Quad II

[Paul Barker]

So the pots shown, shall be double gang linear wired conversely, start point where resistance equal. When imbalance of voltage across matched 10 ohm resistors twiddle 10 turn pot whichever way fixes the imbalance.

All coupling caps changed for pio. All power supply caps polypropylene only cathode resistor bypass cap, pio.

[Paul Barker]

Preamp

[Paul Barker]

That name I lost is Keith Snook.

So if I dont find my ef86’s rather than buy them at todays prices I can convert to Keith Snook’s cascode ecc88 which doesnt require much adaptation and drives the 66’s with more currect, mind thats still only 2 mA.

Did I say I have more than “enough” ecc88’s, and actually I like them.

I find Keith Snook confusing but his ecc88 circuit doesn't need much alteration to the quad original. I appreciate keith is a clever cookie but his presentation isnt suitable for people who after a stroke have only series connected thoughts one at a time. He is all over the place.

That only leaves one problem. GZ30’s are through the roof on price. I thought Derek said on the phone yesterday he had some for £7.50 but cant see them on his website. I’ll have to phone him again and order them that way unless I misheard the price.

I’m actually considering, shock horror, soldering some diodes to the pins. Maybe at least to get me off the ground.

Anyway yes, ecc88 cascodes sound best option up front.

I also learned from
Keith’s written word the reason the single cathode resistor cap exists at all is to limit the bass as the opt’s cant handle much bass. Well that explains a question as to why a cap at all, its there to limit the bass. I knew there must have been a reason for such an undersised bypass cap. So I won’t alter that value. Apparently the opt’s are quite on the limit and it doesn't take much to knock them off their perch. Scary thought. But because ai like the idea of cathode feedback, its the main attraction for me. So I better make them happy.

[pre65]

I thought GZ32 was for Quad 11 ?

[Paul Barker]

I thought GZ32 was for Quad 11 ?

Yes I think youre right. There I go again. I make a lot of mistakes in a row with my post stroke cognitive mind.

[simon]

The price escalation of decent nos 5V rectifiers is painful alright.

If you can bear the compromises there are a lot of decent damper diodes available for cheap still. For instance 6AX4 - I bought them for $1 each, last time I looked they were $4 each I think. They're 6.3V rather than 5V, 1.2A each, will do a lot of volts HT. There's warehouses full of them in the US of Trumpland.

Use two for full wave, or one in series with SS rectifiers, or in the earth line as jc morrison does.

[Paul Barker]

Thanks for the tip Simon. I calculated that if Derek’s are 7.50 after vat carriage they cost £28 for two. So I bid on a pair of Mullards that test good. If I win I’ll pay more £32, if I dont win I wont bid any more. Then I’ll ring Roko to find out if there are some for £7.50.

Im glad I found the answer to my confusion of why the single cathode resistor is bypassed at all.

I recon that bias adjuster circuit I proposed above would fit on a plinth with two vu meters and the dual gang pot, and I can always ensure no dc current. Pat on the back Paul.

Now then, Patric Turner took it all to the split ends of the feedback winding but bypassed without consideration of protecting opt from too much bass. Not quite as clever, but not right to speak ill of the dead.

Didnt find any bargain rectifiers on Langrex.

I could just put two diodes on the valve base and one of what you said on the ground leg. Increase the bias resistor ohms to reduce the current flow as the b+ would still go too high.

[Paul Barker]

One benefit of the 6ax4 on the ground it would reduce b+ 22v at the 144mA current, which might be enough. It would also fit the existing base, just take the pinouts to different places and put two diodes somewhere. 5v heater voltage is probably fine and may provide further b+ drop. All working together.

[Nick]

Im glad I found the answer to my confusion of why the single cathode resistor is bypassed at all.

I cant see where you said what the answer you found was, but I would normally expect it to be because the output stage can and will move from A into B, so the current through the cathode resistor will vary. The cap is there to prevent that creating degeneration.

[Paul Barker]

Keith Snook

“ If it is any higher than say 47µF it could allow any low frequency imbalance current to saturate the output transformer ~ Making C5 higher than 100µF as some other QUADII modification sites advocate could be done but the low frequency response at the input should be tailored to about –3dB @ 30Hz ~ You cannot get more out of the little SPEC 1003A transformer than it is designed fo”

[Nick]

Ok. But none of that says why it's there, just what it's value should be.

[Paul Barker]

I’m still looking for where KSnook put the detail of the roll off, but while looking for that I read what follows which is right up my street. 2nd harmonic more se sounding and well suited to Q57’s.

https://keith-snook.info/stuff/Buttons/NB-G-80.gif

[Paul Barker]

That didnt open though.

Ill try something else.

The QUAD II negative feedback is normally taken from only a third of the output winding [ R11 connected to P ] for the 15Ω tap using only link S–R ~ And and from half of the output winding for the 8Ω tap using links Q–R and S–T ~ Thus the feedback resistor does not need to be changed when the impedance links are changed

Being a QUAD amplifier there is most likely another reason why it is done this way and it is probably to keep the amplifier stable with load ~ Especially if the load is an ESL57 or its predecessor ~ With only part of the winding in the feedback path a low impedance cannot completely shunt the feedback and cause the amplifier to overload sooner

This is the exact opposite of the QUAD I where the feedback was taken from the 15Ω tap and when the 7Ω tap was used the unloaded section of 15Ω still provided the feedback the same as the LEAK TL series ~ There is also rumour of a small value resistor in series with part of the QUAD II output winding and having undone many of them I confirm it is so

I wanted the output taps set to 8Ω so linked S–T and Q–R but then provided overall feedback by connecting R11 directly to the output terminal ~ I also wanted the sensitivity to be increased by applying less feedback ~ With a 2k7 resistor for R11 connected to the output terminal the sensitivity was about 0.5V for 10W and with these amplifiers clipping at over 20W due to the higher HT voltage from the GZ34s and the 60µF smoothing cap this was fine

The main effect of the feedback change is a reduction in output impedance or greater damping factor if you prefer ~ When coupled with ESL57s or ESL63s there was a noticeable lack of Bass most likely due to the lower resistance drive ~ With the normal feedback the output is more current source and the apparent extended Bass could well support the argument that ~ the best amplifier to drive ESL57s is the QUAD II ~ provided it does not have its feedback modified

Distortion with less but overall feedback was actually no worse than with the normal feedback arrangement and in both cases the second harmonic is higher than the third etc. ~ The harmonic distribution is unlike a normal push pull output stage where the even harmonics are usually much lower than the odd due to transformer cancellation

The QUAD 11 distortion characteristic is more like a Single Ended amplifier than a push-pull design and this may be it's appeal to those who listen to music rather than read specifications ~ The prominent 2nd harmonics are due to an imbalance in the drive to the KT66 grids which could be reduced by varying R8 but this would have to be done for each specific pair of EF86s and would probably ruin the 'warm'. . . 'musical' . . . sound of this amplifier !

[Paul Barker]

Ref the -3db frequency of the 22uF bypass cap on the 180 ohm cathode resistor. My calculator shows



Now the link not working. My calculator comess up with -3db roll off 47hz for KT66 Tetrode mode. I did put a link to the maths but it isnt working.

Somewhere Keth mentioned a similar figure but his stuff is hard to find again, I havent found it again.

Oh, then my calculator opened. Go figure.

[Paul Barker]

Keithe said 47uF should be the absolute max cap. My calculator says that is 22hz -3 db, dicing with the output transformer and q57’s wont ever go that low anyway. I’ll just use the spec on the official diagram.