Just worked on a Fender Pro Jr that was modded by Fromel (not a kit installed by a hobbyist).
At first I assumed it was modded by my customer and thought the work was pretty good for a first timer. He said it was working fine then a major loss in volume and very distorted/farty sounding. Traced the problem to a shorted C11 - the 2200pf 1KV cap (in series with R26) across the OT primaries.
For some reason... C11 was changed from a 1KV cap to a 600v orange drop of the SAME value. Since I don't have a 1KV .0022uf on hand I called the customer to see if he may still have the original. That's when he informed me he sent it to Fromel and had them do the mods.
So... I'm no expert, but.... I assume C11 is there to cancel out some high frequencies and possibly prevent some oscillation. Why in the world would a "modder" replace that cap with 600v orange drop? The way I read it (which could be totally wrong) that cap sees ~640v. I also would not consider that to be "in the signal path".
Cap "sees" dc plate voltage plus the ac differential of the two OT legs riding on top of the dc, but it is not grounded or other, therefore it is "floating" and does not see the voltage stress that you think as both cap legs are sitting at the same dc potential. Switching to an orange drop of the same value is merrily voodoo.
There are other considerations, but generally you do not want the sum of DC volts plus peak AC volts to exceed the DC voltage rating of a capacitor. The DC voltage across the plates is ~zero, but the AC peak can easily be 1.5 to nearly 2 times B+, so a 1kV cap is definitely needed there... It says so on the schematic too.
I don't know anything about Fromel, but their web page indicates that they are an FX builder that has expanded into amp mods. Maybe there are still a few holes in their tube electronics knowledge.
A quick peek at Mouser shows a nice selection of 1600V rated film caps 0.0022 uF. All very affordable. Take your pick, swap one in, won't break again, done, fo-gedaboudit.
Cap is on the way - needed some other stuff from CE so just got one from them. Circuit board is cleaned up and the cheap input jack "upgrade" has been replaced.
I'm actually glad I got to work on this amp - I had been considering a trade deal that included a Fromel modded Blues Jr.
Just a dumb question, why is this stuff across the primaries even needed? Why try an not scrap it like the SF kluges and hope for the best? Is it just such a cheap OT that it oscillates? Other problems in the PCB layout?
rp wrote:Just a dumb question, why is this stuff across the primaries even needed? Why try an not scrap it like the SF kluges and hope for the best? Is it just such a cheap OT that it oscillates? Other problems in the PCB layout?
Many times I see ultrasonic parasites riding the waveform in Blues Jr & Pro Jr. Sometimes it can be fixed by lead dress, mostly positioning the OT primaries & ribbon cable. Sometimes not - then I apply the well used 100 to 470 pF 1KV disc cap across drive tube plates as seen in many an old classic 60's-70's Fender. Mayhaps this little zobel is Fender's way of attacking a problem they know they have, slap it on every amp & hope the parasitic doesn't show up. Fromel's subbing-in of an under spec'd OD doesn't speak well for their vaunted "expertise." 1600V rated OD's are still cheap, if you must use one. One of those, it ain't gonna break.
1600V rated OD's are still cheap, if you must use one. One of those, it ain't gonna break.