Quote:
Originally Posted by turretslug
... The Quads strike me as stretching minimalism a little too far, with the well-known KT66 unwisely-high grid resistor drawback being a manifestation of this ...
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Given that the KT66s' anode and screen voltages and dissipations aren't pushed to the maximum in the Quad II, the 680k grid leaks work OK as long as a) they haven't drifted high in value (but after a few decades the Erie carbon comps will have) and b) the interstage coupling capacitors haven't become leaky (but after a few decades the Hunts paper-in-oils will have). The amp can suffer, but it's not really a circuit issue, it's that decades down the line the components are giving up the ghost.
Other manufacturers also worked close to the limit. Stephen Spicer in his book
Firsts in High Fidelity describes perhaps the most notable example of this. Harold Leak wanted to spec his TL/12 Plus amp (replacement for the legendary TL/12) which would be based on Mullard's new EL84 at 12 watts output with, like all the Point One range, 0.1% distortion. Mullard themselves had said in their 5-10 design that a push-pull pair of EL84s could deliver 0.3% at 10 watts or 1% at 14 watts. Leak breezily assumed that they had built the EL84 with the same engineering safety margins that previous manufacturers had always put into their valves (and their data sheets). So to get what he wanted he over-ran them a bit. Unfortunately for him the EL84 was perhaps the first example of a valve which was
really tightly manufactured. You could run it at the design maximum, but if you went even a little beyond that it would fail. Leak already had plenty of TL/12 Pluses and Stereo 20s in the field when they started to come back with catastrophic 'burn ups'. A substantial redesign was forced on the company, including a whole new output transformer. The output spec was dropped to 11 watts and to be honest seeing 0.1% distortion at that power from this amp is a rare thing indeed.
If you want an example from Radford then take the bottom off an STA25. There's a picture of one here
http://hifi70.com/wp-content/uploads/2015/10/top2.jpg part of which I've cropped out and attached. It's had some work done, including replacement of the ringed resistors. Note the scorching on the pcb underneath them. That's quite typical. The resistors are R32 and, on the right hand pcb, R10 as well. R32 is part of the potential divider which sets the screen voltage for the ECF82 pentode. It would originally have been a 68k carbon film Iskra rated for 1 watt maximum. Radford's circuit diagram shows it running with 265V across it, so V^2/R is more than a watt. And if your mains voltage is a little high (or, heaven forbid, your toddler has rotated the all-too-accessible mains voltage selector) and/or the resistance is a little low and/or the ECF82 screen grid isn't drawing quite as much current as it should then the resistor will run hotter still. As the scorching shows. It really should have been a wirewound.
Long story short - quite a few amp manufacturers built up to, and sometimes beyond, the component manufacturers' specs. These specs were often generous and (except for Leak) there was rarely a problem until long after the expected lifetime of the equipment.
Cheers,
GJ