Quad II problem - Rectifier blowing (GZ32)

[cathoderay57]

Providing that you did actually remove the wires from the X, Y & Z terminals before measuring resistance to chassis (thereby eliminating any path to chassis from elsewhere in the wiring) there must be an internal short inside the output transformer. If so, that is most likely the cause of destruction of your rectifier valve. Getting the transformer rewound or replaced is your only option. Jerry

[waynej4]

Originally Posted by cathoderay57

Providing that you did actually remove the wires from the X, Y & Z terminals before measuring resistance to chassis (thereby eliminating any path to chassis from elsewhere in the wiring) there must be an internal short inside the output transformer. If so, that is most likely the cause of destruction of your rectifier valve. Getting the transformer rewound or replaced is your only option. Jerry

I did remove the wires from X, Y and Z, as per your guidance.

Thank you for the advice and help with this (to everyone who joined in). I'll look at options for the transformer. I did get a quote from Majestic. Will look to see if there are any rewind services out there too as another solution.

[waynej4]

And I've taken note of the chastising, so as a start, I'll look at building myself a Lamp Limiter for further testing once the transformer is sorted!

(above comment meant in the nicest possible way, in case it's not clear! ).

[Boulevardier]

Also, very importantly, implement that Quad modification (see your post #32) and fit a 250mA fuse in the HT line immediately after the GZ32 cathode before you power it up again with replacement components.
Mike

[waynej4]

Absolutely. I can work on that out while sourcing replacement OT's.

Another question then, If the mod is done with the HT Fuse, is a Lamp limiter any help? Wouldn't the fuse blow before the lamp limiter would "get involved"? Or are we also protecting some of the AC stuff with the lamp limiter, so doing the mod AND using a Lamp Limiter are essential?

[Boulevardier]

Originally Posted by waynej4

Absolutely. I can work on that out while sourcing replacement OT's.

Another question then, If the mod is done with the HT Fuse, is a Lamp limiter any help? Wouldn't the fuse blow before the lamp limiter would "get involved"? Or are we also protecting some of the AC stuff with the lamp limiter, so doing the mod AND using a Lamp Limiter are essential?

I think the fuse should be a permanent fitting (can't think why it wasn't fitted originally). You could bypass the fuse with a wire while using a lamp-limiter. I'd order some spare fuses anyway.

[trobbins]

Originally Posted by Boulevardier

Also, very importantly, implement that Quad modification (see your post #32) and fit a 250mA fuse in the HT line immediately after the GZ32 cathode before you power it up again with replacement components.
Mike

Mike, I have a passion for fusing so quite interested how you determined 250mA, and what type/model fuse you chose.
Ciao, Tim

[Boulevardier]

Originally Posted by trobbins

Mike, I have a passion for fusing so quite interested how you determined 250mA, and what type/model fuse you chose.
Ciao, Tim

I didn't determine the 250mA - I was just citing the OP's reference in post #32 to the official Quad mod - which he says specified 250mA. I just assumed that Quad knew what they were doing (though not enough to have incorporated a fuse in the original design!). I assume it would be slow-blow to allow for charging current to the reservoir cap.
Mike

[trobbins]

Ta Mike. I didn't see anything related to that fuse on Keith Snooks site. The only reference I can see is a 2010 post by GJ (https://vintage-radio.net/forum/show...t=57331&page=2).

Fuses have changed a bit since the original use of 250mA, so if anyone is keen then the value and type of fuse to use nowadays can be assessed further.

[PJL]

Both the output transformer and the mains transformer appear to be showing bitumen leaking from around the pins. I would check the output transformer before you go any further.

[Boulevardier]

Originally Posted by trobbins

Ta Mike. I didn't see anything related to that fuse on Keith Snooks site. The only reference I can see is a 2010 post by GJ (https://vintage-radio.net/forum/show...t=57331&page=2).

Tim - see post #32 of this (current) thread, where the OP says: "I've also seen a Quad mod where they install a 250ma fuse on the high voltage line after the rectifier. I assume this is also a good thing to do?".
I haven't looked at Keith Snooks' site, so I don't know if he mentioned it.
Mike

[trobbins]

Mike, yes I saw that but there was no specific link, and the 2010 post by GJ may actually have been the same info.

[Gabe001]

Originally Posted by waynej4

Absolutely. I can work on that out while sourcing replacement OT's.

Another question then, If the mod is done with the HT Fuse, is a Lamp limiter any help? Wouldn't the fuse blow before the lamp limiter would "get involved"? Or are we also protecting some of the AC stuff with the lamp limiter, so doing the mod AND using a Lamp Limiter are essential?

Well, if you don't use the lamp limiter and the fault is still there (or you have another fault), you'll be going through a lot of fuses.

Some good info about fusing for amps here. Merlin says 2-3 times maximum transformer HT rating for the HT fuse, and 2-3 times max current for primary fuse

http://www.valvewizard.co.uk/fuses.html

[waynej4]

Originally Posted by trobbins

Mike, yes I saw that but there was no specific link, and the 2010 post by GJ may actually have been the same info.

Yes, that was the post I was referning to with regards the 250ma fuse.

[Boulevardier]

Originally Posted by trobbins

Mike, yes I saw that but there was no specific link, and the 2010 post by GJ may actually have been the same info.

Yes, I see what you mean, Tim. Sounds like it's the same info.

Interestingly, Mullard also specify 250mA for the HT fuses in both the 5-10 and 5-20 - despite the different HT current draws...

Mike

[cathoderay57]

Not sure if either would rewind a Quad but you could ask Forum members Ed_Dinning or murphymad (Mike Barker). Hope your luck improves from now on! Jerry

[Richardgr]

Is it worth pointing out that the issue with the OPT might be a symptom of another issue? Could be that a coupling capacitor was leaky or a tube was gassy, and a tube ran away. Do you know if the output tubes are OK?

There is a guy I met over here (Stockholm) who is sitting on a stash of 6000 KT66 maintenance pulls from Sverige Radio (who ran a sQuad of Quads), but nowadays we do not have the luxury of affordable KT66 replacements. Hence it is not worth being too much of a purist under the hood if that means you are implementing safety and resilience improvements.

To that end, there are the fuses, chassis earth point, possibly blocking capacitor on the input, separate cathode bias for the output tubes and I added a screen grid stopper as well so I could use cheaper NOS tubes (5B/255M).

[sortedradio]

Originally Posted by Richardgr

Do you know if the output tubes are OK?

nowadays we do not have the luxury of affordable KT66 replacements. .

These seem reasonably priced, however I have no experience of using them.

https://brimaruk.com/valves/tetrode/brimar-kt66/

[Richardgr]

Less than £100 for a pair - that does look reasonable! They should last many years once you have chased away the gremlins. Strange that matched pairs are not in stock, but quads are?

Have you considered what rectifier to use to replace the GZ32? There are cheaper options there. I am using Sylvania 5R4GYB tubes that popped up cheap on an auction site, but those Brimar GZ34 should be OK, and also look reasonable. There is another mod you can do to add a couple of series diodes to the anodes of the rectifier so they do not see the peak inverse voltage, to make up for variable modern supply.

Another consideration. If you do have a replacement OPT from Majestic, might it be worth getting a pair so they are matched in construction? I actually have non-matched OPTs in my pair (a very early style and the more common one) and I don't hear it, but it might be a niggle.