Heyulp!!!First Time an Amp has Died on Me????

Bellycaster

My Dr Z Maz 18 gave up the ghost tonight at rehearsal. I used it last night and it was fine.

Can you help with diagnosing the problem? I am guessing it's the Power Valves or Valve (EL84's) but I'll leave that to the experts who know more than me.

Amp was connected to Cab in the correct manner (impedence and all), on Stand-by by for a minute as usual, the power up.....No Guitar Sound, but a slightly noisier hum than usual from Amp, then I got the Guitar sound for 5 seconds or so and then sound faded out again.

Had a look in the back, bright orange power valve, I think the whole plate was glowing orange. Obviously, I only had a quick look, but I'm sure it looked like this, it didn't look normal, naturally I thought this is not good so flicked amp straight off sharpish.

As I flicked it back to stand-by though (I'd just played a chord, I was just randomly strumming in frustration) and the faint, fading sound of the guitar came through as the amp was going into stand-by mode, like it briefly came back on going into stand-by, if that makes sense?

Checked the fuse at the back(near kettle lead socket) and it hadn't burned out. No smoke or funny smells coming from amp either.

The (Stock) valves are only 1 year old, I know EL84's last less hours than other valve types, but I thought they'd last longer. It has done a fair share of gigs and rehearsals though, plus some home use.

Can't do any of my novice diagnosing until at least Saturday, so in the meantime I thought I'd ask on here.

Do you think it is the Valve/s? I'm hoping it's nothing worse, like a failed speaker cable(same one I used night before) or failed speaker jack connection.

Thanks.

SunDevil

I’m no expert, but I’ve seen what you describe myself..

Sounds like one of the valves red plated (too much current for the valve)

Generally it will make the valve which it was paired with do the same.

If you switch off before the valve goes completely, then it will play normally again once it’s cooled off, but will fall over into red plating again within a few minutes.

Catch it early as you did and the amp should be fine.

If it is that, it’s at least a new pair of valves to match the ‘good’ two. 

A failing component could also cause this and (from memory) amps which are prone to it can be tempered by increasing the cathode resistor.

Some valves are tougher than others in this regard - old Mullards and Ei seem to be less prone (but expensive) - check with Dr Z site for recommended brands and ranges

Bellycaster

Thanks @SunDevil ;

You'll have to forgive my very limited knowledge. When you say "Too much Current for the Valve". Does that mean it is something within the amp that's made it get too much current or something from external mains sources?

I know you touched on the "Failed Component" notion. You saying this is not a regular power valve failure/issue?

There are only 2 power valves anyway, so don't mind forking out for those, but can't keep buying them if they fail again due to something else.

These amps are meant to be rock solid component wise (except for valves which can go anytime).

Oh boy, this I don't need

Bellycaster

Is this a problem with Biasing? These amps are not biased manually, they are "self biasing"?

ecc83

Yes, cathode biased but that does not guarantee the valves are running at a sensible current and the mains voltage could have been a bit OTT at the time.

I understand JJ EL84s are pretty tough? I found TADs reliable but in the valve world, things changes every few months!

You might have a problem in the speaker circuit but I doubt it but in any case, BUY A FEKKIN METER! That will allow you to do continuity tests.

With the amp setup back at base, switch it on with speaker connected but nothing else and keep an eye on the valves, best done in a dim room. If you see the slightest bit of dull red down the centre of the anode (the grey thing)  switch off and order a set of 84s . If the replacements go red, get the amp to a tech. Yes, you might have to get the cathode resistor value upped by 10 Ohms or so.

(this is in fact a mod for the VERY early A15 and A30s) .....***t'appen.

Dave.

ICBM

From the description it is most likely just a faulty power valve - either do what Dave said or just buy a new pair anyway - you always need spares. (Mark the one you think is suspect, until you know for sure - the other one will still be useful.)

These amps are notorious for blowing valves, they are just not well designed from that point of view.

DJH83004

The output valves share a common cathode resistor (150ohm / 10watt) so it is unlikely to be problem there (e.g. short) as both valves would red plate, sensible advice from Dave and IC get a spare set of JJs they are not expensive and pretty rugged. I always fit them in EL84 amps that are cathode biased they do tend to last the distance better.  

SunDevil

Dr Z recommend the JJ EL34 in the Maz 38 and a NOS 6N14N (Russian EL84) for the Maz 18 http://drzamps.com/support/tube-info/

Conjecture, but I suspect the 18 is biased hotter and they are recommending the 6N14Ns as the think they are more able to handle the current?

ecc83

"These amps are notorious for blowing valves, they are just not well designed from that point of view. "

IC^ do you have any details re bias resistor value and HT voltage?

SunDevil, I don't think it is a case of some valves more able to handle the current? Anode size is the same? I found that EH 84 would 'dull redplate' in an A15/30 whereas other brands would not  (Ruby iirc but see *) so I think gm must vary a bit or it could be due to excess grid current?

Whatever. 'We' found on A/B test that upping the cathode resistor by 10% or so fixed the problem with all the brands at our disposal and did not make any discernible difference to the sound, clean or mayhem.

*Had some Ruby KT88s at my last incumberence . Total shit! Lasted less than a hour at 2/3 FP in an S1 200.

Dave.

SunDevil

Hi Dave,

All makes sense - just going off my own experience - I have a very 'hot' EL84 quad amp with the original AC30 cathode bias resistor values in it that red plated new JJs and Sovteks in about 2 hrs flat

I even found a company in Germany who would grade and send me low IP ranges but they still got toasted

At the builders recommendation, before I went to upping the cathode resistor, I put a set of Telsa branded NOS Eis in there.

Two years later and issues at all, but quads of those are now well north of £100

I have a Rocket clone with the same resistor values running a set of the 6N14Ns that seems to have held it's own too - they come in at around the £75 mark for a quad

ICBM

ecc83 said:

IC^ do you have any details re bias resistor value and HT voltage?

Not off the top of my head, but when I've worked on them I've always found they run the valves far too hot. I haven't modified them, owners seem to like the sound of them like that and it's perceived as part of the ownership package...

They're also one of the amps we mentioned recently with the standby switch in the wrong place in the circuit, so they frequently blow rectifier valves.

I also seem to remember the little SE one didn't have a screen resistor. (Which I did fix.)

Bellycaster

Thanks chaps, wisdoms all round. Looks like I'll be ordering a set of JJ's. I'll take a look at those NOS ones too @SunDevil ;

jpfamps

A lot of EL84 based amps use 100 ohm screen resistors, presumably because that is the "correct" value because VOX used that value in the AC30. 

It would be wise to increase this is most designs especially as many use higher voltages than recommended for EL84s.

Regarding the red plating of different brands of valves, the two most likely causes of this are higher current draw and thinner plate anode material.

In my opinion too many designers use too high HT with EL84s and thus also the wrong loading.

VOX did get it right with 320 V HT and 4k load with a quad of valves.

ICBM

jpfamps said:

VOX did get it right with 320 V HT and 4k load with a quad of valves.

Which of course is why AC30s are not *that* hard on the valves compared to some modern amps, despite typically running them at around 16W at idle.

Misty

Over on the Dr Z forum there are discussions where it is recommended to change EL84's with a new matched pair/quad after 300 hours max. I've gigged a Maz 38 for years without ever blowing a valve, but I change the power valves after about 150 hours, always JJ EL84s which is what Z has mostly used. I have other Z amps and not had a problem yet....this doesn't help anyone I know, just emphasising the importance of replacing valves before they blow. I also have a NOS Mullard GZ34 rectifier in my Maz Sr instead of the stock Sovtek, purely for reliability, and it's been fine. Dr Z is now using NOS 6N14Ns fpr the Maz Jr, which he says are a more rugged version of the EL84, but I haven't tried them myself. There's a troubleshooting aid on the Dr Z website which may or may not be a help; I've always thought it a good idea to have it with me just in case: http://drzamps.com/info/faq-troubleshoot/

Bellycaster

Thanks @Misty been meaning to have a word with you as I dug up an old thread about Dr Z's and Valves. You said on that you have them the equivalent distortion rating as Groove Tubes 6. Would that be late onset on distortion?

I ordered my JJ EL84's from Karltone yesterday and phoned the guy after to notify him of the Amp type, nice bloke, quite helpful chat and he was appreciative of me telling him what the amp is. He said he would match some with lower current draw, does this sound about right to you. Would lower current draw mean later onset of distortion or am I getting 2 different things mixed up?

It's interesting you mention the Rectifier Valve. I know you swapped yours for the Mullard, but would you recommend changing the R.V at certain intervals of time or is the R.V less likely to go pop?

I did look for the Russian NOS 6N14N's, but couldn't find any on sale in UK, unless the have an alternative name. Are those Mullard R.V's readily available? I will check anyway.

Regarding the hours that my Stock Maz EL84's lasted, I'm not sure, but 150 sounds about right, I certainly never got 300 hours out of them though. I mainly run the amp almost clean with pedals(for gigs) and only a small fraction of it's use at home I have the gain up for the amps OD tone, but even then the master is usually very low. Not a great lifespan for that pair in my amp.

I'll be more on the ball regarding swaps in future though.

Misty

Bellycaster said:

Thanks @Misty been meaning to have a word with you as I dug up an old thread about Dr Z's and Valves. You said on that you have them the equivalent distortion rating as Groove Tubes 6. Would that be late onset on distortion?

I ordered my JJ EL84's from Karltone yesterday and phoned the guy after to notify him of the Amp type, nice bloke, quite helpful chat and he was appreciative of me telling him what the amp is. He said he would match some with lower current draw, does this sound about right to you. Would lower current draw mean later onset of distortion or am I getting 2 different things mixed up?

It's interesting you mention the Rectifier Valve. I know you swapped yours for the Mullard, but would you recommend changing the R.V at certain intervals of time or is the R.V less likely to go pop?

I did look for the Russian NOS 6N14N's, but couldn't find any on sale in UK, unless the have an alternative name. Are those Mullard R.V's readily available? I will check anyway.

Regarding the hours that my Stock Maz EL84's lasted, I'm not sure, but 150 sounds about right, I certainly never got 300 hours out of them though. I mainly run the amp almost clean with pedals(for gigs) and only a small fraction of it's use at home I have the gain up for the amps OD tone, but even then the master is usually very low. Not a great lifespan for that pair in my amp.

I'll be more on the ball regarding swaps in future though.

Dr Z used to advise specifying that equivalent GT distortion rating, and, as I understand it, that would be medium distortion onset, which is how I always order mine from Hot Rox; they normally come with the figures PC 44 and TC 9 or thereabouts on the box. They always sound fine, and I've never had an issue. In fact last time I asked they didn't know what that GT rating was any more, presumably because Groove Tubes were taken over some time ago and no longer use those figures.

Regarding the current draw, I can't say for sure, maybe one of the more technical guys here can help, but if it were me I'd just order medium distortion rating matched EL84s, and not worry about anything else as the amp is self biasing anyway. 

Regarding the rectifier, there's plenty of advice over on the Z forum if you hunt around a bit. Yes my Maz Sr has a NOS Mullard GZ34 which I installed in place of the stock Sovtek purely for reliability, and it's been in the amp probably 7 or 8 years now; I don't expect to change it. I also have two other Z's, one of which is a Maz Jr, both bought more recently with the stock Sovtek rectifier, and I'd probably change that every other power valve swap. 

With my Z's I've never actually tried to get 300 hours out of a pair or quad of EL84s; it's not worth it as they're not that expensive to replace, a new set always sounds better, and it's not worth the risk. I did a half-hearted search for those 6N14N's in the UK, but it looks as though you'd have to get them from the US. Again, there's advice on Z-talk; apparently they will help the amp sound smoother with less peaky highs, and last longer.....http://ztalk.proboards.com/thread/72831/6n14n-selected-tubes-available

Dr Z is very responsive if you email with any questions, he's always got back to me within 24 hours, though I get the impression that the forum is the main source for general information.

Good luck, great amps IMO, not so common here but hugely popular in the US.

Bellycaster

OK thanks @Misty ;

All good advice.

Bellycaster

New valves in and at least a couple of hours out of them this afternoon, so, long may it continue.

Kebabkid

Bellycaster said:

New valves in and at least a couple of hours out of them this afternoon, so, long may it continue.

So, all working ok, J?

Bellycaster

Kebabkid said:

Bellycaster said:

New valves in and at least a couple of hours out of them this afternoon, so, long may it continue.

So, all working ok, J?

Aye, cheers, Cos. Let's see how long they last.

  

Might re-valve the Badger too. Although I'd assume the longevity of EL34's in the Suhr is greater than EL84's in the Maz? Can never be too careful though.