thanks wild bill, I'm just going to respond to a couple questions in your post and then re-read everything else and take it all in.
The twin is a post-'76 silver face...135 watts, master knob. the schematic is at the bottom of this post. There is a hum balance pot and a Output Tube Matching pot on the back. The 60Hz hum I was talking about is not affected by the Hum balance control. The Output Tube Matching control is set to roughly the middle when the hum is totally gone.
Ok, now we know which Twin. There were a LOT of Twins!:tongue:
Hum balance pots in guitar amps never did seem to do anything. As I had said, they are a "hifi" amp idea that got carried over into the guitar amp world. That's why most guitar amps use a pair of fixed resistors or have a centretap on the heater winding that is grounded. Much cheaper and works good enough for a guitar amp.
Your amp has no bias level adjustment! The reason is that when Leo sold his company to CBS Music the new bosses hired some new engineers to change his circuits, in effect "putting their own stamp on things." They changed the cosmetics to the Silver Face look and promptly started making circuit changes, including bias balance instead of bias level controls.
Most if not all of these changes seem to have been "hifi" things. I've come to believe that these engineers were not players but guys with a great resume and reputation from designing hifi amplifiers. It looks like the "suits" that had taken over the company made a very common mistake in thinking that all amplifiers are the same. Music had changed with rock and roll. Why on Earth would a guitarist want a "hifi" sound? It now was SUPPOSED to have distortion!
Not all forms of distortion are pleasing or sound the same to different ears, or are appropriate to a specific song. That's why a guitar amp needs to be different than one used to play recorded music.
History proves this in that Fender's sales tanked after CBS had taken over and made their changes. The company nearly went bankrupt and finally CBS sold the company to the employees. They rescued it with the help of guys like Rivera but lately with the moving of production to China I'm wondering if the ghost of CBS is has started to haunt the halls again.
Anyhow, with your amp they've preset the bias voltage level high enough to lower the idle current to the point where most any new tubes will run safe. Now, push-pull amps like most we use for guitars work by splitting the signal waveform into an upper half and a bottom half (stop cringing you techies! I'm trying to explain to a newbie!). You have a tube handling each half of the wave driving half the primary winding of the output transformer, which combines the signal and drives the speakers. For more power you can strap another tube in parallel on each side, like with your Twin.
The balance control means you can adjust the idle current from each "side" to be equal. This way the tubes will all share the power load equally. That's also why you had to put this control into the middle to kill hum. What you heard when it was way out of range was hum from a big imbalance in the output tranny current. One side was pumping much more current than the other.
Today we can buy tubes already matched so bias balance is not really necessary. Most guys would rather have the traditional bias level control, to enable setting all the tubes to the "sweet spot" of warmth and tone. You can certainly either learn how yourself or have a tech make the circuit change but you might stop to consider...should you bother?
The reason I say that is that this is a Twin! It's different from pretty well all the other Fender amps ever made. It is supposed to be hellacious loud and very, very clean! Your version is what is called the "UltraLinear" model, which is even more so. You're not really trying for a bias "sweet spot". In this mode simple clean is enough. So maybe it would make sense to just leave it alone.
The reason I said "changing some resistors" is there are no trim pots anywhere near the tubes and anywhere else in the amp. Only the Hum Balance and Output Tube Matching pots that are mounted to the chasis. So I thought the "old school" way may have been to use fixed value resistors and to change those to other fixed value resistors when you need a different value. Now I'm starting to think that an amp tech would install a trim pot for bias adjust the first time they re-bias an amp and that this amp just has never been re-biased. Is this the case?
Nope! As already covered, they never wanted one! Most other amps have a bias level adjustment. A few pinned it down with fixed resistors but they expected you to leave things alone and just go with their chosen setting. Of course, the first thing everybody does with such amps is to put in an adjustment control! :tongue:
When you talk about "bias balance" knob is that the same as the output tube matching pot?
By now it should make more sense. You're absolutely right here. Two different ways of saying the same thing.
I hope I've explained this all reasonably well. I've typed it before I've had my morning coffee!