Teething troubles. Advice needed please.
What causes a nasty buzz from the amp tubes when I strum a little harder than a gentle strum?
The amp, when resting, is nice and quiet. No hum, no buzzes, no pops or bangs, nothing is overheating.
If i strum gently, no problems. But if I strum a bit more aggressively, i get these sounds like electric arc-ing: nasty buzzes. This all happens at low volumes. I haven't tried turning it up high yet for obvious reasons.
The EL34s are set at around 28 mA (measured by getting around 28 mV across the 1 ohm resistors I put in the tube grounds) and I am using a brand new set of four balanced JJ EL34s. AFAIK they're all good, bit I don't have a tube tester. I'm not seeing any of them suddenly going bright red and burning out.
The three ECC83s and the single ECC81 in the preamp stage seem fine, and swapping those around with spare ECC83s I have from my Origin50 doesn't alleviate the problem. I've got a brand new ECC81 on the way, that should arrive today.
I've seen YouTube vids by Stuart Smith, a decent UK amp repair guy in England, where he often says JJs, 4 tube Marshall's and high plate voltages can cause "arc-ing over", and I'm thinking that's what I'm hearing. But I'm not sure that's what he means, so a description of this would help.
I also had this noise on my Origin50 a while back, and the Marshall official repair shop in Houston said my tubes were "weak". He replaced them, re-biased and all is now good there.
Also, if I run the Hiwatt clone with just two tubes, I dont get this problem.
I'm going over my wiring again, but is there anything specific anyone can suggest checking?
Also, I've heard comments about running JJs at high plate voltages can be the issue. "They're not as good as they used to be" kind of thing.
I presume the "plate voltage" refers to either the anode voltages (pins 3) or the screen grids (pins 4). The pins 3 are at around 467 volts, the pins 4 around 450-460. I don't know how I could drop the anode voltages as they're connected directly to the output transformer. The screen grids are fed through 100 ohm resistors. I could up those 100 ohm units to, say, 120 or 150 ohms, but I'm not sure that's the right thing to do. The control grids (pins 5) are fed through 22k ohm resistors, BTW. Those could be changed too. All the resistors are metal film 1% tolerance and were individually checked by multimeter before fitting.
