Transformer Failed? Thinking about a TAD Replacement?

martinw

ICBM said:

Can you fit a self-resetting thermal fuse?

Good idea, I think I will. RS have this one:

http://uk.rs-online.com/web/p/resettable-thermal-fuses/1769205/

It costs a bit more, but is still a lot cheaper than a new tranny.

I'd go for a lower temperature than 141 degrees though! That's pretty much a last-ditch backup and might be above the melt temperature of some of the plastics.

I think it is intended as a last line of defence, to prevent fire rather than prevent damage to the amp. I mean, many (most?) mains transformers in valve amps don't have them. The TAD replacement for the Classic 30 certainly doesn't! None of the transformers for other amps I buy off the shelf have them. Modern regs arse covering. The manufacturers put them in the prevent law suits, knowing that at worst they sell more spare parts, or in the case of PCs, control boards etc, complete units.

The other point is, how reliable are these thermal fuses? This one is nearly 20 years old, and has been subjected to countless heat cycles, vibration etc. Did it fail because the temperature went over 141 deg, or just because it was fatigued?

Why not put them on the outside so that they are replaceable?

martinw said:

jpfamps said:

Do you have a Megger to test transformers?

Yes.

Why did you ask?

jpfamps

martinw said

martinw said:

jpfamps said:

Do you have a Megger to test transformers?

Yes.

Why did you ask?

Just out of interest! I do like test gear.

jpfamps

martinw said:

ICBM said:

Can you fit a self-resetting thermal fuse?

Good idea, I think I will. RS have this one:

http://uk.rs-online.com/web/p/resettable-thermal-fuses/1769205/

It costs a bit more, but is still a lot cheaper than a new tranny.

I'd go for a lower temperature than 141 degrees though! That's pretty much a last-ditch backup and might be above the melt temperature of some of the plastics.

I think it is intended as a last line of defence, to prevent fire rather than prevent damage to the amp. I mean, many (most?) mains transformers in valve amps don't have them. The TAD replacement for the Classic 30 certainly doesn't! None of the transformers for other amps I buy off the shelf have them. Modern regs arse covering. The manufacturers put them in the prevent law suits, knowing that at worst they sell more spare parts, or in the case of PCs, control boards etc, complete units.

The other point is, how reliable are these thermal fuses? This one is nearly 20 years old, and has been subjected to countless heat cycles, vibration etc. Did it fail because the temperature went over 141 deg, or just because it was fatigued?

Why not put them on the outside so that they are replaceable?

martinw said:

jpfamps said:

Do you have a Megger to test transformers?

Yes.

Why did you ask?

Indeed the fuse is to prevent the amp catching fire and subsequent law suits.

Other amps do have thermal fuses in their mains transformers including most of the modern Fender valve amps.. The thermal fuse on Fender PTs has its own lead out so can be bypassed (its between the violet and black and red wire), although of course I don't recommend you do this.

For the fuse to be most effective it should actually be inside the windings, which of course makes to impossible to replace

ICBM

jpfamps said:

The thermal fuse on Fender PTs has its own lead out so can be bypassed (its between the violet and black and red wire), although of course I don't recommend you do this.

I'm glad you said that first and not me .

I have done this a few times on amps where the thermal fuse has gone but the amp was either something odd with a transformer that wasn't easy to get, or just not worth replacing it. In every case I've soak-tested the amp at full tilt for a full working day and never found a transformer that felt like it was dangerously hot, so I think in almost all cases it's down to fuse failure rather than a genuine overheating fault.

I haven't tried this with a C30 though...

martinw

jpfamps said:

For the fuse to be most effective it should actually be inside the windings, which of course makes to impossible to replace

Interestingly, the resettable one shown above states:

"P82 Series

High thermal sensitivity - unnecessary to mount directly in windings"

The u/s one I removed wasn't really 'in' the windings, just taped against them.

On balance I've decide that for my own peace of mind I will replace the customer's PT in this case.

Transtronic have them in stock ..... £83 incl VAT and carriage.

ecc83

I would guess that heat fuses are the result of Globalization? Some country somewhere insists upon them so we all suffer!

I would bet the argument is that if someone covered the amp, with a heavy coat say, then it could get hot enough to catch fire. I actually think this unlikely.

For an entirely different purpose "we" completely closed all the ventilation on a Stage 60 and was surprised to find that the internal temperature did not rise very much above normal, an extra 5dgrs IIRC.

In any case, transformer insulation should not support combustion? The result of the winding insulation melting and shorting should blow the mains fuse which is surely always required?

Wall rats and line lumps have heat fuses but then fusing those is impractical and they are very likely to get covered up.

Dave.

martinw

Further developments:

TAD have inspected the returned PT, but aren't able to define why it failed, so are returning it to their winder and will get back to me when they have an answer.

With regards to the other Classic 30 I have in for repair, I opted to replace the PT with a Transamp item (part of Transtronic in Chesterfield), and they have them in stock, so I was able to fit that the next day. As expected, the stack is a fair bit bigger than the original, and it runs a good bit cooler. After 6 hours on the bench, it was just comfortably warm to touch.

The voltages seems slightly lower than the original, probably because it's designed for 240V (my wall voltage is usually around 242). The amp biases up correctly, and when I checked the aux rail voltages (as per @ICBM ) they were spot on with the 180r resistor in R67. In fact by then, I'd jumped the gun while the board was out and fitted a 1k, but this brought the voltages down too low (although the amp still functioned) so I substituted a 220r and the rails are now at 28.5V and 14.5V approx.

I took the board out to fit 1k/5W screen resistors all round to hopefully guard against further strain on the PT.

Fingers crossed this should now be a fully reliable amp. It certainly runs a lot cooler, without the need for a thermal fuse!

DJH83004

Good spot regarding Trans-Amp, I have dealt with their parent company Transtronic in the past with some industrial gear (they are just up the road) and they are very professional. did you just specify the amp make / model or did you have to give them more specific info (I am curious because I can't see it listed on their website so wondered if it was a custom build to your spec).

 

martinw

DJH83004 said:

Good spot regarding Trans-Amp, I have dealt with their parent company Transtronic in the past with some industrial gear (they are just up the road) and they are very professional. did you just specify the amp make / model or did you have to give them more specific info (I am curious because I can't see it listed on their website so wondered if it was a custom build to your spec).

 

It's on their TransAmp website:

http://www.trans-amp.co.uk/products.html?page=3

martinw

Just to wrap this up, I received and fitted a replacement mains transformer last week from TAD. They never got a  reply from their transformer winder, so decided to send  a replacement free of charge. They are to be congratulated for that, even though it would have been nice to get this sensible treatment without all the hassle.

Anyway, I've fitted it and done some tests. The secondary voltages are all on the high side, but with some adjustment to the bias circuit, and the 'R67 1K ICBM' mod for the +/-15 supply, all is well.

I sent TAD a set of voltage readings that I took, before and after the tweaks, and they were grateful and say that they will put a 240V tap on the next batch for UK customers! Yay. Harmony restored.

(The Transtronic PT didn't need any tweaks to the secondary voltages/bias, as clearly it has been designed for UK voltages from the outset, whereas the Peavey and current TAD PTs are obviously set up for 230V).

There you go