How can you tell if a valve is going down?

[Mike Reed]

Have had biassing problems on one amp recently and suspect a valve failing.

Have sought advice, and emailed T de P, but am awaiting a reply.

Tonight I changed over the two output valves (Tungsram 509s) and the biassing LED balancing problem changed as well, telling my aged brain that the valve is the culprit.

As this will be the first time I've fiddled around with valves (though not the first time I've had them), I'm wondering if it's better to replace the set of 5 in each monoblock, start afresh and keep the others as spares. I've no way of testing them.

What do others do?

[DSJR]

509's used to be very common indeed due to TV use (and why Tim desided to use them), so a replacement set hopefully won't break the bank...

I was told way back when, that it is better for both of a pair of valves to be changed together, as the new one may take down the older one.

[Ali Tait]

I guess it depends how much spare cash you have! It's always good to have spare valves,and a new set will give you peace of mind if nothing else.Otherwise,just buy a new pair.

[Mike Reed]

Thanks, Ali and Dave.

I already have an unused set for both amps, so that's not a problem. Each amp has three ECC somethings as well as the two output valves. How does one know when these are tired? Unlike solid state, valves deteriorate, and I imagine the sound does too.

With so many valve amp. users here, someone must have been in this situation, or does everyone have the electronic engineering skills and kit to test their valves periodically?

I take your point, DAVE, in replacing pairs, but I'd feel inclined to replace BOTH pairs (or doesn't it work like that?). Then do I wait for one of the lesser valves to blow up or something? How does one tell the state of (a) valve(s).

This is not rocket science; there must be procedures or tell-tale signs to look out for, surely.

[Mike]

Generally the sound will slowly degrade over time... You probably won't notice it happening. Popping a new set in should soon tell you!

How does one tell the state of (a) valve(s).

With a valve tester!

[DSJR]

Small signal valves just seem to fade away somewhat, although it depends, as with output valves, on how they're driven internally. Tim's designs have proved themselves as well sorted and almost utterly reliable over the decades, so you may just find that the ECC83's are fine.....

I recently gave the phono stage and power supply valves in me preamp a replacement and am pleased with the extra precision it gave. I *think* the un-touched line stage is a bit better as well, although if there's a difference at all, it's very small...

If you doin't mind, do the whole bloomin' lot.......

[Ali Tait]

Small signal valves usually last much longer than power valves.How long have they been in there?
One thing,if you are not sure about the 509's probably best to replace them as if one let's go,it could cause some damage to the amp.I don't know the circuit,Tim may have designed to avoid this,it can be done,but not knowing,I'd probably change them.With power valves in a PP amp,as Mike says,they deterioate gradually,you may also get an increase in hum.Try measuring AC mV at the speaker terminals for each channel,and see if the reading is higher on the suspect channel.

[hifi_dave]

Mike,
Unless there is a catastrophic failure to a tube, which will show, you can't tell the state of them without a tube tester. The normal user will simply switch tubes to work out which, if any of them, is faulty. You have done this with the 509 tubes and now know that one is on it's way out.

It's then up to you to decide whether to change just the one or replace the whole lot. If they cost tuppence each it would be a no-brainer.

If you are confident that the spare set is good and unused, I would put an 'iffy' label on the faulty tube, replace the whole lot and put the old tubes away for emergencies. You can then rest easy and enjoy the music....

[anthonyTD]

hi mike,
the only way to tell if you have a valve problem is to get them tested, the other cause for bias drift could be a coupling capacitor from a previous driver stage leaking DC onto the grid of the output valve in question, if this is the case every valve you put in that position may eventualy get damaged.
hope this helps.
A...

[Mike Reed]

Am obliged, gentlemen, for the above. I didn't know about valve testers, so that's a plus point; shall investigate my local Maplin's on Saturday. However, they sold me a multimeter a couple of months ago specifically for testing continuity. After scratching my head at home for a few days I realised that that was the one thing it couldn't do! Useless oiks! Had to make my own.

Anthony, I doubt there's a problem with the biassing circuit; partly because of a service 10 months ago and partly, as I mentioned, the biassing imbalance shifted when I swapped valves.

Dave W. Sensible suggestion and that's precisely what I'm gonna do. I've no idea where to source further replacement valves from, though. E.A.R., possibly? Or just search the net? Are there valve stockists around nowadays?