Quad II - one KT66 heater much brighter

[AndyW NZ]

Hello all,

Just moved my trusty Quad II's from Melbourne to Sydney, and shortly afterwards seem to have a couple of problems. First, one KT66of the old set (Golden Lions) started to red-plate.

So I changed the whole lot for a new matched set. All good for a few weeks. Now one of the new KT 66's is much brighter, and hotter, than the other. I'm assuming this is the heater. See attached pic. Problem seems to follow the valve when I switch them. And if I put one of the old KT66's back in, brightness seems reasonably consistent across the two.

Two questions - Firstly is this likely to be just a heater problem in one valve? Just unlucky and some shorted turns or some such?

Secondly, if this isn't a heater issue could it be related to the red-plating. Should I consider replacing the coupling capacitors?

Thanks in advance for any ideas or advice.

[Michael Maurice]

It does look like a valve problem to me. Has the amplifier ever been serviced? If it has been properly serviced within the last 10 years or so by a competent engineer then it is likely that good quality coupling caps have been used.

IF IT HAS NEVER BEEN SERVICED STOP USING IT RIGHT NOW UNTIL THIS HAS BEEN DONE.

As the problem follows the valve, I think it most likely that that particular valve is faulty.

[Radio Wrangler]

A faulty heater running too hot might rob quiescent current from the other valve, but the red hot anode may be an indication that you have two faults. One bad valve and maybe a leaky coupling capacitor. If you don't know what sort of capacitors are in those positions, and there is any suspicion, keep the amplifiers off until they've been checked. At the moment you need one KT66 and possibly two capacitors. It would be a disaster to add one or two transformers to that list. That red anode took a lot of excess power, and that power had to come from the mains transformer via the output transformer.

David

[GrimJosef]

Red-plating KT66s in Quad IIs are occasionally due to faulty valves themselves (some KT66s can be prone to gas and the grids are tied to zero volts only by a relatively large resistance, so small amounts of gas current can drive the valve into thermal runaway). But if the original Hunts paper-in-oil capacitors are still in situ then most of the time the problem is with leakage through them. They are much cheaper to replace than the valves, unless you have to pay someone to do the soldering for you. If you do replace them then desolder the originals (don't cut the leads) and put them in a safe place. If you ever sell the amps the buyer may pay more if you also have the removed parts.

If the difference between the valves really is just the heater then there may be nothing wrong. Almost all the light comes from the bits of the heater which poke out of the end of the cathode tube. Valve manufacturers don't care about these bits and as long as they're not really huge they don't make much difference to anything else.

If you are happy to work on the amp live then while it's still unplugged turn it upside down (make sure there's no weight resting on the valves), take the baseplate off, connect a load (speaker or decent-sized 8-15 ohm resistor) and then plug it in and power it up. Measure the DC voltage across the cathode load resistor (it's mounted on two turrets on the choke baseplate). Once the amp's had a minute or two to warm up it should be 26-27V. Then measure the DC voltage between turrets V and U on the output transformer baseplate. This should be around 1.35V (plus and minus 0.15V is OK). Repeat between turrets V and W. These voltages are generated by the cathode currents of the two KT66s flowing through the roughly equal resistances of the two transformer windings. They tell you what the individual cathode current of each valve is. If they are very different then one valve is passing a lot more current than the other and that is A Bad Thing. Keep checking these voltages for 10-15 minutes. If they have stabilised then all is probably OK. If there is gas in one valve then it will probably be fine for the first 5-10 minutes then as the valve gets properly hot the gas pressure will rise and will start to cause increasing current flow. If the voltage across either VU or VW gets above about 1.6V switch the amp off and set about fixing the problem.

Cheers,

GJ

[G8HQP Dave]

Does the pic in post 1 show a very hot cathode, or a very hot g2? If the latter, then check the anode connection.

[arjoll]

Originally Posted by Michael Maurice

IF IT HAS NEVER BEEN SERVICED STOP USING IT RIGHT NOW UNTIL THIS HAS BEEN DONE.

Andy, I notice your profile says Wellington - I know there's a forum member up there, and also several around Australia if you're in the West Island for a while, so they may be able to help with the necessary servicing.

[AndyW NZ]

Thanks for the carefully considered replies all. Greatly appreciated.

I believe the amps have been serviced, but I don't have a date. I'll see if I can find a sticker somewhere. I'll also make the measurements that GrimJosef suggests and see where that takes us, hopefully this weekend.

[DangerMan]

It would be easy to pull out the GZ32 rectifier and see what happens. If the heaters are equal now, this will give you an idea whether it's shorted turns in the valve.

As has been said, the transformers are relatively easily damaged, and if your red-plating valve (or coupling capacitors) has caused one half of the output transformer's primary to go open circuit, this overheating could be the result... a white hot screen grid.
Easy enough to check the anode to centre-tap resistances to check for this. You could always meter from anode pin to anode pin from the top of the valveholders, and should see resistance; if you see open circuit, that's a worry.

Sometimes the valves themselves develop dry joints inside the pins, if the anode pin is affected, that could do it.

[GrimJosef]

The 'problem' follows the valve, it seems, so the output transformer's probably OK. If half the primary is o/c there will also be quite a loud hum from the speaker.

Cheers,

GJ

[DangerMan]

Oops, fancy missing that. Must learn to read properly.

[AndyW NZ]

Pulled the rectifier as suggested by DangerMan and both KT66s appear the same with the rectifier out. When the GZ32 is back in place the KT66s come up looking the same then a second or so later, (when I assume the HT is up), the bright orange glow returns.

So not a heater problem. I'll make the measurements suggested by GrimJosef next weekend once I'm back from working out-of-town.

Thanks again for the advice

[jon_the_id]

Could this be the screen getting hot?

[bikerhifinut]

The problem "Follows the valve".
Seems to me you've got a duff KT66.
I assume you don't have a spare KT66 to try in its place.
I'd just substitute it and if the problem disappears and then as a belt and braces if the voltages underneath check out, I'd consign the duff KT66 to the bin.

[AC/HL]

Originally Posted by jon_the_id

Could this be the screen getting hot?

That's what the photo seems to show, as it follows the valve you could try resoldering the anode pin of the valve. Nothing to lose.

[GrimJosef]

Originally Posted by bikerhifinut

The problem "Follows the valve".
Seems to me you've got a duff KT66.
I assume you don't have a spare KT66 to try in its place.
I'd just substitute it and if the problem disappears and then as a belt and braces if the voltages underneath check out, I'd consign the duff KT66 to the bin.

It says in the original post that, following a move, one of the KT66s started to red-plate. So the OP changed the whole lot for new ones. After a few weeks one of those new ones now seems to be acting up. Failure of single valves does happen from time to time. But two failures in quick succession starts to look like there's an underlying problem. While putting more and more new valves in might keep things going I think it would really be best to diagnose what the problem (if there is one) is. Some measurements while the fault is still there are really needed to do this. The more numbers we know (voltages on every valve electrode would be ideal) the better chance we have of sorting it out.

Cheers,

GJ

[Leon Crampin]

As always when this problem arises, assuming the coupling capacitors and grid resistors are absolutely above reproach, measure the g1 to chassis voltage for each KT66 over a period of time. A rising value much over 200mV spells certain trouble.

The problem is compounded by the single cathode resistor - much discussed and directly contrary to MOV's recommendations. The quality control of MOV KT66s is not above reproach either - grid current is all too common, largely due to gas. I suspect that either materials impurities or poor pumping and gettering are responsible.

The combination of a poor circuit design and components of inconsistent quality makes this problem all too frequent. I acknowledge that these amplifiers are getting on a bit, but there's no excuse for sloppy design - all too frequent from this maker.

Leon.

[G8HQP Dave]

If the problem follows the valve then the valve is faulty - bad anode connection.

If the problem stays in the same place then the circuit or valveholder is faulty - bad anode connection.

In either case the problem is caused by the anode having no supply so all the cathode current tries to go to g2 - which then gets very hot! At first glance this can look like a hot cathode, but the fact that it goes away when the rectifier is absent shows that it is not some obscure heater problem.

[bikerhifinut]

Originally Posted by GrimJosef

It says in the original post that, following a move, one of the KT66s started to red-plate. So the OP changed the whole lot for new ones. After a few weeks one of those new ones now seems to be acting up. Failure of single valves does happen from time to time. But two failures in quick succession starts to look like there's an underlying problem. While putting more and more new valves in might keep things going I think it would really be best to diagnose what the problem (if there is one) is. Some measurements while the fault is still there are really needed to do this. The more numbers we know (voltages on every valve electrode would be ideal) the better chance we have of sorting it out

Sorry I should have paid more attention GJ.
I concur with your opinion so far now that I have read back.

ooops.

[AndyW NZ]

Hello again,

Finally back in Sydney and able to get the QII on the workbench, with the following observations:

No hum from speaker, V4 (the valve nearest the smoothing cap I believe) is very bright.

Making GJs suggested measurements:

Voltage across cathode resistor 14V and appears to be slowly but steadily rising
U/V on o/p transformer 0V
W/V on o/p transformer 1.42V and rising steadily
Faulty valve pin 5 -7V
OK valve pin 5 +0.45V

So clearly something not right at all.

Thought I'd best confirm that OP TX hasn't gone OC, but both U/V and W/V were similar (about 19R I think)

Looking for more facts I measured KT66 dc voltages as follows:

V4
Pin 3 450V
Pin 4 395V
Pin 5 -7V
pin 8 15.8V

V3
Pin 3 406V
Pin 4 381V
Pin 5 0.48V
pin 8 28V

I noticed that the voltages across C5 and C6 stayed stable for many minutes - is this a sign of good caps? C5 and C6 showed a resistance on my DVM of 500k.

I then checked out the resistors, in situ, so maybe not accurate:

R2 1.28M
R3 1.26M
R4 680 R
R5 218k
R6 217k
R7 760k
R8 2.8K
R9 717K
R10 84R
R11 84R

Obviously many have drifted high, but in a fairly consistent way!

So, I can see that the grid voltages are way off, I guess the key question is why? Any ideas....?

Finally, the amps were certainly serviced in 1989, but may have been done later. Most components (e.g. Hunts caps) seem fairly vintage. Photo of inside attached. So beautifully put together!

[Courtney Louise]

Those Hunts caps should probably have been changed years ago. Just my thoughts...

Courtney