Blackstar HT20 blowing HT fuse

DartmoorHedgehog

My HT20 made a "pop" through the speaker while sat in standby warming up, then wouldn't work at all.  Nothing through the speaker out or emulated out.  Power light and channel select light still working.  All valves glowing normally.

The HT fuse had blown.  I reseated all the valves, replaced the fuse and it was OK in standby, but blew again as soon as I took it out of standby.

What's the first thing to try?  Could a valve have given up and blown the HT fuse while the amp was in standby? (I thought standby turned the HT circuit off).  The preamp valves are one per channel, so I could try a known good one in each position and see what happens - is that a good idea?  If that fails, I've got a spare pair of power valves, but I don't want to risk damaging anything else.

Is a dodgy valve the most likely cause of this?  Any known weaknesses in the HT20 that I should get looked at before I just swap valves around?

ICBM

Remove both power valves and try again. If the fuse doesn't blow, it's faulty power valve. If it does, it's a job for a tech since even a preamp valve with a direct short won't cause a fuse blow.

As far as I know, the standby on Blackstar amps doesn't turn the HT off, it turns off the supply to the power valve screens, which means that the plates are still fully powered up and so a valve fault could occur then and blow the fuse. (And would probably make a noise through the speaker.)

If it does prove to be a power valve, you can find out which one (it will be only one) by trying one old and one new valve together. Keep the good one as a spare.

jpfamps

Remove the power valves, replace the fuse and fire up the amp again. 

If the fuse still blows, then the amp will need looking at. If the fuse doesn't blow then you have a defective valve, so replace the power valves and try the amp again.

If it is a shorted valve, it's still possible that something in the amp may be damaged by the valve shorting, but if the amps seems to work OK, then it's fine.

I think Blackstar have done something "clever" with the standby that may not involve removing the HT.

DartmoorHedgehog

OK, thanks folks.  I'll try that later and let you know the result.

DartmoorHedgehog

Sorted - thanks for the help.  It was a dead power valve.  Other one is fine, as you suggested.  Spare pair in there for now, new pair on order

What an excellent forum this is!

(extra thanks to @jpfamps for helping out - I know you're not a fan of all things Blackstar!)

jpfamps

DartmoorHedgehog said:

Sorted - thanks for the help.  It was a dead power valve.  Other one is fine, as you suggested.  Spare pair in there for now, new pair on order

What an excellent forum this is!

(extra thanks to @jpfamps for helping out - I know you're not a fan of all things Blackstar!)

Actually I have no real problems with Blackstar products; they are pretty well engineered and I don't see many of them in for repair (I suppose you could are that this is a "bad thing" 'cos if they were more unreliable then I would get more work!), and whilst I have yet to play through a Blackstar amp I would entertain using live, they do seem have a good following amongst people into "heavier" styles of music.

Blackstar have (sadly) copied MESA Boogie's spurious patent publication strategy, but if they want to waste money on lawyers it's their choice.

I have experienced a couple of Blackstar amps with unusual faults, ie not valve failure etc.

One had a intermittent switching problem that I diagnosed as a dodgy op-amp. The amp has not returned, so I expect I fixed it.

The other had some DC leakage through the PCB around one of the MOSFETs that make up the phase splitter. 

I've taken some snaps of the repair and will (eventually) put it up on the JPFamps blog.

This is an unusual fault as most amps don't use MOSFETs, and I could only cure this by lifting one of the MOSFET legs off the PCB.

The offending MOSFET was in a TO-220 package, which is quite small and doesn't have very wide lead spacing.

DartmoorHedgehog

OK, thanks - sorry, perhaps I got the wrong end of the stick!  I use my HT20 head for everything at the moment, including live. It's got way more gain than I ever want to use (I think you're right that they're popular with the metal fraternity - I'm running a 12AY7 in the drive channel preamp to make it a bit tamer) but I really like the sound of it.  Recently we've got another guitarist in the band and sometimes the dark sound of the Blackstar struggles to cut through against his Fender amp (I'm finding myself running the treble way further up than I used to) so I'm possibly thinking about getting something else, but I still like the HT so will probably hang onto it even if I end up using something different in the band.

It seems pretty reliable - I've had it three years and apart from one suspect preamp valve I've not had a problem with it until now.

ecc83

The HT-20 does not have a standby switch! Sure it is one of "ours"? 

ICBM. HT is always present everywhere all the time! "Standby" is effected by biasing the EL34s off when either the input or speaker jack or both is removed. I suspect an OP valve has blown the HT fuse, you can safely try a new pair since the correct value (315mA OTTOMH?) will blow again well before they are damaged if there is a secondary fault. You can look for red plating anyway as you run it up.

This is about all you can do Dart', after that you need a tech'. BTW the 20 uses a mix of cathode and fixed biasing but it is quite in order to change the valves without a re-bias to check or keep going on a gig. Do get it setup as soon as you can tho.

A trick with the HT-20 (and possibly others in the HT fold) to "punch through" a bit more is an EQ pedal in the FX loop set for about 6-10dB of boost in the 

 mid range. The amps are voiced a bit on the "dark" side some say.

Dave.

ICBM

ecc83 said:

The HT-20 does not have a standby switch!

I was going to question that since I was sure I remembered that they do - and it says so in the OP - but it seems you're right! The other models do… so why not the 20? Seems an odd omission when all the other models do apart from the HT-1, which I can understand on cost grounds.

I thought I had worked on one, but perhaps not. HT-1, 5, and 60 for definite, and I think 40.

ecc83

ICBM said:

ecc83 said:

The HT-20 does not have a standby switch!

I was going to question that since I was sure I remembered that they do - and it says so in the OP - but it seems you're right! The other models do… so why not the 20? Seems an odd omission when all the other models do apart from the HT-1, which I can understand on cost grounds.

I thought I had worked on one, but perhaps not. HT-1, 5, and 60 for definite, and I think 40.

Don't know TBH mate! I doubt it was just the cost of the switch. It might have been a silly thing like they had the front panel all mocked up and the case dimms done and THEN realized "No effin switch!" and by that time too many wheels were in motion for an economic pull out? 

No idea, but fortunately all you have to do is yank the lead out of the front or back. This system does of course give the advantage that the 20, in fact most of the range, can be used as pre amps and you don't have to bother with loads. 

Naturally the omission has caused some stick to be given in forums!

Dave.

DartmoorHedgehog

Yes it's definitely an HT-20.  I didn't say it had a standby switch, but it does have a standby mode - it's got that freaky "standby controlled by the input jack" thing.  Ridiculous idea, but you get used to it

I will set the bias as soon as I get the new valves - it hardly changed going from the original TADs to the JJs I had in there, so I'm sure they'll survive a few home practice sessions without rebiasing (the old TADs are back in there now temporarily).

Fez

I don't find the HT20 dark mind you the other guitarist in my band has a propensity for cranking the middle on his Fender amp. Clean boost or graphic in the loop works great so long as you don't dial in too much gain.

The standby system works fine and saved me from blowing the amp up when I forgot I hadn't connected the speaker and fired the amp up.

ecc83

"The standby system works fine and saved me from blowing the amp up when I forgot I hadn't connected the speaker and fired the amp up."

Great! But...YCPAOTPAOTT!

Dave.

DartmoorHedgehog

I have started running an EQ in the loop of mine recently, and it does help a bit.  I like the voicing of the amp and the weird standby doesn't bother me now I'm used to it - I wasn't intending to come across as negative

BTW, Dave, what is YCPAOTPAOTT?  The only reference I can find to it is from another of your posts on this forum, and I still can't work it out from that...  I suppose it stands for "how do you keep an idiot in suspense?"

ecc83

"BTW, Dave, what is YCPAOTPAOTT?  The only reference I can find to it is from another of your posts on this forum, and I still can't work it out from that...  I suppose it stands for "how do you keep an idiot in suspense?" 

Huh! Can't Please some people! (lil' clue)

Dave.

ICBM

You can't argue with a parrot.

DartmoorHedgehog

Ah... you can't please all of the people all of the time.  Right, I can get on with something else now!

Cacofonix

You can't push an output transformer past an output transformer's tonnage.