• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

My Quad II and 22 restoration

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Should I add capacitors to the inputs? If so what flavour? Do I keep the 1.5M resistor in parallel?

One would not expect dc on any pre-amp output, but anything is possible these days. So for the small trouble, yes, I would put in a series say 0,1µF cap at the input (polyester - as said previously, don't be tempted by expensive boutique components. They have no audio relevance.) You then need to retain a grid grounding resistor. The original 1,5M was that high because of need of non-loading on the particular 'designed for' pre-amp. You could replace with say 470K.

What's a good choice for the 0.1uf caps I need to use to mitigate the lost capacitance from the old metal can PIO caps?

Any decent polyester caps would suit. And while at it, use 630V types; no extra expense, some extra safety.

Referring to the 'lost capacitance', if you mean the C-to-earth, see my post #9. As said I compensate for changed phase effect on NFB by rather going for phase-lead compensation, adding the mentioned 1 nF in parallel with R11 (470 E). BUT: If you live in an area with heavy r.f. transmission all over the show (e.g. in close vicinity to a radio station), that might cause interference back into the input via long loudspeaker leads. The actual capacitance 'lost' when not using metal cans is about 18pF to earth each side; one can then add that to earth if desiring to keep the status quo. As said, the lead compensation gave a slightly better test square wave response.
 
Replacement set of KT66s arrived today, so finally got to hear these running. They sound lovely.

Next job is to fit some proper speaker binding posts, fused and switched IEC power socket and RCA inputs. Then I'm done for now. I'll listen to them for a while and decide if I'm keeping them long term or not. If they're staying, I'll probably chrome or respray them.

I kept mine on the basis that the minimal cost to renovate/modernise gave me a lot for not much. Even a secondhand set of monoblock tubey's or a DIY build will cost $hundreds more!

I resprayed mine with a dark grey (semi-metallic) paint and applied the Snook-method to the perspex name-plates. It resulted in a much more modern look, despite the age of them.
Didn't keep the Quad22 either, but have used a Grounded Grid pre-amp instead.

Just awaiting new chokes from Majestic Transformer Co, as one of them has migrated south in its pot.

Enjoy the ride!

TT
 
Referring to the 'lost capacitance', if you mean the C-to-earth, see my post #9. As said I compensate for changed phase effect on NFB by rather going for phase-lead compensation, adding the mentioned 1 nF in parallel with R11 (470 E). BUT: If you live in an area with heavy r.f. transmission all over the show (e.g. in close vicinity to a radio station), that might cause interference back into the input via long loudspeaker leads. The actual capacitance 'lost' when not using metal cans is about 18pF to earth each side; one can then add that to earth if desiring to keep the status quo. As said, the lead compensation gave a slightly better test square wave response.

What's your thoughts on adding a conductive foil tape to the outside of C2 and C3 to restore that 18pf? I've seen one company online that appears to wrap copper screening tape around them before clamping them back down to the chassis.
 
Finally finished these tonight.

d1cc9f3f9fb1fd4242ac83ec5498703a.jpg


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