Fantastic show, Stuart! Thank you for taking the time to talk through the schematic. It's always nice to understand why. Wonderfully entertaining and educational as well. I'm sure I'll watch this several times. Mike
Another fantastic video. Probably one of the best, clearest demonstrations what does a leaky coupling cap do to the tube bias in the circuit. Few remarks: - the first "signal" 104 cap could have stayed in - its role is exclusively tone shaping (just like caps in the guitar) and doesn't need to block any DC - for the mains plug bracket (and similar): pieces of FR4 are very useful, they are much easier to shape than metal.
The extensive Keith Snooks website on Quad amplifiers shows the use of a 10 ohm resistor and small capacitor between the added earth ground wire and the signal ground, to ground the chassis for safety without creating a ground loop hum.
Yes but they were desgned without a ground and the customer insisted. I think they probably get a ground from the preamps. My guess is they wanted it 'floating'to avoid hum loops.
@@stuartukguitarampguy5830 , the matching mono Quad preamp did not have a transformer or power supply, it got it's B+ DC and heater voltage from the amplifier via an umbilical power cable plugged connecting the two via the 6-pin connector. The Quad preamp would not have had an earth ground wire! The customers preamp, however, might have a grounded power cord; and if it does, its often better to earth ground the amplifiers and lift the ground at the preamp in order to avoid hum, although this can vary from system to system. Anyway, if you reference the Keith Snooks website I referred to, he mentioned isolating audio ground from chassis earth with a 10 ohm resistor, which is often done in pro-grade amplifiers and some big home-audio amplifiers, with audio ground lifted above earth ground by anywhere from 10 to 100 ohms (some pro amps had barrier strip terminals on back where you could remove a jumper and lift the audio ground from the earth grounded chassis by inserting the resistor).
Nice vid however a pet hate of mine is snipping leads and soldering new components to them rather than to the connectors. I’m guessing that comes from my training and job history.
These are classic, valuable and collectible amplifiers and deserve the very best possible workmanship from any repair efforts; anything less devalues them.
@@stuartukguitarampguy5830Well I’ve never worked on passenger jet but I was a radio officer but the removal of old leads in a repair and creating a structural solid reliable joint is part of IPC soldering and inspection standards for reliability. Plus it just makes it look cleaner and more ‘factory’. But if the client is happy 🤷♂️
6:30 input goes to the grids of two EF86's and signal is taken off the plate for both - I don't see any phase splitting going on: both signals are in phase... is a parallel single ended amp? 23:46 what's wrong is drawing conclusions about problems and solutions without any testing... even though you know it's a valuable amp and in no way looks like a guitar amp, you just went in hacking. Why do all 'professional' tube amp repair services have to order each and every resistor they replace?
It's definitely a push-pull amplifier, utilizing a Paraphase inverter. The output transformer is a unique Quad design that only superficially bears some resemblance to McIntosh's BiFilar, cross-coupled transformer windings. Note that the output tube cathodes are further lifted above ground via the third winding on the primary side of the output transformer, and purportedly provide a measure of negative feedback. Stuart didn't spend much time looking into these amps online or he would have found that there's a guy named Keith Snooks who has an extensive website about these amplifiers, with a sensibly redrawn schematic, information about circuit function and specs, various brands of parts used Quad in building these, and so on. Anyway, yes, I share your other concerns and criticisms. PS, I would have simply removed the dual can-capacitor to get more space for mounting the replacement filter caps; It actually looked to me as if it might have been an oil-filled capacitor, and those rarely fail, even with advanced age, but an X-ray photo on the Keith Snooks website shows that they are standard electrolytic paper/foil caps.
Hi Mike Friend of mine made a fortune selling 'intrinsically safe' gear to the coal industry! I vaguely recal some of the principles behind it. Honestly, it's almost as if people have been killed/injured by stray electrical sparks!
I had a pair of these , KT66 power valves I think from memory . They used to provide a fantastic display of frightening purple arcing and never did work , a free gift from an uncle , had the control unit and a tuner . Only QUAD kit I ever did possess !
@@stuartukguitarampguy5830 Hi Stuart , Yes that was probably around 1972 or thereabouts. Your capacitor changeout and circuit analysis was really interesting and entertaining. I’m sure that it may have been the issue with my amplifiers although the Electrolytics might not have been that old then . I wouldn’t want to buy KT66’s now , if available! I’ve got a Fender Blues Junior clone called an Artist TweedTone 20R that has an awful hum which I need to try and get to the bottom of , my background was Telecoms for 40 odd years but I always fancied getting more into proper servicing and getting more Electronics education. I’ve been focusing on the Output Transformer area on this amp , the centre tapped windings read very mismatched DC resistance wise but I’m beginning to think this might be normal depending on how the windings are constructed. A real pain to get circuit diagram for this , I’ve changed all filter caps with no change whatsoever . It does have a pretty awful pcb with ribbon cables and valve bases so that might be worth a look . I will study your videos on Blues Junior faults and perhaps use the Fender Blues Junior schematics to get some ideas on test voltages . I admire your skills and tenacity!
Stewart, had you looked a little deeper online you would have found Keith Snooks' website on these Quad amplifiers, with numerous photos, redrawn schematics, tips on repairing them, component types and values used, audio perfomance specifications, used and so on. There's even *X-rays* showing that the giant can capacitor inside is a standard paper and foil electrolytic and not an oil cap as I might have otherwise guessed. The third primary winding the output transformer actually provides negative feedback and you have probably shifted the amount of feedback significantly by doubling up on the cathode-bypass capacitor value.
@@stuartukguitarampguy5830 not according to the analysis of the circuit as described on the website where he rewired and rebuilt one of these amps. He's clearly got a lot of experience !
@@goodun2974 Hmm I'd need a LOT of convincing. The cathodes are virtual ground (AC) and so feedback will jot affect that junction. Always happy (ish!) to be proved wrong though.
@shocked' - I get it! Well, two things. Firstly the amps were designed not be mains grounded and they were designed well after such safety issues were known about. Secondly the customer specifically requested not to ground it. Finally, these amps are, in gfact, grounded but usually through the signal connection. I think they are designed this way to avoid hum loops.
I'm wondering if the chassis is not grounded to prevent creating a ground loop from pre-amp to the Mono blocks. I'm only 7 minutes in, so maybe you cover this later in the video. SUBSCRIBED
@@stuartukguitarampguy5830 Are you certain that the caps inside the cans were electrolytic? I have some similar looking caps from that era (4uF) and they are oil filled paper.
The Quad could be said to an op amp made with valves using cathode ultra linear feedback to the KT66. By not using triode arrangement the voltage gain is high enough. Always use original valves if you can.
The 6 pin connector shown and referred to at about 43:04 is known here in the States as a Cinch-Jones connector, and it's not only the signal input connector but it also provides high voltage DC B+ and 6V AC tube heater voltage output, to feed a matching Quad preamplifier, which did not have its own power supply. Similar voltage output arrangements for powering a matching bare-bones preamp were implemented on vintage American made hifi equipment such as Heathkit and even McIntosh. Aside from reducing cost of the Preamp, this had the advantage of reducing or eliminating the possibility of ground loops between the preamp and power amp because they both shared the same power supply. This was okay if you had a single amplifier and a single preamp in a monophonic system, but became impractical when people bought a second matching amplifier in order to expanded their system to stereo. As far as I am aware Quad never made a self-contained stereo preamp that could get its B+ and tube heater voltages off of both monoblock amplifiers simultaneously ( ground loops were likely) and so this scheme was apparently abandoned when the whole hifi market switched over to stereo.
I had a message from another listener questioning if those 2 x 16uF Caps in the metal can were, in fact, electrolytics. That started me thinking. Do you happen to know?
@@stuartukguitarampguy5830 , the Keith Snook website I referred you to says they are indeed electrolytic capacitors (it includes an X-ray photograph of the insides of the cap can). Google Keith Snook Quad 405 and you'll find his extensive service notes and photos. PS, RU-vid failed to post my previous reply....
Hi Michael Usually low powered amps are cathode biased. It's kind of like a thermostat, it self regulates. With higher power amps you lose too mucch through that cathode resistor (the one which was already getting hot!)
Perhaps you could have rolled those 16+16 mfd Caps over. Cut the blank side out with a dremel and studfed the new caps inside the steel case. But who knows what nasty chemicals you release by doing that.
Despite doing 2,765 of them over the last 20 years you mean?? Your comment is totally pointless unless you make some useful observations which others can learn from. The amps were not working properly when they came to me. They now test out perfectly at a price the customer could afford. All the work I did, I left to be reversible if anyone wanted to in the future. So what exactly is your complaint?
@@stuartukguitarampguy5830 First of all, the way you handle the amps. was not very reassuring ! The job on the electrolytic was not good, you should have cut the button open, and put the new ones inside. Worst part was the coupling caps., if you have restored many amps. You would know that old coupling caps. need to be replaced immediately (especially Hunts).
@@mikaelchristensen5433 Mikael Thanks for the reply. I’m a bit puzzled though. That can would have been VERY hard to cut open and it would have looked terrible and also it would have been purely for aesthetic purposes. The bill for the customer would also have been much larger. I left the original in case anyone wanted to do that in the future, so surely nothing lost? I also changed all the coupling capacitors and proved that each was leaky. So your complaints are a bit thin really! It amounts to “you should have changed all those caps without discussing it first!” Anyway, thanks for the input.
