Ed;
Hi Dimi,
Wow now you are running hot mate,, good work.
You will be the expert soon.
You posted just as I finished this but I'll post it anyway as it's relevant and may help.
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Number 1;
I think it would be wise to establish what AC voltage is coming from the transformer that powers the heater filaments,, FIRST!
Then move on to other possible problems.
Remove Valves
Set meter to read AC volts;
REMOVE the 2 filament wires that go to Bridge rectifier.
(in the pics I see on the net they might be Orange but you better check)
Insert probes in those 2 wires and read the AC voltage,, then post results.
For AC filaments the no load voltage should be around 7VAC, give or take a few points.
If the transformer was wound for full wave rectification then it should read less,, maybe 5VAC.
Yes you can power up with a power tube in place and read the heater pins on the back of socket, it will only take a minute
If it worries you then you can likely remove the high voltage wires from the pcb but leave the filament wires in place to power the heater,, (looks like all the transformer wires are spade cons so should be easy to remove B+) should only take a minute to read the loaded heater voltage, then turn off.
Regards AC hum,, Again misguided design Xpurts.
The PCB tracks are in the wrong place hence heater hum.
my guess;
To lazy to reroute the pcb with better layout so they just slapped a bridge on the heater supply.
which did help hum but now the filaments are running too hot.
Do they care? I doubt it.
They sell trashy crap with a brand name on it and people buy it cause it has a big name brand on the front. I fixed a couple of Fender pro Juniors (some models had a really bad hum)
The most sensitive signal path in the whole circuit ran a trace right next to the B+ track.
I simply Replaced that track with a shielded cable directly to valve socket and hum was gone.
And that is just FENDER,, other big names make similar F*** ups.
And you pay big dollars for the privilege of their name on the front.
From the RCA Valve manuals; "Triodes should last for 10,000 Hours at least,, power valves 5,000 hours."
If they don't it's usually because of a crap design in the circuit.
I read a report that a uni had a valve computational circuit which run 24/7 for 30 years.
Yet The best names in guitar amp world still don't get the joke,,, but the shops sell a lot of new valves every 2 years. go figure
Marshall are notorious for burning out EL34's because they run the screens way over voltage,, and still doing it.
Regards Amp distorts too soon;
Easy to understand once you study a bit of history.
Back in the 40/50's these type of amps were popular because a jazz player could lift has volume.
Those old guitars had low output PU's and there was enough power to lift the sound to balance against the Brass section.
Now even the cheapest guitar with crappy PU's has many times the output of those old semi acoustic jazz guitars,,, Result = your Amp cannot play clean with a high gain PU's,, Unless you turn right down.
If you want clean then you need a bigger amp to get clean headroom.
Rant over.
Phil.
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