PR is getting the better of me - ideas?

[Chris Scott]

As I posted earlier, I have a Princeton Reverb here that I back-dated to B1270 specs, and hum was an issue before, though slightly less so now.


So to cut to the chase, here's where I am at with this amp:

ALL voltages are within range. When I play the thing, it has a slight low-end distortion on top of the low notes, but otherwise has good power, and rev. and trem. work well. What is repeatable is when I touch the plates on V1 with my meter, it will start up with a constant hum, which sounds like the same frequency as the finger-to-tip of-lead hum, and can be shut off and on by repeatedly touching said connections. It sounds almost like an oscillation, but I might be mistaken.

What I've done:

I've checked/reflowed all grounds, including both inputs. I've replaced all p.s. and cathode bypass caps, as well as all signal caps, save for the .003pf and the .10pf on the reverb circuit.

I've checked all resistors for value, and like I said before, all voltages are within range across the board. I've subbed all tubes, and actually brought the idle current down to about 15mV, as it seemed to be what got rid of the hum issue I was having before.

I've checked fil current, and actually replaced the 2 100 ohm artificial center taps, though they were fine.

I've reversed (and back again) the OT primary leads.


I've considered tossing the damn thing out the window, though however satisfying this might be, I know there's something I've missed.


ANY ideas out there?

 

[jay42]

When I play the thing, it has a slight low-end distortion on top of the low notes, but otherwise has good power, and rev. and trem. work well. What is repeatable is when I touch the plates on V1 with my meter, it will start up with a constant hum, which sounds like the same frequency as the finger-to-tip of-lead hum, and can be shut off and on by repeatedly touching said connections.

This amp worked very well for the better part of 40 years. Reversing the primary OT leads was an odd notion...reflowing them a good notion. I suppose adding 470 ohm and 1500 ohm resistors on the 6V6s would be reasonable. It sounds like a cold solder joint or a funky socket. If you have a stock of known good tubes, do some swapping. Run the amp to an external speaker cabinet to check the low note distortion. hth

 

[Jerry Glass]

I have couple of suggestions:

(1) remove and clean the area where the #1 input jack contacts the chassis. Also look at the #2 jack and make certain it isn't sprung.

(2) make certain your D node (B+ feeding the preamp) is located well away from any sources of strong signal starting at the cap can up to the point it first attaches to the board; this would include being at least an inch from any of the primary leads to the output transformer. Relocate as needed or coil wire around it near any high signal areas with one end of the coiled wire attached to a ground point.

 

[Chris Scott]

Thanks Jay and Jerry.

I tried a pr. of swampers on the octals, with no result. How would adding a pr. of s.g. resistors help? - honestly don't know. I've reflowed quite a few solder unions through the process of the refurbishing, though there are a few I'm sure I didn't catch, and I'll do 'em tomorrow. Sockets have been cleaned and retensioned as well, all tubes have been subbed and I've been running out of a good known spkr.

Jerry, I always go through the inputs on this older stuff, and these have been removed, cleaned and inspected in the manner you suggest. (#2 was in fact slightly sprung, and we all have our little technique for bringing 'em back) Your other suggestion for the D node makes sense, and I had to replace a fair bit of wiring in this area, so I'll have at it tomorrow and we'll see what comes of it.


Thanks again gentlemen!

 

[jay42]

k...remove the output tubes and run the PI input out to another amp. You need to know whether it's the preamp or power amp making trouble.