Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: luthierwnc on June 16, 2023, 02:00:08 pm
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Hi all, I'm trying to put the finishing touches on a rebuilt Premier 90 reverb tank. These things are famous for bad filter caps and ground loops. Almost all the components have been replaced. The full wave rectifier is new. They come with a half-wave . Attached is the schematic I've used. It works as it should except for a little ground loop and a weird chirping I haven't heard before. It sounds like a cardinal calling chicks out of the nest -- starting very high in pitch and coming down. By ear it's 120 cycle but I'll verify that.
I haven't isolated the input and output jacks but will when the washers arrive. The chirping comes at all times whether the tank is plugged in or any signal is coming in. I'm using a replacement tank that is isolated with an aluminum sheet attached to the mounting screws. Both input and output wires are shielded.
Any thoughts on the bird noises would help. Thanks, Skip
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> starting very high in pitch and coming down. By ear it's 120 cycle
"Very high" is probably thousands. 120Hz is a low buzz.
Get away from your cellphone and wifi.
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No phones or wifi down in the dungeon. Plenty of fluorescent lights, though. The chirps occur what seems like twice a second but that's unofficial. The chassis is just a piece of galvanized metal bent in stair-steps. These units are notorious for picking up radio stations if angled just wrong. In the box it might do a bit better. There is a ground loop too but I don't know if that is related to the bird sound. I only smoke-tested it this afternoon so I'm gathering data for fine-tuning. Thanks, sh
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Your schematic shows a blue jumper connecting Node A and Node B of the power supply. It bypasses the 33k dropping resistor. If built as drawn, you really have a 1 node power supply feeding all of the tubes. That could set up weird interactions that I could not predict due to my lack of systematic electronic training... but it looks like trouble to me :smiley:
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The grounding scheme looks unnecessarily complicated to me. I’ve never worked in one of these, but I wonder why a more conventional ground bus with a single attachment to the chassis wouldn’t work.
Would love to hear the bird sounds…
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Sorry gents. I posted the wrong schematic. The current one is in its place. The chirps occur 74 times a minute. I have a nest of cardinals outside my office window -- have for years -- they are a good match when the parents are trying to get the fledglings to fly. The schematic I'm using is comprised of ones found here, DIY and NoiseMikers -- including the grounding idea. I'll pull that the next time I'm downstairs. After that I might have to use a three-prong cord. My friend wanted to keep it stock for home use but I'd really rather it was closer to code.
The only mods are a bridge rectifier with what was thought appropriate filter caps, a bleeder cap on the first pre-amp plate and a few different caps and resistors on the plates. I'll need to dumb the output down too. It is unusable past noon on the reverb pot.
Still chirping. Any idea what that is?
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The grounding scheme looks unnecessarily complicated to me.
It's drawn busy, but really simple. There is First Filter Cap. There is All Amplifier Stages. And there is a loop-breaker to Chassis (and wall-ground?). As drawn, it keeps rectifier pulses out of the amplifier and references amplifier to wall-wiring.
The B+ seems acceptable assuming the 33K is really there and not too high for that screen grid.
A 1CPS chirp may be visible with a voltmeter. What nodes are jumping around?
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Success! Adding a three-prong cord helped -- a little. Moving a couple wires around helped -- a little. Adding a shielded cable from the second coupling cap to the output jack made all the difference. No more birds. And that's quiet in a shop with fluorescents, no shielding on the tank using a telecaster into a 4-watt PA conversion amp.
I'm not quite done. This thing produces way too much gain and way too much reverb. The gain makes the low end farty when the Level is turned past 10:00. Parity loud with the normal amp settings is a shade past nine. Reverb Level past 10:00 is space noise. The coupling cap off the pin #1 has already been reduced from the original .01 to .003 with a little bleeder cap. I'm thinking of reducing the pentode grid-lead resistor down to 270k, possibly ditching the bypass cap on the cathode and dropping the exit coupler to .0047 to tame the beast. The grid leak on pin five of the 6EC7 is also a candidate for reduction.
Any thoughts before I sew it up? Thanks for everything, Skip
Current schemo attached.
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Any thoughts
way too much gain
the 6EU7 has always been a problem child in my experience, kill 2 birds, rebuild with 12AU7, make the output section a CF
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something like this (with the heater wires rearranged)? sh