Well, it isn't all fun and games it seems. Before plugging the STA15 into my 57s I thought I'd plug it in to an oscilloscope, and I'm glad I did as one channel was misbehaving, going into oscillation with the amplitude of the oscillation quickly shooting off the scale. I swapped the valves over between the channels and the oscillation stayed with the channel. Hmmmm. I checked and double checked all my wiring connections both for good solder joints and connecting the right bit to the right bit - at least I had one channel that was working to compare to. But I couldn't find the mistake. I even buzzed out all the wires, all the transformer resistances, double checked for solder bridges on every connection, everything that I could think of.
Strangely, the only different in voltages I could find was between the channels was that over cathode resistors; one channel as about 25.5VDC and the other 21VDC.
I was a little stumped, I mean the wiring is not that complex. So what's the best thing to do in those situations? Go to bed and sleep on it, and I followed my advice to the letter.
Thought I'd have another go today.
I took the misbehaving PCB out by unsoldering the top output connections and the HT connection, but leaving all the connections to the valves in place. When I double checked all the soldering on the PCB I couldn't see anything out of line, all the resistances between the two PCBs where the same, nothing leapt out at me. I did rework one connection between the PCB and the ECF82, but only because the routing of the wire didn't quite match my OCD. Without much hope I re-soldered the top connections and the HT, screwed the PCB in and hooked up the sig gen and oscilloscope and the bloody thing worked fine. Checked all the three speaker taps, still worked fine. Poked around the PCB and the valves with a chopstick, couldn't upset it. So, I am in the dark, but it works...yeah
I took the opportunity to run the amp up to signal chipping to measure the output power. I don't have the ability to measure THD so my measurements are kinda crude.
Previously ill channel (Yellow)
SDS00033 by
Garf Arf, on Flickr
SDS00034 by
Garf Arf, on Flickr
SDS00040 by
Garf Arf, on Flickr
Working all the time Channel (Purple)
SDS00037 by
Garf Arf, on Flickr
SDS00038 by
Garf Arf, on Flickr
SDS00039 by
Garf Arf, on Flickr
Both channels are peaking out at about 13.2VDCrms in to an 8R dummy load which gives a power per channel of (V^2/R) 21W. I would imagine the THD is quite high at that output.
The square wave of the yellow trace does have a very slight "ring" to it but nothing bad. In fact I was expecting to see a much more rounded set of square waves, much like the ones the Quad IIs generate. But the STA15's are very straight up and down.
So, time to put the handles on, the baseplate on and slide the cover on. After that I'm gonna check it again on the bench, just to build some trust between us both.
Tomorrow I see speakers in the mix...
PS. I've ordered a pair of NOS Telefunken ECF82s
PPS. Is there somewhere I could have these Mullard EL34s tested? I would like to know how "fresh" they are.