Hum and Buzz in Cordovox Build

[scstill]

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

[shooter]

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.

[tubeswell]

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.

[shooter]

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

[Platefire]

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

[scstill]

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 ???

[shooter]

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 


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

[Platefire]

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

[shooter]

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.

[scstill]

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

Why did you draw the vol pot before the tone pot? Doesn't 5E3 have the tone first?

Anyway I did correct this tone/vol circuit to match the 5E3. It did not make any difference to the hum.
I corrected this in the original post schematic.

Still hopeful for a solution...

[Platefire]

I'm Sorry!   I sketched the Princeton 5F2A Tone Control.  I thought the 5E3 and the 5F2A tone stacks were the same, but they are a little different

[scstill]

Hey NP, I'm still learning (always)

For me it begs the question what is the difference volume first vs tone first.
Based on what Gemini says (below) I might change to Volume first.
What do you think?
Plus I do not think any of my builds have Volume first

"In a guitar preamp circuit, placing the volume control first allows the tone controls to work with a more consistent and controlled signal, while putting the tone control first can lead to more dramatic or thinner tones, as it applies the filter before the signal is fully boosted, and the later volume control then adjusts the overall output. Essentially, tone-first shapes the signal's frequency content early, while volume-first allows for more subtle tone shaping that is less affected by distortion.

Volume Control First
When the volume control is placed first in the circuit, it acts as the primary input control for the preamp stage. This allows the subsequent tone controls to operate on a more predictable and stable signal level, which can lead to more controlled and predictable shaping of the tone. It can also result in less distortion, as the signal isn't being driven as hard into the first gain stage. Think of it like adjusting a water hose before the sprinkler. If the volume control is first, it's controlling the overall water pressure going into a filter, ensuring the filter receives a consistent stream to work with.

Tone Control First
Placing the tone control first means the signal's high frequencies are filtered out (shunted to ground) very early in the circuit, even before the primary signal level is set. This can result in a more dramatic change in tone, potentially leading to thinner or more muted sounds, as the tone-shaping is done on a very raw and uncompressed signal. This is like adjusting the water hose's spray pattern (the tone) before you've even turned on the main faucet (volume). The filter is applied to a very broad or direct stream, and then the overall output is controlled. "

[shooter]

what Gemini says

so most of the major amps designed/made last century got it wrong according to AI?? 


from a Double E perspective, AI might be correct, from 80 years of guitarist Rockstar's, they might disagree.


when I 1st can to tube amps from the engineering world, I was horrified at the waveforms guitarist said "Now That's how a Great Amp should sound"  If my boss seen that kind of sheet coming from one of my systems, I'd be selling beer bait n gas long ago.

[Platefire]

I almost hate to answer the question because when I do, I prove more and more what I don't know
 
This is the way I think of it. First you have the raw signal divided into two paths. One is the Raw volume path, the other the with the treble cap and tone control pot.Then the two paths are mixed back together prior to going to PI or in a SE amp, the power tube.

So this leaves the question on tone adjustment side of the circuit, rather to put the 500pf prior to the 1M tone control like the 5E3, or after the 1M tone like the 5F2A.

It seems to me the idea of the 5F2A arrangement for the tone control with the raw signal traveling through the 1M tone pot first controlling the amount of raw signal effected by the 500pF cap before being mixed back in with output of the volume pot would offer more control?

The 5E3 way with the 500pF cap first, a 100% of the raw signal is effected and the 1M tone pot just controls the amount of 100% effected signal is mixed back in with the raw volume signal sent to the PI or/power tube.

However skewed my thinking is electronically on this, this is the way I think of it Seems like AI and I tend to agree on this one

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Back to hum Troubleshooting
1- Heater Voltage measures 6.6v; any concern being too high?
2- Bias on 7591 blue (-18.12v)  on 7591 brown (-17.73v); any concern with imbalance?
3- with preamp tube removed hum is still controlled by MV. shouldn't the the hum be in front of MV since it is adjusting hum? but with Preamp removed is there anything in front of MV to create hum?
4- with PI out hum is gone.