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Quad II help needed...
Quad II help needed...
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Old 24th November 2021, 07:07 PM   #21
waltube is offline waltube  Italy
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Please check each resistor for right vaue.
One of the problem of these old stuff are the values of resistors ( but also caps) that have a differences from the original value.
I have done lot of checks on this amps ( but the problem is similar to Leak stuff) and after replacing the reisstors and caps the amp return at the best performances.
Then I give to the owner a little bag with the original components, just to remember!

Walter
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Old 24th November 2021, 09:41 PM   #22
Sorento is offline Sorento  Europe
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Some useful information about Quad-II problems and modding can be found here:

QUAD II Valve Power Amplifier Information

~ QUAD II Valve Power Amplifier Modernising and Modifying ~

quote:
Resistor R12 is under-rated

The R12 resistor fitted by QUAD is rated at 3W but under the best conditions will dissipate about 3.75W and in some circumstances even more ~ Even if it looks okay it is best to replace R12 with a 5W or a 7W resistor like the TT electronics W22 type shown opposite and at the same time replace C5 with a capacitor adequately rated at a.c. and as well as d.c. (see bad examples in buying guide)
:unquote

Last edited by Sorento; 24th November 2021 at 09:47 PM.
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Old 24th November 2021, 10:40 PM   #23
ahankinson is offline ahankinson  Europe
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The heater problem may have been dirty sockets. Get some isopropyl alcohol and some of those inter-dental brushes and give them a good cleaning, if you haven't already.

The Quads don't have a blocking capacitor on the input so any DC from your preamp is likely to pass straight to the EF86s. The inputs are sensitive enough that running it from a phone / cd player directly (to test) works fine.
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Old 24th November 2021, 10:58 PM   #24
ejp is offline ejp  Australia
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Quote:
Originally Posted by Sorento View Post
Resistor R12 is under-rated

The R12 resistor fitted by QUAD is rated at 3W ...
This is often stated, but in fact I've never seen one fail. It is mounted well up into the breeze where convection can occur, and it's a very sturdy looking component bigger than what it usually gets replaced by, so maybe it was conservatively rated in the first place. Clearly in the application it is well able to handle the wattage.

By all means replace it if you like, mounting it the same way, but neither this resiststor nor any other can possible cause a hum.
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Old 25th November 2021, 07:18 AM   #25
funkuncut is offline funkuncut  United Kingdom
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I'm basically happy with the sound, and while I am not at all bothered about originality I don't want to change things that work ok. The hum/buzz really bothers me though - does the solution lie in adding an input capacitor to block dc on the Quad II? Or is there a fault with the 33? Hope this topic is not straying too far!
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Old 25th November 2021, 07:55 AM   #26
OldHector is offline OldHector  Sweden
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I following Patrick Turners recommendations for the basic mods to the Quad II, for example seperate cathode bias resistors ...

Patrick Turner Quad II

I added the DC blocking capacitor to the input, but initially I missed the extra resistor to ground for the input tube and had some nasty instability if I disconnected an RCA.

It is a great read, that site, and I really hope there is a custodian to take over now that Patrick is no longer with us.
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Old 25th November 2021, 08:21 AM   #27
funkuncut is offline funkuncut  United Kingdom
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Thank you - I have had a read of his site, and enjoyed it! I am particularly interested to hear of real experiences - theories and recommendations are one thing, but I am not sufficiently qualified or knowledgable to judge on my own. I wouldn't want to modify extensively or 'improve' the Quads but using modern components seems perfectly logical - I'm sure that manufacturers in the past used the best components available (within the budget dictated by the accountants, of course) I would, however, like to prevent the very loud buzz/hum if used with the 33, and add a safety earth connection which doesn't use the signal cable, so any experiences of following recommendations from Patrick Turner or anyone else would be most welcome.
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Old 25th November 2021, 10:21 AM   #28
waltube is offline waltube  Italy
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Splitting the resistor on cathode of KT66 ( each one doubling the original value + caps) will help also to check the bias current if it is the same for each tube. Pairing the KT66 is a great thing.
It is not a difficult job.



Walter
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Old 25th November 2021, 03:53 PM   #29
ejp is offline ejp  Australia
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Quote:
Originally Posted by funkuncut View Post
does the solution lie in adding an input capacitor to block dc on the Quad II? Or is there a fault with the 33?
No and no. Hum isn't DC. There is only a fault with the 33 if there is no hum without it.

Is it really a hum (50Hz sine wave)? or rather a buzz (100Hz triangle wave)? Does it happen with no preamp connected?
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Old 25th November 2021, 05:21 PM   #30
funkuncut is offline funkuncut  United Kingdom
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It's more of a hum, but really loud. With nothing connected to the input there is just the faintest hum, but only audible with an ear almost touching the speaker cone, and there is no earth connection. No more hum than there is from an old Hacker valve amp connected to a 99db sensitive speaker in my main system anyway - and that uses the 33 with an active crossover between it and the amps. With an iPod connected the faint hum is diminished, but with the 33 connected it becomes really very loud!
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