Non-Reverb Princeton (AA964)

[sliberty]

My build is done, and I am trying to eliminate a few problems.

1. When the trem intensity is at zero, there is an undercurrent of hum (not very loud, but noticeable). As you turn the intensity up, the hum is reduced, and eventually eliminated. Is this normal? Or is there something wrong here?

2. There is a jump up in the noise level when the volume pot hits max. Drop it back slightly, and the noise falls back down again. What would cause this? It is actually happening on another one of my amps as well - a 6G2 Princeton.

3. The bigger problem in all of this is that I am hearing what I think may be blocking distortion. I've added a 470K resistor on the PI, and that helped a lot. I swapped out the 1.5K resistors that I originally had on the power tubes for 33K resistors, and I don't believe that did much of anything. Should I be reducing the .1uF coupling caps that lead to the power tubes as my next step, or is there something else worth trying?

Note - all stock except for the following:
- Deluxe sized iron (OT and PT)
- .02uF caps instead of .01uF in the trem to slow down the oscillator.
- Bias connected to a 50V bias tap instead of the HV tap - resistors changed accordingly

Thanks,
Steve

[cbass]

Swap your grid resistors back 33k is to much. Reducing coupling caps is a good idea . I like to reduce the input stage cathode bypass to around 4.7uf or even 2.2 . this may help . I assume your tubes are biased above 50 percent

Volu!e put sounds normal an audio taper pot does that the taper ramps up at the last bit. Unless you are getting a different noise which could be oscillation

Not sure about the trem possible grounding or lead dress

[sliberty]

CBASS -
Yes, I am probably at about 70% - are you mentioning that because you've found that these amps do better at around 50%?

[cbass]

No I meant being biased to cold can cause crossover distortion I do like to back the watts off though on those poor little 6v6s in fixed biased fender amps though . usually around 55 or 60 % .

[sliberty]

Ah, OK. Then I may not be suffering from that

[cbass]

Try the smaller cathode bypass caps I think it's almost necessary if your gonna play these amps cranked .

[cbass]

Oh and I guess I should point out I've never built or worked on a no reverb but I assume!e the same tweaks will help

[sliberty]

I reverted back to the 1.5k grid stoppers on the 6V6's.

I tried a 2uF cathode cap, and didn't like the results, so I reverted back to 25uF (I didn't have a 4.7uF on hand to try).

Then I reduced the coupling caps to .022uF from .1uF.

At that point, I started hearing some noises that I had not heard before. I chopsticked around a bit, and found that the wire going from the wiper on the volume pot to the grid of the 2nd stage was making some bad noises (oscillating) when moved around. This wire ran through holes in the board along side the input jack wire (which is the standard lead dress for this amp).

So I removed the volume pot/grid wire, ran a shielded cable in its place grounded at the pot end, and went through a different hole in the board that was vacant to keep it away from the input wire. The wire is now nice and quiet and the sounds I perceived as blocking distortion are gone.

I can't be sure that the oscillation was causing those sounds since I also reduced the coupling cap values. But after I return from this week's biz travel, I might put the .1uF caps back in to determine this. Or not - there is enough bottom as is.

This amp has very little gain due to the sparse tube compliment, but it has some great sweet spots, and when pushed by a nice clean boost (or two), it gets really monstrous. I've only played 2 real AA964's, and it was on the same day at a guitar show in Philly. Both were silverface models. I don't remember them being this "weak" as this one on lower settings, but then again, I just don't remember. It is clean (ish) all the way to 10, which I expected from this model. But until you get up to about 7, there isn't much oomph (without a pedal).

I'll keep researching - I'd like to know for sure if this is the way the amp should sound, or if there is still an issue with it. But its gotten a lot closer today.

Thanks for the help!

Steve