Wow, that was a lot more info than I expected. Thanks!
Re Usenet: Yes it is not always polite, and sometimes people cannot cool down in time. But it comes with the "unmoderated" territory. It is what Internet was and is about for those of us who have been around here long. I might not personally like everything I see or hear but, hey, at least everybody can say what they want. So flame me, ha!
(actually dont, Im not here for that purpose)
Back to Quad:
Lots of food for thought. Thanks for sharing the ripple voltage measurements under different PSU configurations. Just to clarify, I use both box type "studio" speakers, 8 ohms, and original ESL57 (rebuilt EHT blocks) with this pair of Quad IIs. Amps are strapped for 9 ohms. No fires yet. But I see why you would want to put warning and shutdown circuits in your amps. If this was something I built for someone else, which as a hobbyist I dont do, then one needs a lot of built-in failsafes. As a tinkerer, I open up my amps now and then and check for any health problems. So I dont worry about fires yet. My fake Williamson looks like a rat's nest inside (a "work in progress", over years) but actually had no issues whatsoever in 15 years of constant use. I caught and changed a dry PSU cap once, before it could make a mess. On the other hand, would I want my kids to inherit this? Uh, not before checking the safety arrangements, and maybe tucking the whole thing into a grounded cage. Which kind of proves your point Patrick, so I think I will look into at least separating the QII KT66 cathodes in the future. Also, these chassies were probably late production, with exactly fitting holes for new IEC power sockets already there.
But the mods I routinely do to old original junk like Quad-II does make them last longer, ie, KT66 life is longer, and their safety is made better. Both issues require circuit changes that exceed what you are doing just by changing the two main electros in PSU.
Well, actually, I had to rewire the whole thing (both of them) from scratch, using some original wire looms from two other junked chassies. The previous owner had gutted my amps to build something else but had given up. I put everything back in place except the dangerous HT wiring at the Jones plug. Nowadays there are standalone preamps that are nicer (but not as good looking!).
OK, you are in love with the crummy old quality of the past.
Yes! It has its charm. And the fact that the Quads use a strange circuit, cathode feedback, pentode inputs and too high grid R's, is interesting. It is very different from the other things I have seen or built for myself. I mostly like tubes like 5687 or ECC88 driving 6B4Gs or triode strapped 6L6 or 807 in SE or PP. But I know I am not in any way an inventor in this field, since I usually just start with a known good base circuit, steal it, and tweak it using whatever old iron and tubes I can find.
Slightly OT: Old Jags apparently had trouble with the Lucas electronic components. Some people gut them and rewire everything with modern wires, LEDs and electronic ignition timing. There are crazy forums for that too online
To minimize hum, the 0V rail must have 5r0 between it and 0V rail, and if there is a B+ fuse, best place is between HT CT and 0V, NOT to chassis "ground."
Original Quads do have a 5r0 between 0V and Chassis. Now have a real good look at your original Quads' earthing method. What do you find?
No, I missed this. There are, AFAIK, no 5R resistors anywhere, unless you are looking at the long wire from the "E" point.How does it work? Was it intentional, like the C to GND of the PIO caps? Or did I rewire the whole thing wrong....
You can have one KT66 operating with red hot anode, the owner is oblivious, and amp limps along for awhile damaging itself, maybe towards a very expensive repair. SO, the ONLY remedy is to have some active circuit which detects excessive Idc flow in output tubes and this turns off the amp and a red LED lights up which tells an owner "I will NOT work until YOU fix a problem".
Sure. But Id rather have a magic eye, neon or at least a funky Nixie lighting up, instead of those LEDs!
Tube amps can be hell to live with. I've repaired and rebuilt tube amps which have nearly burned houses to the ground. Get real, Old junk is always old junk unless proved otherwise, and much must be done to save them from themselves and from dopey owners so that THEN the past can be really enjoyed, with added benefit of better modern parts.
Ive worked on a few old guitar amps for musician friends, with varying results. Original AC30, Fender Twin and such. THAT is a can of worms. They are not at all babying their gear like audiophile guys do. Half broken wooden boxes, redplating tubes and loose wires everywhere from previous DIY attempts. And the owners dont want any new R or C because that would alter the sound :/
I'm preparing a page about how I rebuilt a pair of Quad-II-Forty. These Chinese made souped up versions of Quad-II have many of the same horrid features to be found in original Quad-II.
Patrick Turner.
I just saw that one! Nice writeup! Good to see you got rid of the ugly PCB. I am curious about the thermistor they used on the rectifier in the original version. What kind was it? Did it work? Did they REALLY not bother with a dedicated 5V winding? Crazy.
Martin
(not having nearly as much time as I would like to tinker with stuff or even read here - day job and family comes first)