Have a guess - what's wrong with my '73?

[Quinny]

My 73 JMP is at the tech this week, having gone silent recently. Was due a service and check-up, so dropped off at local chap to give it the once over.... HT fuse had blown, one bad power tube in there which we assumed caused it. New tubes, bias set, general check and test and I picked it up today. 60 seconds in just as I put volume up to around 6, HT fuse blew again. I use a PS100 with my old Marshalls, so took that along with the amp back to tech - this time he let me hang around while he rigged up and did various tests, which was GREAT to see, very interesting. Initially all looked well, amp checking out OK and PS100 ruled out as a suspect. When the volume gets up around 5+ we noted one of the measurements (apologies, I'm not tech enough to know which one) was hovering around 600mA, which of course began pondering as to whether this was blowing the 0.5A HT, but it wasn't doing so during testing. (The initial blown HT was 1A instead of the specified 0.5A, in there from when I bought the amp a year or two ago, so maybe some history there and someone decided to just up the fuse rather than deal with the actual issue.) Various signals input and measurements taken at various stages, all looked rock solid on the various scopes and screens used.

Eventually we plug a guitar in and he says set the amp up like you'd normally play it - for me on a NMV JMP that's around 6 just as the gain is good and not starting to mush. Played a while, all good.... but, played another few chords and as I was playing there was some manner of surge internally, which blew the HT. Was great to see the event captured on the test kit - a big wave from somewhere and fuse gone again. The amp was previously tested with signal generator at full volume in all inputs but for whatever reason that wasn't causing the event to occur (as I type, I wonder if my playing is the problem!!). I've left it with him, he's going to get it happening reliably/consistently then work out what's going on.

Probably a whole heap of reasons this might be, but just for fun was wondering if anyone had seen this before and might have a guess what the issue will turn out to be?

On the plus side, for the brief period it worked earlier today when I had it home it sounded notably better for a service, so I'm excited as hell to get it back. This is the amp - May 73 turret board 50W - my favourite!

Cheers!

 

[FleshOnGear]

I’m not an experienced tech, so I don’t have any really solid ideas. Are your filter and bias caps original? Maybe it’s something with the coupling caps between the phase inverter and the output tube grids? Just spitballing. I wish you luck!

 

[StingRay85]

Were the filter capacitors replaced already?

Did you notice any red plating of the tubes before the fuse went out?

Are the EL34 tube sockets suspect?

PI coupling caps intermittently passing DC maybe?

 

[Quinny]

Tube sockets definitely got a mention. Cosmetically they look in very good shape but he mentioned some specific testing he was going to do.

Lots more stuff mentioned too but way over my head from a technical perspective. Great to see an expert on the job though, normally I just drop my amps there and head home. Tea and biscuits too, all very English.

 

[Quinny]

Hairline cracks and signs of arcing on the EL34 sockets apparently, so replacement planned. Which leads to my next question - any particular recommendations on EL34 sockets for a 73 JMP? These look a reasonable option:

8 Pin Octal Mcmurdo Brown Valve Socket Chassis Mount 3 legs

Original Mcmurdo chassis mount octal 8 pin socket which is ideal for vintage amplifiers. These were used as original equipment in Marshall, Hi Watt, Sound City and Orange amplifiers. These items come direct from military surplus and due to the age the mounting rings do show some light...

www.watfordvalves.com

Though if replacing, maybe ceramic/micalex would be a more suitable option these days?

Thanks!

 

[Dogs of Doom]

the "signs of arc'ing" is a very probable cause...

If there's sediment from a previous arc, that could cause a short between pins & blow back high voltage causing a surge...

I was going to suggest a bad speaker cable, or short somewhere down that line, but, a socket that conducts a short would be a major suspect...

 

[neikeel]

I’m loathe to say that is what I wondered (but I can say that now you have found it!!) I’ve seen and experienced that arcing before (71 Superlead) you get cracking and flashing when you hammer out chords, ie exceed threshold.
You can still get it with ceramic but less commonly.
I’d go with those McMurdos if new.

 

[mickeydg5]

I will go with a failing bias supply. OH I SEE THERE ARE SOCKET ISSUES which could cause that.

It would be good to know the testing measurements taken during fail as well as all voltages around each power tube.

 

[Quinny]

I tend to run the amp pretty hot - we briefly discussed transient voltage suppressors to keep the peaks at bay and maybe provide some protection. I declined that as it sounded like a mod to what's otherwise a fairly stock 50yo, plus with the actual issue at hand fixed it should largely be unnecessary...... but, to those more experienced than I in these matters, is it something I should consider further? I'm moving out of the country this year and reasonable chance I'll sell the amp, so originality feels sensible.... but, it's one of my favourite amps ever so I'm going to try to avoid selling it and if I do it'll be in storage a year or two before joining me overseas, where it'll once again continue life with volume around 6 for the good stuff.

 

[neikeel]

Chief thing is to keep the sockets clean and free from lube and dust (the lube attracts dust and allows arcing. Then you get a carbon track burned into the socket insulation which is impossible to remove. New socket required.
When you open up an old Laney or two (anode voltages over 600v) you will find a blob of Vaseline (yep!) between the heater (pin 2) and HT (pin 3) where you can get flash over. I use Belton on new design amps but no problem using McMurdos (if I can find new ones) as they last 50yrs + if not abused.

 

[Quinny]

Hairline crack on one of the current sockets.....

(loving seeing all this and doing some learning, great stuff!)

 

[Quinny]

Bit of advice please folks......

The original sockets are apparently Millen 33008 with mounting plate: https://www.etsy.com/listing/502414076/millen-33008-steatite-octal-tube-socket

Can't find any replacements and apparently none of these are likely to be direct drop-ins: https://www.watfordvalves.com/products.asp?ID=6&man=83

Before we head down the route of considering washers as some manner of 'conversion kit' type thing to get some new sockets mounted - anyone able to advise on whether there are some readily available, quality drop-in replacement EL34 sockets for a 1973 50W JMP??

Thanks!

 

[neikeel]

I might have some ceramic octals, didn’t think that they were Millen tho’. Iirc spring loaded into the mounting plate by a circlip? I can’t remember if I sold/traded them a while back. I have a set of Cinch ceramics (same as the ‘65 45/100). Let me check.

 

[Quinny]

Thanks, appreciate that @neikeel

In the meantime this is what we're considering. Think the screw/hole distance on the mounting plate was a little short so plate been drilled out slightly to align with the chassis, washer on top to sure things up.

 

[StingRay85]

Looks good. I would roll with it

 

[neikeel]

If they are well made and secure. Afraid some of the Sino import ceramic octals I’ve acquired have not been fit for purpose - rattly and pins sliding out - only good for the bin or sending back.
But if they are good yes I would go with it but use the original screws vs bright zinc cross heads. Ymmv
Of course that is a mock up? As the position is 180 degree out of alignment.

 

[Matthews Guitars]

The correct original style ceramic Amphenol sockets are readily available and should be replaced with the same type.
There is absolutely no reason to switch to a different type, particularly one that requires opening up the holes in the chassis to make them fit.

I've got maybe a hundred or so NOS sockets of this type. So they're easy to get.

 

[Quinny]

The correct original style ceramic Amphenol sockets are readily available and should be replaced with the same type.
There is absolutely no reason to switch to a different type, particularly one that requires opening up the holes in the chassis to make them fit.

I've got maybe a hundred or so NOS sockets of this type. So they're easy to get.

No mods to the chassis, it's the holes on the mounting plate which have been lengthened.

Would you mind linking some Amphenol please? Saw a few, but don't recall any having the correct measurement across the mounting plate holes. I'd be keen to keep everything as like for like as possible, if we can find it.....

Thanks.

 

[Matthews Guitars]

They're the type that mounts with a snap ring.

They look like this. (The one on top with the eye plate) Can't remember the part number, but they can still be found NOS or in like new condition.

3 used Vintage Ceramic 8-Pin Octal Vacuum Tube Sockets Cinch Amphenol | eBay

Find many great new & used options and get the best deals for 3 used Vintage Ceramic 8-Pin Octal Vacuum Tube Sockets Cinch Amphenol at the best online prices at eBay! Free shipping for many products!

www.ebay.com

 

[Quinny]

Yeah saw those on my search - was the only one of that type I could find and is in the USA (I'm in the UK), hence I've ventured down the road of an alternative.