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Actually, I don't know if motorboating is the best way to describe it as there is also a high pitched sound, oscillating at the same rate. It's a kind of spluttering sound. The amp is fine at low volume (Though the sound isn't great on the lead channel at low volume), but as I turn it up, there comes a point where if I hit the strings a bit too hard, this awful noise occurs. Turn it up more, and I have to be REALLY gentle.
(This, by the way, is the state I got the amp from the shop in.)
I had a repair guy look, but he was not a pro, just some guy who fixes simple problems for cash, and he said the signal was clean at the effects send, so the problem was in the output. I'm inclined to agree.
I've tried severely restricting the bass by changing caps. I haven't tried changing the coupling caps yet, as I'm assuming they were new, but I'll probably try that next. I've checked for arcing on the power tubes. (Not scientifically. Just banging the strings in a dark room, looking for sparks) I've swapped the output transformer for one from a 120 watt orange and got the same problem. I've replaced all the tubes with spares to no avail. I'm kinda thinking it's possible that the power supply they put in might not be suitable, since mine is a 100 watt amp with 4xEL34 tube, and I see on their website that they don't offer this option. Maybe someone was half asleep and just working from memory.
If it wasn't for what the repair guy said, I'd suspect those coupling caps (I'll check, anyway) otherwise I'm leaning towards something dodgy in the power supply.
I've also replaced a preamp tube with a 12AT7 to cut the gain, and yeah, I could turn the amp up further on the dial before it screwed up, it still screwed up at the same actual volume. (This is what prompted me to switch output trannies and tubes)
I also notice the amp has a cathode follower circuit in the pre-amp, but the tube filaments are plain 6 volts (ish). I haven't checked the cathode voltage, but I was under the impression that you had to use a biased filament supply with a cathode follower. Am I wrong, or did they miss that? 'Cause if they did, then my mis-matched power-supply doesn't look as far-fetched anymore. Especially since the coupling caps they included in the kit were little electrolytic caps rated for 16 Volts.
P.S. If this noise is a result of bad grounding, what would be the best way to address that? I mean, everything seems grounded in a fairly sensible place.
(This, by the way, is the state I got the amp from the shop in.)
I had a repair guy look, but he was not a pro, just some guy who fixes simple problems for cash, and he said the signal was clean at the effects send, so the problem was in the output. I'm inclined to agree.
I've tried severely restricting the bass by changing caps. I haven't tried changing the coupling caps yet, as I'm assuming they were new, but I'll probably try that next. I've checked for arcing on the power tubes. (Not scientifically. Just banging the strings in a dark room, looking for sparks) I've swapped the output transformer for one from a 120 watt orange and got the same problem. I've replaced all the tubes with spares to no avail. I'm kinda thinking it's possible that the power supply they put in might not be suitable, since mine is a 100 watt amp with 4xEL34 tube, and I see on their website that they don't offer this option. Maybe someone was half asleep and just working from memory.
If it wasn't for what the repair guy said, I'd suspect those coupling caps (I'll check, anyway) otherwise I'm leaning towards something dodgy in the power supply.
I've also replaced a preamp tube with a 12AT7 to cut the gain, and yeah, I could turn the amp up further on the dial before it screwed up, it still screwed up at the same actual volume. (This is what prompted me to switch output trannies and tubes)
I also notice the amp has a cathode follower circuit in the pre-amp, but the tube filaments are plain 6 volts (ish). I haven't checked the cathode voltage, but I was under the impression that you had to use a biased filament supply with a cathode follower. Am I wrong, or did they miss that? 'Cause if they did, then my mis-matched power-supply doesn't look as far-fetched anymore. Especially since the coupling caps they included in the kit were little electrolytic caps rated for 16 Volts.
P.S. If this noise is a result of bad grounding, what would be the best way to address that? I mean, everything seems grounded in a fairly sensible place.
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