Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: luthierwnc on November 30, 2023, 10:11:18 am
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Hi all, This isn't my first stab at this gizmo. I've been trying to restore a friend's Premier 90 Reverberation box. They are pretty famous for ground-loops owing to the chassis being a single piece of bent steel. I've rebuilt it three times with all new components, star-grounded it, shielded almost everything and the inside of the cab too. It still hums like it doesn't know the words. I haven't gone to DC heaters yet but that's next.
Looking past that, are there in-line filters that can specifically reduce hum coming from a single component? Most of what I've seen is designed to fix loops with multiple devices acting together. In this case there is only one villain. Hum-busters seem to come in a variety of sizes and prices but I'm not sure if any of them are built to combat a single noisy device.
Thanks for any ideas or if anything comes to mind on the rebuild. I can repost pic and schematics if you like. sh
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Is it the schematic attached below?
I just found that with those mark-ups & the ground scheme on that drawing looks pretty similar to a 6G15 I built & was very quiet.
I isolated everything & also implemented Merlin's hum-block circuit for the main ground to chassis. (also included on the schematic below)
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I built a 6G15 (Fender reverb unit) years ago and it hummed like mad with a three-prong chord. It was suggested to me that I remove the green earth connection from the power cord to the reverb unit chassis, or just use a polarized two pronger ... the thought being that ground protection would be provided through the unit jacks from the amp itself, keeping both the reverb unit and the amp at the same ground potential. I ended up putting the ground on a switch and this totally eliminated the hum.
Obviously don't do this if you're using an old amp that hasn't had it's power cord updated.
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I built a 6G15 (Fender reverb unit) years ago and it hummed like mad with a three-prong chord. ...
The original 6G15 had a 2-prong cord, so it never developed a ground-loop with the amp via the power cord.
The modern 63 Reverb Reissue (https://el34world.com/charts/Schematics/files/Fender/Fender_63_reverb_manual.pdf) isolates the "Circuit Ground" from "Chassis Ground" (and power cord 3rd prong) with R22, R23, CR5 and CR6. That's enough isolation to kill loops (though the jacks are grounded to the circuit-ground, not the chassis).
(https://i.imgur.com/Izj4Ypt.png)
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Here's how I used the hum/ground loop blocker in my latest 6G15 build.
/Max
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I've used this with great success - very quiet noise floor.
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I've used this with great success - very quiet noise floor.
Why do you connect the heater CT to the 6K6 cathode? and not to the red star point?
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I've used this with great success - very quiet noise floor.
Why do you connect the heater CT to the 6K6 cathode? and not to the red star point?
Heater elevation
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Why do you connect the heater CT to the 6K6 cathode? and not to the red star point?
Heater elevation
More:
Not every tube will exhibit the defect of heater-to-cathode leakage, but when (mainly preamp) tubes do they can couple hum from the heater into the audio.
One mechanism for this is the cathode behaves like "a plate" and the heater behaves like "a cathode" because the cathode is more-positive than the heater (at least half of the time). The heater then has emission at the AC frequency of the heater winding, kind of like a tube rectifier. So referencing the heater winding to a. voltage more-positive than the cathode "reverse biases the heater-cathode diode" and stops the emission (and hum).
NOTE: Some will try heater elevation as a "hum cure" only to find out they still have a humming amp. It can only cure hum when that hum is caused by this form of leakage. Other hum-causes will still contribute their noise to the mix.
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On this unit, there's also a slight advantage in elevating the heater by 28V to reduce the h-k voltage on the CF.
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No every tube will exhibit the defect of heater-to-cathode leakage, but when (mainly preamp) tube do they can couple hum from the heater into the audio.
If its a defect why elevate the heater and not just replace the defective tube?
How to know? Does this leakage show in a tube test?
Or is there some other way to assess, like swapping tubes?
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> why elevate the heater and not just replace the defective tube?
You may not want to replace the tube if a buck's worth (or less) of parts will make it satisfactory.
The excuse I heard was that in *factory* work this trick greatly cut-down the number of amps which failed final play-test and had to be re-worked (tube-juggled) by higher-paid workers.
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would this imply that we should always elevate the heaters?
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would this imply that we should always elevate the heaters?
Not in my world. Everything I have built (or worked on) has had the filaments referenced to chassis ground, either via a direct center tapped heater winding or an artificial center tap using two 100Ω resistors. ***EXCEPT*** my November project from AX84. I did use a B+ voltage divider to provide a voltage for heater elevation in that one just because the smart people at ax84.com designed it that way. That was 20 years ago and I was experimenting with a lot of the web voodoo. Years later when I built a very similar 6V6 Plexi I just referenced heaters to ground.
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Not in my world. Everything I have built (or worked on) has had the filaments referenced to chassis ground, either via a direct center tapped heater winding or an artificial center tap using two 100Ω resistors.
Ok, but don't you have a bucket full of good hey-day American made tube pulls? :icon_biggrin:
New tubes have problems on a much higher percentage than the tubes you have.
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Ok, but don't you have a bucket full of good hey-day American made tube pulls? :icon_biggrin:
No. But I do have a cabinet full of NOS mil-spec tubes. :icon_biggrin:
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would this imply that we should always elevate the heaters?
Not always necessary. But it can be a good idea for prolonging tube life in applications with high h-k voltages (like cathode followers). YMMV
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If its a defect why elevate the heater and not just replace the defective tube?
How to know? Does this leakage show in a tube test?
Or is there some other way to assess, like swapping tubes?
> why elevate the heater and not just replace the defective tube?
You may not want to replace the tube if a buck's worth (or less) of parts will make it satisfactory.
The excuse I heard was that in *factory* work this trick greatly cut-down the number of amps which failed final play-test and had to be re-worked (tube-juggled) by higher-paid workers.
would this imply that we should always elevate the heaters?
A little anecdote:
A boutique amp-maker was building an amp based on the 5F6-A Bassman (https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a.pdf), but wanted to use a cathode bypass cap smaller than the 250µF found on the original input stage in order to lose some of the mud. But he encountered a hum he couldn't resolve. He thought he ruled out the preamp tubes as the cause of the hum, because he tried 5, 10, 20 different tubes and the hum persisted. But tacking a 220µF cap across the cathode resistor killed the hum dead.
The problem was all 100+ tubes in the case-lots he got had the same heater-to-cathode leakage defect, so the build hummed unless an over-size cap was used on the input stage. I don't recall (this convo happened ~2005), but that builder may have used heater-elevation to avoid the problem in future amps while still being able to use the small cathode bypass cap to shave bass (this might have even been a switched voicing-option for the model). Further, the builder was buying tubes by the hundreds, and somewhat at the mercy of whatever his vendor shipped (gotta get those sold-amps out the door).
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This experience with that builder convinced me that the reason Fender used 12AY7 (https://frank.pocnet.net/sheets/135/1/12AY7.pdf) for the "low noise ... hum" that the manufacturer bragged about on the data sheet, and then went overboard with the Bassman's cathode bypass cap to be doubly sure to kill hum in the input stage (in a "belt & suspenders" sort of approach).
Where possible, the very-large bypass cap will do the job.
When the very-large bypass cap cannot be used, heater-elevation may be a solution.
Not in my world. Everything I have built (or worked on) has had the filaments referenced to chassis ground ... ***EXCEPT*** my November project from AX84. ... I was experimenting with a lot of the web voodoo. Years later when I built a very similar 6V6 Plexi I just referenced heaters to ground.
... I do have a cabinet full of NOS mil-spec tubes. :icon_biggrin:
I've got a couple-hundred 6BC4 tubes made by RCA. They weren't "a dime a dozen" but maybe only $1.20/dozen.
I used them in an amp build that has a switchable bypass cap for a bass roll-off. Wouldn't you know, about half of them hum when the small bypass cap is used. Enough so that the amp & cap-switch is a strong qualitative test to sort 6BC4s into "leakage" and "no-leakage" piles.
If the primary application for the tube didn't force the manufacturer to control some characteristic (like heater-to-cathode leakage) then they didn't focus effort on it (unless perhaps the manufacturing techniques to control the issue were already being applied elsewhere in their products).
Found that was true for microphonics in "computer tubes" like the 5965 (https://frank.pocnet.net/sheets/137/5/5965.pdf) (because the computer has no speaker, and the tube normally switches between cutoff & saturation). If anyone thinks, "that Mu of 47 might make this a good 12AY7 sub"... just don't.