Hoffman 5F2A Kit Squealing

[humbug]

Hi all,

On another thread I chatted about designing a chassis for a Hoffman 5F2A kit with the tubes upright. I am new to amp-building, though a lifetime ago I did some electronics and know how to solder, bleed caps, etc. Well, the amp's all done, including the 7/8" cedar box with finger joints, etc. The amp looks great and has a brand new 4 ohm Weber 8" speaker.

However, the amp gives out an ear-piercing squeal. The squeal is constant with or without a guitar plugged in. The volume/tone controls change the squeal pitch a little but not much. The guitar is amplified, but I have to wear earplugs because of the painful squeal. Here's what I've done:

1. Read Hoffman's link on tracing out a circuit problem. I checked it all and traced it for the 4th-5th time with yellow highlighter. It all checks out.

2. Read the link for squealing troubleshooting by GeoFEX who suggests: tube microphonics. The tubes are all brand new purchased with the Hoffman kit. I did have another 12AX7 tube and tried it but the squeal is the same. Would brand new tubes be bad?

3. One common cause of the squeal is reversing the OT main connections. However, I have them attached just as the kit instructions say: Blue goes to V2 (6V6) pin 3 and Red goes to V3 (5Y3) pin 8 which is then jumpered to the board (B+). I have not reversed them, because it's contrary to the kit instructions. If I reverse them, it means the MOJO OT has switched their wiring colors (???).

4. Another possible problem is having the PT and OT set up with the windings parallel. I *think* they are not parallel. They are not that close on my chassis, which is about twice the size of the classic Fender one, so it's relatively roomy.

5. As you can see from the pic below, I have the feedback wire from the speaker jack crossing the B+ jumper. Could that cause the oscillation? How to I check this?

Here's a link to Hoffman's schematics and building pdf for the kit.

http://el34world.com/Hoffman/files/Hoffman_5F2A.pdf

Below are two pictures of the amp set up on a stand. Is there an obvious problem? I'm really at a standstill here and appreciate any help you can give.

Thanks in advance,

Steve

[labb]

Disconnect the negative feed back circuit..Reverse the OT primaries regardless of what the drawing shows.....If one of those two things don't solve the problem them you probably have a wire routing problem...Chop stick and see if you can find it.

[Willabe]

Agree with Labb.

The 6V6 and 12AX7 are really close together. That might be a problem.

I can't see but in the 2 blue circles are those 2 pins next to the heater pins wired together? Pins 4/5 and pin 9 should have nothing else wired to them.

You have a few extra ground connections you don't need. I don't think this will stop your squeal but they might make the amp buzz.

The wires in the black circles are not needed and will cause a ground loop. The pot buss is grounded to the chassis at the input jack chassis ground lug connection. The 2 chassis connections/bolts by the PT are already grounded together through the chassis, same with the OT ground, the OT output jack is grounded to the chassis through the jacks ground lug/jack threaded sleeve.

The red wire in the yellow circle probably should be shielded cable, you flip/floped the volume/tone pots and that made the red wire even longer then it would have been.

[Willabe]

4. Another possible problem is having the PT and OT set up with the windings parallel. I *think* they are not parallel. They are not that close on my chassis, which is about twice the size of the classic Fender one, so it's relatively roomy.

The PT is what they call a lay down transformer, it's core is not parallel with the OT's core.

[Willabe]

5. As you can see from the pic below, I have the feedback wire from the speaker jack crossing the B+ jumper. Could that cause the oscillation? How to I check this?

I don't think so, but I would probably move that wire, in orange circle behind the bolt and under the front edge of the board.

The wire in the purple circle is the power tube input grid wire and might need to be shielded wire.   

[sluckey]

3. One common cause of the squeal is reversing the OT main connections. However, I have them attached just as the kit instructions say: Blue goes to V2 (6V6) pin 3 and Red goes to V3 (5Y3) pin 8 which is then jumpered to the board (B+). I have not reversed them, because it's contrary to the kit instructions. If I reverse them, it means the MOJO OT has switched their wiring colors (???).

You will have to reverse something. If you don't want to reverse the red and blue leads you can reverse the black and yellow leads on the speaker jack.

The big lesson here might be "don't cut the OT primary wires until you are sure the NFB is phased correctly." Or don't trust the instructions when it comes to NFB and OT wire colors. This is a common problem with new builds.

[humbug]

Thanks so much for the ideas here. Much appreciated. I'll get on the amp tomorrow and let you all know what happens.

(And Willabe, sorry the pic is not clear, pins 4-5 in V1 are wired together but to nothing else for the heaters and pin 9 is heater only, so that shouldn't be the problem. But I appreciate you--and the others--checking the pic so closely! It's great not to be alone on this problem.)

Later,

Steve

[Willabe]

Steve, no need to apologize, just trying to get to the bottom of it. 

I think your build is going to fly high!
 

[EL34]

I see a couple possible problems in your pic

1 - you have a big ground loop from your pot bus to the main ground
you do not need that green wire I circled in red
The grounds make their way from the pots through the chassis
That is the ground path
You have two parallel ground paths

2 - I don't see where you have grounded your filter caps?
See the big red line
I don't see a wire connecting the backside of your caps to ground
most likely you did it under the board?
Just checking to make sure

[humbug]

Yes. YES. YEEEESSS! 

No more squeeeeeel! And the hum is a very civilized undertone. Just enough to let you know there are real live tubes in this beauty. Thank you all so much for taking the time to look over my build. I know as a newbie I was taking a big chance designing my own chassis, but, dangit, I just found it so much fun to do that part too. And now I have a real live amp to show for it. <chest thump>

Here's what I did:

1. Reversed the OT leads contrary to instructions. Doug, it looks like MOJO switched the lead colors from previously. Their web site does say they may do that. sluckey, I will not cut OT leads in future builds before testing. Good advice. AND I won't solder things so heavily, so that de-soldering is easier! :)

2. I cut all the extra ground wires as Willabe and Doug both pointed out. The instructions say "Ground Bus" with the ground symbol, and I took that to mean "attach to the ground lug" but it just means that the wire across the pots IS a ground bus. Now I know and understand how the input jacks provide the ground to chassis.

3. I followed Willabe's advice and replaced two of the red wires with shielded wire and I moved the feedback wire under the board. Good thing I ordered an extra foot of that shielded wire when I ordered the kit!

[P.S. Doug on your question #2: yes, the cap ground wire is under the board as per instructions.]

I'm attaching a pic of the amp components as they stand now and a couple others to show off the wood (it's the same dimensions as the original tweed except an inch thicker).

A big thanks again. When I was choosing a Princeton kit, this forum was a big selling point because everyone seems so helpful. And Doug's kit instructions are really clear and helpful despite my boo-boos with the grounding.

All the best,

Steve

[EL34]

It looks good, Glad you got it sorted out

[sluckey]

That cleaned up nicely! And the cab looks great.

There's still one thing you should do under the chassis. Remove those two 100Ω resistors from the indicator lamp assembly. They are not needed since your PT has a real center tap lead (green with yellow stripe) that's connected to chassis.

[Willabe]

Very nice!     

[humbug]

Thank you gents!

Sluckey, OK on the 100Ω resisters. Is there an audio effect having them installed along with the center tap? Just curious.

Best,

Steve

P.S. Now I have to get back to playing guitar. I've spent my "guitar time" on the amp not the axe. :)

[sluckey]

OK on the 100Ω resisters. Is there an audio effect having them installed along with the center tap?

Probably not. It's more in the category of "why do you have two on/off switches on that amp?".

[humbug]

Hehehe. That works for me. :)