Princeton Reverb and unwanted Distortion

[Telebuckers]

Hey all first time poster and longtime lurker here. Anyway, I've got a fender Princeton Reverb here I'm working on that the guy says blew a couple of fuses and had some unwanted distortion, but it wasn't THAT bad. The amp is still working and it hasn't blown a fuse since I've been working with it. It had a 5AR4 rectifier so I swapped it with the specified 5U4GB as well as all other tubes. The distortion became worse, so I put all the old tubes back in and the problem stayed the same- intolerable distortion. The rectifier tube socket was really corroded- one of the pins pulled out with the 5AR4 when I originally removed it- it was pin 6 so it wasn't connected anyway- but you get the idea. So I changed the socket but nothing has changed about the sound.

I'm wondering if the power transformer is going because I'm not getting any voltage on pins 2 and 8 on the rectifier from the secondary. I'm actually reading between -4 and -1 volts. When I take a reading from those pins to the center tap I'm getting 162 ohms.

On the power tubes I'm getting these readings:
pin 2- 00.7v (isn't this really low? heaters are still lighting up)
pin 3- 408v
pin 4- 400v
pin 5- -36v
pin 7- 00.7v
pin 8- 0v

Any help would be greatly appreciated. Thanks!!

[52 Bill]

On the power tubes I'm getting these readings:
pin 2- 00.7v (isn't this really low? heaters are still lighting up)
pin 3- 408v
pin 4- 400v
pin 5- -36v
pin 7- 00.7v
pin 8- 0v

These voltages are not all that out of line. As for pin 2 and 7 are you reading dc or ac volts?

Have you checked plate and cathode voltages on all of the tubes? Anything odd there?

[Telebuckers]

DC Volts. The voltages I listed were on the power tubes. I haven't checked the preamp voltages yet.

[Robert M. Martinelli]

Voltage AND current measurements

Hi,
when measuring heaters' voltage your meter should be set to measure AC voltage, the correct heater voltage for 6L6s and the like and the 12AX7s/7025 ( parallel heater wiring ) is 6,3 VAC - you should check the PT output to the 5AR4/GZ34 at pins 4 ( GZ34 anode 1 ) and 6 ( GZ34 anode 2 ) with the meter set to ACV, this is the the high AC voltage out of the PT. If the PT and the GZ34 work properly you will find +HT ( with the meter now set to DCV ) between pin 8 ( GZ34 cathode ) and GND, not pin 2.

The voltages you listed are more or less acceptable ( except for the heaters reading, as Bill stated, there' s no DC to be found there ), it is normal for the screen grids ( pin 4 on the 6L6 ) to be supplied with a voltage which is slightly less than the + HT found on pin 3 ( anode or "plate" ). Check all the power tubes, you should get the same readings under quiescent conditions ( no signal applied ). If you have the chance, apply an audio signal before the final stage and check if the unpleasant distortion is still there, if not, then you have something wrong in the preamp section. I would also measure all the voltages on all the preamp tubes to see if you have something strange there, like a resistor/cap out of spec, and don' t overlook the PI stage either, as this stage is responsible for the symmetry of the signal fed to the power tubes.If you have a scope it should be pretty easy to see where the signal gets "polluted".

A last word about sockets... I know maybe it' s a dumb question, but.... have you checked the other sockets ? If you have +HT voltages on the plates but one or more of the power tubes has a bad contact on a cathode, or oxide on a pin then this will act like a series resistor, less current will flow through that tube both in quiescent and working conditions and you are likely to have cross-over distortion. Having a voltage at one point doesn' t ensure you that current is actually flowing.....

To check for this, disconnect the cathode lug ( pin 8 ) of the power tubes' sockets from GND, and insert a 1 Ohm resistor ( 1% if possible to get accurate readings, 5% just to get an idea ) between each cathode and GND. This way, with your multimeter set to low DCV measure the voltage drop across each one of the four 1-Ohm resistors. You' ll get a reading of maybe 30-40 mV DC, and this will mean 30-40 mAmps flowing through that cathode ( thanks to Ohm' s law, I=E/R ). This is the total current flowing through each tube, which is the sum of plate and screen grid currents, and it should be more or less the same for all the power tubes, if you find a big difference or one of the tubes does not read any mV across the 1-Ohm resistor, then you' ve probably found the reason for the unpleasant distortion. Another possibility is that the power tubes are biased way too cold... If measuring across the 1-Ohm resistor you find all the 4 values to be under 30 mAmps ( and the sockets ARE good ) then the plate quiescent currents are about 25 mAmps ( supposing a screen grid current of, say, 5 mAmps, to get the real value simply check the voltage drop across each screen grid resistor and divide it by that resistor' s value to get the screen grid current for that tube, e.g. voltage drop 4VDC, resistor 470 Ohm ----> screen grid current =8,5 mAmps ) and your tubes are probably biased too cold and that is another reason for the cross-over distortion to occur. I don' t know how familiar you are with biasing procedures, but in case there' s a need we can deal with this later...

Hope this helps

Good luck!

Bob

[Amp Kat]

Your checking the heaters from ground. Go across pins 2 & 7 with your AC volt meter and you'll find your 6.3 volts but it will more likely be closer to 7 volts with our higher wall voltage. I'm always leary about amps that have been worked on before me and I go over them using the schematic and layout to make sure everything is connected right and all the right values are legit. It's a tad of work but well worth it in the long run. When I find distortion like that it's ususally a bad connection or wrong component value. Another thing I do is disconnect the feedback wire as to see if it changes anything and sometimes it does.

[Telebuckers]

Hey thanks guys- I knew I was doing something stupid when I wasn't getting the correct heater voltages. Now I guess it's time to start trying to figure this thing out.

[Telebuckers]

Ok well I'm back again after having had a chance to go through this thing some more. Still no breakthrough.

My bias seems to be good as I'm reading 17.4ma on V5 & 18.7ma on V6. I've gone through and checked most of the resistors and they all seem to be good.
The only weird thing to me is that I'm getting some voltages on V4 that are cycling but I'm not sure if that's normal or not. I'm going to list all the voltages on the preamp tubes to see if someone has any ideas on where this could be coming from.

V1 - V2 - V3 - V4
1- 176v - 301v - 156v - cycling between 175 and 350v
2- 0 - 0 - 0 - cycling between -2.6 and 1.8v
3- .96v - 5.23v - 1.25v - " " 1.34 and 2.65v
4&5&9- 2.7v - 2.7v - 2.7v - 2.7v
6- 181v - 305v - 162v - 198v
7- 0 - 0 - 0 - 14.4v
8- .88v - 5.2v - 1.2v - 41.8v


Thanks in advance for anyone that can help!!!

[Enzo]

Turn off the trem and see if your cycling stops...

Or rotate the speed knob and see what happens.

[Telebuckers]

The trem knob is all the way off but I'll see what happens when I turn the speed
and intensity knobs. I'm guessing that has nothing to do with my distortion problem though.

[Telebuckers]

Anyone have any ideas?

[Enzo]

Apply a sine wave and scope the signal path stage by stage.

In fact, just scoping it lets you see the nature of the distorted waveform, and that is often a clue to the source of the distortion.

[Telebuckers]

I wish I had a scope.

[sloblues]

scoping distortion

Apply a sine wave and scope the signal path stage by stage.

In fact, just scoping it lets you see the nature of the distorted waveform, and that is often a clue to the source of the distortion.

Enzo, this sounds interesting, could you describe what different types of distortion 'do' to the waveform view on a scope? I generally inject a 800hz .125VAC signal into my amps - I measured pickup output and it appears to be around 100-130mvac so that is why I use .125vac as the test signal.
thanks.

[Enzo]

Nah. Look at the distorted output. The nature of the distortion might be apparent from the waveform, but regardless, if you can see the distortion there, then trace the signal through the amp to find the stage where it takes on that distortion.