Belonging to a gentleman named Glenn, this little baby Princeton was just not sounding right anymore. He had already had numerous things changed in it, looking for the restoration of the quality that it had before, as well as the removal of a slight buzz that he could hear.
The main smoothing cap had been changed for a genuine Fender replacement, but still to no avail. The small buzz was still present when it arrived with me.
Many good quality components had already been changed looking for the buzz & "lack of tonal quality" issues.
On the test gear I personally couldn't see a power or tonal quality issue. However, I could hear this "gritty buzz" right in the back of the speaker. I therefore did two mods on this amp:
Firstly, an issue that I have never been a fan of is "multiple ground (earth) points". On any amps that I design & make, I always do my grounding order in a "buss fashion" with a final, single connection to the chassis at the input....... the correct way to do things! Most Fenders do use multiple ground points, and in particular this can be a problem when the main smoothing caps-can houses 3 or 4 caps. What happens here is that they all share the same common negative, grounded at the mounting point, which in the case of this Princeton is at the corner of the chassis. The first two caps in the can are either side of the choke. Fine, but the third cap in here is through a 10k resistor and then supplies the splitter stage via a long wire. The bottom end of the splitter components are grounded across the other side of the chassis, on the brass controls plate. Doing things like this causes a small "eddy current" (earth loop) hence the slight buzz. I removed the 10k resistor from the can & strapped this third cap to the second one, increasing the main smoothing a bit. I then fitted the 10k resistor on the main board & added a single smoothing cap in here too, grounded at the same point as the splitters grounds & guess what........no more buzz!!
The second mod is an absolute must to any fixed-bias tube amp in my
opinion! I fitted 2 x 20 turn Cermet pots; one for each output tube.
Then, with the addition of a 1 ohm 1% resistor in each output tubes'
cathode, the output tubes' bias can be set up absolutely perfect, and
individually. This also eliminates the need for accurately matched
tubes. Glenn was well pleased with the mods and tells me that the
individual bias adjustments work superbly!
Both these modifications can be clearly seen in the above photo.