Four Terminal Pot?

[apehead]

Hello,

A friend of mine has a 1976 Fender Quad Reverb with some issues that I've been commissioned to repair. The screen grid resistors on two of the four output tubes released their magic smoke! Both of them were on the "brown" lead of the output transformer. I traced back the circuit to the "Balance" pot. It appears that someone has modified this section of the amp in the past. The electrolytics have been replaced, and it seems that whoever did this failed to re-attach the surrounding components to the "Balance" pot properly. The pot has four leads on it, and on the schematic all four seem to be used. On the amp, only three of the four have any components soldered to them, although the fourth has solder and flux residue, like something was soldered to it at one point. I have never worked with these four terminal pots before, so I'm not sure how they physically correspond to the schematic. Here's how it is attached at the moment:

Photos of "Balance" pot

The second photo has labels explaining all of the connections. Here is the schematic for the amp:

Fender Quad Reverb Schematic

Could this cause one side of the push-pull to burn up its screen resistors, or am I just chasing a red herring? Even if it was connected properly as is, could this possibly happen if the "balance" was adjusted all the way to one "half", causing those two tubes to run much hotter, eventually causing enough screen current to burn those resistors up? My friend is notorious for accidentally leaving the amp on overnight on several occasions.

[Bruce / Mission Amps]

It is not unusual to see a 4 lug bias supply balance pot wired as a variable bias voltage "idle current level" setting pot. The level control will make all the tubes go up or down in idle current.

What you need to do is take all the power tubes out and turn the amp back one, then measure the negative DC voltage on each socket, lug 5 and see what is happening when you turn the pot. CAREFULL...big nasty high voltages on the tube sockets. You are looking for a negative voltgae, ... -50 to -65 volts or something like that.
When you turn the pot clockwise or counter clockwise, (it takes a little while to come back closer to zero, meaning more positive with a light load on the bias supply) you'll see the voltage changes.
If the voltage rises and falls equally on all sockets, it is a power tube idle current level setting control but if the voltage goes more positive or negative on only
two left or two right sockets, ... then it is bias voltage balance control.

[lowell]

I agree w/ Bruce that it's most likely been rewired as a variable bias supply. Better than a balance pot anyway IMO. One other thing to watch for is the coupling caps from the PI to the power tubes. I've had amps in the shop where one of those caps was leaky and was cancelling out the bias to half of the power tubes, reaking havoc on the quiescient operating point of two tubes. Just something to check... especially when you retube and power up look to see if 2 tubes on one side begin to "redplate" after throwing the standby switch into play mode.

[apehead]

That wouldn't explain why just the "brown" side fried its screen resistors, and the "blue" side seems completely fine, no signs of burning, or anything. Also, the 3.3K resistor that provided the fixed bias wasn't removed. So all this possibly modified control could do is turn the bias down (more negative). It doesn't look like this was the tech's intention to me, and if it was, he should find another profession! This would explain the way both of the 68K resistors are wired to a common point, although there aren't any unused terminals on the board where they are located, implying that this is how it was wired at the factory... whew! this is going to be tougher than I thought! O.K., let's put it another way. What would cause half of a silverface amp to burn up its screen resistors?

[apehead]

One other thing to watch for is the coupling caps from the PI to the power tubes.

What does "PI" stand for?

[apehead]

Also, before the amp stopped working, none of the tubes' plates were glowing red, nor was there that characteristic hum of an over-biased tube.

[lowell]

if the tubes were not glowing and there was no hum then it would not be a bias issue. I believe that bad screen resistors are usually due to bad tubes... usually. PI stands for phase inverter.

[rf7]

What does "PI" stand for?

Phase Inverter.

The pot you've shown is re-wired as a bias adjust pot. The 3.3K resistor in series with the pot and the 15K resistor to ground are there to provide a voltage divider with the output on the middle tab of the pot. Those 3 parts together, are used to set the bias voltage. Google voltage divider with resistors for the math on how this works.

You need to turn on the amp, with the power tubes removed and standby set to play mode, and measure the voltages on the power tube sockets with your DVM. See if there are any differences between the two pairs of output tubes. Write those voltages down for reference. Be very careful, because you're looking at 475V or more on the plates. Do not do this, unless you know what you're doing, you could get severely electrocuted if you don't know what you're doing. Any major voltage difference would indicate a problem.

Lowell's comment above references one possible scenario which could cause your problem. The PI splits the signal into two halves (positive and negative), and feeds each pair of power tubes from a different coupling cap. One leaky cap would only feed 2 power tubes and could be trouble for those 2 tubes.

I used to have one of those amps in the '70's. I've only seen one or two since then.

[Enzo]

This is another example of simple tube failure. The two tubes on the left are wired together, and the two on the right are wired together. When one tube on the brown side failed, it shorted the grid out and killed the bias voltage there. SInce its neighbor tube was wired to it, that tube lost its bias as well. So they burnt up. Burnt tubes burn out their screen resistors often as not. Happens all the time, it is not a matter of bias adjustment. Once the screen resistors open, the tubes no longer conduct, so they won't be red plating anymore.

Your pot has been rewired into a bias level control instead of a bias balance control, a VERY common modification. Turning it all the way over to one end will either maximize or minimize the level of bias. As it sits, it would not selectively alter one side over the other.

[apehead]

Just for educational purposes, how do these pots work? Is the fourth terminal stationary at half of the total resistance? Are there two sets of wipers? Is the middle terminal on the "three" side still the variable one?

[Enzo]

It is a conventional pot with the extra lug being a center tap on the resistive element. That sounds like your first description is correct. You can use a tapped pot like this in place of a common one any time you want. The tap would then be invisible to the circuitry.

[apehead]

I have tested the four 6L6GC's on the Hickock Model 6000B tube tester at work. I used the 6L6 settings in the test data book (I guess 6L6GC's didn't exist back when the test data book was published). According to the tester, there are no shorts in any of the tubes. Could this mean the short exists in the amp, rather than the tubes themselves? Any hypothesis or advice would be much appreciated.

[apehead]

A new development for the silent majority riveted to their PC's by my ongoing trouble-shooting adventure... having exhausted the idea of a bad tube, I went back and tried to find shorts in and around the sockets. Mystified as to why only half of the tubes blew their screen resistors, I visually inspected pin six (a blank pin used as a soldering post to jumper all of the screens together) on all the tube sockets, until I came across a suspect solder connection. On pin six of the V9 socket,the wire jumpering the V10 screen to the V9 screen wasn't even soldered!! It was just sort of pressed against the hole in the socket pin! That makes sense, considering both of the screen resistors after that connection blew out, but the other two prior to it are fine! I didn't find any shorts, but I suppose this would cause some serious problems! I've ordered replacement resistors, and will update on the results.

[apehead]

O.K., I replaced the two old, burnt screen resistors with new ones, and replaced the old screen jumpers on the those two sockets with new wire. Replace tubes. Energize... [I]Poof![I] another old screen resistor sprays yellow crap everywhere, this time on the "blue" side of the OT (the other time, both of the other burnt resistors occurred on the "brown" side of the OT, which is just a reference to the color of the leads on either "side" of the OT's HV winding). Now I begin to think that despite the test results on the Hickok, that the problem resides in a bum tube. I replace the resistor and jumper on the third socket, and number the tubes with a sharpie. Replace tubes ("#1" on far left, "#4" on far right, even though I think I initially numbered them right to left, adding another stupid variable by switching the interior tubes' positions! ). Energize... [I]Poof![I] I blow a new screen resistor, on the interior "brown" side socket. Tube "#3" is the culprit! Marked it bad, got rid of it. Still, I do not know if Tube "#2" is bad as well, because of the confusion when I re-installed them. I am now running the Quad Reverb in a 50 Watt config., using tubes "#1" and "#4", with no problems. Here are some questions I have from the problem:

I tested all the tube with a good tube tester, and a multimeter for shorts, and found none. Has anyone seen this condition before? Could the tube be arcing due to the high voltages in the amp, that were not present in the test equipment? Could this be due to using the "5881" setting on the Hickok, rather than proper "6L6GC" settings (lower average screen and plate voltages)? Is there a way to test tube "#2" under a high voltage (without replacing more screen resistors )? I have a megger at work (possible HV test pwr supply?).

[Bruce / Mission Amps]

I have lots and lots of tube testers. I use to collect them as a hobby.
17 of them are 532s, 533s, 539s, 752s, 600s, 800s, 6000a, etc etc) and none of them run power tubes at anywhere near real B+ voltages and currents.
That is why your tube tester isn't telling the whole story.

I would say that I find 95% of the time a screen resistor is blown, it is the power tube.
I think all you needed to do was start the amp up with no power tubes and see what happens... everything is OK until you insert the bad tube(s), causing the screen circuit current to flow beyond the wattage of the screen resistor.
By the way, EL34s are the worst for this! Yes, 6L6s and 6V6s can do it too but EL34s can pull a huge amount of screen current compared to a 6V6/6L6 type tube...