Help required - amp head and cab ohmage gone wrong

beed84

Okay, I've dimwittedly plugged my 16ohm cab into the 8ohm input of my Blackstar HT5R head. It has played fine for the past few days until last night. I put it on standby and shortly after it started popping.  Immediately I turned it off and thought nothing of it, hoping that'd it be okay. Total denial.

Anyway, after figuring out the problem and connecting it correctly, all it does is make a hissing noise, the guitar faintly coming through.

Yes, I confess I'm and idiot and should've been more careful, but what I'm worried about is that I've truly goosed it. Any ideas?

Maynehead

Hopefully it’s just a blown 12bh7 power valve...

beed84

Maynehead said:

Hopefully it’s just a blown 12bh7 power valve...

Hopefully, although it still lights up?

Now it's making zero noise at all. Could it be the speaker?

ICBM

beed84 said:

Maynehead said:

Hopefully it’s just a blown 12bh7 power valve...

Hopefully, although it still lights up?

Unlikely to be the 12BH7 since it's used in push-pull, hence the amp would make some sound - although distorted - unless both halves are dead, and for that it would need to not light up.

It could be the 12AX7 though.

beed84 said:

Now it's making zero noise at all. Could it be the speaker?

Unlikely but possible. Do you have any other cabinet, any other amp, or any headphones?

@ecc83 (Dave) will be able to offer more help than me!

It's probably not related to the impedance mismatch though - that would normally be safe, if not ideal. May be just an unlucky coincidence.

jpfamps

You won't have the blown the speaker; valve amps deliver less power when the impedance is not "correct".

Most likely the 12BH7 has shorted and blown the HT fuse.

I have to explain this on almost a daily basis to customers, but here goes again:

Just because the valves light does NOT mean they are OK.

beed84

ICBM said:

beed84 said:

Now it's making zero noise at all. Could it be the speaker?

Unlikely but possible. Do you have any other cabinet, any other amp, or any headphones?

Good call. Just tried it with my headphones and it worked  Just a speaker to replace then?

(You will have to humour me; I'm a complete novice when it comes to stuff like this)

jpfamps said:

I have to explain this on almost a daily basis to customers, but here goes again:

Just because the valves light does NOT mean they are OK.

I will never forget this 

Maynehead

beed84 said:

Good call. Just tried it with my headphones and it worked  Just a speaker to replace then?

(You will have to humour me; I'm a complete novice when it comes to stuff like this)

The headphone out only uses the preamp stage so even if the output valve/transformer has blown it would still work.

At least it proves the preamp stage is ok and the problem does lie in the output stage or the speaker.

ecc83

You could well have buggered the OP transformer Beed. I only ever had one fail but they are very small and although IC is quite right, the mismatch should have been ok, failure is a possibility.

You can try a new 12BH7* but I hold out little hope. Where will you go to get it fixed?  That the cans out works shows the ECC83 is ok and the HT fuse has not blown (sodding Wickman innit!)

*It will need biasing correctly IF it works but the bias will be close enough for a minute or two's test.

Just thought! The speaker 'mismatch' COULD just be a co incidence and the fault back in the PI stage.

Dave.

ecc83

Might sound silly this but stand the amp, off, in front of a radiator for an hour then try it and report back.

Dave.

ICBM

ecc83 said:

The speaker 'mismatch' COULD just be a co incidence and the fault back in the PI stage.

Unfortunately that was my second thought from the fault description...

(First was the 12AX7, but that would stop the headphone output working.)

Crackling/popping and loss of signal but still putting out background noise for a while doesn’t sound like an OT failure.

ecc83

ICBM said:

ecc83 said:

The speaker 'mismatch' COULD just be a co incidence and the fault back in the PI stage.

Unfortunately that was my second thought from the fault description...

(First was the 12AX7, but that would stop the headphone output working.)

Crackling/popping and loss of signal but still putting out background noise for a while doesn’t sound like an OT failure.

No, it doesn't IC and, not to take work away from you chaps but, Blackstar's service charges are not that high AFAIK and they will bring the amp up to date for any mods.

Oh and, mention was made of a failed speaker? Very unlikely, the amp  can only put out ten watts even if you beat the ***t out of it! Certainly the 40W early combo Celly is bombproof. Don't know what the later 12" was but even an old Goodmans could surely cope with a Five?

Dave.

ICBM

I don’t think a ‘blown’ speaker is possible given the low power of the amp, but I wondered if it could just possibly (very remote chance) have failed by itself. I’ve come across one with a stuck cone which sounded similar, in that it appeared to produce background hiss quite well, but almost no signal.

The magnet appeared to have shifted due to the amp being dropped though, so I very much doubt it’s the case here - but worth checking if the OP has another cab, just to be able to exclude it completely! I very much doubt it is the cause though.

beed84

Well I've managed to try another cab and that didn't work either - @ICBM was right.  TBH, I've had it less than a week, so I'm hoping to get my money back. My only worry is it was written off stock and sold as seen. I just can't be bothered with faffing around it being sent off/getting repaired/possible replacement/additional costs etc.

Maynehead

Are you sure you want to give up on it before even trying another valve?

ecc83

Maynehead said:

Are you sure you want to give up on it before even trying another valve?

He will have to take the chassis out to change the valve and I doubt he has a 12BH7 just 'hanging around'?

If he CAN get a refund, way to go. Now, IF the amp was bought from a regular dealer as 'B' grade it still falls under being of "Merchantable quality" and he should be ok. If a private sale for monkey money? Bit stuffed.

Dave.

ICBM

From the fault description I think it’s unlikely to be the valve. For it to be, it would have to not light up at all or be physically cracked - the top of the valve will be white rather than silver - since the two halves of the valve operate independently. If one half had failed by itself you would get about a quarter of the output power (still quite loud) with some odd-sounding distortion.

If it’s within a week it should be possible to get a refund unless it was a private sale for cash - I think Ebay would accept that as faulty goods even from a private seller. If it was a dealer then it falls under the Sale Of Goods Act regardless of being sold as B-stock.

beed84

Got a refund, which is a relief. Thanks for all the help though guys, much appreciated. I even learnt some stuff along the way