Amp PT troubles

hywelg

My Ceriatone OTS started blowing mains fuses a couple of weeks back so I brought it home from the rehearsal studio and got it on the bench yesterday.

Rigged up a light bulb limiter as I was running out of fuses (2a slo blow) I've put a 60 w bulb in the line, it glows brightly when I flip the power switch ( I have tried a 100w bulb and being a clear one it started smoking internally so I swapped back, my wife will be furious if I kill one of the 100watters, they are getting difficult to find now!). So I proceeded to disconnect all the primaries to make sure there was nothing wrong with the various amp circuits. I have lifted all the earths to the secondaries too. I am left with the 240v primary, the 0v primary and the earth connection still connected that is all and still the bulb lights up brightly. I have also swapped the 240v primary for the 220v primary , still the same.

I have measured the resistances of the primaries and the 0-240v reads 7R2 and all the others read logically, nothing seems completely shorted. OC from all of them to earth.

I cannot see how this can be anything other than a transformer problem fuses blow within 1 second without the limiter, other than if the Carling mains switch was somehow dumping AC to earth through the body , but that would trip my RCD I would think (it is quite sensitive) .

ICBM

You are correct. If you've disconnected everything except the primary winding and it still pulls anything like that current, it's dead.

The primary winding may meter more or less correctly (although 7R2 is a little low for most I can think of), but if it has even a single shorted turn (or one of the others does - you still won't be able to measure the drop in resistance) it will act as an extremely low impedance load when you put AC through it.

The 100W bulb won't come to any harm even if the amp is a dead short by the way - it's not under any more stress than it would be connected directly to the mains.

hywelg

No but these Chinese bulbs are crap and if I kill it I will be in deep dudu!! We seem to have loads of 60watters but only 1 100watt.

Thanks, I couldn't understand why it would pull so much current if it was still reading logically. Nik did suggest 7R2 wasn't far off standard.

ICBM

Basically if you have a transformer winding with 1,000 turns and one is shorted to the next, the DC resistance will only fall by 0.1% so you'll never measure the difference - but the single shorted turn will still produce a near-zero-impedance load, so even if it's in a secondary winding (it usually is, almost always the HT) it will still draw a huge current in the primary.

The bulb-limiter test with nothing else connected to the transformer is conclusive unfortunately.

martinw

I've had a quite a few mains transformer failures on Ceriatones.

Ossyrocks

martinw said:

I've had a quite a few mains transformer failures on Ceriatones.

The OTS PT's are vulnerable to failure. I've been in discussion with Martin Garton recently concerning one he had in for repair. Dead PT. We really didn't see any other option than to order another one from Ceriatone, as they are quite unique, 5v relays taps and all.

Nik quoted me $154 for a 50w PT inc shipping. Seems very reasonable, but I might have been quoted at discount as I have bought a lot of stuff from him.

Rob.

ICBM

Would a Mesa Dual Rectifier PT work? That has a 5V winding for the rectifier valve filaments.

It won't be cheap, but at least it's unlikely to fail again.

hywelg

I've found a number of replacements. Not all exact but not far off.

http://www.inmadout.com/download/pdf/TA34M50.pdf

https://www.edcorusa.com/xpwr173

My Ceriatone original has a 6v tap rather than a 5v , and the relays are run off this. I'm guessing that the 1v drop won't be significant. Its going through a a dedicated rectifier and a 7812 voltage regulator. A 100v bias tap rather than 60 will just be a resistor swap?

WOndering if there's any mileage in swapping the OT at the same time for something better? Not considering Mercury BTW.

RiftAmps

100w bulbs are still easy to find, any electrical wholesaler will stock them. They're called 'Rough Service' or similar.

exocet

There's also the Heyboer (Mojotone) option. MOJO762EX which is a bassman PT with Euro primary.
The Edcor option appears to have a German retailer?

https://www.don-audio.com/Edcor-XPWR173-Power-transformer

ecc83

The acid test for any mains traff regardless of purpose is the primary magnetizing current. This is remarkably constant for all transformers of around 100VA and even larger and smaller ones at 70-80mA iirc. Best way I find to measure mains current is a 1 Ohm 10W in the neutral circuit and clip DVM across that.

This is of course with no secondary loads and mains at 240ish volts. The MC rises very sharply as you exceed about 260V!

OPTs can be checked for a shorted turn by feeding heater volts into the speaker winding. Bit of maths will tell you what the anode to anode voltage SHOULD be, watch pinkies!

By association, if of an experimental bent, you can use a low voltage secondary mains traff as an OPT if it has a 115-115 primary.

Dave.

normula1

If a new one is very expensive, you could always getting your original PT rewound.

These guys did an output transformer of mine.

hywelg

It's not of significant value to consider a rewind. I would expect a rewind to be more expensive than a Hammond. And it doesn't seem to be too difficult to find something that's nearly correct. I might need to add a relay transformer but that's no big deal.

Still trying to figure out the correct spec. I know the voltages but not the required current draw.

exocet

Bassman Power Tx is correct but lacks the additional 5 volt tap to drive the relays so separate Tx required for that unless you buy the original from Ceriatone.....or the hugely expensive Mercury version.

hywelg

I'm going to see whether I can get a correct spec one first. But the bassman will be a possible with an extra 6v tyranny for the relays. Good to have options

hywelg

After much fruitless searching and laughing at shipping prices from the USA I ended up getting a Hammond 291FEX from Bluebell Audio, on the basis its a drop in fit (Bassman was too small, this one is for a Twin Reverb) and much bigger iron that the original (which i could probably have had rewound).

Not an exact spec, 320-0-320 instead of 345-0-345, and no spare filament tap to run the relays. So had to get a 266JB6 filament transformer to run them, Just room to squeeze it in next to the the PT.

Now a question for anyone who might know. I need to create an artificial centre tap for the heater circuit since the PT does not have one. Does anyone know where you can get 100R 1W 1% resistors without paying an arm and a leg in postage. Mouser have them, but under £33 its £12 postage!! I will get some 3W metal film R's at the same time as I might have to adjust the dropping string due to the lower HT, but still not 33 quids worth! Unless anyone has some good ideas for what I might usefully need and buy from Mouser?

4114Effects

http://www.ebay.co.uk/itm/10x-100-ohm-100R-1W-1-Watt-Metal-Film-Resistor-/200654817736?hash=item2eb7f589c8:g:lbMAAOxyj4hTJHXn

Same seller does 2 and 5 watt resistors.

ICBM

Why do you need 1% resistors for a virtual centre tap? Any 100R 1/2W resistors will be fine.

If fact you don't want to go any heavier on the power rating either - if a valve shorts to the filament, even fairly high-power resistors will very likely burn out even with an HT fuse fitted, and the larger the resistor the larger the mess it makes. I don't like using resistors as fuses - if you need a fuse, use a fuse - but in this case it's the best way to avoid the most damage.

(A typical HT fuse in a 100W amp will be 1A, so before the fuse blows the two 100-ohm resistors - 50 ohms between the filament and ground - will try to dissipate 50W, or 25W each… so 3Ws will end up just as dead as 1/2Ws.)

hywelg

Only going by what is suggested on TAG. So would ½W Carbon be OK in that position? I have plenty of them. Or better to go metal film?

martinw

I've always used standard 1/2W or 1W carbon film.

ICBM

It doesn't matter, 1/2W carbon is fine.

It's one of the few cases where it's best not to over-spec a resistor. Its also worth not fitting them "too well" - ie wrapping the leads through the terminals etc - since there's a fair chance you may have to change them one day.