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Torres DM200 motorboats on loud notes

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  • Torres DM200 motorboats on loud notes

    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.
    Last edited by xgabrielx; 11-05-2010, 05:20 AM.

  • #2
    You should probably contact Torres Engineering.

    I'm sure he could help you resolve this. If it's an amp fault he'll probably just take care of it. If it's not a problem consistent with the original design he'll still be reasonable.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      It wasn't from Dans shop in the US, but more of a "franchise" here in the UK. Sending it back isn't really an option. Just out of principal. But also, it doesn't seem logical to pay money to guys to fix a problem that they allowed to leave their shop unnoticed. If I get outside help, there are probably techs who live much closer and charge much less.

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      • #4
        You do not need a dc biased heater supply with a cathode follower, I know the transformer specs for the UK Torres PTs and they don't run any more plate voltage that the tens of thousands of perfect working amps with CF driven tone stacks. Their 100W PT has a 345-0-345VAC primary, 300mA dc B+ winding & 6A heater current...same as a Fender Twin, should handle EL34 fine with 3 or four preamp tubes. I'd expect to see a little over 6VAC at the heaters? Their transformers work fine, I have used a bunch of them. Electrolytic bypass caps on the preamp tubes don't really need to be above 16v typically.

        Have you tried running with just 2 power tubes & correcting speaker load (move cab to next lowest jack e.g. plug a 16ohm speaker in the 8ohm out), or tried 4x6L6/5881 (to see if the heaters rise any)?

        I'd lean more towards it being a layout/grounding issue, did they give you a schematic? Do you have any pics?

        Daisy chaining grounds on the board can be problematic, as can using a single ground point for the whole amp. Preamp cathode grounds & preamp/PI filter caps should all run to the 1st input jack ground lug. Power tube grounds, main & screen filter caps & bias supply ground to the PT end of the amp, along with the B+ centre tap.

        Whereabouts in the UK are you?

        You can PM me if you are worried about publicising any "proprietry" designs. How long ago did you buy the amp?

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        • #5
          Dan might be able to give you email or phone support if it's an issue that he's aware of.

          May just be a lead dress issue, With a chopstick and the amp live try moving some of the grid wires around an away from other wires that may cause interferance.
          Helping musicians optimize their sound.

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          • #6
            I'm a long way off to try to give a diagnosis, especially since I have no idea what a Torres DM200 is, But .....

            Sending it back isn't really an option. Just out of principal. But also, it doesn't seem logical to pay money to guys to fix a problem that they allowed to leave their shop unnoticed.
            I agree with your last sentence, but completely disagree with the previous. As a matter of principle the guy who built it should fix it, or at least be given the opportunity to make it right. You really should contact him and give him that chance.

            The problems you are describing all seem to fall into the category of instability in the power amp section, anywhere from the PI section on to the OPT and NFB loop. Could be as simple as a bad cap on the grounded side of the PI tube to ..... yikes.

            I'm no Torres fan, at all. But, anybody can have a part fail, no matter how conscientious he is. No reason to not use the warranty either explicit or implied.
            My rants, products, services and incoherent babblings on my blog.

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