Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SnickSound on July 21, 2023, 08:06:19 am
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In my experience, it works. I've built a few high gainers and adding DC elevation always cures all hum I can hear, and I don't have to roll tubes. I mean AC heaters but with an elevated DC reference (30-50VDC or so).
I've googled it and saw a thread here from 10 years ago, and consensus seemed to be "cause it adds complexity and costs". Ok fair enough.
But then you see the SLO100 moving to DC heaters, which to me adds more parts to the circuit... for the same result (at least in my experience). And DC heaters are common on Mesa, Friedman, and other modern high gain amps. DC elevation is 2 resistors and a small cap, couple bucks at most, maybe less for on a production line.
And not only does it cure hum, it makes life easier on Cathode Follower circuit.
Maybe the question is more: why are OEMs choosing full DC heaters instead of DC elevated AC heaters? (Which is cheaper, just as good IME, and makes life easier on CFs)
Am I missing something?
Also... that diode between grid and cathode to prevent the grid from going half B+ above the cathode that Merlin suggests. I always do this now and have never had a failed cathode follower tube since (it did happen once before, ruined a gig). No more need for a standby switch now. It's one diode and one resistor. Litteral pennies, to potentially save a tube later down the line if the user flips the standby too quick. Yet no OEM seems to do it. If it changes the sound, I've never heard it.
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Why...
Inertia, customers not requesting the feature, cost.
The mistake is thinking that they want to build the world's most reliable amps.
I'm not really knocking the major companies, most of whom build good sounding amps at reasonable prices -- but their priorities are not the same as mine or yours.
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I recall some recent Marshalls have elevated heaters.
It’s tricky to find their schematics online though to check :dontknow:
Heater elevation pretty much eliminates heater buzz that’s caused by leaky insulation between a valve’s heater and its cathode.
Mostly that’s an issue with partially bypassed or unbypassed early stage valves, eg many Marshall type lead spec models.
So elevation makes selecting 12AX7 for such amps much easier.
But such heater buzz typically isnt an issue if preamp cathodes are fully bypassed.
It can’t help with heater hum, or with HT ripple, contamination of the signal path.
By heater hum, I mean the magnetic field around AC heater wiring causing interference in the signal path, typically due to other wiring being too close to it. Mains frequency hum.
HT ripple contamination might referred to as ground loop hum, ie twice mains frequency.
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DC elevated AC will still hum on PCB because no twist.
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In my experience, it works. ....... I mean AC heaters but with an elevated DC reference (30-50VDC or so).
I like it, put it in all my builds/rebuilds. Combined with a hum dinger pot. (Feed the dc standoff voltage to the wiper of the pot, 2 pot ends feed the heater wind ends the vdc.)
Kevin O'Connor recommends 70vdc to 80vdc stand off. Sometimes 35dcv to 50dcv is not enough to knock out the noise.
Our friend Ed did an experiment on a build of his that had old style octal preamp tubes. He started at ~35vdc and kept going up until he knocked out the noise. IIRC, he ended up at a little over 70vdc.
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Kevin O'Connor recommends 70vdc to 80vdc stand off. Sometimes 35dcv to 50dcv is not enough to knock out the noise.
Sometimes the amp has a cathode follower, split-load/cathodyne inverter, or long-tail pair where the cathode is at an elevated voltage. So the heater needs to be at ((cathode voltage) + 35 to 50 volts) to get the hum-reducing effect.
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Why...
Inertia, customers not requesting the feature, cost.
The mistake is thinking that they want to build the world's most reliable amps.
I'm not really knocking the major companies, most of whom build good sounding amps at reasonable prices -- but their priorities are not the same as mine or yours.
This interrogation came after I listened to Mike Soldano go in depth about why he went with DC heaters on the new Boutique Amp Distribution version of the SLO100, which came down to: it was getting hard to find a V1 tube that didn't hum too much. The problem was clear, and the only solution he ever envisioned was DC heaters.
So I get it, customers don't request elevated heaters, they request a quieter amp, and Soldano delivered it that way. But did Mike just not know about elevation, or is there a valid reason he couldn't go that way . That's what I'm wondering.
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DC elevated AC will still hum on PCB because no twist.
That makes sense, that's a good point.
Coming back to the SLO100, the tube sockets are not PCB mounted though... BUT they do use parallel wiring instead of twisted. But I am doing that on my builds as well and elevation has fixed all hum so far.
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Kevin O'Connor recommends 70vdc to 80vdc stand off. Sometimes 35dcv to 50dcv is not enough to knock out the noise.
Sometimes the amp has a cathode follower, split-load/cathodyne inverter, or long-tail pair where the cathode is at an elevated voltage. So the heater needs to be at ((cathode voltage) + 35 to 50 volts) to get the hum-reducing effect.
That's a good point. Marshall style CF will have the cathode at around 140VDC IIRC, so not really feasible to have the heater higher than that. But elevating it at least reduces the strain from having the heater 140VDC BELOW the cathode.
Noise wise though, usually doesn't really matter by the time you get to the CF. It's really just those first couple stages (if everything else is properly designed). I kind of remember the Dual Rectifiers having only V1 running DC heaters.