Simple P-P Motorboating Really Bad

[Einric]

HELP, I have a Tubelab Simple P-P that is motorboating really badly.
A screaching put-put after warm up.
I am using a Triad choke and Motor Run capacitor in the P/S section.
All tubes are new.
I have tried messing with the feedback wires and rearranging to Triode and UL mode all with the same result.

 

[kevinkr]

Sounds like output transformer is phased incorrectly. Try swapping the plate leads, and UL taps if used.

 

[Einric]

Ok, so you want me to try and swap the plate and screen wires(Blue Pair & Brown Pair)?
O do you want me to change the positions of the blue and brown pairs on the circuit board (put the blues where the browns are)?

Thank You

 

[kevinkr]

Put the blue pairs where the brown pairs were, and vice versa.

 

[Einric]

Ok so I swapped the blue and brown pairs, now the screeching put-put is not as obnoxious. But it's still there. With the inputs shorted it makes a loud low pitched sound like a whale.

 

[kevinkr]

Do you have a scope?

 

[Einric]

Not exactly, I do have one but it's at work and I can't take it home.
I may be able to borrow one from a local HAM radio friend.

 

[kevinkr]

That will make it much easier to diagnose. The problem also does not appear necessarily to be transformer phase related.

Can you post some more pix of your build? Perhaps there will be something obvious. I don't see safety ground and it doesn't appear that the top plate is grounded to the audio circuitry. I don't see feedback or grounds on the outputs, and see a skinny brown wire that runs all over the place - what is that?

 

[Einric]

Updated Photos

Here are some updated photos, the feedback is hooked up, the top plate is grounded.
The brown wire was an extra ground going from the RCA grounds to the safety ground point, that wire is no longer there.

 

[ronaldw441]

Restore output transformer wiring to what is shown in your first photo

Left to right (with rear of board toward you)
White/Brown
Brown
White/Blue
Blue
Red
Feedback from Speaker – (connected to white wire on OPT secondary)
Feedback from Speaker + (connected to yellow wire on OPT secondary)
Green (6.3VAC)
Green (6.3VAC)
White/Brown
Brown
White/Blue
Blue
Red
Feedback from Speaker + (connected to yellow wire on OPT secondary)
Feedback from Speaker – (connected to white wire on OPT secondary)

Then remove C101 and C201 (the caps in parallel with the feedback resistors). My SPP with 25W Edcors would oscillate with almost any value cap in this position.

Report back to us.

 

[Einric]

Is the feedback + feedback 01 or 02?

Now with the C101 & 201 removed I have just a good strong normal hummmm.
The hum goes away when inputs are shorted.

 

[Einric]

Looking at the board I see the side of the feedback that is attached to the ground "ring".
I have my feedbacks hooked up correctly.
I don't have an input ground to safety or spk ground to safety, I do have a top to safety ground.

 

[ronaldw441]

So what happens when you connect a source to the inputs?

 

[Einric]

A good strong normal hum.

 

[ronaldw441]

I wouldn't consider hum normal. I have 3 SPP's and none of them hum.

 

[Einric]

Normal as in it doesn't sound anything like screeching, just a normal sounding hum.

 

[Einric]

Thank You Everyone that is helping me, this has been a thorn in my side for quite some time.
I hope I just have a grounding issue to deal with now, I will be back on tonight for another round of troubleshooting.

Thanks Again,
Rich

 

[Einric]

Thank You again for all of your help, after last nights troubleshooting I had my amp down to a strong humm.
Today after work I ran a clip lead ground from one SPK- to earth and it is dead quiet and OH SO SWEET.

This has truly been a frustrating but very rewarding project and I'm glad it's over.