Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: scstill on September 02, 2025, 02:19:59 pm
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I have a frustrating noise issue, both Hum and Buzz in my Cordovox Conversion project
The build is somewhat complicated in that there are three chassis and an umbilical cable connecting them.
I have tried everything - haha - except for the solution. Hoping you might provide some wisdom.
What "good practices" I have done...
1a) all signal cables are coax, grounded only one end
1b) all electrolytics replaced, some coupling caps replaced (others have no dc leakage)
2a) all grounds are star with only one chassis connection for each star
2b) some of the original design grounds were kept (indicated in ground schematic)
3) good cable layout, twisting, separation
4) Hum increases with MV Hum with buzz; Buzz increases with Vol and Tone
5) design calls for 3.2 preamp stages 16v on 7591 grid/ 50mv guitar input = 320
What I have done to troubleshoot
6a) tried different tubes except for 7591 (no spare)
6b) Bypass umbilical with Audio - no change
6c) Bypass umbilical with AC line - no change
7) Bypass original jacks with another jack - no change, less buzz without PreAmp1 grid stopper, no change with lower grid stopper
8) Bypass PreAmp1 & Vol & Tone - no change, hum lower makes sense w/one less stage
9) Bypass PA2 & MV - no change, hum controlled by Vol
10)Bypass PA1 and PA2 and MV hum low but so too is the audio,
11) bypassed the unusual speaker circuitry - no change
12) bypassed the 6v CT circuitry - no change
the diagram had the 7591 cathode cap incorrectly drawn edited below
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that's a lot to unpack!
got a scope?
drawing error on the cap? or is it wired wrong like schematic?
add a 1 ohm R to the PA cathodes n verify current, not just grid vdc.
Is the Buzz audible inside the amp or just at speakers?
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I really tried not to bring this to the forum, trying the stuff learned here, but I'm at wits end, not giving up...
The buzz is clearly at the speakers and increases with MV and to lesser extent with Vol. So getting into audio somehow, but cables are shielded. If it was a amp buzz like mechanical transformer plates would be present with audio down. Because MV controls hum and buzz it suggests that it occurs before MV and V but then when I bypass those stages and pots it is still present.
The cap is drawn wrong - thanks.
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When the volume/MV is all the way down there is a very slight noise coming from the PT.
When I add finger pressure the noise changes, so there is some slight electromechanical noise from PT.
When I briefly disconnect speaker that "PT noise" goes away.
But would that slight noise be amplified to uncomfortable levels with volume up? Its really loud.
I cannot change the pitch of the audio noise at all with finger pressure on PT.
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drawing error on the cap? or is it wired wrong like schematic?
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drawing error on the cap? or is it wired wrong like schematic?
Cap is wired correctly, drawn wrong. Will change the diagram.
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with amp on and annoying, measure both AC n DC on BOTH sides of the 100 ohm between rectifier n 1st PS tap.
do the ohms law for current across that 100 ohm.
post it all
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100ohm
rectifier side: 0vac; 442vdc
1st tap side: 0vac; 428vdc
14v drop; 0.14amp; ~2w
no voltage change whether MV low or annoying
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Just some things to check:
check continuity form Signal 5 to B7 on 9 pin connector
Try disconnecting NFB
Check continuity on V2a from junction of 1m & 1.2K resistors to G4 Ground
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Just some things to check:
check continuity form Signal 5 to B7 on 9 pin connector
Try disconnecting NFB
Check continuity on V2a from junction of 1m & 1.2K resistors to G4 Ground
Continuity check; checked all resistors on V2, all check fine. Voltages check fine too.
Disconnect NFB - when disconnected noise gets slightly louder
Interesting that even when removing preamp tube V1 the noise persists implying it is in power tube section
But with V1 removed MV controls the noise volume which implies kin the preamp section ???
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you need to "standardize" your hums n noises
ie;
I have 60hz hum
I have white noise
otherwise I just glaze over thinking about the humming bird that buzzed me the other day and the noise the wings made :icon_biggrin:
I think you're fighting on 2 fronts, maybe 3 but
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Sorry thought I described noise in the first entry...
There is both buzz (sharp like when you say buzzzzzzzz) and 60hz hum (mellow like when you hummmmm)
I think most of the buzz might be from LED lights as when I'm in the dark it is mostly gone. But the Hum is always present and increases with MV. Even though I'm plugged into AC noise filter. Have also tried outlets at the other end of house with same results.
The hum is noticeably reduced going directly into PreAmp 3, but that could be because two stage amplification is gone too.
BTW - the guitar tone is very nice. Just the hum is quite annoying.
bench setup below. All three chassis have ground continuity. I was careful to keep star grounds as shown in the ground schematic in first entry
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I don't know if this is contributing to the noise but it appears to me the 5E3 tone stack is wired wrong on the schematic?? Double check that anyway
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Even though I'm plugged into AC noise filter.
60hz hum
the amps 60hz can couple into the signal path from INSIDE the system, a noise filter only keeps the outside noise out.
60hz is filaments "based" so anywhere filament wires and sensitive signal get close, crosstalk can happen, I know you used shielded wire, is it properly connected, grounded at one end only.
do filaments n signals pass through your connectors at pins close to each other?
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Thanks for the ideas, I will check these ideas next week as I'm off to the mountains for the weekend.
yes signal lines shield are only grounded at one end.