Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on October 13, 2013, 12:53:00 pm

Title: Slight thump from tremelo with all controls down
Post by: punkykatt on October 13, 2013, 12:53:00 pm

Hello guys,  Have a 1969 Twin Reverb AB568 on the bench. Just did a filter cap job on the power supply and bias board, removed the silverface caps and resistors on the power tubes, replaced the 47k resistors in the PI with 82k and 100k, and changed the bias balance circuit  to adjustable bias.  The amp works great, good reverb and good tremelo. I noticed a slight thump coming from the speakers even with all the controls down(off)the speed control does control the speed of the thump but the intensity control does not make the thump louder. If I turn the roach off with the ft sw the thump is gone.   I tried grounding the non grounded lugs of the intensity pot with no change. All grounds test good with the meter. If I turn off my hearing aids I cant hear the thump, BUT the owner can hear a piss-ant trip a mile away.  Is this slight thump normal for this amp or can I do something to get rid of it completely. Any help is much appreciated.
Punky

Title: Re: Slight thump from tremelo with all controls down
Post by: sluckey on October 13, 2013, 01:17:25 pm

I don't think it's something to be concerned about. If you are not using trem just turn it off with the footswitch. If you are using trem then turn it on with footswitch. Surely the intensity will be set to something other than zero.  :wink:

However, trem ticking or thumping can be caused by tired filter caps. It can also be caused by a slightly conductive board around the trem optocoupler. And this service bulletin may apply to you.

Title: Re: Slight thump from tremelo with all controls down
Post by: punkykatt on October 13, 2013, 02:18:35 pm

Sluckey, thank you so much for that service bulletin.  On that add  .01 cap Mylar only. Do you know why Mylar?

Title: Re: Slight thump from tremelo with all controls down
Post by: sluckey on October 13, 2013, 02:46:11 pm

Actually I've never noticed that. And I don't know why they would specify mylar.

Title: Re: Slight thump from tremelo with all controls down
Post by: punkykatt on October 13, 2013, 06:32:42 pm

Thanks again, I have some .01 Mallory and Orange Drops I will clip in tomorow.

Title: Re: Slight thump from tremelo with all controls down
Post by: alerich on October 13, 2013, 10:10:20 pm

I think I have seen people lift some of the component junctions of off the eyelets (basically a sky wire arrangement) in the area of the tremolo circuit to fix this. If I recall correctly it has something to with the fiber board itself becoming conductive with age and the tremolo circuit is more sensitive to this.

Title: Re: Slight thump from tremelo with all controls down
Post by: sluckey on October 14, 2013, 08:23:01 am

I bet you saw a post by Rich Koerner on AGA several years ago and a link to this web page...

http://timeelect.com/jbl-twin.htm (http://timeelect.com/jbl-twin.htm)

... second pic look familiar?

Title: Re: Slight thump from tremelo with all controls down
Post by: alerich on October 14, 2013, 09:49:43 am

Yep, that was it. Lots of good stuff on his web site. Outstanding luthier skills as well as tech chops. There are lots of references to the conductivity of those Fender fiber boards on the web. I've even seen a few threads about it here over the years.

Title: Re: Slight thump from tremelo with all controls down
Post by: punkykatt on October 14, 2013, 01:25:43 pm

Did the skywire bridge trick with the roach and added the .01 cap good by thump. Thanks Guys.  Punky :worthy1: