Strange low frequency oscillation in Fender Princeton.

[Recycled_Brains]

I have a reissue Fender Princeton amp that is creating a really weird low frequency oscillation on certain notes (mostly D and E).

It occurs when I hit the notes on any string, at any octave, and even when said notes are contained within chords.

It's tough to describe the sound, but if I had to, I'd say it's like bending a string on a bass, starting from the top of the bend, then settling into pitch. Also, it's always the exact same pitch.

Any ideas what could cause this? Microphonic tube maybe?

It's driving me crazy.

[Andy Peters]

I have a reissue Fender Princeton amp that is creating a really weird low frequency oscillation on certain notes (mostly D and E).

It occurs when I hit the notes on any string, at any octave, and even when said notes are contained within chords.

It's tough to describe the sound, but if I had to, I'd say it's like bending a string on a bass, starting from the top of the bend, then settling into pitch. Also, it's always the exact same pitch.

Any ideas what could cause this? Microphonic tube maybe?

It's driving me crazy.

Possibly feedthrough from the tremolo oscillator.

-a

[Boogdish]

What kind of princeton are we talking about? Tweed, Brown, Blackface, Reverb? [EDIT: nevermind, I just reread your post, dumb me]

In any case, I'd check it in this order.
1.It could be a speaker problem. Try plugging a differing cabinet into the speaker output.
2.It could be a microphonic tube acoustically coupling with the notes or some other tube problem. Try replacing with new tubes or swapping for a set that works well with another similar amp.
3.It could also be time to replace your filter caps. That what you're hearing is just fluctuating DC somewhere in your power supply which is most noticable with these notes (though usually it's a B or Bb that you hear this on). One warning sign is if you open up the amp, and see battery acid looking stuff leaking out of the capacitors. If you've got experience with a soldering iron, you can do this yourself, just make sure to drain all of your filters before you start so you don't shock yourself. [EDIT: but if this is a new amp like this it's very unlikely that these would be worn out yet]
4.It could also be a parasitic oscillation (accidental positive feedback within the circuit). Which you'd probably want a tech to look at if you're inexperienced with this stuff. [EDIT: now that I know this is a PCB amp, getting a parasitic out without really major modification doesn't seem that likely. Small parasitics are often hardwired into PCB amps]
5.It could be something less obvious.

[Recycled_Brains]

2.It could be a microphonic tube acoustically coupling with the notes or some other tube problem. Try replacing with new tubes or swapping for a set that works well with another similar amp.

Sorry for the delayed response. Finally got around to investigating the problem a bit...

I suspect this to be the issue.

Both pwr tubes make a low-pitched rumble when tapped with my finger. They also "ping" if I use a small screw-driver to tap them. I also found 1 12AX7 that was microphonic, so I swapped 2 out with a matched pair of EH 12AX7s.

Yesterday I was playing around with some different tube combinations in the preamp section, and with a 12AY7 in the V1 position (aiming for a bit more clean volume), the problem was reduced (i.e. quieter), but I suspect that's just because of the lower gain tube feading the pwr tubes.

Overall, with the preamp tube changes, I've found the overall tone to improve, but the problem still exists. It is just less pronounced.

I'm planning on ordering a set of Tung Sols to replace the stock 6V6s. With proper biasing, I'm hoping that'll do the trick.

I'll let you know how I make out.

Appreciate the replies.

[Recycled_Brains]

OK.

So I got a pair of Tung-Sol 6V6s from Tubedepot (who rule, BTW) to replace the suspect stock 6V6s.

A friend of mine just picked up a bias probe, and we were planning on attempting to bias the amp (along with his Deluxe).

I'm as green as they come with this sort of thing, so it occured to me that I have no clue what the correct bias point is for a Princeton RI.

Can anyone shed some light on that for me? Any recommendations for tech. resources I could consult?

[Milkmansound]

you probably don't need to bias them.

if the amp sounds ok, they are probably fine.

[Mudcloth]

Also, it's the the bias point for the tubes you need, not the amp. Tung Sol 6V6 have a max. plate dissipation of 12 watts. If you bias them at 60% of m.p.d. [ a good starting point] you'd get 7.2 watts. Divide this by your plate voltage and you'll get the cathode current you'll want.
If your plate voltage is 315V you'd want the cathode current at .022 or .023.
Your tubes can live anywhere between 50% and 70% of m.p.d. at idle [no signal, volume and tone controls set at 1], so use your ears as well.
Also, you'll probably get more than 315V on the plates because wall voltages are so high nowadays. Still use the formula [x]% x 12 / plate voltage
I like using the crossover notch bias method in conjunction with the cathode current method, but that requires a scope and dummy load.

BTW, if you pull the phase inverter tube and you still hear the low frequency oscillation, then you know it's you power tube section. If it goes away, you know it's in you pre-amp section. Start pulling pre tubes after that to further isolate the problem. Hopefully, new 6V6's you bought will solve the problem.

[Recycled_Brains]

you probably don't need to bias them.

if the amp sounds ok, they are probably fine.

Interesting. I was under the impression that this is NOT a fixed-bias amp, and would require adjustmens. I just consulted the manual and the only thing it said re: tube replacement is:

When replacing output tubes, please note the specific tube
markings on the original tubes factory supplied with your
amplifier. For example, 6L6 output tube types used by Fender
include, but are not limited to: 6L6GC; 6L6WXT; 6L6C; 6L6GE;
etc. The installation of a different type may require significant
adjustment by a qualified technician to avoid reduction in
performance and tube life.

The Tung-Sols have the "GT" marking, as do the stock tubes. Does this then mean, that they're the same, and won't need adjustment? The stock tubes are Groove Tube branded EH 6V6s.

Also, it's the the bias point for the tubes you need, not the amp. Tung Sol 6V6 have a max. plate dissipation of 12 watts. If you bias them at 60% of m.p.d. [ a good starting point] you'd get 7.2 watts. Divide this by your plate voltage and you'll get the cathode current you'll want.
If your plate voltage is 315V you'd want the cathode current at .022 or .023.
Your tubes can live anywhere between 50% and 70% of m.p.d. at idle [no signal, volume and tone controls set at 1], so use your ears as well.
Also, you'll probably get more than 315V on the plates because wall voltages are so high nowadays. Still use the formula [x]% x 12 / plate voltage
I like using the crossover notch bias method in conjunction with the cathode current method, but that requires a scope and dummy load.

BTW, if you pull the phase inverter tube and you still hear the low frequency oscillation, then you know it's you power tube section. If it goes away, you know it's in you pre-amp section. Start pulling pre tubes after that to further isolate the problem. Hopefully, new 6V6's you bought will solve the problem.

Thanks for the helpful information. Would you concure that there may be no need of adjustment?

[Mudcloth]

Thanks for the helpful information. Would you concure that there may be no need of adjustment?

It's possible that you could just pop the new 6V6's in. There's an audible difference in how a tube biased at 50% m.p.d and at 70%m.p.d. sound, though. There's no way of knowing without measuring what's going on right now. Before you pop the new tubes in, take readings of where the old tubes are living at and compare that with the new tubes. If you have access to a bias probe or bias-rite, you should use it. Better to be sure than to guess, if you can help it.

Of course, we're talking HIGH VOLTAGE in these things so if you are doing this with the chassis open PLEASE BE CAREFUL. And by BE CAREFUL I mean don't monkey around in there if you've never done it. There's a protocol in safety that you should be very familiar with when working on amps, including using an isolation transformer, probing with your left hand behind your back, working on a non conductive surface, wearing rubber soled shoes, not standing in a bucket of water, etc.

If you can make bias measurements with a bias-rite and adjust the bias pot without opening the chassis, you'll be fine. Again, if this is the case, take plate voltage and cathode current readings of where the old tubes are. You can then dial the new set of tubes to match up.

You'll probably be in safe parameters, but dialing in how you want the amp to sound will most likely require adjustment. The way to make sure you don't cherry up those new tubes is to know you're in that safe parameter.


I don't know what Fender set their bias at on these Princeton re-issues. I do know that they push the tubes in the Pro Jr.'s and Blues Jr.'s way too hot, 17 watts idle plate dissipation, usually, for an el84 which has a max plate dissipation of 12 watts! Crazy talk, I tells ya!
They do this because the amp sounds bad-ass that way, at a cost to tube life.
Last year a guy brought a new Super Champ into the shop I was working at with burned, blackened, and cracked power tubes. He had the amp for 3 months and Guitar Center wouldn't take it back because he didn't buy the extended warranty.


You can always take it to a reputable amp repair shop where they should charge you a reasonable rate for a bias adjust. If they're cool, they'll do it in front of you and show you a thing or two

[Marc Alan Goodman]

Doesn't sound like a bias problem to me. If it only happens when you hit notes and it's not the tubes I'd look to the filter caps. Probably time for a tech. Don't get burned!

[Recycled_Brains]

Doesn't sound like a bias problem to me. If it only happens when you hit notes and it's not the tubes I'd look to the filter caps. Probably time for a tech. Don't get burned!

I'm not suspecting the bias. I suspect the pwr tubes. They're microphonic when I tap on them. I replaced 2 of the preamp tubes that were also microphonic with better ones, and that didn't solve it. I'm only asking about the bias, because I have a pair of new tubes to try out. Hopefully that'll do the trick.

I have a hard time believing it could be bad caps. The amp is brand new.

[Recycled_Brains]

You can always take it to a reputable amp repair shop where they should charge you a reasonable rate for a bias adjust. If they're cool, they'll do it in front of you and show you a thing or two

Alas, that doesn't exist where I live. There is a great little guitar shop nearby, but their tech has returned poor work to me on several occassions, so I don't bother anymore. Not just me either. I have several musician friends who have sworn off this particular tech. for turning out sub-par repair jobs (in some instances, your amp comes back worse than before).

There is a great tech within 2 hrs. driving distance, that I go to for repairs, but he is in high demand, so turn around time is slow, and like I said, he lives 2 hrs away, so the drive is a pain in the ass.

I'm hoping to avoid having to go to those lengths to get a pair of output tubes biased. That's why I'm hoping that my friend and I can accomplish this ourselves.

[Milkmansound]

I have a hard time believing it could be bad caps. The amp is brand new.

that doesn't necessarily mean anything.

However, when it comes to tube amps, always suspect the tubes first. There's a reason the world shifted away from using tubes 50 years ago!

I still think you should just put the new output tubes in and call it a day. If you still have a noise problem, try subbing out other tubes. If you still have problems, and it is in fact brand new, contact fender because it should be under warranty.

[Recycled_Brains]

OK, so a friend of mine got himself a bias probe, and has offered to bias the new tubes. Thing is, he's a bit of a noob, so I'm a little nervous about it. I want to make sure it's done properly.

In regards to the bold area below... how do you make those measurements?

If you can make bias measurements with a bias-rite and adjust the bias pot without opening the chassis, you'll be fine. Again, if this is the case, take plate voltage and cathode current readings of where the old tubes are. You can then dial the new set of tubes to match up.

Thanks.

[The Scum]

You use a tool like this:

http://www.tubesandmore.com/scripts/fox ... TBIASMETER

It plugs into the tube sockets, and then the tubes plug into it.