What causes the HT fuse to blow?

rico

Set up my 2204 last night at a gig and came to sound check and some strange noises and then silence. Bugger. 

The HT fuse is fried and I didn’t have a spare but luckily I had the DSL401 as a backup so used that instead. 

My question is what would cause the HT fuse to blow? I used the 2204 a few hours before the gig and it was singing beautifully. The only thing I can possibly think of as a factor was a kettle lead in the back of the cab and the earth pin on the plug shorting the speaker connections?!

ICBM

Usually a blown power valve.

The mains plug contacting the speaker terminals will definitely not be anything to do with it.

rico

ICBM said:

Usually a blown power valve.

The mains plug contacting the speaker terminals will definitely not be anything to do with it.

That’s quite annoying! It only had a service and new power valves about 4 months ago...

What would cause the power valve to fail? 

ICBM

rico said:

ICBM said:

Usually a blown power valve.

The mains plug contacting the speaker terminals will definitely not be anything to do with it.

That’s quite annoying! It only had a service and new power valves about 4 months ago...

What would cause the power valve to fail? 

Being a valve... being new... being a modern-production valve... they just do sometimes.

Things where the valve itself is not the cause:

Bias being too hot - common on old Marshalls when the bias caps have started to leak.
No screen resistors in the amp - not fitted on most 70s Marshalls.
Shock/extreme vibration, especially when the valve is hot.
Being used with an attenuator like a THD Hotplate which has the wrong impedance curve.

It's also possible that it's not a valve - a faulty impedance selector (also common on old Marshalls, if the original pull-out type is left in place), speaker cable or jack can also blow the HT fuse, as can an arc in the OT or a failing filter cap.

rico

I should say that it’s a Ceriatone 2204 clone which is only a few years old but in any case I’ll get it to the tech - thanks for your advice @ICBM ;

hywelg

rico said:

I should say that it’s a Ceriatone 2204 clone which is only a few years old but in any case I’ll get it to the tech - thanks for your advice @ICBM ;

In that case it might be the power transformer. Mine went on my OTS and that just kept blowing fuses.

But more likely just a valve. Get yourself a multimeter and learn how to bias it yourself, then you'll know how hot/cold it's been set.  

rico

@hywelg I have a multimeter and i'm not averse to trying it but what are the risks? 

hywelg

Seeing as the Ceriatone has dual bias measurement points and an external bias pot, pretty low risk. Get and order a pair of matched EL34's (I personally like Winged C's).

Read up about biasing and give it a go. If the fuse blows again with the new valves you'll have ruled out that as the problem. 

Modulus_Amps

if it is the HT fuse that has blown it won't be the Power Transformer as the HT fuse is after the Power transformer. Like ICBM said it is most likely a valve issue