5e3 noise after large transients

[afinitemind]

Hi friends,


I'm repairing a 5e3 for a buddy. Somewhere along the line it started making whooshing noises after a large transient: listen here - Most noticeable after the 2nd hit in this short mp3 (ignore the ringy microphonic tele bridge). Maybe we're hearing the PS caps fill back up?


I've tried known good tubes to no avail, and the problem still happens with an external speaker. Thoughts on what might cause this sound?

[jjasilli]

I hear a ringing, not a whoosh.  To me it sounds like a microphonic tube.  See, e.g.:  http://www.rivera.com/checking-for-microphonic-tubes/

[John]

I've tried known good tubes to no avail,

Try holding onto the tube while someone hits the power chord. But, since you've tried good tubes...


How old are the filter caps? What uF rating?


For fun, hook an analog meter up to the plate supply of the power tube while you hit the power chord. See how far the needle drops. If nothing else, it's fun to do. 

[afinitemind]

I hear a ringing, not a whoosh.  To me it sounds like a microphonic tube.  See, e.g.:  http://www.rivera.com/checking-for-microphonic-tubes/

Thanks! Sorry for the confusion, that's why I said to "ignore the ringy microphonic tele bridge"


Maybe I need to grab a different guitar and make a new sound sample.


There's a big noticeable whoosh that sneaks in after the transient.

[afinitemind]

How old are the filter caps? What uF rating?


For fun, hook an analog meter up to the plate supply of the power tube while you hit the power chord. See how far the needle drops. If nothing else, it's fun to do. 

It's a newer build, past 5 years or so. Looks to be a Weber kit, not my handiwork but definitely well built.


All PS caps are 16uf 450V. They look to be cheapies so maybe I'll just replace em. I have some 20uf F&T's on hand.


Really does sound like a cap filling up to me, but I'm not sure how that noise would get into the audio path. Could it be plate resistors or dropping resistors letting noise through?

[pdf64]

I think you can expect that sort of 'compression when overdriven' thing - HT sag, cathode squish, bias shift when overdriven - especially with old designs; it's part of the charm surely, how the amp interacts with the musician?
I don't see any reason why replacing the HT caps would reduce the effect.
Reducing the value of the 0.1uF coupling caps may help, but trying to get it super stiff and tight would be starting afresh but with the wrong platform, as fixed bias and beefier iron would really be called for.

[afinitemind]

Agreed, squish & sag are the name of the game for the 5e3! If you want stiffness, this isn't the platform for it! I don't think that's what I'm hearing...there's a noise in this amp that's not present in my own 5e3 build.


Here's a better MP3. No ringy bridge, and I used a bass (low frequency) so you can more clearly hear the (higher freq) whoosh noise (or crackle?) after the transient attack.


I'll keep at it - let me know if you have thoughts/suggestions cause this one has me stumped. Thanks for pitching in on it, y'all!

[pdf64]

Do you mean the 'rustling amongst dry leaves' noise immediately following the note being muted?
Unfortunately, the description '5E3' gets used for pretty much anything.
Compared to the original Fender schematic and layout, are any changes?
What is mains wall Vac, and the HT voltage at idle?

[afinitemind]

Yep, that’s exactly the sound I’m hearing.

Thankfully, This time 5e3 means 5e3. Unchanged from the Fender scheme except for a 300 ohm power tube cathode resistor & a 5V4 rectifier.


Wall voltage is currently reading 113Vac, amp is idling at 396VDC.

[pdf64]

And V1 is a 12AY7? Lead dress becomes quite significant in keeping oscillation at bay if there's a 12AX7 there.
Have you tried a 5Y3? I think everything drops into place nicely with a HT ~360Vdc.

[afinitemind]

Hmm, interesting. I’ll chopstick the lead dress some more & see what I can find.


Interestingly, the original builder (Schreyer Audio) specs a 5V4 on the tube sheet. I do have a 5Y3 on hand, though.