trouble shooting a Princeton Reverb Reissue

[mike_mccue]

Hi,
I stumbled into agreeing to help a friend of friend with a Fender Princeton power tube swap.

It turns out it's a Princeton Reverb reissue and something is funky and that's why the owner wanted new tubes.

If I can't help the amp will probably have to be shipped out to someone who can.



I played the amp a bit and there is a fitzy, farty white noise that piggybacks the basic sound.

I pulled each tube and re seated them to clean up the contacts in the sockets. (no spray cleaner... just mechanical movement)

All the pots on the front seem clean.

I noticed that all the preamp tubes have been replaced with NOS JAN made in USA GE 12AX7 tubes from the last runs in the 1990s?. Which means that there's no 12AT7 at the driver and no "7025" at the first stage.


I placed my CompuBias meter in the sockets and measured the factory stock EH Groove Tubes 6V6 tubes. I noticed that the bias readings are moving around too much. FWIW, I have the tremolo controls at 1 but it isn't switched out because I don't have the switch.

I would take a reading and see something like 11ma and 14ma draw and then I play a chord and hear the noise and see that the meter would change to something like 16ma and 10ma.

The replacement tubes also fluctuate and they are running so hot they red plate... so I pulled them imediately and want to settle the real issue before I worry about the new tubes.

I have read that at high draw the tremolo circuit will act funny... but an average 13ma or 14ma draw seems typical for what I am reading about this amp.



I have not touched the bias adjust pot and it is my intention to ask a few questions and study the reissue circuit before I proceed further.

I am also aware that I may not want to actually work on the amp if it needs a fresh component on the circuit board... it seems like the Reissue isn't made to be repaired easily. My friends know that I am just a old fashioned hard wired type of hobby and maintenance guy... so I can bow out... I just want to see if I can help and if I can't we'll move on.



Does anyone have any thoughts or ideas about what I should look for?

My plan is to place a real 12AT7 in the driver and maybe pull the chassis and start tracing voltages.

I thought perhaps there might be a known weak spot on these reissue amps that I should focus on.


Thanks very much for any info you can share.



best regards,
mike

[Enzo]

Well yes, get the right tubes in it, otherwise, why should it work right? COnsult the tube chart on the inside wall of the cab, don;t assume, know which type tubes it wants.

Isolate the problem. PLug a different speaker into the amp. SOund the same now? And connect the existing speaker to some other amp. Is the speaker OK?

The ONLY tube current readings that mean anything are taken at idle. That is why it is called idle current. NO sound coming through the amp.

I don't know why the board wouldn;t be servicable, ten screws loose and three small ribbon connectors off, and the board is loose, flip it over for soldering.

Pull the power tubes. You get a nice steady -40v at pin 5 of each? Verify the 1 ohm resistors from the pins 8 to ground. Verify the 470 ohm screen resistors.

With the tubes in and powered up, how do your DC voltages compare with the ones on the schematic? The square ones are the DC voltages.

Your runaway tubes could just be faulty. But also, did you try setting the bias to factory spec?

Everybody just HAS to have point to point wiring, but that part is the weakest aspect of these amps. I trust the circuit board a lot more than I do the hand wired stuff. Look CLOSELY at the power tube socket wiring. Are all the pins actually soldered?

[mike_mccue]

Thank you Enzo.

I'll go through your suggestions and follow up in detail.



I forgot to mention in the OP that the plate voltage seems stable at 450-455vDC.

I did play through each of 2 other known good speakers to check and the problem persisted.

The only reason I might elect to not service the board is that I am not a pro bench tech and everyone concerned with this amp knows it. While I can solder big stuff and have familiarity and experience with I don't know much about the modern small connections. If it needs a real tech the unit will likely need to be shipped to someone simply because there's no one here who does this sort of work professionally. If it really is a few screws and ribbon connectors I'll try to diagnose the issue and make a decision when it is time.

I haven't tried setting the bias yet because it appears to be averaging what I thought to be factory spec. 13-14ma. The issue I was noting is that it isn't balanced well and the balance seems to flip flop.

I didn't specifically play through the amp to test the voltages and draw but I did notice that it changed and the numbers I reported were after it settled down and the amp was quiet.

Thanks again. I'll look into each of your suggestions.


best regards,
mike

[g1]

The reverb driver should be a 12AT7, all other preamp tubes 12AX7 (or 7025 or ECC83). Bias is shown as 23mVDC at each test point. It will take the bias quite some time to settle back down if you play it (tubes & OT get hot and have to cool back down).
Your bias is a bit low, could be causing crossover distortion which to some may sound like "fitzy, farty white noise that piggybacks the basic sound"?
Other possible suspects are poor solder connections or noisy preamp tube plate resistors. Freeze spray is a big help for finding noisy resistors.

[MikeH]

Freeze spray is a big help for finding noisy resistors.

Interesting - how does it work?

[mike_mccue]

Hi g-one

Thanks very much for the schematic and test point layout.

best regards,
mike

[mike_mccue]

Here are some answers to the questions posed by Enzo, as well as some other basic measurements.

The values in parenthesis are the values indicated on the Fender schematic.

I was mistaken there is a 12At7 in the reverb driver spot. The rest are 12Ax7.


Measurement taken with 6v6 power tubes removed and amp powered up.

TP13 Tremolo -41vdc (-40vdc)
pin5 V5 -40.4vdc
pin5 V6 -40.4vdc


R39 screen resistor V5 is 465ohm (470ohm)

R40 screen resistor V6 is 465ohm (470ohm)

Voltage at R39 and R40 is 435vdc on one side and 434vdc on the other.


R20 1.1ohm (1)

R32 1.1ohm (1)

TP19 negative bias supply -33.2vdc (-40)





Test Point measurements with amp powered up and all tubes installed:

Factory EH Groove Tubes 6V6 and the proper preamp tubes.

TP30 442vdc (445)

TP31 435vdc (425)

TP32 266vdc (250)

TP2 plate of V1-A 175vdc (165vdc)

TP35 cathode of V1-A 1.35vdc (1.2)

TP5 plate of V1-B 175vdc (165vdc)

TP4 cathode of V1-B 1.35vdc (1.2)

TP25 plate of V3-A 172vdc (130)

TP7 plate of V3-B 177vdc (125)



TP16 cathode pin8 V5 26mvdc (23mvdc)

TP17 cathode pin8 V6 22mvdc (23mvdc)


TP15 plate pin3 V5 445vdc (440vdc)

TP18 plate pin3 V6 445vdc (440vdc)



Voltage at R39 and R40 is 435vdc on one side and 434vdc on the other.


TP19 negative bias supply -40.6vdc (-40)
pin5 V5 -39.79vdc
pin5 V6 -35.40vdc




I'm not sure what to make of it... most of it seems normal, but the balance doesn't seem ideal and one tube seems to be running a bit hot.


Any thoughts?


best regards,
mike

[mike_mccue]

I forgot to add that I used a chop stick to tap around the amp and it seems like everything is somewhat micro-phonic when I tap it.


Thanks.


best regards,
mike

[mike_mccue]

I just read this thread which seems to describe the sound I've been concerned about:


http://music-electronics-forum.com/t20937/


After considering all the readings I posted, I'm wondering if I should just adjust the bias a little colder and hope the amp owner understands that the panels controls will let you distort the amp enough to cause the noise... so you just should not do that.



edit to add:

I've reconsidered and feel that the noise is excessive.

The board still seems slightly microphonic. The first power supply cap seems especially sensitive.


I made a small adjustment to the bias and brought the tubes down to 21-22ma draw. The difference between the two tubes settled down and everything seemed even.

Then after a while V6 started slowly and steadily climbing. At 40ma the plate started glowing and as I was going to shut the amp down the current quickly dropped back down to 22+/- and everything seemed stable. Then a few minutes later V6 started climbing in draw again and I shut the amp down before V6 got too hot.

I hesitate to put any other tubes in it until I figure that out.



Also, I am wondering why the plates on V3 are so much higher voltage than the Fender docs indicate I will find.



best regards,
mike

[Jazz P Bass]

Sounds to me like the tube that is red plating is bad.
Swap the tubes, if the problem follows the tube, then it's the tube.

[g1]

Could be V3 itself. Swap it with one of the others and see if the bad voltage reading follows the tube or stays with the socket.
Regarding V6, sounds like the bias problem has appeared with more than one set of power tubes, is this correct? If so, the problem is in the circuitry, likely either a bad solder joint somewhere between TP13 and V6 pin 5, or a leaky coupling cap C16. Also check the pin 5 contact in the V6 tube socket.

[mike_mccue]

Yes, thanks, swapping tubes in position is the first thing to do today.

I'll post the results shortly.


best regards,
mike

[mike_mccue]

With regards to the power tubes I swapped the factory EH 6V6 tubes and used the CompuBias to observe that more or less the extra draw did indeed follow the tube, but when I swapped the tubes back to check again the tube that draws more current immediately ran away in the socket it was in originally.

Sometimes everything is sitting sweet at 22/22.


I have adjusted the bias a little and so TP13 is at -41.5vdc.

The voltage at R20 and R32 seems nearly identical at 22+/-mvdc.

The plates are at 440-445vdc.


One tube always starts up cold right at power up. On the Compubias it may hover Ik19 for a while but then it gets to Ik22 and seems fine as the other tube is right about there. Then it slowly rises to 30ma and stabilizes in the socket for V5.

The voltages at R20 and R32 still seem to match even though the current draw is not. The actual voltages at Pin5 on each tube do seem to reflect the imbalance.


I swapped the tubes back and when the one that draws more current was in it's original position, V6, that same tube seems to want to run away. When it was warm and I just powered up it spiked so quickly I just shut the amp off asap.

I measured R20 and R32 again they seemed a bit less resistive than when I measured them yesterday. I'm using a recently purchased Fluke 179, yesterday R20 and R32 measured at 1.1ohm. Right now they measure at 0.7ohm?

I guess it is time to look at R8 and R33.


Any Thoughts?


edit to add:
I switched back again and the tube is definitely transporting the problem. I think it's time to try some other tubes.




With regards to the plate voltage at V3.

The voltage at TP32 is about 20v high at 270vdc.

R36 and R37 see that 270vdc. They both test ok while hot at 102k and 100k. The voltage on the plates of V3 is 170vdc and 175vdc which is about 40v higher than spec. I wonder if this extra gain is the source of the farty noise... perhaps at the next stage?



Thanks for all the tips and suggestions.


best regards,
mike

[Jazz P Bass]

When you are testing a triode, keep in mind that the plate voltage is not the full story.
You really should measure the cathode voltage as well.
Both of these readings will tell you how well the tube is conducting.

[MWJB]

Change the 6V6 tubes for a new pair & recheck. You seem to have identified that one of them is faulty so don't keep putting it in there.

Your voltages are not unusually high, they might be a little higher here and there than the schematic, but I have seen up to 470vdc on the power tube plates with a GZ34 and 23mA per tube idle current in these amps.

You should not need to drop the plate current down below 22mA at idle, certainly if you need to go below 20mA something is very wrong. As you reduce plate current, idle voltage will increase. You could try a 5V4 or even a 5Y3GT rectifier to reduce plate voltage.

R36 R37 are in the vibrato circuit, it is very unlikely that they are contributing to the noise. You can short out the tremolo foot switch by making up a shorted RCA plug, or inserting a small crocodile clip into the footswitch jack to short the footswitch socket.

A 5751 tube in V1 should reduce noise overall. It will drop gain a little, otherwise stick to the Sovtek 12AXWA that came with the amp, these are lower gain than many other 12AX7 types.