5f6a PT touching speaker and rattling

[jlatrace]

My Jenson PR-10 (X-bell) are in contact with the PT and I get an unmusical distortion. I used a Mojo cab and Mojo PT ordered several years ago. Amp plays and sounds great on the bench and with chassis on top of cab. However, when I load it the non-musical distortion returns.

Questions - should I dent the speaker cage, OT bell or is the a more sophistiated solution I should try?

Thanks in advance for the help.

[Richard]

I wouldn't dent the speaker, all the parts are in precise alignment and you may ruin it. Without seeing it, I can't be sure of exactly how the PT is bumping on th speaker, can you loosen the bolts on the speaker and PT and slide them at all? If that doesn't work can you mount the speaker in front of the baffle? Can the chassis be slid to the side? I would try to exploit all the wiggle room of the components first but denting the PT bell probably wouldn't hurt if there is clearance on the other side. Cutting a new baffle may also be an option.

[jlatrace]

Thanks for your response. Also I will see if there is more clearance on the bottom row of speakers and, if so, try turning it upside down. If all else fails I will cut a new baffle.

[TD_Madden]

Don't the bells come off those top speakers? That's what Fender did.....

[Tom Phillips]

Don't the bells come off those top speakers? That's what Fender did.....

That's the recommended solution.
Here's a picture of an original 1960 5F6A showing the speakers as installed by Fender.
Regards,
Tom

[jlatrace]

The bells are not on the speaker. Jensen’s can be purchased without bells. The PT is touching the metal bracket that covers the ALNICO magnet.

I have some room to slide the chassis maybe far enough away to avoid contact between the speaker and the PT. I'm going to drill a bigger mounting hole in the cab to allow me to slide the chassis sideways.

[Tom Phillips]

OK. There isn't much clearance between the PT and the speaker in a stock Bassman. There must be something a little different about your cab, chassis or the PT. It appears that you are a victim of tolerance buildup. Is it possible that your baffle board is thinker than standard for a 5F6-A? I'm assuming that rotating the top speaker doesn't solve the problem. Right?
Does your arrangement have the support bracket at the top of the PT.
Regards,
Tom

[jlatrace]

I've already tried rotating the speaker, it helped, but didn't completely resole the problem.

Its clear that Mojo skimped on the cab I purchased from them. My PT is pretty standard sized for a bassman, (a lay down stlye) and I'm using a Weber chassis purchased a few years back.

I'm about to redrill and move the chassis over about 1/4 of an inch. I'm hopeful this will solve the issue.
Les

[jlatrace]

Tom,
Mine does not have a support bracket for the PT. Tell about this? I may need to put one in to hold up the PT off the speaker, eh?

I suppose that a bracket is mounted on the side of the cab that the PT an rest on when the chassis is put into the cab, right?
Les

[Tom Phillips]

Tom,
I suppose that a bracket is mounted on the side of the cab that the PT an rest on when the chassis is put into the cab, right?
Les

Not exactly.
In an original 5F6-A there is an angle bracket on the top of the PT that connects to a bolt through the top of the cab. That third bolt is just like the two chassis bolts. Without it the weight of the PT and resulting torque on the chassis bolt will cause the chassis to sag and often crack the cab pine wood near the chassis bolt at the PT end.
I'll look for pictures to post.
Tom

[jlatrace]

Thanks Tom.
I my PT not longer touches but I'm still getting some noise on certain notes and only in the normal channel. For example, its a crackle when I hit a D at the 7th fret. It started out pretty bad but as I plasyed and the amp warmed up it lessened but was still there. It's there even when I clean the amp up. I swapped out V1 with a new tube and it still was there.

Since the noise is only in the normal channel, do you have any suggestions of where to look?
Les

[Richard]

I had a similar crackle that mainly appeared on certain notes, it sounded like something was rattling when something vibrates and touches something else. It turned out to be a poorly soldered capacitor. Try turning the knobs, if it goes away then it's probably something connected to that knob, you can start tracing from there. Does whacking the amp with your fist do anything?

[Tom Phillips]

In addition to Richard's input I'd say to look in the normal channel circuit for the loose part or solder joint since you only hear the problem when playing through that channel. Also wiggle the Normal channel tube in the socket while paying a note to see if it's a bad connection at the socket.
Tom

[jlatrace]

Thanks Tom and Richard. I will try these things the next time I'm in my shop.

Building amps is like raising kids, just when you've worked through one thing up pops another.

Thanks for all your help.
Les

[jlatrace]

Tom,
I am going to add a bracket to my build to hold my PT in place. Do you know where I could see an example?
Les

[stokes]

I would also check the plate resistor on that channel.Often a source of crackling,either a bad solder joint,as pointed out already,or a bad resistor,especially a carbon comp type,the lead inside the body of the resistor sometimes gets loose.

[jlatrace]

Thanks I will. The plate resistor is carbon film but I will chek it away.

[Tom Phillips]

Les,
Attached are two photos. One of an original 1960 bassman showing the stock bracket on the PT and the other of a 1958 bassman cab showing the third chassis screw location on the top of the cab.
Regards,
Tom

[Bruce / Mission Amps]

I got tired of buying big, thick L brackets at the hardware store, drilling the holes, grinding away on my bench grinder to get those to fit the bell cover of the PT and then having to remove one of the laminations bolts to install it so I did this instead:

http://www.missionamps.com/images/Parts/Stiffener1.gif

or here, under the pilot light wrench...

http://www.missionamps.com/hardware.shtml

[jlatrace]

Thanks Bruce, just went to your site and ordered one. Also threw in a couple of your 3 position impedance selectors (good buy, Hoffman has them at $9).
Thanks for the help.
Les

[Greeny]

I have build several 5F6's and most of the time there is that problem with the PT and 1 speaker. When you have done all the movements that are possible between chassis and speakers, I put a little piece of hard rubber between the PT and the magnet of the Jensen, so no rattling noises are to be heard.
Also I will mount an extra bolt on the PT and the OT against the inside of the cab to give more support to the chassis.
As far as rattling noises form the chassis, that is a problem with some cheaper versions of 5F6 chassis that are too thin. There are many vibrations going on while your playing a 5F6. I have spend many hours searching for all kinds of rattling noises from and in those chassis. Sometimes it is caused by bad joints in the corners of the chassis.
I also put a bolt through the backpanel (just in the middle) right into the chassis to keep it more tight.