Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kasperbergholt on February 27, 2025, 10:47:08 am

Title: Help identifying heavy hum
Post by: kasperbergholt on February 27, 2025, 10:47:08 am

Clever people,


Looking for inputs as to diagnose heavy hum - both sides - in a KSLM77 (https://bergholt.net/clone) preamplifier clone (Chinese-made).

I've tried different power cables, interconnects, driver tubes & rectifier tubes. Result remains the same, heavy hum.

I also get a lot of noise from when turning the potentiometer.

Here are some photos, let me know if there are some sections I should focus on for further investigation.

Thanks in advance!

(https://bergholt.net/wp-content/uploads/2025/02/ksl-gigaDSC_6676-gigapixel-standard-v2-1.2x.jpg)

(https://bergholt.net/wp-content/uploads/2025/02/ksl-gigaDSC_6677-gigapixel-standard-v2-1.2x.jpg)

(https://bergholt.net/wp-content/uploads/2025/02/ksl-gigaDSC_6678-gigapixel-standard-v2-1.2x.jpg)

(https://bergholt.net/wp-content/uploads/2025/02/ksl-gigaDSC_6679-gigapixel-standard-v2-1.2x.jpg)

(https://bergholt.net/wp-content/uploads/2025/02/ksl-gigaDSC_6680-gigapixel-standard-v2-1.2x.jpg)

(https://bergholt.net/wp-content/uploads/2025/02/ksl-gigaDSC_6681-gigapixel-standard-v2-1.2x.jpg)

(https://bergholt.net/wp-content/uploads/2025/02/ksl-gigaDSC_6682-gigapixel-standard-v2-1.2x.jpg)

(https://bergholt.net/wp-content/uploads/2025/02/ksl-gigaDSC_6683-gigapixel-standard-v2-1.2x.jpg)

(https://ae01.alicdn.com/kf/Sa2bedf158d324d418bce6098a28a033aO.jpg)

(https://ae01.alicdn.com/kf/Sdc989a63b34f41f08b8dae020ab03cb1q.jpg)

(https://ae01.alicdn.com/kf/Se2b13fbd69be43b684def6b86fbacd9fi.jpg)

Title: Re: Help identifying heavy hum
Post by: triode on February 27, 2025, 01:17:29 pm

I had to delete my personal opinion comments like 10 times.

Let me just say, I am a hifi person first and guitar amp manufacturer second. This is what a real M77 looks like. (https://6moons.com/audioreviews/kondo/m77.html) I am putting this here as this preamp is not a clone.


I am not sure why you did not just do a tag board, hoffman turret board or PTP on that simple schematic you have there, better yet, you could have done an SRPP from tubecad.com and would probably have a much better preamp... nevertheless, uhm, did it ever work or is this a "I wired it up and it has bad hum?" OR is this a "I purchased a factory wired unit and it has bad hum?" Not that would matter that much, but it might.


Before we go any further...

• Is the power cord a three prong? If so, can you lift the ground on it?
• Do you have a simple DMM or volt meter (even if its like a Simpson 260) and can check for DC voltage on the volume pot?
• Do you have a scope? (not totally necessary but could make life easier)
• I am guessing from the looks of it, they are trying to put DC on the filaments, can you check what the DC and AC voltage is on the 12AX7 filaments?

Title: Re: Help identifying heavy hum
Post by: Merlin on February 27, 2025, 01:23:38 pm

The chassis appears not to be earthed. It should be earthed to provide hum shielding.

Title: Re: Help identifying heavy hum
Post by: kasperbergholt on February 27, 2025, 02:24:28 pm

Quote from: Merlin on February 27, 2025, 01:23:38 pm

The chassis appears not to be earthed. It should be earthed to provide hum shielding.

I think you're right - the hum sounds like an un-earthed component to my ears.

What's the easiest & safest way to earth the chassis?

Title: Re: Help identifying heavy hum
Post by: Merlin on February 27, 2025, 05:07:53 pm

Run a wire from the IEC inlet to a mounting screw?

Title: Re: Help identifying heavy hum
Post by: kasperbergholt on February 28, 2025, 09:35:50 am

I got the following 'diagnosis' in another https://hifi4all.dk/forum/forum_posts.asp?TID=109567 (https://hifi4all.dk/forum/forum_posts.asp?TID=109567):

"The electrolytic capacitor (yellow circle, bulging at the top) is damaged and needs to be replaced. The reason? The manufacturers in China installed it the wrong way around. Look at the stripe marking the negative terminal (green arrow). It’s facing the opposite direction compared to the reference images (the first two pictures in your post).

Because of this, the capacitor is essentially acting like a short circuit, putting strain on the voltage it was supposed to smooth out. And yes… it’s going to hum like crazy. Do not power it on.

Next thing that’ll happen? The capacitor will explode, sending a bunch of foul-smelling paper fluff everywhere."

The capacitor in question is indeed bulging, and it's not firmly connected to the circuit board like the other components.

(https://hifi4all.dk/forum/uploads/Boydk/2025-02-28_044821_H4A-2.jpg).

Title: Re: Help identifying heavy hum
Post by: Merlin on February 28, 2025, 10:07:07 am

Someone has good eyes!

Title: Re: Help identifying heavy hum
Post by: tubeswell on February 28, 2025, 01:28:28 pm

Quote from: kasperbergholt on February 28, 2025, 09:35:50 am

I got the following 'diagnosis' in another https://hifi4all.dk/forum/forum_posts.asp?TID=109567 (https://hifi4all.dk/forum/forum_posts.asp?TID=109567):

"The electrolytic capacitor (yellow circle, bulging at the top) is damaged and needs to be replaced. The reason? The manufacturers in China installed it the wrong way around. Look at the stripe marking the negative terminal (green arrow). It’s facing the opposite direction compared to the reference images (the first two pictures in your post).

Because of this, the capacitor is essentially acting like a short circuit, putting strain on the voltage it was supposed to smooth out. And yes… it’s going to hum like crazy. Do not power it on.

Next thing that’ll happen? The capacitor will explode, sending a bunch of foul-smelling paper fluff everywhere."

The capacitor in question is indeed bulging, and it's not firmly connected to the circuit board like the other components.

(https://hifi4all.dk/forum/uploads/Boydk/2025-02-28_044821_H4A-2.jpg).

 :huh: