My Very First Ever Build AA764 Vibro Champ. (Cab is here)

[M Fowler]

Tribi,

I have been following along but knew you were in good hands so I lurked behind the scenes. Glad you got your amp going now take a step back and button it up nice and solid and it will last a long time.

Next build maybe a better layout/schematic is need such as a marshall style 18watt TMB.

Mark

[tribi9]

Mark that looks a lot easier to read than the Fender layout's I've seen so far. This amp left me pretty broke, but that looks like an excellent suggestion for a second build. Believe me, it's not hard to tempt me.

I just noticed I missed grounding the volume knob. I had that volume issue wiring a guitar once. When I ground it it took care of it.

This is a lot of fun. I'm a little rough around the edges but I got the willingness.

Thanx again all for the support.

[tribi9]

Disconnect the pot. Put your ohm meter on the wiper and one outer leg. The do it with the other outer leg. You are looking for the ohms to be zero with the knob in the extreme postion, left or right, depending on which outer lug you are attached to. If it doesn't zero, replace it.

I tried this and the pot is fine. It zeros out on the opposite leg.

This is weird. There's only a certain range that I can have the volume knob at for the amp to give me usable sound. to work. I would say from like 3 to about 6.

Anything below 3 and it squeals in a really high pitch. Anything over 6 and same thing. A really awful high pitch sound. The lug is grounded straight to a lug where also the the input jack and the buss ground meet. ( I drilled a hole right under the input jacks and put a bolt and a nut there.

[M Fowler]

Tribi,

Looks like that pot wired to the board? That black wire coming of the left side looks like it is going to the board where the 47 ohm and 1M ohm resistors meet or is that photo trickery?

You need to run all the grounds off the board to a spot on the chassis or use buss wire or copper wire (romex). Solder the volume pot pin 3 to the pot itself or run a wire to the ground lug on chassis.

Mark

[M Fowler]

Here is a different layout but as you can see it provides grounding options.

Mark

[tribi9]

Mark, the third leg on the Volume pot. (Starting from the input jack side) Is now wired onto itself. (But not to ground) I had it wired to a nut/bolt I drilled near the input jacks but I changed it after your last message.

It is still squealing at low volumes and there' only a certain usable range.

This is how it is right now. Should the pot leg be attached to the pot itself then to ground?

All the grounds go to that solder ball that's suspended up there. The black wire goes straight to the ground eyelet.



[IMG:800:600]http://i114.photobucket.com/albums/n271 ... C02050.jpg[/img]

[IMG:800:600]http://i114.photobucket.com/albums/n271 ... C02053.jpg[/img]

[Phil_S]

The volume pot is not wired correctly.

The black wire to the wiper on the treble pot is correct. I assume the blue wire on the wiper (middle) runs to the grid input on V1 pin7.

Cut/remove the connection between the wiper and the other outer lug. The other outer lug (left side in your picture) should be grounded.

There is a 15K resistor between the bass pot and it's ground. You can run the ground wire to the ground side of that 15K resistor or to that cluster of black (ground?) wires that appear to gather the preamp grounds.

[tribi9]

The picture makes it look like the wiper is attached to the lug but it's not.

I've tried both with the left lug wired onto just the back of the pot itself and also tried wiring the left lug to the ground eyelet.(without touching he back of the pot)

The 15k resistor is there, You can see it on the bottom pic. It attaches to ground (not at the cluster) but it's attached to the same bare wire that goes to that cluster.

[Structo]

As for the squealing, I'm not sure it matters on a single ended amp like yours but on a push/pull amp the usual fix is to reverse the primary wires of the output transformer.

But then again this is a single power tube amp so it may not matter.

You are getting an oscillation, that is what the squealing is.

Try the suggestion of putting the 68K grid resistors right on the tube pin 2.

Be sure to only ground one end of the shielded cable from the input to the resistor.

[Phil_S]

Try swapping the primary leads on the output transformer. This amp has a negative feedback loop. If the OT is out of phase, you have positive feedback = squeal. That is why swapping the primary is suggested. This would be true of both push-pull and single ended amps. Those darn electrons just like to run in a particular direction and it's up to you to make sure they are working for you, not against you.

Before you swap the leads, which is not a great deal of work, it is simpler to disconnect the feedback loop. Lift one end of the 2.2K resistor that ties into the speaker jack or disconnect it at the jack. This is easier than futzing with wires on tube pins. If the squeal stops, you can be pretty sure the OT is wired backwards, regardless of the color of the leads. If this doesn't work, chances are it is oscillation; in the case of your amp, likely a lead dress issue.

First, properly ground the volume pot to the buss. Wiring that outer lug to the back of the pot does not ground it.

[tribi9]

Be sure to only ground one end of the shielded cable from the input to the resistor.

Sorry, Im going to sound dumber than usual here. I'm not sure if this is what you mean...

Ground only one end? So the shielded cable would have two wires and a ground right? Wire each end to the tip on the jacks on one side of the cable. The other end wire each end to each 68 K resistor. Join the two resistors and wire them to pin 2.

Now the ground wire... Do I just connect that to ground on the jack? Only on one end. Is that what you mean?

[tribi9]

Try swapping the primary leads on the output transformer. This amp has a negative feedback loop. If the OT is out of phase, you have positive feedback = squeal. That is why swapping the primary is suggested. This would be true of both push-pull and single ended amps. Those darn electrons just like to run in a particular direction and it's up to you to make sure they are working for you, not against you.

Before you swap the leads, which is not a great deal of work, it is simpler to disconnect the feedback loop. Lift one end of the 2.2K resistor that ties into the speaker jack or disconnect it at the jack. This is easier than futzing with wires on tube pins. If the squeal stops, you can be pretty sure the OT is wired backwards, regardless of the color of the leads. If this doesn't work, chances are it is oscillation; in the case of your amp, likely a lead dress issue.

First, properly ground the volume pot to the buss. Wiring that outer lug to the back of the pot does not ground it.

Sorry you posted as I was tying my other reply. Thx, Ill try this now.

[tribi9]

Yes, disconnecting the NFL wire got rid of the squealing. The volume pot works perfect now.

[tribi9]

Now the amp is truly done. I connected the NFL back, but reversed the primaries on the OT. Squealing is gone, volume pot works, got sound. I will clean up a bit on the pot side and make it a lot neater than it is. I gained lots of valuable experience and my second build should go a lot smoother.

I can't thank you enough guys for all the support. I'll post some pics of it once I get the cab sometime this week.

[Structo]

Be sure to only ground one end of the shielded cable from the input to the resistor.

Sorry, Im going to sound dumber than usual here. I'm not sure if this is what you mean...

Ground only one end? So the shielded cable would have two wires and a ground right? Wire each end to the tip on the jacks on one side of the cable. The other end wire each end to each 68 K resistor. Join the two resistors and wire them to pin 2.

Now the ground wire... Do I just connect that to ground on the jack? Only on one end. Is that what you mean?

OK, no you use a shielded cable similar to a guitar cord, but much smaller.

It has a center conductor and an outside shield.

You would connect the center wire to the input jack and ground the shield there as well.

The other end where it connects to pin 2 or better yet to the 68K resistor that is connected to pin 2.

What you do is strip back the shielding about 1/2 inch.

Trim it even with the outside insulation jacket.

Then strip about 1/4" of the center wire insulation.

I like to take a short piece of heat shrink tubing of the correct diameter and put it on the end of the cable so just the center wire is exposed.
This keeps the shielding from touching anything.

So then you solder the center wire to the pin or resistor.

It is only grounded at the jack. This helps prevent what's called a ground loop.

If you don't have any low pf cable, you can get by by scrounging a RCA patch cord to a stereo.

Or if you have any George L cable around that would work good.

One not of caution, unless the cable has teflon insulation, it's easy to get the shield or center wire too hot when soldering and melt the insulation.

Then it could short out. No big deal because you will know if you do this.

I nice quick soldering job is what is needed here to avoid melting things.

As to the squealing.
Sounds like you found the problem when you lifted the NFB.

So now you should reverse the two primary wires of the output transformer, just swap them from what they are now.
Then re-connect your NFB and everything should be cool.