Quad II Mods/Tweeks

[johnm]

I think those amps MAY have been serviced by Keith Snook. He does the same thing with the original name plates (rubbing the paint off the back) as shown at the bottom of this page:

QUAD II Valve Power Amplifier Modernising and Modifying

I can't tell from your photo but it looks like the green resistor under the choke is a 5W Kiwame job? In which case it is good quality, but might benefit from some kind of addition heatsinking. Failing that a heatsinked 180-Ohm wirewound resistor would be worth fitting for peace of mind:

http://uk.rs-online.com/web/search/s...duct&R=1073942

- John

P.S. Regarding KT66s the Shuguang KT-66R (R for retro) are based on an original KT66 valve which was sacrificed to enable the design phase of this valve. I used them for 3 years with no problem at all. It IS Chinese, but made to a UK spec I believe:

Tube-Shop.com - Vacuum Tubes for Hi-Fi, Guitar and Radio amplifiers

The Gold Lion KT66 is also meant to sound fantastic:

Gold Lion KT66

[pred]

I was thinking of using something similar but not sure where to bolt it to the chasis. I thought to explore the 6l6 just since the shape is different and the tubes are similar.

Regards,
Pred

[stevieg]

[QUOTE][P.S. Regarding KT66s the Shuguang KT-66R (R for retro) are based on an original KT66 valve which was sacrificed to enable the design phase of this valve. I used them for 3 years with no problem at all. It IS Chinese, but made to a UK spec I believe:/QUOTE]
I was told that the Chinese bought the whole Mullard package including engineers to set up the plant. I've certainly really enjoyed these valves too. They look the dogs with the maroon base.

Don't get too radical with any mods - they sound fine as they are. It's like turbocharging a Velocette motorbike. It's certainly possible but you lose the charm that attracted you in the first place.

[Johan Potgieter]

Pred,

Some of your questions were answered in my posts #36 and #39, and others. As said, 6L6GCs could be used with advantage.

I looked at the 'DC - daylight" site. OK, the guy does say "modernising and modifying" but I prefer to just restore (refurbish, whatever). Using a Quad II chassis and components in another circuit no longer constitutes a "Quad II". If the wiring is quite different ... well, one can argue, I suppose. With due respect, the wiring of the worthy author of the above site was untidy. Did it make a difference to the sound - probably not. Did it make a difference to the character - definitely. I would not purchase his Quads.

But enough of that. The arguable R12-C5. Heavy weather is often made of 'the smallest dc influencing low frequency response (of the OPT); or effect of tube unbalance. May we just take a look at the original specs (1955 Quad booklet page 21): "Valve mismatching up to 25% (introducing 2nd harmonic) not to cause distortion to exceed 0.18%. Total distortion at 25 c/s not to exceed 0.25%.".

I have found that among some 30 6L6GCs or WXTs, the spread in plate current at 75mA was no more than 8%. Among EH 6L6GCs it was down to
6% and among 12 Tung-Sol 6L6GC-STRs, 5%.

I use a quite higher C5-value not for stability reasons. At maximum output the Ia will increase, even though one has class-A conditions. Under signal conditions a high enough C5 will simulate fixed bias conditions; tube data shows the improvement. That is why I use 2200µF (35V) for C5. (That is the one change I will confess to.)

Regarding R12 it was mentioned that you could move it to where C5 is situated at present, and mount C5 across the present tie tags. Keep R12 in the air; I use a 10W type which distribute heat somewhat more evenly because of its size. To repeat: Keep R12 away from C5 and from under the choke.

If not mentioned before, the h.t. voltage will rise to about 355V by using a GZ34. This is still within the maximum dissipation ratings - 25W for KT66, 30W for 6L6GC.

[pred]

Can't sleep, I'll be regretting it tomorrow at work

was happy reading your replies. Well with GZ34 it will come to 24W a lot close to maximum dissipation, now I am not sure if I even should try it with my GEC tubes?

I was asking about some NOS 6l6wgb since they are cheaper (I guess guitar people can not use them). Did you notice much of a sound change with 6l6gc?

I saw some 180 thick film resistors that look like TO220 package. I thought it would be not bad to screw them to the chassis using rectifier socket, but I guess your examples are easier to implement... I see that resistors are not that expensive so I will have to wait some time to collect enough of "things" so shipping will not hurt. I will listen for the moment hour or so.

My worry about 300mV at KT66 grids is no more. I read dc-daylight site and he writes it is perfectly ok. So I will leave it for the moment, find some nice resistors and order them to replace all of them in the amp, just to feel more comfortable.

Thank you guys again for help and I will report if I do something new (I replaced RIFA's that I have with some Vishay MKP1836 I think, no more difference but I feel more relaxed since it is a better capacitor).

Regards,
Pred

[Johan Potgieter]

Pred,

Oh - yes ... forgot that the rest of the world is not as fortunate as us having a public holiday tomorrow!

No difference in sound. The circuit topology irons out a lot of small differences, as demonstarted by quoting the specs. The dissipation comes to 22W total. Remember that Va is between anode and cathode, and that total current includes screen also.

And I might make some enemies, but don't waste money on over-expensive boutique brand components. E.g. polyester/polycarbonate capacitors are quite fine. [I will not upset members by touching on the 'audio quality' (sic) legends.]

Wishing you well with your venture!

PS: Wise words by Stevieg.

[pred]

Originally Posted by Johan Potgieter

Pred,
Regarding R12 it was mentioned that you could move it to where C5 is situated at present, and mount C5 across the present tie tags. Keep R12 in the air; I use a 10W type which distribute heat somewhat more evenly because of its size. To repeat: Keep R12 away from C5 and from under the choke.

After looking into my amp I am now convinced that the R12 is responsible for the choke high temperature. I want to change it but am not so sure on how to approach this.

What I like a lot are thick metal film resistors like these ones, and are not sooo expencive, 12$ a piece.

Arcol NPR2 TO-221

and I look for the place to bolt them since they need heat sinking. I indetified two transformer screws that could be used for the job, like in my picture. What do you think about them?

I must add I like metalic ones wirewound but they seem heavy to be hanging from the leads for a long time. What types you guys use? Can someone send some photos just to have an idea?

Regards,
Pred

[Johan Potgieter]

Hi Pred,

Very nice resistors indeed! We down here in RSA are not priviledged to have that kind of thing stocked locally. But it is unnecessarily sofisticated. I have used ww resistors similar to the type mounted there at present (a 10W version). There is also the possibility that a 25W Arcol might simply be used without heatsink, as the dissipation is about 3W only. Perhaps someone else can advise whether you could mount the Arcol without heatsink in this case.

If you do make that move, the best place could be the rectifier tube socket screw where C5 is situated at present. The contact area will be limited because of the limited socket tab area, but as said above. The screw (one of two) holding down the choke again brings heat a little closer to the choke, but not as seriously as before.

Hanging a 10W resistor by the leads is no problem. If you are worried, perhaps wind the leads a turn or so round the terminal before soldering. (And that little C5 shown in your photo, is rather scrawny. Try get a larger type, higher capacitance as explained before.

[perfrej]

Just a few notes... I did read some of the posts above, but beeing a newbie here on Diy there is so much to read.

I own a pair of Quad II's and they are my main source. I have fixed them up and learned a few things along the way, some even from Erik Andersson (Audion etc).

1) KT66 original GEC are the best tubes for the Quad II. I have tried a number of alternatives before I stumbled on a fair supply of early 1970's made in original boxes - all GEC. They are snappier and they are cleaner.

2) Capacitors. They need to be changed to modern ones. I even had a breakdown of the big double one, so I carved it out and mounted modern capacitors in the old box. Looks good!

3) GZ32. Not GZ34. Erik was very specific about that, but I do not remember what his point was. As I happened to have two new Mullard from the 60's in original boxes, I took the 34's out and put original 32's in. Anyway, he looked at the spec for 34 and said that the 32 is a much better fit, regardless of power handling and the like (I trust him).

4) Tube matching. I matched all my KT66's (a few in the box ) in pairs and use two pairs that where in the same range. Same with the EF86's - match them!

5) Resistors. I took out all resistors carefully and measured them before storing them. New metal film ones went in. Some values of the original ones were off by 30%...

6) Tube sockets. Put new ones in and lots of crackle and noise disappears.

I do not think that there is much sense in modifying the design itself - it's pretty good.

My Quad II's currently drives a pair of Yamaha NS1000M's in mint condition. Great combination. I also have a pair of mini monitors with Lowther DX2 with the PM6 cone assembly. Fantastic speakers and a perfect match to the Quad II's!

All the best to all, and keep them Quad II's in good shape!

/p