another question about my twin RE:biasing

Marcus0000 said:

haha.. yeah I've always heard conflicting opinions. Mostly I read that new power tubes always need re-biasing but I've also heard people say it's not always necessary...depends on the amp. I took a super reverb to Gar Gilles (RIP) from Garnet once and asked him to re-bias it and he laughed at me (as he often did) and said he'd gladly take my money but that it wasn't necessary.

From what I understand all that's involved is measuring voltages from the tubes and changing resistors... is it more involved?

Thanks for the concern. The plan is to learn and know what I'm doing before I start. i gotta get experience some how right?

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You've been getting good advice so far. As far as biasing, when Gar was pioneering amps tubes were much more consistent in manufacture. Odds were that when you replaced output tubes the bias adjustment was "close enough."

Modern tubes are much less consistent today. It's a real crap shoot as to how much idle current will flow through the new tubes. If you're lucky it won't be so high that the tubes won't last long but it can be - i've seen it often when replacing tubes for customers. If the idle current is too low that only affects tone. Your amp may sound lame and you don't know why...

The factory setting from Fender these days seems to bias on the light side - maybe they're worried about tubes failing during warranty. Whatever, warming them up to the "sweet spot" sure helps.

I suggest you go to http://www.aikenamps.com and poke around at his tech faqs. He's got some great ones about biasing.

If your Twin has built in bias test points you can simply measure the B+, do some calculations and measure/set the cathode current. If not, you'll either have to put some in (1 ohm 1/2 watt resistors from each 6L6 cathode to ground, after breaking the existing ground wire) or use what's called the shunt method. This involves measuring the dc resistance of each side of the output tranny primary winding to the centretap, while the power is OFF! Then, when the amp is powered up you can measure the voltage drop across each side to the centretap and use OHM's LAW to calculate the plate current that way. Since a Twin uses a pair of tubes on each side you'll be measuring the combined current of the pair so take half for your reading. As long as the tubes are reasonably matched you'll be close enough and ok.

The shunt method is a little scary 'cuz you have full B+ voltage from your test leads to ground, even though the drop ACROSS the winding is only a few volts. I usually 'gator the centretap and then 'gator first one side and then the other, cutting the power between each reading. You'll have to calculate the desired drop at the amount of idle current you want through the tube(s). It will be very different from one side to the other. This is because of the way output trannies are made. Each side has a much different dc resistance reading 'cuz the windings rarely have the same length of copper wire.

You confuse me when you talk about "all I gotta do is change some resistors." You shouldn't have to change ANY resistors to bias a Twin! They have an adjustable trim pot that you turn to set the bias to give the desired idling current.

If your Twin re-issue has a bias balance pot like the SilverFace years instead of an overall bias voltage adjustment then you won't be able to set the idle current to the "sweet spot". The "Bias Balance" thing was a hifi idea. It is NOT to kill hum! That's the function of a "Hum Balance" control and I don't think Leo Fender ever used one. If you use a bias balance control to kill hum you'll probably fry some tubes! Balance controls work to make the idle current the same with the tubes on each side of the output tranny primary winding. Cranking it too far one way or the other may kill hum but it can result in one pair of tubes doing all the work. While the other pair get to "loaf" the working pair will burn out early...

If you tell us exactly which Twin circuit your amp is using we can better advise you.

As far as a 470 ohm 1 watt resistor, if 1 watt is called for then you must replace with 1 watt or BIGGER! If you use a 470 ohm rated for only 1/2 watt then it won't be able to handle it and will overheat and likely burn out.

PLEASE read Mr. Aiken's faqs! Amps can hurt or even kill. If you've educated yourself first you'll be ok. It would be better if you've got an experienced buddy handy to stay with you the first few times to make sure you're ok. Diving in and learning from getting shocked is neither efficient or pleasant...

Of course, working on amps is a lotta fun and well worth it! If you survive, of course... :tongue:

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.

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?

When you talk about "bias balance" knob is that the same as the output tube matching pot?

Marcus0000 said:

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.

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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.

Marcus0000 said:

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?

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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:

Marcus0000 said:

When you talk about "bias balance" knob is that the same as the output tube matching pot?

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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!

Addendum for Marcus

Marcus, you may find that 2 watt resistors are not that available. The blown resistor you have is almost certainly what's called a screen dropping resistor. If a tube went bad or the balance was cranked to make that tube and its partner do too much of the work load that probably blew the resistor.

It might be easier to find 470 ohm, 5 watt wirewound resistors. They usually look like rectangular white "logs". As I had said, going up in wattage rating for resistors never hurts anyway as it gives more safety factor.

Whatever, in this area I'd suggest changing all 4 resistors. That's one screen resistor per output tube. Whatever stressed the burnt one likely has hurt one or more of the others. When you're dealing with cheap parts like these I always replace them all for the customer. The lion's share of the labour is in opening up and closing the amp. To replace one in this instance is like replacing one brake shoe in your car. The extra 3 resistors won't add up to the cost of a good beer!:food-smiley-004: