Fender Princeton '65 Bias (?) Problem

[John R Smith]

Good afternoon from Cornwall. I am new here, but I have called in to pose a question regarding my little Fender Princeton '65 Reverb reissue amplifier. I purchased this about two and half years ago, so it is well out of warranty. Other than testing it briefly when I first had it, I have never used it because health issues prevented me from playing guitar. Just recently I have at last used it three times in all, for about an hour each time at very low volumes in my studio.

Last night I had been playing for about an hour when I noticed the amp smelled hot. Looking in the back I could see that both 6V6 power valves were red-plating (glowing anodes). I powered down and put the amp on my bench for investigation. With no signal applied, but the amp warmed through, I could just see the same problem, but when I turned up the vibrato intensity from zero it became much more obvious. The brightness of the red anode glow seemed to be directly related to the vibrato intensity setting.

So I stripped out the amp chassis and set it up with speaker connected and no input. Checking with my meter at TP16 (V5) I have 51mv rather than the 23mv specified by Fender for the bias setting, even with the bias trim pot R22 at its minimum setting. On TP17 (V6) I have 46mv. If I then bring up the vibrato intensity pot from zero to say 4 or 5, the reading at TP16 will climb to over 100mv and the anodes start glowing on V5 and V6.

Looking over the PC board I can see no obviously over-heated or burned components. The amp is still working fine and when you play through it there is no distortion or noticeable problem to the user. At this point I am stumped - any ideas would be most welcome.

[Leo_Gnardo]

Cornwall, hey I know where that is! Now about that Princeton. First thing to know, the vibrato is obtained by "shaking" the bias voltage - making it swing up and down. As bias voltage Vb heads deep into negative numbers (say -40V) tends to choke off current in the output tubes, which reduces signal, there's your vibrato. However that bias voltage also swings into fairly low numbers for part of its journey, say -20V or so, that permit more bias current than you would need, and has negiligible effect on volume. What you have is a pair of 6V6 that are already passing too much bias current. Superimposing the low frequency vibrato on this condition only makes things worse.

Two routes: 1. you could select a pair of output 6V6 that are rated with low Pc values. You'll probably find you can adjust your bias to the appropriate level with them. Such tubes will have been tested (first good thing) and selected to be in the low range of emissivity therefore will not tend to "run away" into red-plate condition. If I'm not mistook tubes tested and rated this way can be ordered from Antique in USA and TAD in Germany. I'm sure the UK parts distributor has some. Now route B: you'll have to alter the bias supply in your amp to run at a slightly higher voltage. As I don't have the RI Princeton schemo ready at hand, can't say exactly what to do but it will probably involve changing one resistor, or parallelling one resistor with another, no big deal either way.

[audiopete]

Tremolo (Vibrato as Fender erroneously calls it) will goose your power tubes because it raises and lowers the amplitude of the incoming signal or, in this case, varies the bias voltage. The Princeton routes the bias voltage through the Vibrato circuit (Vibrato pot) on the way to the power tube grids. I would take a reading of the negative voltage on the power tube grids, and if not right, I would trace it back. I would also look at C10 and C11 for DC leakage, and C22 in the trem circuit also for leakage. If any of these caps are leaking positive voltage, it will cancel out your negative bias voltage and your tubes will redplate. Also disconnect the bias supply from the Vibrato pot (no power tubes in!) and check for proper operation. To check caps for leakage, lift the low voltage leg and measure with a voltmeter to ground while the amp is on. Anything over a few millivolts is unacceptable. I'm not going to give you all the warnings - either you can work on amps or you can't and will take it to a tech. Cheers.

Schem - http://support.fender.com/schematics..._schematic.pdf

[audiopete]

Cornwall, hey I know where that is! Now about that Princeton. First thing to know, the vibrato is obtained by "shaking" the bias voltage - making it swing up and down. As bias voltage Vb heads deep into negative numbers (say -40V) tends to choke off current in the output tubes, which reduces signal, there's your vibrato. However that bias voltage also swings into fairly low numbers for part of its journey, say -20V or so, that permit more bias current than you would need, and has negiligible effect on volume. What you have is a pair of 6V6 that are already passing too much bias current. Superimposing the low frequency vibrato on this condition only makes things worse.

Two routes: 1. you could select a pair of output 6V6 that are rated with low Pc values. You'll probably find you can adjust your bias to the appropriate level with them. Such tubes will have been tested (first good thing) and selected to be in the low range of emissivity therefore will not tend to "run away" into red-plate condition. If I'm not mistook tubes tested and rated this way can be ordered from Antique in USA and TAD in Germany. I'm sure the UK parts distributor has some. Now route B: you'll have to alter the bias supply in your amp to run at a slightly higher voltage. As I don't have the RI Princeton schemo ready at hand, can't say exactly what to do but it will probably involve changing one resistor, or parallelling one resistor with another, no big deal either way.

This all assumes there is no problem with the trem or bias circuit.....I don't see this as a "mod the bias" issue when the bias current is almost double factory recommended. I see this as a malfunction with the amp.

[John R Smith]

Many thanks for the input so far. Measuring at TP13 I have only -29.7V, rather than the -40V specified. So it looks as if something is wrong in the bias/vibrato circuit, perhaps?

[stokes]

I would first check the negative bias voltage with no tubes installed.If you are unable to get the -40,ideally you will have a range around -40.If the best you can get with no tubes is the -27 you stated,go to audiopetes steps.If you get the desired range of neg voltage,it is safe to assume the problem is in the tubes.

[John R Smith]

Thank you, Stokes. I have just measured the negative bias voltage at TP13 and TP19 without the power tubes installed, and I still only get -30V. So it looks like the trouble is on the board somewhere, not in the tubes.

[Tom Phillips]

http://music-electronics-forum.com/t38333/

Many thanks for the input so far. Measuring at TP13 I have only -29.7V, rather than the -40V specified. So it looks as if something is wrong in the bias/vibrato circuit, perhaps?

Edit: I see there was activity while I was writing the following offline. Hopefully there is some additional helpful information in my post.

The information that you posted gives very good troubleshooting clues. Based on the history of the amp since new we need to be open to the possibility that the amp may have left the Fender factory in this condition and you just noticed the problem during your third playing session with it.

In addition to the information already provided I offer the following troubleshooting process:
1) You have already determined that the power tubes are under biased and are therefore running too hot. As has been pointed out in that situation the bias wiggle tremolo just makes the problem worse. However, I think it is likely that the tremolo circuit itself is working OK. Therefore, let’s work to isolate the problem. Do this by turning the tremolo oscillator off with the footswitch AND turning the intensity control to minimum. Leave it that way for now while we do the tests below.
2) Now evaluate the bias circuit.
a) You already measured -29.7V at TP13. Measure the voltage at TP19. I suspect that TP19 will also be -29.7V. Please verify and report back.
b) You said “Checking with my meter at TP16 (V5) I have 51mv rather than the 23mv specified by Fender for the bias setting, even with the bias trim pot R22 at its minimum setting.” Does the reading change at all when you adjust R22? Note: You can also remove the power tubes to troubleshoot the bias supply. You won’t get any readings at TP16 or TP17 but you will be able to determine if the bias supply is working by monitoring the voltage at TP19 as you adjust R22. I suggest you do that and report the range of voltage you read at TP19 as you sweep R22 through its full range. The bias voltage normally should be adjustable over a wide range such as -20s to -40s.
3) If you can’t select a wide range of bias voltage numbers then we need to start by troubleshooting the bias supply circuit.
a) Start by visually verifying the values of the individual components. D4, D7, R2, R22, R28 & C25. The problem could be cause by an incorrect part value installed during manufacturing. D4(1N5370B) is a 56.5V Zener and appears to be there to limit the maximum negative swing to -56V. If it is faulty or an incorrect part value then it could be limiting the max negative voltage swing. The voltage response you observe at TP19 while rotating R22 will give us a clue about that.
b) If all looks OK then we will need to measure parts values to find the faulty part(s).

Tom

[John R Smith]

Hello Tom, many thanks for your lengthy reply. I have checked a few things as you asked -

2) (a) The voltage at TP 19 is -30V

(b) With power valves out and measuring at TP19 while sweeping the bias pot R22 throughout the range the reading is from -26V to -31V. Not good.

3) (a) A visual inspection reveals no problems - all the components you mention are there and look intact.

I have to go out now, so that will be the end of the story for today, but any further thoughts you have will be most welcome. Thanks again to all for your time and trouble -

John

[Tom Phillips]

If the amp were on my bench I'd measure the actual values of the resistors in the bias circuit. If they match the schematic values the next thing I would do is lift one leg of D4 thus taking it out of the circuit. Then I would do the R22 sweep test again and determine if the voltage range readings at TP19 improve.

[g1]

Also check for presence of AC voltage at TP19.

[John R Smith]

Also check for presence of AC voltage at TP19.

There is no AC voltage at TP19. I have also checked a few other things -

* The supply voltage was 246VAC this morning.

* The B+ voltage at TP30 is 426V

* Plate voltage at TP15 and TP16 is 422V

* Everywhere I can check it, the bias voltage is no better than -31V with power valves removed

* The power valves start to glow red on the plates within 5 mins of firing-up

At this point I think I have reached the limit of my abilities, so it looks as if I will have to find a local amp tech to check the components and find the faulty one. Many thanks for all your help just the same, it is greatly appreciated!

[g1]

At least try disconnecting D4 as Tom suggested in post #10. If you don't want to pull the board to unsolder it, just cut one leg so it is disconnected.
It's a very cheap part, so no big extra expense if it is not the cause of the problem.

[Tom Phillips]

When you did your visual parts verification were you able to determine that D4 was a "1N5370" part number?

[John R Smith]

When you did your visual parts verification were you able to determine that D4 was a "1N5370" part number?

Yes Tom, it is a 1N5370B part number. I suppose I could cut the leg in the centre so I could re-solder it again across the break, rather than have to take the circuit board out to access the rear.