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Might as well jump in with both size 13s and no life jacket. In no particular order, and based on my direct experiences:
a) If all four amps were assembled in sequence, there is every possibility that the same 'mistake' was made on all four units. There is no shame in this as if the instructions are not absolutely clear, the first mistake is made. And the amp passes the most basic preliminary tests - NEXT! It may be 5,000 miles between *you* and him - but it is likely not even 200 km between him and another decent tube guy. What I am suggesting is that a 3rd party go through the amps and the schematics.One who has never seen them before. This person will have no preconceived notions and no expectations. Writing for myself, a neighbor had put together a tube guitar amp - which was behaving very badly, even though all voltages were present and correct, the bias set properly and the amp was dead-silent with no input. He fought with it for almost two weeks. Then he called me over.... I looked at it for less than 5 minutes and asked him about a specific pin-bridge that was not in the schematic - though others were. That was it. One clip with a pair of mini-cutters and he was in business. Like that.
b) Wire Dressing: Are the wires to be shielded properly done? *Twisted Pairs* are typically to be connected only at one end. Make sure that the are not connected at both ends. And are any shields missing? Are adjacencies correct? This may take a bit of experimentation - and in that case do only one (1) amp at a time until it is right (if this happens to work) before applying to any of the others.
c) Pinched wires - you would be shocked and awed at how often this happens. And if a chassis layout is particularly open to such a problem, no reason at all it could not happen four times. Really. The Dynaco ST120 had a real problem that way as one common example.
d) I am working on a Scott LK150 at this moment that pins the bias voltage meter on the left channel - even though the reading at the test point is correct, and the amp plays perfectly well. I am working backwards from the meter - but here is a situation where appearances are not what they suggest. I expect to find out a mis-connected wire, or a wire that looks good, but it is the insulation holding it to the pin - the broken conductor being hidden. Anything wired with solid wire will often have this issue. So check *EVERY* connection. Use a dental pick or similar to pull on the connection if deep in the chassis.
e) Look for common parts to _all_ the amps, and that are in the relevant circuits. Even if they came from separate suppliers, even in different countries, if they all came from an ultimately common source as already noted they could all be equally defective.
Repeat: In no particular order!
I have my "weirdness" stash. One is a GE 5AR4 that tests perfectly well on a very high-end tube tester (Hickok 539B), and will play perfectly well in an amp - for about 15 minutes. Whereupon it goes cold. No heater at all, cold. I have reheated the pins and all that. But, there it is. Give it about 2 hours to cool down - repeat. Does not matter in which-of-three amps.
I have a 20uF@500 V, 105 C (Panasonic) cap that operates at about 400 V in-circuit. It will run for hours in a vintage radio at low volume (6F6s in PP output). If I boost the output it goes open - and that can be ugly! Lower the output, it comes back. Don't ask me how long it took to figure that out.
So, don't be surprised if you find something strange.
Best of luck!
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