Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on January 06, 2024, 11:02:36 am
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I see online that Hum is an issue with the AC4HW1. I have one and just started looking inside.
Question! What are the (2) blue wires from the transformer attached to the heater filament Green leads for?I can see the (2) 100 ohm filament resistors. I would have thought the center of those (2) resistors 100 ohm would be going to ground but their not.I have pulled the preamp tubes one by one and the hum is still there. Looks like a decent build except for the filament wiring. Attached PDF with schematic.
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Question! What are the (2) blue wires from the transformer attached to the heater filament Green leads for?
Is this really a question??? :think1:
Vox was never known to paint the wires the color you like. :icon_biggrin:
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My Bad. The heater filament wires are Blue and go through a fuse. I feel like an idiot!!! :dontknow:
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Question! What are the (2) blue wires from the transformer attached to the heater filament Green leads for?
Is this really a question??? :think1:
Vox was never known to paint the wires the color you like. :icon_biggrin:
Yes i just notice the blue are the filament wires. Didn't expect a inline fuse for them.
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here, this might help, although you have to apply the colors :icon_biggrin:
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Each resistor reads 49.5 ohms.Center of 100 ohm resistors on diagram are grounded. Mine are not.??? Tried to ground for a second and heard a hum.
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Yes i just notice the blue are the filament wires. Didn't expect a inline fuse for them.
Never hurts to LOOK AT THE SCHEMATIC. :icon_biggrin:
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Each resistor reads 49.5 ohms.
Exactly what they should read in circuit. Guess why?
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Each resistor reads 49.5 ohms.
Exactly what they should read in circuit. Guess why?
In circuit with heater leads.
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No Continuity To Ground With Meter.
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or 2 100's in parallel with each other ='s 50?
se the note next to the circled ground? says heater ground. so now measure heater ground to ground ground(chassis) n see if they are zero ohms apart or...
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... Center of 100 ohm resistors on diagram are grounded. Mine are not.??? Tried to ground for a second and heard a hum.
No Continuity To Ground With Meter.
Start with the complete AC4HW1 schematic (https://i.imgur.com/UpnHTq7.jpg)
100Ω resistors are tied to "Heater Ground" which ≠ "Chassis Ground."
Skim the schematic, and you see that "Heater Ground," "Tone Ground," and "Ground" are all unique and separate from Chassis Ground.
Look over to the Input Jack area of the schematic to see why: R29 and C19 separate circuit-ground from Chassis Ground, almost certainly as a ground-lift and to kill RF interference.
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... Center of 100 ohm resistors on diagram are grounded. Mine are not.??? Tried to ground for a second and heard a hum.
No Continuity To Ground With Meter.
Start with the complete AC4HW1 schematic (https://i.imgur.com/UpnHTq7.jpg)
100Ω resistors are tied to "Heater Ground" which ≠ "Chassis Ground."
Skim the schematic, and you see that "Heater Ground," "Tone Ground," and "Ground" are all unique and separate from Chassis Ground.
Look over to the Input Jack area of the schematic to see why: R29 and C19 separate circuit-ground from Chassis Ground, almost certainly as a ground-lift and to kill RF interference.
Ok i will get there and look it over.
I think this may help. I pulled on the Heater Filament wires from the EL84 going under the board 7" roughly and this is what it looks like. The wires are just laying under the board and connecting to the filament resistors at the far right end of the board. I am going to run them down from the filament resistor connections to the EL84 tube filament pins away from the OT wires. Twist them good and should eliminate 3" of wire and as far away from the board and OT as possible. The wires are barley even twisted together.
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Wow! What a difference! Killed about 80% of the hiss / noise. Now onto the input later. I am wiped out. And yes Steve the wire color coding in this amp sucks. I cant believe it is this quiet now! :think1:
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or 2 100's in parallel with each other ='s 50?
se the note next to the circled ground? says heater ground. so now measure heater ground to ground ground(chassis) n see if they are zero ohms apart or...
Heaters to ground. I find no continuity from center resistors to ground. Why if it's going to ground?? Here is the reading from center heater resistors to ground.God i have to clean my meter!
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100K IS a reference to ground, seems high though, I would expect ohms to small K-ohms
the schematic doesn't show anything worth while for the ground scheme. suspect it's follow the wires time.
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100K IS a reference to ground, seems high though, I would expect ohms to small K-ohms
the schematic doesn't show anything worth while for the ground scheme. suspect it's follow the wires time.
I m going to try and see if i can get a small dental mirror and light under the board to see if the center resistors that should be going to ground is wired to anything. So far all wire searches with meter show no continuity to them or any ground. I hate this thing.
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Used dental mirror and light. Dead end. Just a blob of solder in the center of the board where the filament resistors connect together.Dam this is not possible! Those (2) center 100 ohm resistors must go to ground! That's the whole point in them being there!
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Positioned all grid wires up and away from any other wires. It's as quiet as it's going to get which is 100% better than it was.Especially the heater filament wires being in a bundle off the EL84 & away from the OT. Shortened the OT EL84 plate wire as well. Done!
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Ciao
If you want to fix in a better way the proplem you must put your hand on your iron
so why not to arrange a better situation in your AC4 ?
instead to connect the junction of the resistors to ground why don't arrange a Voltage Divider from the PS and elevate the ground on 40 - 60 V DC ? Usually this is a better way to avoid Humm (remember to add an e-cap)
(http://i.imgur.com/8RmN9e2.jpg)
or simply connect to the cathode of the Power Tube
(https://i.imgur.com/6MWux0g.jpg)
Other thing you can do is to remove the two 100R resistors and mount a 500 R pot as Humdinger (you can connect the wiper directly to ground or to an elevated DC source as a Voltage Divider from PS or to the Cathode of the power tube)
Franco
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Thankyou much Franco. Good info to know. I will go over this.
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I would simply disconnect the conjunction of the 2 100 ohm resistors from wherever it’s going and wire them to the + side of the cathode resistor of the power tube ( like Franco hinted)
This will elevate the heaters and is a proven tactic to reduce hum in cathode biased circuits.