Do I need to Re-Bias the "same" tube?

[Hella1hella]

Hello everyone! One of the 6L6 power tubes in my Super Reverb just happened to stop working. It wont light up at all! I have put the tube in similar amps just to see if it would light up (to make sure it was not my SR) and nope! Nothing! I emailed the company about it because there warranty is for 90 days, however it only had MAYBE 8 hours on it? They have no responded and now I am needing some new tubes. The solution? Buy some 6L6s and take it to my tech for biasing. Here the issue... I try to make very limited trips to the tech because he is almost 2 hours away, and its hard to get a hold of him right now because of his seasonal job that is busy right now.

The question: Is it THAT important that I have the tubes rebiased even if im replacing them with the same type? The package says that I do not need it if im replacing it with their BRAND and TYPE but says nothing about the type in general. They are Groove Tubes and I would like something else because I had not heard back from their support but if I can buy a pair and just throw them in without bias I would do that to save the hassle. So to make a long story short, would I be able to switch them with another brand but the same Type without biasing?

Also on another note, is Biasing HARD? I don't know a ton about what is involved but I know its something with a multimeter haha!

Thanks!

[Alan0354]

If you buy a new tube and plug it in, if it does not hum I won't worry too much. But that's just me, maybe some will say you need to bias them.

The adjustment is very simple..........if you know what you are doing. But you need to open the chassis. If you have no idea with electronics, it can be dangerous as there is high voltage inside. But the adjustment is very easy after you pull out the chassis. There is only one trim pot close to the power transformer, you use a flat screw driver with insulated handle and tweek that back and fore to get the lowest hum and you're done. BUT again there might be some disagree and claim you need to measure the idle current of each tube!!!

I am not a perfectionist, I never bought matched pair!!! When I pull tubes out, never label them. BUT again, that's just me!!! Fender is easy, it has the pot. Some amps don't have it and you really need the match pair.

[Gtr_tech]

The graded GT stuff is matched for overdrive characteristics via that numbering system as far as I know. I dunno how the bias requirement plays into that. Normally tube suppliers that match tubes will attach a little sticker to the base that details the plate current and transconductance ratings, so they could just send you a replacement with the same ratings and yer off to the races.

Just a note here, the filament may have opened up, but I've seen newer prod tubes with questionable solder on the base pins which I've had to redo when I had one get abnormally noisy when subjected to vibration or the heater quit working. You may wanna have a look at that...

[pdf64]

'There is only one trim pot close to the power transformer...tweek that back and fore to get the lowest hum and you're done'
You may be getting the 'bias balance' control, as fitted to some Fenders on the 70s, mixed up with the 'overall bias' control, as fitted to most other Fenders (eg AB763).
The 'bias balance' control doesn't affect the overall bias setting.
Many current production tubes require a higher bias voltage than the tubes that 70s Fenders were designed for, resulting in them over dissipating in a stock amp.
The method suggested in post #2 is not appropriate for regular 'overall' bias control. Lowest hum may result in both tubes being biased past cut off (assuming sufficent range was available).

'BUT again there might be some disagree and claim you need to measure the idle current of each tube!!!'
Why the triple excamation marks? It's hardly an onerous procedure, and has been the manufacturer approved method on the current 'pro series'.
Tubes are expensive; why risk shortening their service life?
Bias affects tone; why leave that variable to chance?
Pete.

[exclamationmark]

Even tubes of the same type and brand can have a significant difference in current drawn at a particular bias voltage (though I don't have numbers - it's quite noticeable to someone armed with a multimeter). The counterpoint to this fact, is that even with a quartet of tubes, people share the bias feed between all 4 tubes. And they will all bias/amplify somewhat differently (even when 'matched').

Adjusting the bias really only affects the class A load line (ie, when both tubes are amplifying), and if you don't play at high levels of volume it will never even make a difference on how you bias (assuming you don't do something like bias it so cold it becomes 100% class B), because it will NEVER go into class B operation. The hotter you bias, the more power your amplifier will produce. In addition to this, it will stay in class A operation at higher volumes compared to a colder bias.

As you can imagine, when installing new tubes in place of old ones, they can either run hotter or colder with the same bias (this 'hot' or 'cold' thing is in reference to idle dissipation). I've never come across a situation where a fresh tube runs dangerously hot without rebiasing - and although this could theoretically happen, I would think it unlikely (I'm not a tech though - Might have to ask others). Assuming you play with your volume up halfway, then yes, a re-bias would be wise, otherwise if it's solely a home practice situation where the output power doesn't exceed a couple watts, I might just leave it, as a re-bias isn't likely to do much.

Biasing isn't hard - assuming you have 1 ohm cathode resistors installed. It's possible without one, but just more difficult. Another thing about biasing is that it really depends on EACH amplifier, as 70% idle bias might be 60% for another amplifier - in the end it doesn't make a huge difference but can reduce tube life somewhat. To set the bias, what you want to do is measure the voltage across the 1 ohm cathode resistor (with the amp turned on obviously - nothing in the input jack). For example a voltage of 0.5v measured across the resistor indicates a current of 50mA through the tube (this includes both plate AND screen current). You can measure the voltage across the screen resistor as well, and find the current flowing through the screen using ohm's law. This step is optional, as it doesn't make a huge difference. Subtract the screen current from the cathode current and you have the plate current. Multiply the plate current by the B+ measured at the plate of the output tube, and you get idle dissipation. Looking up max idle dissipation values for your specific tube, it should be anywhere from 60 to 80% of max plate dissipation. If it's not (usually in the case of new tubes), find the bias potentiometer and tweak it (normal safety rules apply when sticking metal objects in live amplifiers - I've blown up many a circuit by accidentally shorting multimeter probes to things). Repeat this process as many times as it takes to get the desired bias.

For an amp without 1 ohm cathode resistors, you will have to measure the DC resistance (with the amplifier turned off) between the centre tap of the OT and one of the plate taps. Turn the amp on, and measure the voltage drop between the centre tap (where the B+ is attached to), and the plate of a power tube. Use ohms law to calculate the plate current (don't have to fiddle with the screen current with this method), and fiddle with the bias potentiometer, until you get idle dissipation where you want it to be.

If you don't feel comfortable or confident working on live amplifiers with several hundred volts running through it, do yourself a favour and take it to a tech - saving a hundred bucks isn't worth getting electrocuted over.

[Alan0354]

'There is only one trim pot close to the power transformer...tweek that back and fore to get the lowest hum and you're done'
You may be getting the 'bias balance' control, as fitted to some Fenders on the 70s, mixed up with the 'overall bias' control, as fitted to most other Fenders (eg AB763).
The 'bias balance' control doesn't affect the overall bias setting.
Many current production tubes require a higher bias voltage than the tubes that 70s Fenders were designed for, resulting in them over dissipating in a stock amp.
The method suggested in post #2 is not appropriate for regular 'overall' bias control. Lowest hum may result in both tubes being biased past cut off (assuming sufficent range was available).



'BUT again there might be some disagree and claim you need to measure the idle current of each tube!!!'
Why the triple excamation marks? It's hardly an onerous procedure, and has been the manufacturer approved method on the current 'pro series'.
Tubes are expensive; why risk shortening their service life?
Bias affects tone; why leave that variable to chance?
Pete.

Does Super Reverb has newer versions from the 70s and has other bias control? I thought these are the old ones only has balance control to match the pair. The old ones don't allow bias adjustment, only balance. If they change it, excuse me!!!

[Hella1hella]

Maybe I will just try to find a tech close to just bias it for me. Im not really worried about price. Biasing an amp doesnt seem like it should cost a ton anyhow.

[JoeM]

Does Super Reverb has newer versions from the 70s and has other bias control? I thought these are the old ones only has balance control to match the pair. The old ones don't allow bias adjustment, only balance. If they change it, excuse me!!!

Early BF Fender (i.e. older ones) had a bias pot, not a bias balance pot, that change occured in the SF era. Most techs would change it to an adjustable bias (not Balance) (NOT the hum balance control)

[Alan0354]

Early BF Fender (i.e. older ones) had a bias pot, not a bias balance pot, that change occured in the SF era. Most techs would change it to an adjustable bias (not Balance) (NOT the hum balance control)

That's the problem I left this field in the 70s!!! Just got back lately and missed most of the time in between.

[Chuck H]

Adjusting the bias really only affects the class A load line (ie, when both tubes are amplifying), and if you don't play at high levels of volume it will never even make a difference on how you bias

Hmmm? For PP operations bias also affects how much swing will cause the tube to saturate or cut off. That's pretty significant to exclude.

The hotter you bias, the more power your amplifier will produce.

I really don't want to get into a big stinky techno pissing contest here, but this is really thr opposite of the truth. A hotter bias may produce more power power for a given voltage swing, but... That can be taken out of context. The limit to how much voltage swing a tube will amplify is more limited with a hotter bias. So the actual cieling is lowered. Grid swing has a very important relationship with bias even though they are often treated exclusive in guitar amps.

I've never come across a situation where a fresh tube runs dangerously hot without rebiasing - and although this could theoretically happen, I would think it unlikely (I'm not a tech though - Might have to ask others).

I'm not a "tech" either. But I have seen it happen. Especially with tender tubes that are typically run hot even in factory amps. I'm talking about EL84's of course. Never seen it with 6l6's.

Biasing isn't hard - assuming you have 1 ohm cathode resistors installed. It's possible without one, but just more difficult. Another thing about biasing is that it really depends on EACH amplifier, as 70% idle bias might be 60% for another amplifier - in the end it doesn't make a huge difference but can reduce tube life somewhat. To set the bias, what you want to do is measure the voltage across the 1 ohm cathode resistor (with the amp turned on obviously - nothing in the input jack). For example a voltage of 0.5v measured across the resistor indicates a current of 50mA through the tube (this includes both plate AND screen current). You can measure the voltage across the screen resistor as well, and find the current flowing through the screen using ohm's law. This step is optional, as it doesn't make a huge difference. Subtract the screen current from the cathode current and you have the plate current. Multiply the plate current by the B+ measured at the plate of the output tube, and you get idle dissipation. Looking up max idle dissipation values for your specific tube, it should be anywhere from 60 to 80% of max plate dissipation. If it's not (usually in the case of new tubes), find the bias potentiometer and tweak it (normal safety rules apply when sticking metal objects in live amplifiers - I've blown up many a circuit by accidentally shorting multimeter probes to things). Repeat this process as many times as it takes to get the desired bias.

For an amp without 1 ohm cathode resistors, you will have to measure the DC resistance (with the amplifier turned off) between the centre tap of the OT and one of the plate taps. Turn the amp on, and measure the voltage drop between the centre tap (where the B+ is attached to), and the plate of a power tube. Use ohms law to calculate the plate current (don't have to fiddle with the screen current with this method), and fiddle with the bias potentiometer, until you get idle dissipation where you want it to be.

I would only say that not every Super Reverb has a bias adjust pot. In fact, some have no adjust pot and only a "balance" pot. Things get a lot more complicated at that point.

[Alan0354]

Actually if hotter biasing means lower the negative voltage on the grid, this will give more idle current. This make the tube run hotter, more towards the class A mode. But the total power you can get out of the amp is not going to be higher. Pure class A burn as much power whether you have input signal or not, deep class AB burn very little power when there is no input signal. You don't get more power from class A.

[g1]

As far as the original question about the groove tubes, the whole point of them is that you replace them with the exact same model of groove tubes of the same GRADE and you do not need to rebias. This is assuming you had the bias adjusted for the groove tubes you are replacing.
Without getting into arguments about how important bias is or whether you can get away without it, if you had your amp biased for them, it will be biased correctly with exact replacements of the same grade (however they grade them now, number or colour or low, med. high whatever).
Modern tubes of the same type and brand can differ in their bias requirements from tube to tube. That is why they sell matched pairs. Differences between tubes of the same type but different brands can be even larger.
A different brand of 6L6's may be ok in your amp without rebiasing, but maybe not.
If you want to be sure your bias stays the same when replacing output tubes, that is what graded tubes are all about. But you pay a premium for them.

[Enzo]

Please dont overlook what Gtr_tech said. Look at the tube pins closely, particularly pins 2 and 7. See if either needs to be resoldered.

The wires come out of a 6L6 through the glass just like on a 12AX7, but they are then put through the hollow pins of a tube base to make the octal bottom. If the solder inside the hollow pin fails, so does the tube.

If you can resolder the tube and restore it to function, then the question of exact matching becomes moot.

[exclamationmark]

Considering an amplifier designed for class AB operation, isn't it always true running AB always increases the power over running pure class B (which is the basis for my statement - perhaps this was misconstrued). I'm happy to learn something new, if anyone wants to explain though

[Steve Conner]

I'd say no. To a good approximation, tinkering with the bias doesn't affect the power output at all. The reason is: when the amp is making its maximum clean output, the condition at signal peaks is that one power tube is conducting as hard as it can and the other one is cut off.

"Conducting as hard as it can" is determined by plate voltage and screen voltage, the bias setting doesn't come into it, because by adjusting the input signal for maximum clean power you drive the grids to the onset of grid current.

And, you can never bias the same tube twice, for new electrons are ever flowing through it.