Leak Stereo 20 and TL12 Plus in need of recommissioning. Help and Guidance needed.

[Radio Wrangler]

Just to eliminate an outside chance, check that the grid leak resistor is OK and is soldered OK.

David

[frankmcvey]

I certainly concur with what other have said re replacing the Mullard EL84s. The RS range of valves are reputed to be sourced from the Shuguang factory (the EL84s are Part No 678-4120) and sold as premium badged valves from other suppliers. At £7 or so apiece from RS, they won't break the bank and RS quality control is pretty good.

Wrap the Mullards up carefully and save them for your pension pot!

Cheers,

Frank

[bikerhifinut]

That's cheap enough £7 + VAT.

I have used Chinese valves in my Leak and I'll bet they are the same as RS. No problems over the years with them. I can't remember if RS do matched pairs of EL84 but fortunately that's not an issue with the Leak cathode bias circuit, its never bothered me with this circuit.

Good point David (RW) about the grid leak resistors, an open circuit or resistor gone high there would give problems.

Hope you all had a good Christmas.

Regards.

A

[GrimJosef]

Originally Posted by bikerhifinut

... Do this with no output valves plugged in or even with no valves except GZ34, this will eliminate a leaky coupling capacitor which could cause the valve to pass more current than its rated for and cause it to fail ...

This is what Sir Humphrey would call a 'very courageous' test. The Leak circuit diagram says that with all the valves in place the GZ34 will deliver 334V at the reservoir capacitor, C13, and the HT will then drop to 318V at the first smoother, C12, 300V at the second, C11, and 220V at the third, C10. More to the point the small-signal valves will have roughly 160-220V on their anodes.

If you take all the valves but the GZ34 out you will generate a lot more than 334V on C13 and with no current draw the very same voltage will be applied all the way down the HT rail and onto all the valve sockets' anode pins, and therefore across all the coupling capacitors. If all your couplers are now 630V film caps and your black can electrolytics (C10-C13) are 450V rated then you might well get away with it. But it would be sad if something that wasn't previously bust is destroyed by too severe a test.

You'll also be applying this huge voltage to the primaries of your precious output transformers. They ought to be OK, after all when the music's playing the voltages on much of the windings swing well above the average HT level and the transformer will be hotter (=electrically more fragile) too. But if there's some weakness in the interwinding insulation right at the point where the centre-tap wire goes in then there will be a risk of a punch-through there which would be a tragedy.

As far as the original fault goes, in my experience it's rare for a power valve cathode resistor to fail and bust the valve, especially if there's no sign of overheating on the other three resistors. If there is a fault then it's much more common for the valve to short internally and to burn out the resistor. Rarer, but still more common than the resistor failing, is for the electrolytic capacitor across it to go short circuit, switch the valve on so hard that it fails which puts so much current through the cap that its short is cleared at which point the fault current diverts through the resistor taking it out too. Both the Stereo20 and the TL/12 Plus run their EL84s pretty close to the edge. I wouldn't be surprised by a valve failure.

I don't know how far the Shuguang EL84s can be pushed before they start to fail, but some of the Russian 6P14P valves are essentially an uprated EL84 and they have a very good reputation for durability. The whole designation of these can be hard to work out - they're labelled in Cyrillic characters, so where I've written P the Russians would have printed the character which looks like a capital Greek Pi. Sometimes western sellers write this as capital P. Sometimes they write it as lower-case n (not a capital N - that's a different valve - confusing isn't it ?). The 6P14P comes in several versions denoted by suffixes and the strong ones are the 6P14P-EV and 6P14P-ER variants. Here's some information about the first one http://www.thetubestore.com/lib/thet...Data-Sheet.pdf which is also sometimes written with a B instead of the V at the end. I would choose the -EV or the -ER rather than the plain 6P14P.

Cheers,

GJ

[vishalk]

Thanks Guys

Heat wasn't the issue as it was switched on from cold, but I have placed feet on it to raise it.

So I replaced the cathode resistor R15L (all the resistors are around 6mm high away from the circuit board).

The corresponding coupling cap was checked and found to be in value, but just to be sure I replaced it.

I also checked the grid leak resistor which was also fine, after that I double checked all the soldered joints and added more solder just to be sure (made sure it melted all the way through) then lightly tugged on each component.

After that I rechecked all the voltages with and without the valves, can you confirm that the voltage across R14 and R15 to pin 3 please. I measure around 9.5-11VDC.

I replaced the 12AX7's with some brand new MESSA 7025 and new Matched Tube Technology Sovereign EL84's.

http://www.tubetechnology.co.uk/ttsovervalves.htm

Can anyone recommend a new type GZ34? I have Mullard and Tungsram but would prefer to buy new ones.


Shame about not using the Mullard EL84's (surely the vintage 12AX7's will be ok to use?). Like Frank said I'll probably just wrap them up and keep them to gain value and sell later on.

So it was a faulty valve that caused the issues, please see picture. I'm not to sure why it's cracked at the bottom near the pins, as there was no physical damage to it. It just seems to have cracked on its own from blowing, would this have effected the other valve as well I wonder? Unless the wife dropped something on top of the valve, cracking it and causing it to blow? hhhhhmmm I'll interrogate her later.

Thanks for the support! I'm about to start the TL12 Plus now, got a few days off before work starts. Usually I'll sit in my PJ's and watch old movies sipping on a single malt whiskey, but I have a project to finish!

[Radio Wrangler]

Probably the crack came first, starting from a pin in the base button, It could have sat there for years, waiting. Then the amp gets used again and thermal cycling gets the crack moving. Air starts leaking in, slowly, but faster than the getter can accommodate. With very low pressure air, it ionises, lights up and takes considerable current. The cathode resistor burns out protecting your transformers and the high heat really opens the crack. The hot getter is more effective and turns to white powder in all the air.

If the glass is flawed, any disturbance can be the trigger, any thermal shock, which makes a bit of a joke of mucking about with cryogenics.

David

[ToneArm]

I’ve had no luck with new production GZ34, unfortunately. I’ve got one old mullard GZ34 in a mono block amplifier in constant use that has been going strong for years. The other channel has seen a selection of modern chinese and russian valves that don’t seem to last long. I’m tempted to try a mullard but when the time comes the price always scares me off! Hang on to those old rectifiers.

[GrimJosef]

The Stereo20 doesn't put much of a load onto its GZ34 (300-0-300V from the transformer, 170mA average current drawn, 32uF reservoir cap) so I would expect modern ones to work pretty well. I've run both JJs and Sovteks in Quad II amps without trouble.

Cheers,

GJ

[AC/HL]

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