AB763 Deluxe reverb build Lamar MV and reverb hum

[JGk]

Hi all I finished a AB763 single channel build last year and am in the process of fine tuning and tinkering now. It was build along deluxe reverb lines and sounds really good but I have hum in the reverb circuit, its quiet until around 4 and then its gets noisy also the reverb is also pretty overwhelming at this point. I noticed that the layout doesn’t use a 220ohm mixing resistor and was wondering if installing one would make a difference.

I also added an additional filer cap 30uf prior to the mains any thoughts on if this is a good or bad mod?

I was also wondering if I could install a Larmar type 2 master on it with the bias vibrato and how would I hook it up with the bias wire to the master and then to the vibrato pot?

Thanks for any insights JGK

[Mike_J]

Hi all I finished a AB763 single channel build last year and am in the process of fine tuning and tinkering now. It was build along deluxe reverb lines and sounds really good but I have hum in the reverb circuit, its quiet until around 4 and then its gets noisy also the reverb is also pretty overwhelming at this point. I noticed that the layout doesn’t use a 220ohm mixing resistor and was wondering if installing one would make a difference.

I also added an additional filer cap 30uf prior to the mains any thoughts on if this is a good or bad mod?

I was also wondering if I could install a Larmar type 2 master on it with the bias vibrato and how would I hook it up with the bias wire to the master and then to the vibrato pot?

Thanks for any insights JGK

I had a problem with the reverb on my previous AB763 build.  When the pot was turned above three it would oscillate.  While searching for solutions on the forum and on other sites it appears that in addition to the oscillation issue there is a tendency to hear hum when the reverb is turned up on many AB763 amps.  Fender grounded the RCA jacks at the jacks on the back of the chassis.  Apparently this can cause ground loops in some of the amps.
The reverb jacks should be isolated from the chassis and grounded along with the 220K resistor on the jack to the ground side of the cathode cap (in this case V2-b).  The vibrato jack should be isolated from the chassis and grounded to the ground side of the cathode cap at V4-a.  This should lower the likelihood of hum in the reverb channel.
You need to be careful when filtering the B+ coming off a rectifier tube.  Don't want to exceed 50uF for a 5ar4 if my memory is correct.
For the LarMar you need a 250K dual pot of course.  The wiper of the vibrato intensity pot is tied to the output of the master volume pot.  The output of the intensity pot goes to the bias circuitry.  I made a separate board for the bias circuitry and am placing it on the front panel next to the volume pot.
Thanks
Mike

[JGk]

Thanks Mike I was experimenting yesterday.

I put the 220k mixing resistor in line and it seemed to help some with the hum , also added a 25k mid and raw control. Like the mid really didn’t care for the raw control muddy and I did get some squealing feedback when it was maxed. It also adds allot of wire to the amp.

I’ll look into grounding issues today and maybe try the lamar I currently have a type 1 PPIVMV (train wreak pages) installed and it works fairly well just want to see the difference with the lamar. JGK

[Willabe]

......also added a 25k mid and raw control. Like the mid really didn’t care for the raw control muddy....

What value are your preamp cathode (K) bypass caps?

[Mike_J]

I put the 220k mixing resistor in line and it seemed to help some with the hum.

I don't think you need the mixing resistor with the one channel AB763 circuitry.  The mixing resistors were in the original AB763 schematic in order to mix the normal channel with the vibrato channel.  I am guessing but you might have lower hum because the 220K resistor is reducing your gain.  It would be better to isolate the jacks in my opinion.


Thanks
Mike

[JGk]

Thanks for all the great info bypass caps are 25uf, spent the last day experimenting and tweaking things. Did the Larmar MV and it works great much less muddy than the type 1 no noise or problems really seemed to open the amp up.

Removed the tone stack bypass pot and replaced it with a 10K mid pot nice like roll off and the gain.

Did a mid boost switch with a 750 pf cap in parallel with the 250 pf like it a very usable boost.   

Put a 5K pot on the feedback loop jurys out on that not sure if I like it (need it) we’ll see it does get a little rawer and hits the high mids.

Reverb still hums once its past 3-4 I will isolate the inputs tomorrow and see if that helps it also gets really wooffy past 3 I’m thinking of changing some caps maybe

Tried a few different speakers mainly a Red Fang and a Texas Heat really like the TH the Red Fang has maybe a bit to much bit and never gets that alnico sweetness with this amp.

[Mike_J]

A couple of guys on the web had the same hum problem you are talking about.  If you already isolated the reverb jacks you can do what these two did. One of them created a shell around his tank out of mu steel, all six sides.  Apparently the mu steel is very expensive and all six sides have to be covered or it doesn't work reportedly.  The other guy bolted a steel plate to the open side of his reverb tank to reduce PT interference and wrapped two layers of aluminum foil around all six sides of the tank and placed it in a vinyl bag. He too reported success with his system.


Thanks
Mike