Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on February 11, 2019, 01:47:02 pm
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I am replacing the single can-style filter cap with separate capacitors and resistors. I have labeled the filter caps 1, 2, 3 and 4. There is another ground lead coming T1 of a 6V6 tube labeled 5. It used to be connected to a ground on the can-cap where the cathode resistor and transformer center tap met. I have seen elsewhere to put the center tap the same spot as the IEC ground. I am also planning on moving the Heater CT to before the cathode resistor to raise the potential.
Any advice on cleaning up the grounding scheme? I could always connect 1,2,3,4 & 5 to the center of the tag strip, or to the chassis ground point (which is actually a bolt on the PT) but I have seen diagrams where the ground wires were connected at different places but I'm not sure of the reason why they go one place or another.
Thanks for the help.
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> center tap the same spot as the IEC ground
No. PT HV CT/return should go RIGHT to the - end of the first filter cap. *Then* it can be jumpered to whatever else needs ground.
The IEC ground *only* goes to a nearby chassis bolt (preferably dedicated so it need not be disturbed in later hacking).
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I was looking at this... https://el34world.com/charts/grounds.htm
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That's a proven excellent plan. The only thing I do differently is I put the power cord (or IEC connector) ground wire to a separate ground lug near the entry point. Usually a PT bolt, but sometimes a dedicated screw. Nothing else is connected to this lug.
Sometimes there is a need to deviate slightly from that plan, but the main idea is KEEP THE PREAMP GROUNDS SEPARATE FROM THE POWER GROUNDS. And keep your power components and grounds near the power transformer. Keep the power grounds short and don't let them ramble near input jacks, preamp tubes, tone controls, etc.
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Second try. This time I ran a lead from the transformer ground lug to the first filter cap. I will run all the other leads there as well.
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That's a proven excellent plan. The only thing I do differently is I put the power cord (or IEC connector) ground wire to a separate ground lug near the entry point. Usually a PT bolt, but sometimes a dedicated screw. Nothing else is connected to this lug.
Sometimes there is a need to deviate slightly from that plan, but the main idea is KEEP THE PREAMP GROUNDS SEPARATE FROM THE POWER GROUNDS. And keep your power components and grounds near the power transformer. Keep the power grounds short and don't let them ramble near input jacks, preamp tubes, tone controls, etc.
You posted while I was typing... let me try again.
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How about this.. first filter cap and cathodes go to their own ground near the PT. The other filter caps and the mystery ground are attached to the middle tag which is connected to the chassis.
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That green wire should be red/yellow.
You need a jumper between terminal lugs 1 and 2.
Terminal lug 5 will be your power ground chassis contact point so be sure to use a kep nut or star washer.
I would move the 10F preamp filter cap to a location in the preamp circuitry. It will work as you show it but it would be better if the cap were moved.
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That green wire should be red/yellow.
You need a jumper between terminal lugs 1 and 2.
Terminal lug 5 will be your power ground chassis contact point so be sure to use a kep nut or star washer.
I would move the 10F preamp filter cap to a location in the preamp circuitry. It will work as you show it but it would be better if the cap were moved.
There are two wires that I assume are center taps on my transformer. The yellow wire in my diagram is really red/yellow. The red/yellow wire is twisted with the heater wires so I assume it is the heater center tap. I assume the green wire is the CT for the HV. I'm not sure why I was thinking the last two caps were for Preamp, because I labeled it "to 6V6" so it make sense to jumper lugs 1 and 2.
In the diagram I had a lead coming from a PT bolt to the lug 1 and 4 for the ground. I cannot move the last cap without a lot of trouble so I was thinking of using lug 5 as the ground for the preamp circuitry.
Does having the leads from lug 1 and 4 make them "long and rambling?" They don't go near any of the signal wires/inputs, etc. I could connect them to lug 5 then run a lead from lug 3 somewhere else.
I'm probably overthinking this given that originally all of these leads were connected to lugs on the can capacitor.
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The red/yellow wire is the center tap for the red wires. It should connect to the power ground. The green wire is likely the center tap for the two yellow filament wires. However, the schematic does not show a filament center tap wire but a hi-rez pic I have does show a center tap. I would connect the heater CT to the power ground.
With a true point to point amp such as the 1482 you often have to deviate from an ideal grounding scheme, especially when the filter caps are all in a can. Look at the attached pic for a suggestion about locating the preamp cap...
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Thanks... I have been totally off with this layout. For some reason I was thinking the yellow/red was twisted with the yellow wires, but I re-examined and you are correct it is with the red wires. Ugh.
So I will keep the last voltage dropping resistor (27K I think but I've been so off with this I will have to verify) back near the power filter caps and connect the red wire to it. Then I can put the last filter cap over where you suggest. Brilliant... I didn't want to drill new holes, etc so now I don't have to.
Thanks.
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In my 1482, the Filament secondary CT is Red. In fact it's not a CT per se, but rather 2 enameled winding wires form ea inner side of the windings. Anyway I managed to somehow burn out a filament winding -- which I since learned is a common issue -- so then used a supplemental filament tranny.
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In my 1482, the Filament secondary CT is Red. In fact it's not a CT per se, but rather 2 enameled winding wires form ea inner side of the windings.
I noticed there were two enameled wires... Does this mean that they I should use 100R resistors connected to the wires to create a pseudo-center tap instead of connecting the wires to ground?
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It was common in a lot of old transformers to actually have two separate 3.15VAC windings for the filaments so you would actually have four wires coming out of the transformer for the filaments. Then the proper two wires would be soldered together and enclosed in some insulation tubing to act as a single center tap wire. Connect your filament center tap wire to power ground.
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Yes, per sluckey. The 2 enameled "CT" wires in the red sleeve connect to ea other at their exposed ends and are soldered to a grounding point. When this is done they function together as a CT, so no ground reference resistors should be used.