Rebuilt Gibson GA-8T Loud Hum at 120hz.

[SoulFetish]

I don't see a reason to suspect a ground loop problem when the hum can be stooped by adding 10µF of filter capacitance.

I agree. I missed that earlier in the thread. His description of ground connections sounds fine as well

[g1]

Posts #12 & #13 sounded like you were referring to pictures. There are no pictures showing in the thread.

[Steve Blackdog]

Posts #12 & #13 sounded like you were referring to pictures. There are no pictures showing in the thread.

I think there must be a problem with the forum software- I can’t see any photos in any threads - where have they all gone?

[g1]

I think there must be a problem with the forum software- I can’t see any photos in any threads - where have they all gone?

Scroll to the top of the page. Are you in 'posts' mode, or 'latest activity' mode? (tabs under thread title)
Pictures are working elsewhere. In 'latest activity' mode, you will see most recent post at the top of page, but attachments (and maybe pictures?) will not show up.

[Steve Blackdog]

Scroll to the top of the page. Are you in 'posts' mode, or 'latest activity' mode? (tabs under thread title)
Pictures are working elsewhere. In 'latest activity' mode, you will see most recent post at the top of page, but attachments (and maybe pictures?) will not show up.

Thanks G1

[One10]

I recieved the two 20uf 45v Kemet Caps, one was defective and the other (when installed) didn't make a difference in reducing the 'hum' at 120hz, maybe 3dB at best?
Spectrum analyzer shows -47dB to -51dB, sometimes -56dB.

It turns out that the two old Sprague TVA-1903's from 1982 & 1984 are able to drop the hum over 40dB! and they still test as very good on the LCR meter, but one has a 'bubble' on the end. I wouldn't use them any ways, they were take-offs from another amp and over 30 years old.

Such a shame that the modern PEG124 Kemet capacitors do not work in this application.

From what little I understand from the spec sheet about the 'new' capacitors I purchased, they were supposed be superbly better the sprague atoms...not much good if they allow massive amounts of hum through.


$45 mistake.

I am now thinking that after I remove the Kemet caps and replace with the older style original specs, there will be neglible hum.

What am I not understanding about these? According to below, they should work great?


"Overview KEMET's PEG124 is an electrolytic capacitor with very longlife and outstanding electrical performance. The device has a polarized all-welded design, tinned copper wire leads, a negative pole connected to the case, and plastic insulation. Long-life and very high reliability are achieved by dimensioning of the capacitor, careful selection of materials/methods, and discipline in quality control allowing operation up to +125°C/+105°C. The PEG124 winding is housed in a cylindrical aluminum can, with a high purity aluminum lid and high quality rubber gasket. The sealing system is designed for electrolyte leakage-free operation and a very low gas-diffusion rate of electrolyte. The low ESR is the result of a low resistive electrolyte/paper system and an all-welded design. Applications KEMET's PEG124 is a high performance axial electrolytic capacitor. Typical applications include smoothing, coupling/ decoupling and energy storage in telecommunication, power supply system, data processing, process control and measuring, where long life and high reliability are of paramount importance."

"FEATURES

• Up to 37,000 hours of operational life
• High and ultra-hight CV
• Extremely high ripple current
• Up to 28A_RMS ripple current, continuous load
• High vibration resistance

• Polarized all-welded design
• Outstanding electrical performance
• ESR stability over lifetime
• Some series are AEC-Q200 qualified

APPLICATIONS

• Automotive applications with demanding requirements
• Mobile and aircraft installations with up to +150°C operating temperature"

[The Dude]

Maybe they sat on a warehouse shelf too long and your new caps aren't actually "new"? That aside, there isn't a 20uF/450V cap made that I'd pay $22.50 for.

[One10]

Maybe they sat on a warehouse shelf too long and your new caps aren't actually "new"? That aside, there isn't a 20uF/450V cap made that I'd pay $22.50 for.

Haha, I agree. I actually purchased 5 total capacitors, including the shipping twice= $14.99 + about $31.00, etc. $45 for all 5. Yes, they were made in 2019 & 2020. They are supposed to have a 10yr. shelf life.

[The Dude]

FWIW and probably too late: You can get high voltage good quality radial caps cheaper than axial caps these days, and there's plenty enough room in there to fit them.

[Enzo]

And I think you are assuming a straightforward caps=smoothing. That is the primary thing. yes, but ripple currents return through the grounding circuit. And if your preamp filter shares copper with other cap returns, it can impress that 120Hz ripple on the preamp. Where exactly are your filter caps grounded? and where is the HV center tap grounded?

[One10]

The Dude, yes - that is correct, but I was looking to use quality, long life capacitors (22,000 hrs.+) and the axial's happened to be in stock.


Hello Enzo, (I tried to explain this in post #21, the real reason there is 120hz hum), the filter caps neg. side is connected back to the B+ ctr. tap, which also is not grounded, as per the factory wiring. I put in 39+ year old sprague capacitors in their place and the hum went away, only with the 'new' and improved Kemet's did the hum appear & get louder/worse. There is no 120hz hum with the 'old' capacitors (discarded from another amp). Thanks.
Basically:

No hum = A Sprague 20uf and two TVA1903's (really old, used 39+ yr. old. capacitors discarded from another amp rebuild).
Loud 120hz hum = Brand new 'high' quality, 10yr. shelf life minimum, excellent spec'd Kemet Peg124 type capacitors.

[One10]

What I am now trying to understand is how & why almost identically constructed parts can work, or not work in the same application they were both designed for.
When I read and compare manufacturer 'A' & manufacturer 'B' capacitors blurbs - they both generally claim the same thing, (as far as suitablility for use and general specs), but one works and the other doesn't in my amp and I don't know why.
Maybe the Kemet PEG124 type are not a good choice for use in guitar ampilification in my case.

[Helmholtz]

I recieved the two 20uf 45v Kemet Caps, one was defective ...
]

I hope you mean 450V.

Kemet has excellent caps and of course they are well suited for tube amps.

Can you measure ESR (= series AC resistance with your meter)?
If that's higher than a couple of Ohms, the caps are out-of-spec and you should return them.
Most likely reason is that the cap were stored for years at the dealer's.
Long time storage can age ecaps faster than in use.
Reason is that they need to be charged to full voltage from time to time to re-form and keep the ESR and leakage low.

What are the capacitance readings of the Kemet caps?

But, as Enzo said, it's important to connect the caps' negative ends to the same ground spots as the original ones.

[One10]

I hope you mean 450V.
Yes I meant to type '450v'. can't edit/correct it unfortunately

Kemet has excellent caps and of course they are well suited for tube amps.
That is what I had read about them and was not sure if I picked the right type, for the right purpose.


Can you measure ESR (= series AC resistance with your meter)?
Yes, ESR measurements and more as follows:
Kemet PEG124YH2220QL1 & PEG124YF2100QL1 Capacitors
22µf x 450v & 10µf x 450v / µf@120hz. All the rest are @100hz per specifications sheet.

(120hz hum is at -45dB or more using these. see old capacitors for noise comparison)

22.62µf; ESR = 2.0Ω; RP = 2.45k; D = .029; ⍬ = -88.3˚; Q = 34.8

23.03µf; ESR = 2.0Ω; RP = 2.27k; D = .030; ⍬ = -88.2˚; Q = 33.0

22.69µf; ESR = 2.0Ω; RP = 2.36k; D = .029; ⍬ = -88.2˚; Q = 33.7

9.930µf; ESR = 7.2Ω; RP = 2.45K; D = .054; ⍬ = -86.8˚; Q = 18.38

9.977µf; ESR = 8.8Ω; RP = 2.80K; D = .056; ⍬ = -86.7˚; Q = 17.77

Compared to 39yr. old, used Sprague capacitors:

(120hz hum is at -89dB or better with these.)

11.009µf; ESR = 9.9Ω; RP = 2.08K; D = .069; ⍬ = -85.9˚; Q = 14.42

10.061µf; ESR = 11.3Ω; RP = 2.16K; D = .072; ⍬ = -85.8˚; Q = 13.77


If that's higher than a couple of Ohms, the caps are out-of-spec and you should return them.
Most likely reason is that the cap were stored for years at the dealer's.
Long time storage can age ecaps faster than in use.
Reason is that they need to be charged to full voltage from time to time to re-form and keep the ESR and leakage low.
Ok. I have already permenant instructions with the seller/vendor to not send caps over a year old, hasn't seemed to work yet, though.
Also what is odd, is that the new Kemet capacitors take quite a long time to get a reading, they act if they are 'open' and it takes 30 seconds to a minute to get a reading, sometimes they won't 'read' at all even after a minute.

What are the capacitance readings of the Kemet caps?
I put that info above with the ESR readings

But, as Enzo said, it's important to connect the caps' negative ends to the same ground spots as the original ones.

. Yes, I agree and have left them connected exactly as the factory did and as explained earlier, perhaps I cannot communicate as clearly as I would like.

Thank you for your help.

[Helmholtz]

The 22µ caps are ok, all 10µ caps are borderline regarding ESR.
From the measurements there's no reason why the old caps should work better.

But there's an additional cap parameter, namely leakage resistance (or leakage current) which can't be measured with an ESR meter.
Leakage often improves with re-forming.
That can be achieved by running the amp (with the new caps) on a bulb limiter for some hours.