5F2A Build - Nasty Squeal

[SkipJohnson]

I have a new build of an 5F2A design. I have done this build many times and only have one change on this build.
The standard 5F2A design has a 1.5K V1A Cathode Resistor and 25mF Bypass Cap. 
I usually add a .47mF Bypass cap in addition to the 25mF Bypass Cap and install an On/Off/On "Boost" switch to provide some sonic variety with Off being only the 1.5K Cathode Resistor, and the two different Bypass caps being selected by the two different On positions.
In this build I eliminated the .47mf Bypass cap (it never really gave much sonic difference) and replaced the On/Off/On switch with an On/Off switch.
 
Problem: 
•   With the "Boost" switch ON (25mf Bypass Cap enabled), the amp works fine in all respects.
•   With the "Boost" switch OFF (25mf Bypass Cap disabled), the amp squeals at higher volumes.  Tone knob increases also make the squeal appear sooner.
I tried removing the NFB resistor and that made the problem a little better, as in it takes more volume/tone to have the squeal appear.
I have chopsticked the amp thoroughly and did not find any clues.

Does anyone have any ideas or thoughts on where I go next?
Appreciate your help.

Added the chassis layout and pictures

[shooter]

put a 500pf to .01Uf in parallel with the plate R for V1A, make a change?

[kagliostro]

The first thing I've think is a problem with the NFB connection


Usually we solve the problem reversing the primary winding connection of the OT


but you are saying that the problem is there also if you disconnect the NFB


May be some photo of your build can be of some help on understanding better


Franco

[shooter]

Franco, the 5F2 is an SE, not PP

[DummyLoad]

Franco, the 5F2 is an SE, not PP

doesn't matter - applies to SE or PP - NFB polarity still can be positive feedback if the output transformer windings were wound out of indicated phase or connected out of phase, e.g. blue lead to B+ and red lead to plate.


--pete

[shooter]

showing my lack, I have never used nfb on SE 
but I should known better

[SkipJohnson]

put a 500pf to .01Uf in parallel with the plate R for V1A, make a change?

Sorry, but I do not know what "plate r" means.  Also no .01uf in my design.
I am uploading my Chassis Layout and some pictures.  Hopefully those will help a bit.

[shooter]

V1A plate resistor.  IF there is a parasitic oscillation this is a ""cheat" for the problem.

I would "flip" your primary Output Transformer Primary wires 1st though.  So Note what wire color is connected to B+ and what color is connected to tubes plate.  Unsolder both, then swap locations and re-solder.  test, still got a problem, wire in the cap, test.

[SkipJohnson]

….May be some photo of your build can be of some help on understanding better

Added the chassis layout and pictures to my original post.

[Colas LeGrippa]

Put a 500 pF capacitor accross the plate resistor ( 100k ). That s the B+ supply for the plates ( 2 plates in a 12 ax7 ). 12ax7 is a preamp tube
 A tube is an electron valve in a glass envelope....
Good luck

[sluckey]

Your layout drawing and actual board wiring is messed up. You need to compare with the schematic. Look closely to that black 8µF cap negative lead and that black wire connecting that cap to the cathode resistor/cap.

[SkipJohnson]

V1A plate resistor.  IF there is a parasitic oscillation this is a ""cheat" for the problem.

I would "flip" your primary Output Transformer Primary wires 1st though.  So Note what wire color is connected to B+ and what color is connected to tubes plate.  Unsolder both, then swap locations and re-solder.  test, still got a problem, wire in the cap, test.

I tried the 500pf in parallel to the 100k plate resistor, no change in the problem.
I flipped the Output Transformer Primary wires and things got worse.  I had problems in both boost switch positions.
I then removed the NFB resistor again, and the problems went back to what they were:
- Boost on: things work good in all volume/tone positions
- Boost off: Squeals with volume and/or tone past 60%.

I am currently with the Output Transformer Primary wires flipped (brown to B+ and Blue to V2 pin3 (tube plate)).  I also have the NFB resistor removed.

[SkipJohnson]

Your layout drawing and actual board wiring is messed up. You need to compare with the schematic. Look closely to that black 8µF cap negative lead and that black wire connecting that cap to the cathode resistor/cap.

It looks right to me?
Black negative lead jumper going from the negative 8uf cap to the V1A cathode resistor/bypass cap.  See attachments.
It is a little hard to see the jumper connection to the 8uf cap, buts it is there.

[shooter]

I am currently with the Output Transformer Primary wires flipped

put it back to original since it didn't fix.

here's the schematic, you have the layout, grab a highlighter and trace on BOTH schematic n layout, note ANYTHING that seems wrong or odd.

https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5f2a.pdf

[SkipJohnson]

I am currently with the Output Transformer Primary wires flipped

put it back to original since it didn't fix.

here's the schematic, you have the layout, grab a highlighter and trace on BOTH schematic n layout, note ANYTHING that seems wrong or odd.

https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5f2a.pdf

Appreciate the help, but there are a lot of differences between the original 5F2A and mine design which is based on the Two-Stroke from Dave Hunter's book The Guitar Amp Handbook (the Updated and Expanded Edition) (see attached).  Although I have morphed the design over the years a bit.  I also have built this design many times, the only difference with this build is that I moved things around to get the tubes up front for visual effect.

[sluckey]

It looks right to me?
Black negative lead jumper going from the negative 8uf cap to the V1A cathode resistor/bypass cap.  See attachments.
It is a little hard to see the jumper connection to the 8uf cap, buts it is there.

Ain't right. The negative lead of that 8µF should be conned to ground. And there is no jumper over to the cathode of V1.

Compare your layout to the last layout you just posted. Better yet, compare to the schematic.

[DummyLoad]

also, you're missing a resistor - 1M grid-leak of 1st amp. see attached,


--pete

[DummyLoad]

listen to slucky - your 8uF filter is wired incorrectly. see attached - where you have the (-) lead connected now is to the cathode of V1a... 

--pete

[SkipJohnson]

Ain't right. The negative lead of that 8µF should be conned to ground. And there is no jumper over to the cathode of V1.

You are right.  I actually have the jumper, it is just going to the wrong side of the cathode resistor.  This is a design mistake from when I converted the design from Top Load to Frond Load. I have corrected this in my design doc as well as on the build.
Thank you for your help as always.

[SkipJohnson]

listen to slucky - your 8uF filter is wired incorrectly. see attached - where you have the (-) lead connected now is to the cathode of V1a... 

--pete

Yup, listen to slucky.  Thanks for your diagram that made it crystal clear to me what was wrong.  All is well now.  Thanks for your help!!!!

[sluckey]

Did that fix the original problem?

[SkipJohnson]

Did that fix the original problem?

Yes.  I am sure that was the problem all along.  Thx!!!!