Quad II Problems

[slurb.net]

Hi again Guys

My friend has given me a pair of Quad II's to try work out a problem he has with them.. (well.. one of the pair!)

They are setup exactly like mine, same components, valves etc..

I have checked all the value's of the components, and all are within tollerance ..

The problem is abit strange.. Both amps are fired up and work normally ... but after about 10-30 mins, (im asumeing reaching a certain temp?) One of the amps goes very weird.

Basically one of the KT66's gets red hot, and the sound gets highly distorted and a very noticeable hum apears from the speaker.

On first inspection, I just asumed the bias cap and resistor needed replaceing.. that I did to no joy...

If you let the amp cool down, then try again ... Starts off perfect then dies again as above?

Any Suggestions guys?

Thanks again in advance


David

[cerrem]

Try the coupling caps into the KT66 valves...
Substitute BOTH of them...

Chris

[EC8010]

If it's only one of the KT66, then it is most likely to be a leaky coupling capacitor as suggested above. You could probably detect this by running the amplifier upside down and having a voltmeter across the affected valve's grid-leak resistor. Even before meltdown begins, you will probably see a small positive voltage when you should see 0.00V

[cerrem]

One additional thought regarding bias on pin#5 of KT66...
I have seen amps on a few ocassions that exhibit similair situation with no bad components... It was found that the the octal socket had a short....ie, pin#4 leaked over to adjascent pin#5...... Run amp with volt-meter on the bias circuit, as previously suggested..

Chris

[slurb.net]

Hi again guys

Thanks for previous help once again!

Another quickie.. I want to replace alot of the old internal wiring in a Quad II amp, should I be using any specific wiring? Is it just normal singal strand stuff ? or Silver wire ?

Thanks again in advance,

David

[EC8010]

Only change wiring or components in a Quad II if you don't mind destroying the resale value.

[analog_sa]

Internal wiring is a pretty cool solid core and appears to be the last possible item to improve.

[Johan Potgieter]

Hi David,

I have refurbished a number of Quad IIs and really, replacing the wire was never on my list. Without desiring to step on nerves, there is a lot of cult faith in fancy wiring not supported by practical (blind) tests. Especially with tube amplifiers where currents are comparatively low, any effect of wire characteristics is orders below audibility.

However, when the component board or wiring elsewhere is moved, the danger of brakages is real with the solid wire. (And how often I have CURSED when people wound leads round terminals before soldering - one is not constructing an army tank!)

This is not your question, and I do not know whether you had previous advice about this, but perhaps I would be forgiven for mentioning that I firstly replace resistors with 1/2 watt metal film or similar types, and then capacitors with polyester types. People faithfully also replace the twin TCC power supply capacitors. I have not found any of them faulty, which is an achievement after so many years. But with modern developments decent 100 + 100 uF types are available which would give a distinct advantage e.g. at low frequencies.

What is essential, is to get distance between the cathode bypass capacitor and that very hot cathode resistor! I have not yet removed an original cap that was not fried! (I replace with a 2200 uF cap there.) Also get the resistor away from right under the choke; it cooks it!

Then what EC8010 said is true, but it is often a question of having to choose between vintage value or degraded performance. I have found some resistors to have gone out by as much as 40%.

A last remark since I am carrying on about this concerns the Quad 22 pre-amp. Nowadays it mostly fetches quite degrading remarks, but it can be refurbished to an excellent unit. But for a funny reason some capacitors in there have changed value by as much as 3 times which totally destroys response. Anybody interested could ask re the same.

Regards.

[slurb.net]

Johan,

Thank you for the reply

As for the wiring, Its not at all for upgrade purposes, Its because the wiring is very old and fray... and turning to dust @ places! I dont want to replace all of it, but the wires that look dodgy I just wanna renew.

My question was just - did Quad use any specific wire, or just normal singal strand copper..., From inspecting it earlier (when I started to replace) It seems pretty much just copper wire to me.

On another point, you mentioned you replaced the bias cap with a 2200uf , is that instead of the 25uf? (I used a chunky 47uf 450v..), Is ther any advantage in sound quality with the 2200?

And yep, as you point out.. I replaced the bias resistor with a chunky 7watt 220r, as far away from the cap as poss!

Regards..

[slurb.net]

Hi ! (again)

I checked all the grid caps, and reisistors, all seemed fine.

Unfort, (and btw, these are a diff pair of II's!)

It seems to be one of the valves. I swapped out the KT66's and the GZ32 from another block ... and its been running for 4 hours without problem!

The GZ32 i took out looks like an original , and had better days.. and i think thers a tiny crack in its base, Altho its one of the KT66's thats getting extreamly hot.

Im not sure what effect a dodgy gz32 would have on the kt66's.. but I fear whateva it is, It might of also damaged the KT66's as well.

Ive just found a nice local supplier for the Original GEC replica's.. spose to be as good! (will have to see.. they seem to be cheap nuff to try)

Hoepfully a new set of valves should solve the prob.

Cheers for your time and replay (again!)


regards,