Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on May 02, 2018, 09:06:21 am

Title: Slight crackling when hitting high note
Post by: Diverted on May 02, 2018, 09:06:21 am

HI all,

Getting ready to start Tolexing the cabinet for the Fender 5E3/Reverb amp I built for mystepdaughter. In it's tiny pine cabinet (18.5'x16") it sounds cabinet with a vintage Jensen P12Q I pulled out of an old Hammond tone cabinet (I have two).
Anyway the amp is rock solid except for an issue she may not ever deal with as she does not play loud. But I'd like to address it: With volume up I often get a slight, static-ish sound when I hit a high note sharply. It's pretty consistent  in that I can make it happen pretty easily. Anyway, solder joint? Etc? May reflow all the solder joints that don't look awesome ...
As I said it may never be an issue for her but it is (slightly) for me. Thanks!
Also sorry for the scribbling on the schematic ... can't find the clean unmarked version.

Title: Re: Slight crackling when hitting high note
Post by: SILVERGUN on May 02, 2018, 09:56:04 am

It might help to increase the value of the grid stop resistors on the output tubes. I wouldn't hesitate to go to 10K.

Is it the crossline Master that you are increasing when this happens?

Here's a cleaner version of your schematic, if it helps:

Title: Re: Slight crackling when hitting high note
Post by: shooter on May 02, 2018, 09:58:21 am

Quote

Anyway, solder joint?

good start, socket seating, sloppy jacks, left field ideas, coupling cap breaking down, pot at it's extremes flaking out.  did you change guitars n cables?

Title: Re: Slight crackling when hitting high note
Post by: Diverted on May 02, 2018, 10:54:42 am

Tried a few different cable.s And sorry, that master volume has since been removed from the circuit, so there is simply one volume pot per channel.
Will try upping those grid stoppers and see what happens.
Thanks!

Title: Re: Slight crackling when hitting high note
Post by: jjasilli on May 03, 2018, 11:38:18 am

Comments:

*  always suspect tubes first
*  you need to isolate the problem
*  use a scope or a listening amp to determine where the problem originates (likely after the vol. pots)
*  does it happen with both channels?
*  maybe disconnect the reverb circuit
*  maybe a bad speaker, try a different one
*  a snubber cap after the PI might help
*  NFB may help.  There appears to be something wrong with your NFB circuit.  It appears to be wired and/or drawn incorrectly.  Please explain.

Random or intuitive approaches work now & then.  It's seductive, but quickly becomes a waste of time if not producing an immediate fix.  I recommend a logical, step-by-step diagnostic approach with the right tools.

Title: Re: Slight crackling when hitting high note
Post by: jojokeo on May 03, 2018, 02:08:50 pm

Could easily be a parasitic oscillation? Try JJ's suggestions first then also be sure to shield sensitive wiring. Then if necessary start putting small value caps (small pF values) across plate-cathodes, grids, etc... which will "warm" the amp up (shaving the highs) in doing so. I hate having to use these but sometimes it/they can be the only remedy short of remaking an amp with better component placements, layouts, and grounding techniques.