5F2A Oscillation

[mworthi717]

I built one of the Champ/Princeton Hoffman boards (http://el34world.com/Hoffman/files/Hoffman_5F2A.pdf) as a Princeton in a Mojotone Princeton chassis, which is narrow--the volume and tone pots are very close to the B+ end of the filter caps.  (http://www.mojotone.com/amp-parts/chassis-fender-tweed/Narrow-Panel-Tweed-Princeton-Style-5F2A-Chassis#.VFa7Toex_ls).   Rather than buying a kit, I put this together from what looked like the best parts available.

The problem I am having is that the amp is clean and sounds great from volume 1-3--there is no problem until the volume is turned up to about 4, then the amp oscillates.  The tone control changes the pitch of the oscillation, so I temporarily converted the amp to a Champ, which lacks a tone control but is the same board, just to see if it makes a difference.  It still oscillates at the same threshold, but you can't change the pitch.

I moved every wire with chopsticks, changed most of the wiring to V1 except heaters to single-end-grounded RG174 (including the unshielded wire from the vol pot to C2) and stumbled on the observation that the wire from pin 8 of the rectifier tube to C7 (first filter cap) when moved closer to the volume pot reduces the threshold for oscillation.  So it seems the B+ is parasitically coupling to the pots, allowing a route for oscillation.  The pots have the Hoffman buss wire across the back, and before the oscillation, the amp has no hum.

Neither my EE-student son or I could find any wiring errors, including using a magnifying lamp to look carefully at the tube socket wiring. 

I haven't run shielded RG174 from the rectifier tube to the first filter cap, because moving the high voltage wire away from the front of the chassis doesn't change the threshold for oscillation. 

The fiberboard that Monotone sells for this is the stock Fender layout which has the filter caps flipped 180 degrees (high voltage facing the tubes; ground of filter caps faces the tone controls).

http://www.mojotone.com/amp-parts/fiberboards-tweed/Mojotone-5F2A-Tweed-Princeton-Fiberboard_2

The turrets on the Hoffman board would more or less allow that if I finesse how R15 is installed and change jumpers.   There's not a lot of room there to shield the pots with a bent piece of some sort of metal, but I suppose that is an option.  Is there another solution I am missing (like a cap somewhere on the power supply to give the stray signal a path to ground)?  I could still put the RG174 from the rectifier tube to C7, but I worry about the stray capacitance in the circuit already, and since moving the wire away doesn't change anything, I'm not sure that would do anything..

Thanks in advance,

Mark

[sluckey]

Oscillation may be due to incorrect phase for the feedback signal from the OT secondary. Reverse the two primary wires and see if the oscillation goes away.

There was an error on that turret board layout that was just discovered and corrected on 10/28/14. Download the revised schematic/layout and see if it applies to you. Feedback resistor R13 is shown on the schematic but was not on the layout.

[Willabe]

I haven't run shielded RG174 from the rectifier tube to the first filter cap,

I don't think RG174 or most other shielded cable has a high enough voltage rating to use it for B+dcv.


                  Brad     

[mworthi717]

It's not the phase of the OT.  If you do that it oscillates wherever you have the knobs--in a much bigger way.   

I'll check out the board correction. Thanks!

[mworthi717]

Is the change to the board the addition of the 22k feedback resistor with it's own two lugs?  I soldered that to the end of the feedback wire and put heat-shrink tubing over the junction, then soldered it to the end of the turret at the termination of R9 (1.5k).  I figured that was how that was done. 

I appreciate the help.

[sluckey]

then soldered it to the end of the turret at the termination of R9 (1.5k).

You probably meant R8. That will work.

Did you build your own board? Here's a link to the short discussion and the revised drawings...

http://el34world.com/Forum/index.php?topic=17747.0

[mworthi717]

In the same place as on the revised board.  No, I used Doug's board and parts. I guess going to try shielding the front end (jacks and pots) with some sheet metal first and see if that fixes it. 

[Willabe]

It's not the phase of the OT.  If you do that it oscillates wherever you have the knobs--in a much bigger way. 

No, that's not always true. 

Did you try what Sluckey said?


                   Brad   

[sluckey]

Lead dress and grounding scheme can be a source of oscillations. Can you post some high rez pics of the inside of the amp? Maybe we can spot something.

A 5F2A is a low gain amp and such a simple circuit that it usually doesn't have many problems. It's usually more common to have hum issues due to poor filtering of the B+. The originals used a cheap speaker that couldn't reproduce low frequency very well and that masked the hum somewhat. But put a good speaker on it and the hum is unveiled.

[mworthi717]

Sorry, Brad, I wasn't clear...yes, I tried that, which is how I know.   Sluckey, I will when I get it back together.  I have already started to disassemble it to shield the front end and also see if moving the board makes a difference (I was going to try to turn it vertically away from the front end), and then the day job (I'm a doctor) kicked in. I appreciate all your help; I won't be able to get back to it until probably next Sunday afternoon at the earliest.