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
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
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
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
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
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.
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.
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..
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..
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,
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,
David,
OK, I misunderstood; sorry for carrying on. No, no fancy wire that I know of was used at the time, rather stuff with superior covering (you might have found that it is difficult to strip). You might find that solid wire makes a neater dressing job, while stranded does not break so easy. (And please excuse the spelling mistake there earlier!) An all-stranded wire loom could become unwieldy in places even with thin wire - the choice is yours. Again, as said, I am not a supporter of exotic wire. Rather spend your money elsewhere.
The very large cathode capacitor. This is a longer story. It is an effort to maintain a semblance of fixed bias under most music conditions, where the capacity is high enough to not appreciably charge to a higher voltage during brief periods of peaks where higher current is drawn, as would a small capacitor do. (You would know that fixed bias operation renders somewhat higher max. output and lower distortion than cathode bias conditions.) This is more prevalent in higher power class AB operation and marginal in class A, but for the price of a capacitor, why not.
If I can be a further rebel, consider a future change from KT66 to 6L6GC (I will be killed by some for this; the original KT66s did look quite classy! But can you still get them?) The characteristics are so similar that the Russians often use the same innards for both, and the 6L6 draws 30% lower heater current. As you would have realised the power transformer gets quite hot.
Regards and success.
OK, I misunderstood; sorry for carrying on. No, no fancy wire that I know of was used at the time, rather stuff with superior covering (you might have found that it is difficult to strip). You might find that solid wire makes a neater dressing job, while stranded does not break so easy. (And please excuse the spelling mistake there earlier!) An all-stranded wire loom could become unwieldy in places even with thin wire - the choice is yours. Again, as said, I am not a supporter of exotic wire. Rather spend your money elsewhere.
The very large cathode capacitor. This is a longer story. It is an effort to maintain a semblance of fixed bias under most music conditions, where the capacity is high enough to not appreciably charge to a higher voltage during brief periods of peaks where higher current is drawn, as would a small capacitor do. (You would know that fixed bias operation renders somewhat higher max. output and lower distortion than cathode bias conditions.) This is more prevalent in higher power class AB operation and marginal in class A, but for the price of a capacitor, why not.
If I can be a further rebel, consider a future change from KT66 to 6L6GC (I will be killed by some for this; the original KT66s did look quite classy! But can you still get them?) The characteristics are so similar that the Russians often use the same innards for both, and the 6L6 draws 30% lower heater current. As you would have realised the power transformer gets quite hot.
Regards and success.
http://www.watfordvalves.com/product_detail.asp?id=1070
Under £30 for a KT66 which sound brilliant, and cant tell them apart from the original GEC's...
Regards
PS. That site also does kt77, kt88 in same series but also get original GEC if u really want to PAY For em lol
Under £30 for a KT66 which sound brilliant, and cant tell them apart from the original GEC's...
Regards
PS. That site also does kt77, kt88 in same series but also get original GEC if u really want to PAY For em lol
The main problem with the QuadII is heat. They become to hot after a few hours because of:
1) the 2 KT66 are too close from each other.
2) if the output transfo is very good, the main transfo is a calamity. Even when good, they become very hot, and with time, the inside isolation get out the trasformer when it become hot. It is the black-brown thing, and more this append, more the transformer become hot.
What I do is to command new and modern transformers on a local company that made them, and install them not at the place of the old ones, but in the proximity of the amps. The amp become less warm even after a while.
A fan can help to cold down those amps too. In fact it is a necessity if you want to use a such amp more as 2 or 3 hours at a time, because the 2 KT66 are to close from each other.
To rewire it, you must use wire with high isolation voltage. At least 1000 volts, I would recommand even more for the anode circuit of the KT66.
1) the 2 KT66 are too close from each other.
2) if the output transfo is very good, the main transfo is a calamity. Even when good, they become very hot, and with time, the inside isolation get out the trasformer when it become hot. It is the black-brown thing, and more this append, more the transformer become hot.
What I do is to command new and modern transformers on a local company that made them, and install them not at the place of the old ones, but in the proximity of the amps. The amp become less warm even after a while.
A fan can help to cold down those amps too. In fact it is a necessity if you want to use a such amp more as 2 or 3 hours at a time, because the 2 KT66 are to close from each other.
To rewire it, you must use wire with high isolation voltage. At least 1000 volts, I would recommand even more for the anode circuit of the KT66.
Quite so, D-F.
I once had a failed power transformer rewound on C-core to fit into the same enclosure, unpotted. That worked out cooler, but I also used 6L6GCs, which came in a smaller glass bulb - somewhat further from the cans. GZ34 also works a little cooler.
Still, I understood that 130 000 units were sold over the years - wonder if nobody complained and if so, why they did not upgrade......
And Frank, I have some new RCA black-plate 6L6GCs, (bought in the good old days) and I do hang onto them! Dropped one of my new original GECs the other day, and it broke.
Regards
I once had a failed power transformer rewound on C-core to fit into the same enclosure, unpotted. That worked out cooler, but I also used 6L6GCs, which came in a smaller glass bulb - somewhat further from the cans. GZ34 also works a little cooler.
Still, I understood that 130 000 units were sold over the years - wonder if nobody complained and if so, why they did not upgrade......
And Frank, I have some new RCA black-plate 6L6GCs, (bought in the good old days) and I do hang onto them! Dropped one of my new original GECs the other day, and it broke.
Regards
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