Low Bias Voltages on Fender 140

[bobshbob1]

Hey all, I'm trying to repair my Fender 140 head that I screwed up, and am looking for a little troubleshooting help. When I first got it, I took it into a tech for a cap job, and he said he had some trouble getting the amp to bias up right (not sure what he meant because the bias is not adjustable, although there is an output tubes matching pot on the back which balances the bias voltage between the two pairs of tubes). So when it came time to put new tubes in, I had planned on adding 1-ohm resistors to the power tube cathodes so I could at least measure the bias current, and maybe add in a bias pot if necessary. I'd done this on another amp, a Peavey Valveking, and it was easy enough.

Of course, while taking my first plate voltage measurement, my probe slipped and arced to the 1.5k grid-stopper resistor (D'oh!) and, when I jumped back, also the heater wire on pin 2 (D'oh!), immediately toasting the resistor and the hum balance pot. I've replaced that tube's socket, grid stopper resistor, and screen resistor, as well as the associated burned wires, and replaced the hum balance pot with two 100-ohm resistors.

Now the problem at hand: in LOW power mode, no power tubes installed, voltage on pin 5 of all power tube sockets seems normal (-21v on one side and -24v on the other). In HIGH power mode, I get voltages in the -16v to -18v range, which seems way too low. Tracing it back, I get around -70v from the bias filter cap down the chain to the 100k resistor right after the high/low power switch. On one side of that resistor I get -72v. Schematic says -58v so that seems kind of high. On the other side I get -27v in low power mode and -19v in high power mode. The resistor tested okay, but I swapped it anyway with two 220k in parallel, same result.

Being kind of a n00b at this I'm not sure where to go from here. Maybe replace the high/low power switch? In low power mode, on the side that switches bias voltage, I get 0 ohms from the middle leg to either side, and in high power mode I get 10k ohms and rising to either side. This doesn't seem right, but it also seems weird that the switch would fail at the same time as these other mishaps. Being my first serious repair on a tube amp I'd like to make sure everything is right before I fire it up, and any advice or insight would be much appreciated. Thanks!

fender-140-schem.pdf

[52 Bill]

On one side of that resistor I get -72v. Schematic says -58v so that seems kind of high. On the other side I get -27v in low power mode and -19v in high power mode. The resistor tested okay, but I swapped it anyway with two 220k in parallel, same result.

When the hi/low power switch is in high position, the switch shorts across the 100K resistor thereby bypassing it. If the voltage stays low check the switch itself to see that it does short out the resisitor. You may need to clean the switch or replace it.

[bobshbob1]

Okay thanks; haven't been able to access this site for a couple days but I did replace the switch. Now voltages around -65v on one side and -58v on the other in high power mode, so much better there, and I'm guessing with the output tubes matching pot I can get those more equal. According to what I've read, -60v is pretty much the upper limit for grid voltage, so would it be wise to try and lower these numbers? If so how would I do it, like is it as simple as adding more resistance before the tubes matching pot?

[Enzo]

Upper limit? The bias voltage is used to set tube current. It is not done in isolation, the plate and screen voltages matter too, because power tube dissipation is the issue, not voltage. And I tend to think this amp was not going for a hot driven power stage. It had to be ready to shift down to half power by halving the B+ as well as reducing the bias. Plus it is an ultralinear design. Amps do not have to be biased up to the verge of meltdown. Guys like to do it, but you don;t need to. Screaming gain monsters like the PV 5150 come out of the factory running maybe 12-14ma through the tubes, far cooler than the 40ma guys might set it to.

With the matching pot, don;t worry about getting them equal. Worry about getting the two tubes running in balanced fashion. Equal conductivity does not necessarily result from equal grid voltages. Turn the amp on hig power, no guitar, turn all the preamp controls to zero, we want to onl;y hear the power amp noise. Now sweep that balance control back and forth to find the spot with the least hum and noise. THAT is where to set it, regardless of how similar the grid voltages turn out.

As to dropping the voltage, I'd recommend getting to know the amp as it came before thinking about changing it. Remember, any change you make to the bias circuit has to work in both power level modes. If I had to do it, I'd probably be changing the resistor under the balance pot rather than above.

[soundguruman]

"Of course, while taking my first plate voltage measurement, my probe slipped and arced to the 1.5k grid-stopper resistor (D'oh!) and, when I jumped back, also the heater wire on pin 2 (D'oh!), immediately toasting the resistor and the hum balance pot."

Oh no, oh God...not this again.

You "could" put a piece of electric tape around the probe so that only the very tip of it is exposed. But, no.

[Gtr_tech]

Shrink tubing works better....

[bobshbob1]

Oh no, oh God...not this again.

You "could" put a piece of electric tape around the probe so that only the very tip of it is exposed. But, no.

Curse you, Captain Hindsight! Done, now I feel much more comfortable poking around inside this rat's nest.

Upper limit? The bias voltage is used to set tube current. It is not done in isolation, the plate and screen voltages matter too, because power tube dissipation is the issue, not voltage.

That makes sense. I guess as long as the tubes aren't drawing too much power I'll leave it alone; I like the way the amp sounds now so any adjustments would be for the sake of tube life, not changing the sound in any way.

[Enzo]

You were concerned the bias voltage was too high earlier. DO you realize that the greater the negative voltage, the colder the tubes run. BY lowering the bias voltage, we run the tubes hotter, and their life will be shorter. SO the stock setup ought to give you good tube life.

[soundguruman]

When they turn just a "little" (bitty) red, is when they sound the best...

[bobshbob1]

Well, it's all up and running with a new high/low power switch, sounds great too. I don't like it much for guitar but as a bass amp it blows high-wattage solid-state stuff out of the water, which is why I keep it around. Tubes are running on the colder side, between 17 and 19mA, with the plates at just over 530v, so drawing around 10 watts each at an idle. I see now what the bias voltage does; I just got that -60v "upper limit" from reading that it should be "around" -45 to -60v, but I guess being more negative tends to be a good thing as far as tube health. Thanks for the tips all, I'm definitely still learning but this was a nice step forward.