Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Esquirefreak on September 19, 2022, 10:24:50 am
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I have a noise issue with the LP/HP filter section of my "6G* center volume brown based single channel" build. It used to be dead quiet so it's having me a bit confused...
After leaving only the PI and power tubes in the noise appears when putting in the driver tube for the filters. Pulling or inserting the LFO/LFO PI tube and v1 makes no difference.
After poking around I found that a couple of caps (circled in red) are quite microphonic. However, having no frame of reference I don't know how much of that is to be expected. Using Jupiter caps and have tried a fair amount of my "v1 stash tubes" to no avail.
Any good ideas or suggestions before I start replacing caps?
Best regards from Sweden
/Max
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I have been repairing amps for a long time.
I can't understand what you write. Is it me or....? That would explain the lack of answers maybe ?
Do you use translation ?
What do you mean by :
".....inserting the LFO/LFO.."
".... "v1 stash tubes"..
"... putting in the driver tube for the filters.."
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Nah, looking back I was pretty unclear. I see that now...
By "v1 stash tubes" I mean my known good quiet tubes that I use in parts of amps that are most prone to pick up noise and microphonics. Like in the first gain stage or in a reverb recovery stage.
By "LFO/LFO inverter" I mean v3A/B on the schematic.
The tube driver for the filters being v4 on the schematic.
My build differs from the schematic in that there's no "normal channel".
The amp is working really well, but the idling noise bothers me. It's a hum rather than sizzle or hiss. I tried swapping out the filter cap for that section, but no luck there.
Also moving the ground junction of the 250nF cap circled in my first post didn't really do anything to help.
I suspect a ground loop... Maybe?
Please be patient with me... :w2:
/Max
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To clarify - v3A/B and v4 according to the schematic rather than my layout.
So v2A/B and v3 on my edited layout version
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After a lot of reading up and thinking I solved my noise issue, wich turned out to be a groundloop being picked up by the HP/LP filter section.
This issue did not appear with my previous 6G4 build. Maybe because I used an aluminum chassis for this one?
I, therefore, redid the grounding scheme of my build.
I had a piece of thick wire that I made a bus wire out of. Grounding points circled in red. Note that point [Y] is tied to a stand off, but not making contact to chassis at that point.
The well trained eye may note that I rebuilt the tonestack to the later, A-revision, version.
Sorry for taking up everyones time while posting initially. I did my homework and came back just a little bit wiser :)
Best regards,
Max
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Nice job, congrat !
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...I rebuilt the tonestack to the later, A-revision, version...
So, you're using a 350K-70K tapped Treble pot?
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Correct. Although I changed the 250pF treble cap value to 390pF.