Cornford Hurriance blown tx again

[blindboybenton]

Hi

I had a cornford Hurricane on my bench last week. With a smoking mains tx. Ordered a new one from the maker here in the uk. Fitted it and anew set of Sovteks EL84. Working a treat,
Dropped it off to the customer. He liked it the said " It aint sounded so good"
He called me 12 hrs later saying the tx was smoking again and its dead

The Original tx had secondary s/c. It looks like the same thing has happened again

The replacement was exactly the same inc the same voltages/colour wires etc, The New Tubes were checked on my TAD Bias meter and both reading 25ma as were the 2 yr old tubes

The question is " Is this an issue with these cornford hurricane amps " Anybody had the same issue before?

Ive been in touch with the tx maker in the uk and he wants to see both of them.But i cant get the amp till weekend .. I think i have been unlucky ?? any thoughts

BBB

[pdf64]

It's cropped a time or two before, eg http://music-electronics-forum.com/t30046/
Pete

[bob p]

too bad I can't read Spanish.

I have no experience with the Sovtek EL84. Do they have a high infant death rate?

At this point i'd be trying to sort out whether the amp just died because a set of new tubes failed (bad luck) or if it died because there's a problem that remained in the amp that wasn't fixed the first time. If that's the case then maybe you fried the 2nd PT for the same reason you fried the first one.

[Chuck H]

Looking around it seems that PT's blowing isn't rare for these amps. I read on one forum that they are wound by a small magnetics shop to fit in tight quarters. Maybe wire gauge is iffy? Or insulation thickness? Spacing? There is a Hurricane on Ebay right now that has a Mercury PT in it (no doubt the original needed replacing). So it seems that Mercury makes an appropriate PT for that amp. Since Mercury has a good reputation I might trust their product over the OEM part. Mercury is typically expensive. But in this case I wouldn't be surprised if their part costs less than the stock part.

I'd also back Bob re: what caused the first two to blow? Obviously they worked and tested properly as far as you could tell. Sans some circuit problem, if the owner likes to crank it up in volume and gain, voltages sagging and spiking, etc... Who knows? If the PT is the weak link, cranking the amp and really pouring the coal to it is a good way to blow it up.

[bob p]

With two failed PT, I'd start adding fuses.

The Immortal Amp Mods ? Pt. 2 - Premier Guitar

[blindboybenton]

Hi All

Many thanks for the info. The amp was working well i test both sets of tubes all seemed to be good. But as the original set were over 2 yrs old i recmd that i change the power tubes. I dont think its the power tubes. I spoke to the firm that made the tx's in the uk for Cornford. They said they made 1000's and didnt have any trouble. I will be sending both back to them. I was thinking of adding a fuse to protect the PT. I tested it didnt seem to have any issues. I didnt turn it up. The amp is very clean, nothing looked stressed out ( Apart from Me) All the usual checks. Ill keep you all informed

BBB

[J M Fahey]

Modern trend which I *hate* is to wind transformers with "self soldering wire" which means it has a special kind of enamel that **evaporates** without residue (no kidding) at way less than 270ºC (solder temperature) and to boot acts like some kind of flux, so soldering is easy and quick.

BIG problem is that said enamel is too thin (much lower breakdown voltage) , physically weak, and on any thermal overload, any "hot spot" deep inside a winding evaporates it and some turns become shorted.
Once some turns are shorted, the transformer Nukes itself.

That enamel does have some legit applications, such as in pickups, where hair thin wires are almost impossible to scratch and solder without damage, Radio coils, etc; and in very cheap disposable transformers , such as small Wall Warts, where saving a few seconds per soldered wire is an important part of the cost, but it has no place inside a MI amplifier , much less in a tubed one, hot and with very high voltages, and even less in an expensive "boutique" class amplifier.

Yet I have found that wire in JCM900 OT and Mesa Boogie Triple Rectifiers.
They are CRAZY .
So it's not surprising to hear what you say.
Old transformers used high temperature anamel, personally I wind mine with *electric motor* rated wire, both because the 180ºC *guaranteed* prformance, and because they are expected to be subject to high G forces, vibration, dust, high humidity, etc.

Cost?: about the same as the "self soldering" type, the real "cost" is that it's a PITA to strip it, usually means burning the wire red hot (the thin one with a lighter, the thick one requires a plumber's propane torch, a gas stove burner is very slow) and then applying some emery cloth ... but I'm very comfortable with that.

I *hope* that MM uses the high temperature wire.

The evaporating enamel is usually dyed bright cherry or green to avoid confusing it with the "strong" type which is brown , but lately the weak one is coming in transparent enamel.

[blindboybenton]

Thank you JMF

Now ive got a better insight to tx making. The Firm that made the tx have been doing it since 1948, So im hoping they know what they are doing. I did notice that the replacement tx had thinner wire from both primary and secondary, When i spoke to them they told me that cornford got them to reduce the gauge to allow the wire to be feed though the chassis better ??. Im picking up the amp tomorrow for investigation. Then will wait to see what the tx maker says.

Going to add a HT Fuse and monitor the current draw on a dummy load If the tx maker says its not the fault of his tx. I will get another one from a different supplier, The amp inside looked clean. Biasing was ok @ 25ma on both tubes. I did all the usual checked on resistors. So im stumped !!

Many thanks to you all for your help on this and every post i put up

Blind Boy Benton

[J M Fahey]

Fine.
Hope you solve it this time and forever.
Not sure whether you refer to thinner wire on the actual enamelled copper winding or the external connection flying leads.
The first is bad; the second is no big deal.

Just ask the TX makers "I *hope* you are using high temperature epoxy enamelled wire and not "modern" "self soldering/stripping" wire .
To make it clear tell them you refer to the wire where the enamel evaporates to simplify soldering.

If they tell you "it's the modern way" ... ugh !!!
FWIW my own supplier thinks me slightly nutty when I order "motor winding" wire and tells me "everybody else uses "transformer winding" type" .... which nowadays means the weaker type.

Problem is, most popular transformers expect 120V at the primary , gentle 50/60 Hz sinewaves, and lower voltage secondaries.

A tube OPT sits at 400/500V *at idle*, gets twice that end to end with "clean" playing and when overdriven into a crazy indictive load as a speaker those 1000V squarewaves can easily have ringing or peaks beyond 1500V .
Much worse if you forget to connect the speaker.

That's why old style warriors often survive playing without a speaker for a few seconds (until you notice something is wrong) and modern ones often do not.

Of course, who would be so crazy as to overdrive and distort a tube guitar amp ?

[Mick Bailey]

At one time I only saw burned out transformers as a result of an identifiable fault condition. Nowadays it's often the only component to fail and this probably explains why. I see more than my fair share of transformers with shorted turns. A gripe in the UK is that the universal transformers are 2 x 115v and our supply is 240v - in some areas marginally higher.

Also, just look at how small some of these OPTs are, such as the matchbox-sized ones in the Marshall 20/20 or Laney VC30. No wonder they go pop all by themselves.

[blindboybenton]

Hi Mick

Yes i have to agree.. Why is this tx got a 110v winding when made in the uk for a uk version of the hurricane. The tx is fairly small and rated at 130ma. Ive started a new tread on this one looking for some advise and help from you more experienced valve amp techs.... Posted the tread below

http://music-electronics-forum.com/t33183/


BBB