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Old 21st January 2006, 10:38 PM   #1
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Talking Quad II Mods/Tweeks


Firstly, Like to thank everyone who posts on these forums, Ive just spent hours reading loads of posts and learnt tons! Cheers guys

I have a pair of Quad II mono blocks, In ok condition and working fine, but they are all original specification. They have newish valves in.

I have read alot of sites where people have claimed to have given these amps a "new lease of life" with afew modifications..

What I cant seem to find is much info on what these mods are!. I have done all the usuall to them, Putting in a phono input, Bannana jack out, IEC mains plug etc, but nothing to the actual circuitry.

If any of you guys have any suggestions to whats possible I would be very greatfull for any input

Thanks again guys, Keep up the good work!

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Old 21st January 2006, 11:12 PM   #2
SY is offline SY  United States
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It's worthwhile reading the circuit analysis of the Quad II in Morgan Jones's book "Valve Amplifiers". He suggests replacing the cathode resistor and bypass cap. Other mods mentioned are increasing the B+ by going solid state on the power supply and rewiring the output stage to accommodate tubes other than the rare and expensive KT66.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 22nd January 2006, 01:33 AM   #3
anatech is offline anatech  Canada
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Hi slurb.net,
You can use 7581A for KT66. The Electroharmonix may well stand up to this amplifier (6L6EH). I know those tubes are tough enough.

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Old 22nd January 2006, 08:09 AM   #4
Zombie is offline Zombie  Sweden
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Not much to modify is it?
Change the old elcos to new ones or PP caps. The old PIOs for new ones or even Russian teflons. Get NOS original valves. Experiment with different resistors types.
There are discussions about the crap SS diodes send into a circuit, why add SS rectification to a Quad?
Experiment with the feedback resistor, increase it to 2,7k for example (warning: might get unstable).
Increase the cathode R to 220R.
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Old 22nd January 2006, 04:39 PM   #5
SHiFTY is offline SHiFTY  New Zealand
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New manufactured KT66 are dirt cheap and pretty good. Why would you want to change to another type when these are easily available?

If I were you, I would leave it mostly alone, maybe add a bit more capacitance to the power supply, and replace old capacitors and out-of-spec resistors. It is a classic design...

Get yourself some mullard or telefunken EF86s and some NOS rectifiers.

By the way, don't cut any metal, you will lower the resale value a lot. Reversible changes only!
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Old 22nd January 2006, 05:09 PM   #6
Zombie is offline Zombie  Sweden
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shifty: I can agree with that, Saratov KT66 are good value for money, more slam and "raw" sound compared to NOS GECs,, but hey, the price difference...
The GZ32 can't take more than 47uF, though. Read somewhere that the second psu elco can be three times that size, but haven't dared to test it.
A non-inductive cathode R might be nice to try...as is less feedback
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Old 22nd January 2006, 10:02 PM   #7
EC8010 is offline EC8010  United Kingdom
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Originally posted by Zombie
The GZ32 can't take more than 47uF, though. Read somewhere that the second psu elco can be three times that size, but haven't dared to test it.

A non-inductive cathode R might be nice to try... as is less feedback
The second HT capacitor can be almost any value you like.

A non-inductive cathode resistor sounds like a very good idea. 10W thick film power resistors are available in this value and would make an ideal choice.

I assume everybody has seen this picture?
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File Type: jpg quad ii xray 1.jpg (54.3 KB, 1637 views)
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
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Old 22nd January 2006, 11:50 PM   #8
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Thanks guys for all your suggestions, Shall let you know what I end up doing!

Just another quick question regarding the GZ32 rectifier, One of my amps has the original GZ32, and the other has a replacement which is actually a GZ34.

I notice that GZ34's are much easier (and cheaper!) to get hold off, but is there any advantage/disadvantage with using them in the QuadII ?

Thanks again,

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Old 1st February 2006, 02:42 PM   #9
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The GZ34 is an evolution of the GZ32. It can handle more current with less voltage drop on it, so it is a very good remplacement for the GZ32.

You will get more output power with it, without the need to use solid state diodes.

A thing you must be aware on the QuadII is at it become too hot with time, because the 2 KT66 are too close from each other. So, if you want to use it more as 2 or 3 hours at a time, you MUST install a fan in the proximity of each amp. Click the image to open in full size.
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Old 1st February 2006, 07:07 PM   #10
cerrem is offline cerrem  United States
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I have used the Russian KT66 with good sucess in the QUAD II amps.... One main reason is the low plate voltage of the QUAD is kinder on these new production tubes....
I used the Groove Tubes KT66-HP and tried various strengths to measure agianst an original as well as figure which one sounds best in the QUAD II... It happens to get the #8 ratting from Groove Tubes for the KT66-HP for best Hi-Fi results....

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