Al. / Skywave.

[AlanBeckett]

Al,
I admit defeat - your semantics are better than mine
I suppose I could/should have said '... is to disconnect from the mains ....'

Alan

[jazzy]

Well i was told that the wiring loom had been cut away in this amp as it had burnt out,When i checked it i found that all 8 of the 1n4004/1n4007 was shorting out.
I have replaced these with bigger 1n5408 now, the transformer seems to be ok when i checked it (not under load) 740v on one feed and 516v on other,this is straight from the transformer.

[Skywave]

Just a quick thought, Jazzy . . .

Assuming that there are two strings of series-connected diodes departing from each end of the sec'y. of the mains transformer, are there equalising resistors across each diode? Also, if there isn't an RC snubber network on the transformer's pri. (or sec.), it would be a good idea to fit one.

Assuming that the diodes' PIV's were adequate and that there were enough of them, and apart from serious shorts or overloads on the diodes, the omission of these two preventative measures (equalising Rs and RC network) are a common cause of diode failures where there are strings of them in series.

(If you are aware of all that - some people aren't - then please accept my apologies for presuming of your ignorance!)

HTH

Al. / Skywave.

[glowinganode]

Hi all, sounds like an old CB linear (BRL500?). There's a diagram for a similar sounding amp here;
http://www.rmitaly.com/download/manu...al_rel_400.pdf
Certainly sounds like it has suffered a catastrophic rectifier failure due to an overload of one form or another. I'd recommend using Semikron Ska 3/17 rectifiers, these are the only one's that survived my Vacuum Tube Tesla coil antics. Rugged device, well worth 72pence. You should be ok using one of these to replace a pair of paralleled 1n4007's (asking for trouble) and certainly fit sharing resistors to even out the PIV's. 1n5408 still has only 1000v PIV rating, I think 3 in series would be a safer bet than 4 in series/par. Check and reform your filtercaps and check valves for gas and emission. I would disconnect the heater supply while you sort the power supply, and connect a lamp bulb or two (in parallel) in series with primary to limit the current. Don't let the primary voltage exceed 180v for any length of time with no load. HT could be up to 1500v so be careful, make sure you have -ve bias as well.
Best of luck, Rob.