EL84 Red plating on blues Jr. Advice appreciated!

th22

I bought two new Groove Tube EL84r power tubes for my blues Jr about a month ago and one is now red plating when ever I use the amp for more than a couple of hours or so. The volume also drops quite a bit once it starts red plating. Once I turn the amp off and allow it time to cool it's fine again until the cycle happens again.

Any advise would be much appreciated.

Modulus_Amps

Blues JNR are biased way to hot out of the factory and they are set to run off 230v not the 240 - 250v we get in the UK, This makes it all worse. Take the amp to a tech and asked them to set the amp for 240v and cool the bias, A Good tech will do this as matter of course.

There may be another issue in the amp too, so best have them consider that as well.

ecc83

Not all EL84s are the same! I found a cathode biased design was quite happy with one brand but red plated slightly with another. The fix was an increase of the cathode resistor from 100R to 120 R. The value you need depends on the actual HT and as Mr Mod says, get it on the highest mains tap.

Dave.

ICBM

th22 said:

I bought two new Groove Tube EL84r power tubes for my blues Jr about a month ago and one is now red plating when ever I use the amp for more than a couple of hours or so. The volume also drops quite a bit once it starts red plating. Once I turn the amp off and allow it time to cool it's fine again until the cycle happens again.

Any advise would be much appreciated.

The first thing to do is to swap the valves round and see if the redplating follows the valve. If it does then the problem is *largely* the valves.

However Modulus_Amps is absolutely right about setting the amp for 240V and not the supplied 230V - it's a very simple change, you should be able to do it yourself quite easily, only two push-connectors need to be swapped. I always do this, and I've never yet had to re-bias one - for good valves - once it's been done.

There's also a more insidious problem with stability which can cause high-frequency self-oscillation (outside the audio range, you won't hear it) which is the most likely cause if the redplating *doesn't* follow the valve.

th22

Thanks everyone, that’s very helpful. As advised I’ll start by switching the valves to see if the problem follows. Otherwise will take it to an amp tech. Thanks again.

RiftAmps

https://ratvalveamps.com/fbj-lifesaver

ICBM

RiftAmps said:

https://ratvalveamps.com/fbj-lifesaver

To be fair, that’s a bit exaggerated. It won’t kill a set of valves in two hours. Most people use completely stock Blues Juniors, complete with wrong mains voltage setting, for years before anything bad happens. Techs do see a lot of faulty ones, but it’s apparently the biggest selling valve amp in the world...

... although I still do find that almost as baffling as the unfathomable popularity of the Nissan Juke .

I’m not saying they’re particularly well-designed or that they don’t run too hot and have a slightly alarming tendency to catch fire, but they’re not *that* bad .

bencowling81

Are all Fender amps set up for 230v in the UK or just FBJs?

ICBM

bencowling81 said:

Are all Fender amps set up for 230v in the UK or just FBJs?

All amps sold in the EU (still including the UK) must be by law.

Many don’t have a separate 240V option, most Fenders at least do.

ecc83

ICBM said:

bencowling81 said:

Are all Fender amps set up for 230v in the UK or just FBJs?

All amps sold in the EU (still including the UK) must be by law.

Many don’t have a separate 240V option, most Fenders at least do.

ICBM said:

bencowling81 said:

Are all Fender amps set up for 230v in the UK or just FBJs?

All amps sold in the EU (still including the UK) must be by law.

Many don’t have a separate 240V option, most Fenders at least do.

This is a problem that should not be. The company I worked for designed, tested and specified amplifiers, valve and solid state, for 230V mains* However, each design was then run at mains + 10% =253V for some hours at 1/2 rated power, 5 mins on five off. Sometimes a valve would fail and take out a fuse. It is very unlikely of course that anyone would get top mains volts for an extended period because if they did all sorts of things would be popping off!

*The tested power delivery would of course be down on spec' at say 210V but there is little you can do about that. The kit meets spec' at specified V in!

Dave.

ICBM

ecc83 said:

It is very unlikely of course that anyone would get top mains volts for an extended period because if they did all sorts of things would be popping off!

Try central Glasgow near the railway arches...

I’ve never measured under 250V in the shop. And yes, a lot of things seem to have a rather short life expectancy, that wouldn’t normally do - they even had a Weller soldering station die, which I’ve never heard of before. The studio round the corner used to go through amps on almost a weekly basis until they finally accepted what I’d been saying about getting a large industrial power conditioner. They complained multiple times to Scottish Power, who eventually sent an engineer round to measure it and said it was within limits - but if it was 253V it still would be.

I agree that it’s perfectly possible to design a valve amp more conservatively than many manufacturers do though - especially with EL84s, most seem to have a real thing for hammering them as hard as possible, both for dissipation and voltage... there is no need.

ecc83

ICBM! "We had the same problem at the first building we moved Bstar to. If you left a couple of 500VA Variacs plugged in and then switched on a 100W amp the breaker would trip. Found mains was at around 254V, got the utility peeps in to change the tappings on the sub. Get that done, we pay the fekkers enough! 

Oh, just seen you tried. Change power company? And yes, the EL84s were well over their dissipation limit in the first batch of Artisan 15 and 30s. To be fair to the designer he just went 'by the book' for Rk. Maybe the mains transformers were a little overwound? Mains traff design is always a bit iterative.

Dave.

ICBM

ecc83 said:

ICBM! "We had the same problem at the first building we moved Bstar to. If you left a couple of 500VA Variacs plugged in and then switched on a 100W amp the breaker would trip. Found mains was at around 254V, got the utility peeps in to change the tappings on the sub. Get that done, we pay the fekkers enough!

You might have had a bit more leverage as a fairly large company. Our lot just said it was within limits and that was that - it might even be true, just. Running gear at just under the safety limit continuously isn’t really good though, even if properly designed.

ivansc

Sounds like time for an isolaton transformer - I used to get this sort of issue with a USA origin MusicMan amp I had. On one of my converters it was fine but not on the other. No difference in Amps or watts capacity, so I assume it was because one was set to do 110 to 240 & the other 120 to 240 or something similarly different.  P.S> Nice to see you on here, Dave!

ecc83

ivansc said:

Sounds like time for an isolaton transformer - I used to get this sort of issue with a USA origin MusicMan amp I had. On one of my converters it was fine but not on the other. No difference in Amps or watts capacity, so I assume it was because one was set to do 110 to 240 & the other 120 to 240 or something similarly different.  P.S> Nice to see you on here, Dave!

Old style RS 'tool transformers' had 10V tappings so you could get a more precise output voltage. Some auto transformers do the same but they are likely a bit more expensive.

Dave.