Bringing the QUAD II power amplifiers back to spec

Monday, 24 May 2010 20:26

Last Updated on Wednesday, 06 November 2013 21:10

Written by Jeroen

Sometime in the year 2004, I bought a pair of QUAD II power amps. They came along with a pair of bronze/gold ESL's. Both the amps and the ESL's were totally off spec and were really screeming for help. The following story is about what I did to get them fully functioning again - and how they are still functioning at the time of this writing (jan 2009).

One of the QUAD II's had a GZ34 and a EL34 + KT66 (GEC) fitted. The other had a GZ32 and a pair of ValveArt KT66 retro tubes - which were in full use by the owner at that time, so were the ESL's. Along with them came a third QUAD II for spares which had a faulty mains transformer (which I found out) and had a diode rectifier build in. This amp had been abused so bad, the mains transformer lost about 50% of the tar inside, and leaked all over the components and wiring underneath.  On the picture to the right it has been cleaned partialy already to see if the poor machine was still working.  Also note the two huge 100Ω resistors in series on the shoke.
The mains transformer, although a bit empty, still works and is in use.

First things first - I bought two pairs of retro KT66's.  Along with a spare GZ32 I could now try them out at least.  But discovered this was not going to cut it.  They needed a serious work out.  So I replaced all resistors and capacitors except C6 - hoping it was still alright as a lot folks seem to do.  That of course did not work for me!  QUAD GmBH - Germany - has replacements including a solid mounting plate, not cheap but the difference is very well audible.  Well recommend by everyone who wants the best - go replace them.  I have also seen more advanced replacements popping up Ebay from Korea. Anyone tried those?

Anyway, after replacing all parts I decided a better pair of EF86 and GZ32 could not do much harm and found a nice NOS Phillips set for cheap.

On the picture to the left, the QUAD II's are working very well again but looking like a pair of dug up army tanks from WW2!  Although I like things as 'original' as possible, these deserve a paint job to be honest.

 

I took the cover of a FM1 tuner to the paint specialist to match the best possible color car paint.  All tree QUAD II's had different colors despite original and the FM1 was just in between those.

 

 

The covers on the right shows 'real' color vs the matched car paint color.

On left you see one resprayed QUAD II, done with the same paint.

They were all sprayed by myself, by the way.

 

 

 

 

Along the way, I bought a cheap but crippled QUAD 22 controller from Ebay - sold as still working, of course.  But not the mother finest example (More about that later).  The QUAD II's are now in daily use - swapping them with a QUAD 33/303 pair each month or so.  I rarely use the valves in the summer as they simply produce too much heat.

A lot of valueble information on the QUADs can be found on www.keith-snook.info which is well maintained and regulary updated by QUAD entousiast Keith Snook... Looking on his website again - he has a nice list of real replacements for C2 and C3.  I (as most who replaced them) are now using modern film types for those and add a little C (22pF-33pF) from V1, V2 anodes to ground.

 

If your wiringloom had to go out because of the case respray, they might become somewhat sloppy and too short here and there.

 

 

 

To fix that you got the choice of rewiring point to point, which is good but not very original - or make a new wiring loom. I chose to give the last option a go and used a piece of wood and a bunch of nails to create the loom on.

To ease the work down a little and avoid faulty wiring, i made a multilayer image on the PC and draw each wire on a separate layer along with color info etc. By making the layers visible one by one - it becomes very easy to replicate them. Once all wires are up, they need to be stitched with waxed thread using 'Clove Hitch' (?) - mastworp in Dutch. I made the waxed thread myself using beeswax and a piece of thin nylon kite rope.

 

 

But what about a 'good' matching control unit? Check this out.