Dropping Resistors

[octal]

For some reason, I find that PI transformer tone control circuit amusing. Probably works just fine thought.

[J M Fahey]

To tell the truth, I rarely repaired them.
They were the typical, found everywhere PA system used (for the last 30 years) in zero budget, poor/working neighbourhood "Club Social y Deportivo -------------" , the typical local Club where everybody in the area played Football (Soccer), Basketball, old retired Italian guys played Bocce and various card games, chess, domino, and every Saturday night had some Ball/Tango dancing party; Family type of course, were hawk eyed fat mothers watched how close their daughters got to their boyfriends and did not hesitate to cross the sea of dancers and slap in the face the "insolent guy" who was hugging the girl too hard.
One of these amps, 4 sheet metal horn speakers mounted in poles or trees, a crystal microphone shared between "the singer" and "the announcer" and a crystal pickup turntable to play the latest Hits, were enough to keep the neighbourhood awake until 4 AM.
The typical complaint was that they hummed and buzzed a lot (dry capacitors), plus the sound distorted horribly and had some kind of noise gate (leaky capacitors wreaking havoc with bias).
I just sold them "revolutionary" SS amplifiers, 2x2N3055 transformer driven by a third one, with 4 inputs (count'em), similar to what Standel used.
The selling point was the silence, plus the clean power.
Only (slight) problem was that I had no 500 ohm output transformer but I wired series-parallel the 16 ohms horns until I got a suitable load.
In the few cases where they preferred to have the amplifier repaired (usually: "repair it cheaply so we can have a few more dance parties to raise the money") I changed filter and coupling capacitors.
I could get good Siemens (brown, rectangular, candy size) or Philips (cylindrical Mustards) polyesters to replace the leaky black cylindrical paper-in-oil ones, and Tesla Czechoslovakia screw mount electrolytics.
I hated wax covered capacitors.
The tubes still had some reasonable life left.
As KM6 says, tubes used "by the datasheet" live a lot.
If I had to replace power tubes, Philips Holland EL34 ruled.
6L6 were considered a lower quality tube, must have been because the few "good" ones were impossible to buy Sylvanias (so nobody used them), and most available were US surplus glass or metal 6L6 (plain, no extra letters)
Oh well, the old times.
Only test equipment, an old bakelite case Simpson or Triplett clone , later "upgraded" to a Central 200H 20000 ohms per volt multimeter, plus a monster 100W soldering iron, which could probably solder anything to a ship hull.
My dream test piece was a ¿Hansen? multimeter, complete with a 15 or 30 KV, foot long probe.
(ok ok, maybe it was 6 inch long)
Could never afford one.
VTVM? only on "american" Radio and TV magazine ads.
Scope? ... it's that funny thing which looks like the radar screens we see on WW2 movies, isn't it?
Or what Professor Mad has in his "let's conquer the World" Lab.