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Topic: Weird Voltage Fluctuation in Fender Princeton |
Alan Kirk
From:
Scotia, CA, USA
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Posted 4 May 2005 3:30 pm |
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Attempting to bias a 61/62 Princeton (6G2) I put an accurately measured 1 ohm resistor between the tied-together pins (1, 7 &  and ground (on the power tube sockets). I'm getting a voltage fluctuation from between .08 and .14. It takes about 3 seconds for it to go from .08 to .14 and back to .08 again.
Any ideas what component might be causing this?
Thanks.[This message was edited by Alan Kirk on 04 May 2005 at 04:31 PM.] |
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Jay Fagerlie
From:
Lotus, California, USA
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Posted 4 May 2005 5:01 pm
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Change out the power tubes and see if it stops.
Sounds like "electron shedding"
Jay |
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Alan Kirk
From:
Scotia, CA, USA
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Posted 4 May 2005 5:25 pm
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Jay,
I tried different power tubes. Same thing.
Thanks for the suggestion.
--Alan Kirk
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 4 May 2005 8:09 pm
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Look at the 6G2 circuit (it's online here: http://www.ampwares.com/ffg/schematics.html). You can see that the tremelo circuit works by modulating the bias voltage, point Y on the diagram. Even if the intensity is all the way off, there might be a bit of bleed. Try switching the tremelo off using the footswitch, or simply put a shorted-out RCA plug into the footswitch jack - this shorts the oscillator circuit to ground.
This was the typical pre-Blackface method to get tremelo on Vibrolux/Tremolux and lower-end amps. Beginning with Blackface, it was changed except on the Princeton and Princeton Reverb, which kept this bias modulation approach. I prefer this type of tremelo, to me it has a 'smoother' sounding waveform. My favorite sounding tremelo was on a '61 White Tremolux which some yutz painted black years ago. But, man, was it a great sounding amp, yet another good one I wish I had back.  |
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Alan Kirk
From:
Scotia, CA, USA
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Posted 4 May 2005 9:09 pm
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Dave,
Thanks. I'll try check that out. Seems logical.
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John Daugherty
From:
Rolla, Missouri, USA
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Posted 5 May 2005 5:30 am
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Alan, the voltage will change when signal is applied to the tube. turn the gain controls to minimum and see if the voltage is steady. A noisy tube ahead of the output stage can cause this. Remove the tube just before the output stage to see if a signal is getting to the output tube grids.
Also touch that resistor to see if it is hot. It can generate thermal noise if it is hot. You may have to use a higher wattage resistor.[This message was edited by John Daugherty on 05 May 2005 at 06:33 AM.] |
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Alan Kirk
From:
Scotia, CA, USA
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Posted 5 May 2005 5:44 am
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Dave,
I shorted the pedal input. No change. Voltage still fluctuates.
John,
I'll try your suggestions this evening (although I have been measuring it with the volume control full off). Thanks.
--Alan Kirk[This message was edited by Alan Kirk on 05 May 2005 at 06:47 AM.] |
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Alan Kirk
From:
Scotia, CA, USA
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Posted 5 May 2005 2:43 pm
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John,
The resistors are not hot.
I've removed the 7025, the 12AX7, and the power tubes, one by one, with no change in the oscillation. Is it okay to remove the 53YGT for diagnostic purposes?
Thanks.
--Alan Kirk
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Bill Crook
From:
Goodlettsville, TN , Spending my kid's inheritance
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John Daugherty
From:
Rolla, Missouri, USA
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Posted 6 May 2005 5:41 am
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Alan, I finally looked at the schematic. Pin 8 and 7 are connected to ground. Pin 1 is a blank pin. Pin 7 has to be grounded to connect the filament supply. If you are trying to bias the tube by adding a cathode resistor, you must first disconnect the wire connected between pin 8 and ground. Then connect the resistor from pin 8 to ground.
It looks like you are measuring the voltage drop across the wire from those pins to ground. Any resistor you connect to that point will not affect your measurement. |
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Alan Kirk
From:
Scotia, CA, USA
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Posted 6 May 2005 8:13 am
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You guys are great!
I'll dig into this thing again over the weekend.
Thanks.
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Alan Kirk
From:
Scotia, CA, USA
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Posted 7 May 2005 11:30 am
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John,
Do you mean I need to disconnect Pin 8 from Pins 1 and 7 and give Pin 8 a separate path to ground, through the resistor?[This message was edited by Alan Kirk on 07 May 2005 at 12:32 PM.] |
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John Daugherty
From:
Rolla, Missouri, USA
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Posted 8 May 2005 5:40 am
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| Set me straight on exactly what you are trying to accomplish. Is this a modification where you wish to add a cathode resistor, or are you just trying to measure the bias voltage? |
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Alan Kirk
From:
Scotia, CA, USA
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Posted 8 May 2005 6:12 am
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John,
I want to put a 1 ohm resistor from cathode to ground so I can measure the cathode voltage through the resistor, for biasing, as per the "Cathode Resistor Method" of biasing described at http://www.duncanamps.com/technical/lvbias.html
Thanks for your response.
--Alan Kirk[This message was edited by Alan Kirk on 08 May 2005 at 07:15 AM.] |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 8 May 2005 9:21 am
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Actually, Alan, the point here is to measure the cathode current, not the cathode voltage. By inserting the 1-ohm resistor, it doesn't significantly change the plate resistor, but X volts dropped across a precisely measured 1-ohm resistor indicates precisely X amperes of current. Usually we're talking milliamps of current, so you want to measure millivolts.
The questions are, 1) Do you have these 1-ohm resistors hooked up correctly? and 2) How many millivolts is this voltage oscillating? A typical 6V6 in a circuit like this draws around 20-25 ma per tube, depending on the applied bias voltage. If this oscillation is a small fraction of a millivolt, I wouldn't worry about it. Bias voltage and cathode current drift slightly due to many factors like line voltage variations, heating up of components, etc., even when the amp is run quiescently, no inputs, etc. |
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John Daugherty
From:
Rolla, Missouri, USA
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Posted 8 May 2005 12:29 pm
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Yes,Alan, If you want to measure cathode current,break the wire connected to pin 8. Connect the one ohm resistor from pin 8 to ground.
You will probably not get the fluctuation after doing this. With a voltmeter across the 1 ohm resistor, volts=amps. 20ma will read .02 volts (20mv) on a voltmeter. |
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Alan Kirk
From:
Scotia, CA, USA
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Posted 22 May 2005 7:40 pm
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I provided Pins 7 and 8 with separate paths to ground. That ended the fluctuation and allowed me to get a rock steady reading over the 1 ohm resistor between Pin 8 and ground.
Thanks for all the help![This message was edited by Alan Kirk on 22 May 2005 at 08:42 PM.] |
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