Silverface 135 UL 'output balance pot'

[oahudog]

Hi All,
New here, and looked for an 'introduce yourself' place..no joy, so I'll start right in!
'79 UL, 'output balance pot' has been removed (replaced with a dead pot to fill the hole) and bias pot (single) installed. I've had NO luck finding the 4 lug pot (listed on the schem. as '10K LT 010437'). Does anyone know of a 'work-around' for this?
Thanks! dave

[The Dude]

To my knowledge, that pot is no longer available. That said, you don't really need it if you run decently matched tubes. I believe Fender did this so tube matching wasn't as critical. The amp is probably fine as is and was probably modified this way because the last tech couldn't get the pot either.

[Leo_Gnardo]

Hi All,
New here, and looked for an 'introduce yourself' place..no joy, so I'll start right in!
'79 UL, 'output balance pot' has been removed (replaced with a dead pot to fill the hole) and bias pot (single) installed. I've had NO luck finding the 4 lug pot (listed on the schem. as '10K LT 010437'). Does anyone know of a 'work-around' for this?
Thanks! dave

Never have seen the 4-lug pot for sale, we have to deal with the ordinary 3-lug. Fender puts a second stage bias filter cap on the extra lug. You can choose to put a second stage filter on any of the lugs you want. All it does is give you better filtered DC, and with the typical pair of 70 uF in a silverface, it's overdone. 47 uF/100V is plenty for both. And you can't find 70 uF caps anymore.

[oahudog]

Thanks! I was thinking that's what happened.. I was hoping to restore the amp to 'original spec', but I guess an adjustable bias is good (maybe 4 of them?)! Seems they used an STR tube in them originally, so being able to adjust the actual bias is probably better than balancing the legs...but I wanted to do BOTH!! waaaah Seems someone would have done a work around at some point, but that may be moot with a proper bias..Thank you for your time! dave

[The Dude]

If you wanted to, you could install two separate pots for matching. However, I don't think it's really necessary and it would require more modification.

[oahudog]

Soooo....I could use the 3 lug instead and ignore the bottom (missing) lug?

[oahudog]

I'm hoping to reduce the amount of modification. I play pedal steel, so this amp, as advertised, is what I wanted...didn't get that, but would put back if I can! Thanks! dave

[The Dude]

It's not going to change the sound of the amp any- undoing what's in there now. IMO, it's more important to be able to bias the amp than to balance the two halves assuming reasonably matched tubes. You could do something like a dual 5k pot with modification or two separate pots to get tube matching, but, I don't see the gain. Of course there are some amps out there with separate bias controls for each pair, but the majority don't have this feature.

[oahudog]

True! If I had to choose one over the other, I would choose biasing over balancing...so..I guess I got my way! Thanks! dave

[Enzo]

Devil's advocate here.

The stock bias level is relatively cool and should accept most tubes without issue. The balance control was used to reduce hum, wwhich frankly I value more than a control to heat up the tubes more than stock.

Here is a schematic:
http://bmamps.com/Schematics/fender/..._135_schem.pdf

The center tap of the pot was where the bias voltage was fed in, then either end lug went to a grid through resistors. The wiper of the pot went to ground through a 33k resistor. That makes a variable voltage divider that adjusts the voltage side to side.

But in the absence of a tapped pot, a plain bias adjust is a perfectly fine alternative.

[Enzo]

Devil's advocate here.

The stock bias level is relatively cool and should accept most tubes without issue. The balance control was used to reduce hum, wwhich frankly I value more than a control to heat up the tubes more than stock.

Here is a schematic:
http://bmamps.com/Schematics/fender/..._135_schem.pdf

The center tap of the pot was where the bias voltage was fed in, then either end lug went to a grid through resistors. The wiper of the pot went to ground through a 33k resistor. That makes a variable voltage divider that adjusts the voltage side to side.

But in the absence of a tapped pot, a plain bias adjust is a perfectly fine alternative.

[oahudog]

Thanks! What started this whole thing was a hum...so I figured if i could restore the 'balance' circuit, I might lose the hum by 'just' restoring that circuit. That's why I was wondering about a 'work-around' for the 'output matching' pot. The 'hum balance' pot is also disabled..has resistors on the pilot light. I've ordered a new one and it'll arrive next week. Since I started this thread, I've found another that addresses the hum, and I'll try that info as well. Looks like the 'matching legs' thing is not gonna happen, but it does have a single bias pot now...man, ya ever wish people would just leave sh*t alone? I already have a Blackface..(end of rant!) Seems an 'un-molested' amp is a unicorn these days! Kinda like the layout drawing for this amp..
REALLY appreciate everyone's help! I'm 'on the curve' and I'm sure I'll lose the hum by the time I get through the really great posts on the subject. Thanks again! dave

[Enzo]

Here is the deal on hum. There are MANY sources of hum, or at least potential sources, in a guitar amp. And each of those sources has its own cures. Hum can come from power supply ripple if filter caps are weak. Hum can come from heater current interacting with a preamp tube cathode. Hum can come from a poor grounding scheme. But all the additional filter caps in the world will do NOTHING to help heater hum. And elevating heaters or running them on DC will do NOTHING to help power supply ripple hum. So restoring that power tube balance control would ONLY help hum if the hum you hear is being caused by power tube imbalance.

Honestly, those two (probably 100 ohm)resistors to the pilot light are probably just as effective as the original balance pot. Frankly I'd just leave it unless you can determine that is actually affecting hum levels.

Hum that comes from power supply ripple will be 120Hz. Same note as 60Hz, but an octave higher. They can be difficult to tell apart due to overtones, but it is an important distinction. Ripple can be simple lack of filter cap action, but it can also come from filter cap return currents sharing a ground path with a gain circuit. One exception to power supply at 120Hz is the bias supply. The bias supply in this amp is a half wave rectifier, and so ripple hum from it would be 60Hz.

Hum from heater current will be 60Hz, as will hum from poor signal grounds and lack of shielding. Any coupling from the power transformer will also be 60Hz.

Knowing which frequency hum you have helps determine its source.

Isolate the problem. Does turning ALL controls to zero turn the hum down? DO ANY of the controls affect the hum in ANY way? (tone, loudness) Does turning down the reverb affect it? Does unplugging the footswitch cords affect the hum?

Do read the other threads, but the key here is not to find someone else with the same complaint and do exactly what they did hoping it will work for you. They key is to locate the source of your hum - or hums, really - and take steps to correct that.

[potatofarmer]

The way Fender implemented ultralinear/distributed load, there's only one filtering stage for the plates and screens. The screen grids are much more sensitive to ripple than the plates are, so if it's not adequately filtered out it shows up on the output. Fender tackled this by adding the "output tube matching" bias balance control - when adjusted correctly, the ripple on one pair of tubes will cancel the ripple on the other pair.

Earlier silverfaces had this bias balance control, but in those amps it's more a matter of convenience. Here it's basically critical to keeping the amp reasonably quiet.

There are other (better) ways to reduce the ripple, namely filtering it out before you supply the plates/screens. Dynaco/Sunn used a CLC filter before the plate supply, but an adequately-rated choke will be huge and expensive. You could try a CRC filter, but the resistor's going to have to deal with a substantial amount of current.

On mine the humdinger ("hum balance control") helped considerably compared to an artificial center tap.

Enzo is right; of course. This is just one (possibly the only) case where the bias balance is very very helpful. Compare the noise with the master at 1 to the noise with the master at 10.

[Shawnobi]

Do you use Facebook at all? There is a group for silverfaces that I frequent. It's a fairly common mod to change them to a traditional bias circuit. If someone there has done it they may have the old pot kicking around.
Have you tweaked the hum balance pot?
I have a UL Pro Reverb that I was sorting the hum on this week. Fantastic clean machines.