'78 Princeton(non-reverb) Humming

[Helmholtz]

Yeah. It makes me think that one or 2 of the new caps is bad. I can't think of any other possibilities. But I'm new to this stuff, and I still have alot to learn.

Pull preamp tubes one after the other and monitor voltage at circuit point D.

[cyborg_stew]

Will do. I'll get back in the chassis tonight and check the voltages without the tubes.

[Helmholtz]

Will do. I'll get back in the chassis tonight and check the voltages without the tubes.

Sorry, I forgot to mention: Please first lift one end of each 18k resistor and measure resistance, if ok reconnect and make sure they are well soldered.

[Chuck H]

Anybody else notice the schematic is very likely incorrect? Typo!?!

First, there's no node between the two 18k resistors. Why would they do that? And I just ran some simulations and the voltages don't add up at all. The voltage indicated for the preamp stages would be consistent with ONE 18k resistor and no additional node. But it's more likely that Fender ran the concertina/split load inverter from one node and then decoupled the preamp from the following 18k rail resistor. Which would mean the voltages for the preamp node are inaccurate and would be a bit lower than indicated on the schematic. In any case IT'S WRONG!!! And how most of these amps were actually wired is a mystery. Because people are very rote animals and it's probable that the schematic was copied by Fender themselves. So we're actually in dark territory here as far as what's right and wrong.

With the exception that the current draw from the final stage is clearly huge by comparison to what it should be. About a hundred milliamps? So yeah, that's a lot for a preamp stage I think there must be an error there as well. Probably a bad can cap or it's connection.

But I hope we can keep in mind through the course of this diagnosis that the schematic is clearly flawed and we don't know what original preamp voltages should actually be (other than speculation and simulation I suppose).

[g1]

The numbers all work out for me? Roughly 3mA through both (18K) resistors for approx. 110V across 36K, 1mA used by each of 3 triodes on node B.

[Chuck H]

The numbers all work out for me? Roughly 3mA through both (18K) resistors for approx. 110V across 36K, 1mA used by each of 3 triodes on node B.

Ah! You're right. I ran my sim at 6mA. I tend to run two anti phase triodes from each node in my own designs and I brain farted and ran the sim at 3x2mA rather than just three triodes.

So it's 180V across the first 18k resistor and 165V across the next. Keeping in mind that there's a filter between them the imbalance seems suspect. I might replace the whole can.

[Enzo]

I think the unseen and unused C node is just a legacy from earlier design. Maybe shared eyelet board layouts. The PI seems to run from the preamp node on a number of other Princeton circuits. Besides I think that two stage pi filter might keep the preamp node cleaner than one stage with twice the cap value.

[cyborg_stew]

Thank you all for your help! It's greatly appreciated, and nice to know there are people who are willing to share their knowledge with others.

After spending alot of time trying to diagnose where the hum was coming from, then replacing the cap can and fixing that problem, only to have another occur; I decided to take the amp to my local tech. I'm in over my head, and I thought it best to leave it to the pros.