Help debugging a 5F2A build.

[Jonny toetags]

Built a slightly modded 5F2A and I'm having an issue.
First up, I built it using Classictone OT and PT, but I got the upgraded 15W OT this time.

Fired it up with no tubes, all good ,no shorts.
Stuck in a rectifier, so far so good.
Put a powertube, nothing for a few seconds then a loud squelching starts.

I basically pretty much copied the wiring on my 5F1 build (also used Classictone OT/PT)
One thing that I;m never sure of is what to do with the grn/yel and red/yel wires from the PT. On the couple 5F1's I built, I grounded them.


Some build pics:

[g1]

Try reversing the OT primary wires.

[Jonny toetags]

Try reversing the OT primary wires.

Right now I have the COM BLU going to my powertube on pin 3 and the 8K RED going to the board to the firts 16uF cap.
Are you saying reverse those or use the 5K BRN instead of the RED?

Some readings if it helps:
Rec pin8/2: 473VDC
Red pin4/6: 347VAC

Heaters: 3.5V

Powertube pin3 475VDC

Preamp pin 1/6: 241 VAC

[mort]

try swapping the OT primary leads. It's always a 50/50 chance to get it wrong.


edit: I see g1 got it.

[Jazz P Bass]

Right now I have the COM BLU going to my powertube on pin 3 and the 8K RED going to the board to the firts 16uF cap.
Are you saying reverse those or use the 5K BRN instead of the RED?

The first statement is correct.
Reverse the Blu & the Red wires.
Red on plate/ Blu on first 16uf cap.

The reasoning is thus:
There is a chance the the COLORS are not correct for the Primary OT winding.
You have an output section that utilizes a feedback from the OT secondary.
If the OT Primary is wired backwards, you end up with Positive feedback, not Negative.

Want to check it?

Monitor the Volts AC at the speaker with a meter.
Bang away on a chord with the amp on the loudess clean setting.
Measure that voltage.

Now lift the feedback resistor & test again.
If the output voltage goes up, that is the correct OT primary wiring for negative feedback.
If it goes down, then the OT primary is wired backwards.

Note: if you do not have the negative feedback circuit, the wiring is irrelevant.

[Jonny toetags]

Looks like that was it!
No squelching...no noise...amp sounds good!

[Jazz P Bass]

Looks like that was it!
No squelching...no noise...amp sounds good!

And you will never forget this one.

You could have read about it one thousand times.

Until it happens and you fix it, it probably would never stick.

[mort]

And you will never forget this one.

You could have read about it one thousand times.

Until it happens and you fix it, it probably would never stick.

And to add to that it's not always the same type of noise. Sometimes it can be low output, alot of times it will be just a big BBBBRRRRRRRRRRRRRMMMMMPPPPPPP, and sometimes it will squelch. I've learned to consider my OT primary connections to be temporary until it's been powered up and always leave some slack for swapping them and trimming them down once the polarity is confirmed.