# Biasing an amp...I'm scared



## cdub66 (Dec 13, 2007)

Sorry if it's been asked before, did the search thing and came up mostly empty.

I've been thinking I should join the ranks of the tube amp owners, doing the research, learning what I can about them-now I see if you replace tubes you need to re-bias the amp. 

What's that about now? Can I do that myself or will I need to take it to someone to get it done.

Kind of scaring me off a bit, any help, experience, tips, pointers much appreciated!

:food-smiley-004:


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## Wild Bill (May 3, 2006)

cdub66 said:


> Sorry if it's been asked before, did the search thing and came up mostly empty.
> 
> I've been thinking I should join the ranks of the tube amp owners, doing the research, learning what I can about them-now I see if you replace tubes you need to re-bias the amp.
> 
> ...


You have two options. You can learn how to safely poke and prod inside your amp with test meters, either installing cathode resistors on your power tubes (safest method) or measuring the voltage drop across your output trannie while the amp is running, which is the scariest 'cuz that's where the highest voltages are.

This will take more than 5 minutes of reading tips on an internet forum! It's really for those who want to become a true tech.

Or you can zip over to http://www.thetubestore.com and buy one of their bias meters. They work very easily. You just unplug a power tube and plug an adapter into the socket, then plug the tube into the top of the adapter. Then you can measure what you need.

They're not cheap but they don't require much education to use.

Have fun!

:food-smiley-004:


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## Gizmo (Aug 7, 2008)

cdub66 said:


> Sorry if it's been asked before, did the search thing and came up mostly empty.
> 
> I've been thinking I should join the ranks of the tube amp owners, doing the research, learning what I can about them-now I see if you replace tubes you need to re-bias the amp.
> 
> ...


How about buying an amp that doesn't need biasing!
My Reverend Hellhound uses a matched pair and doesn't require biasing. There are others.


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## greco (Jul 15, 2007)

Wild Bill...once you have got the reading from the bias meter, do the instructions for the meter tell you how to adjust the bias to the correct level?

Thanks

Dave


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## cdub66 (Dec 13, 2007)

Looks like I need to buy a book (or two) :banana:


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## greco (Jul 15, 2007)

cdub66 said:


> Looks like I need to buy a book (or two) :banana:


Do you have any skills using a soldering iron and a multimeter. 

If you don't have either of these items, you might want to consider getting them and learning how to use them....add a few hand tools (general and specific to electronics) and you could start to build some basic stuff and do some simple mods to guitars, pedals, etc. ....if this interests you.

There are lots of tutorials available on the internet for basic electronics and there is a never ending list of books on electronics.

My apologies if you are much more advanced in electronics theory and construction.

Cheers

Dave


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## Budda (May 29, 2007)

i bought new tubes, installed them myself - never biased my amp outside of adjusting the bias trim pot in the amp.


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## greco (Jul 15, 2007)

Budda said:


> i bought new tubes, installed them myself - never biased my amp outside of adjusting the bias trim pot in the amp.


Please don't be offended and/or take this as an insult, because I'm just curious and would like to learn:food-smiley-004:

How did you know/determine when the bias was correct?

Thanks Budda

Dave


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## Budda (May 29, 2007)

i dont know for a fact that the bias is correct *L*.

but having talked to a forum member on sevenstring.org who has the same amp as me and bought the same tube kit as me, he had his tubes checked and biased and i asked him what his trim pot was set to, and set mine to about the same. a third member who got the KT77's installed and had his amp biased runs 38mA as well. so i think mine is at about 38mA - i would like to get it properly biased at some point.

sounds good to me


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## greco (Jul 15, 2007)

Budda said:


> i dont know for a fact that the bias is correct *L*.
> 
> but having talked to a forum member on sevenstring.org who has the same amp as me and bought the same tube kit as me, he had his tubes checked and biased and i asked him what his trim pot was set to, and set mine to about the same. a third member who got the KT77's installed and had his amp biased runs 38mA as well. so i think mine is at about 38mA - i would like to get it properly biased at some point.
> 
> sounds good to me


Thanks

Dave


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## Greenbacker (Mar 29, 2007)

What amp are you using?


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## Wild Bill (May 3, 2006)

greco said:


> Please don't be offended and/or take this as an insult, because I'm just curious and would like to learn:food-smiley-004:
> 
> How did you know/determine when the bias was correct?
> 
> ...


Here's the scoop!

Tubes work by having a cathode heated up until it glows. At that point electrons will literally boil off the cathode. By themselves they would just form a cloud around the hot element but when we put a plate nearby with a high positive voltage charge the electrons will be powerfully attracted and flow from the cathode to the plate.

This is a diode! Current flows only in one direction, from the cathode to the plate. Electrons can't go from plate to cathode 'cuz electrons have a negative charge and like charges repel. The interesting thing is that if you pull the negative electrons away from the cathode you leave a "hole" behind. By taking away the negatively charged electrons you create a positive charge in it's place!

If you put an AC voltage into the loop the effect created is to have only positively charged pulses appear at the cathode! The negative charges are blocked 'cuz the electrons can travel only one way and they've all vamoosed towards the plate.

Now, a diode conducts in its forward direction as much as it possibly can! In that direction, it looks like a short. In the reverse direction it appears to be open. This is why with a solid state diode you can check if it's ok by putting your ohmmeter across it. In one direction it will show a very low resistance, perhaps a few hundred ohms. Reverse your meter leads and it will show a very, very high resistance of hundreds of thousands if not millions of ohms.

Long ago a Prof Fleming discovered that if you put a bit of metal screen or mesh (grid!) between the cathode and the plate you could control the current flow through a diode tube by putting an adjustable negative voltage on it. This voltage needed practically no current power at all. Just having a voltage was enough. Crank up that negative "bias" voltage and the current could be made to stop! Make it positive and the tube would conduct even more heavily!

This was the first "Triode" tube. This was an important discovery because you can make a triode amplify! If you set the bias voltage so that the tube is conducting in a safe range and will not burn itself out you can put a tiny signal into it through a capacitor. The capacitor will block the DC bias voltage from leaking back into your driver circuit but the signal will be superimposed on the bias voltage at the grid. As that signal swings a bit positive the cathode to plate current will swing exactly the same, only LARGER! When it swings negative the plate current will dip in a much larger negative fashion.

That tiny input signal which swings positive and negative will be amplified big time through the tube!

The important thing is to have the bias voltage set to the right spot to allow the resting idle current through the tube to be the right amount for the Class of operation, or circuit conditions for the tube.

Some circuits don't use a negative voltage on the grid. They put a resistor in the cathode instead. The current through that resistor causes a voltage drop, making the cathode more positive with respect to ground. The tube sees that as the grid being more negative with respect to the cathode and so the idle current is controlled the same way. 

This is why you don't have to bias preamp tubes. They are self-biased with cathode resistors.

Tubes vary in characteristics and modern tubes vary even more than from the Golden Years. This means the bias voltage will not likely be exactly correct from one tube to another. Some designers just fix the bias voltage high enough so that no tube will ever likely run too hot. The problem is that for the best warm tone and most output power there is a "sweet spot". You need to be able to measure that idling current and adjust it for new tube(s).

Finally Dave I'm getting to the answer to your question!:smile: With most guitar amp circuits it works like this:

Every tube type has what is called its "plate dissipation" figure, or Pd. It will be listed in the data sheets. This is the maximum amount of watts the plate can handle from that current flow energy. With the 6L6 family it's usually 30 watts. EL-34's run 25 watts. This is NOT the output power of the tube! It's how hot that tube plate can get while putting out that output power!

So we get out a bias meter or we put in some 1 ohm cathode resistors on the output tubes or whatever method lets us know the current through the tube. If it's a matched pair the figure should be close in the other tube. If we know the current then we can just measure the plate voltage and use Ohm's Law for Watts, which is Power=Volts x Amps. The safe range for the tube is less than this in most amps. The generally accepted figure is 70% of the Pd number.

So let's take a 6L6 with 400 volts on its plate. It's a new tube that we just installed. The Pd listed in the data sheets says the figure is 30 watts. Now we re-arrange the formula. 30 watts with 400 volts means the current would be 30 divided by 400 = .075 amps, or 75 milliamps. 70% of that gives us 52 ma. So the tube should be set to an idling current of 50 ma.

You can see how important it is to know the plate voltage. If it's higher the proper idle current will be lower, and vice versa.

It's like setting the idle speed with an older car that has a carburator. Too low an idle means poor performance. Too high and the car runs hotter and strains everything. 

When you change output tubes it's a total crap shoot as to how close the old bias setting will mate to the new tubes. How lucky to you feel?

Hope those interested followed along ok. I wrote this before I had my morning coffee!

:food-smiley-004:


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## cdub66 (Dec 13, 2007)

Thank you Wild Bill-that's an informative post right there. :bow:
I (we) appreciate it very much!

I'm looking at starting out with a small amp (likely a Blackheart Little Giant) with just one pre-amp and one power tube, should be a good way to get my feet wet!

I think I'll buy a bias meter and a book from the tubestore as well.

Thanks everyone!! :food-smiley-004:


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## greco (Jul 15, 2007)

*Once again...Many, many thanks Wild Bill*

Your post (tutorial) really helped me to get closer to understanding this concept. I say "closer" because I need to go over your post several times before it will sink in. 

One thing I can't understand...... is how you could write that in the morning BEFORE COFFEE !! 

The time you spend helping us is certainly very much appreciated.

Dave


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## Budda (May 29, 2007)

Greenbacker said:


> What amp are you using?


are you asking me or the OP?


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## bischbd (Mar 14, 2006)

Some good biasing information can be found here:

http://www.eurotubes.com/euro-i.htm#4.
and
http://www.eurotubes.com/euro-video.htm

I can't guarantee all the information is accurate but the guy does seem pretty knowledgeable about amps and tubes.

Good luck.


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## cdub66 (Dec 13, 2007)

Budda said:


> are you asking me or the OP?


I think you, I don't have one yet, just asked the question as I'm attempting to learn a bit about them before I make the leap.


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## Lemmy Hangslong (May 11, 2006)

guys... for anyone looking to get further into maintenance and repair of tube amps... I highly recommend this book...

http://www.amazon.ca/Guitar-Amp-Han...087930863X/ref=pd_sim_b_1/190-8925592-8006006

I've been doing my own work and work on other's amps for some time now... I have a background in Electronics and I'm an Electrician by trade, I also have Instrumentation experience... i know my way around several testing/metering devices... scopes and the like... this book still proved an invaluable resoarce.
What I liked most was it's written in language thats easilly understood by anyone wether you have formal training or not and it takes you through actual ccts... dividing them up into thier various components... starting out simple then getting more complex.

Great book... if you are interested then i'm sure it will be the best $35 you will spend.

Craig


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## nitehawk55 (Sep 19, 2007)

cdub66 said:


> I think you, I don't have one yet, just asked the question as I'm attempting to learn a bit about them before I make the leap.


Get a cathode bias amp and you don't need to adjust bias . you simply buy and install a matched set when it comes time to replace your tubes:smile: . Lots of good amps out there with that feature , what wattage range you looking to get ?


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## cdub66 (Dec 13, 2007)

nitehawk55 said:


> Get a cathode bias amp and you don't need to adjust bias . you simply buy and install a matched set when it comes time to replace your tubes:smile: . Lots of good amps out there with that feature , what wattage range you looking to get ?


Actually I'm thinking of wetting my feet with a Blackheart Little Giant, one pre-amp tube, one power tube. So perhaps I'm working myself up for nothing, but I tend to overthink most everything I do :tongue:


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## greco (Jul 15, 2007)

cdub66:

Did you see this?

http://www.guitarscanada.com/Board/showthread.php?t=17674

Nice long-term projects

Maybe you could convince the seller into shipping...if this appeals to you.

Cheers

Dave


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## cdub66 (Dec 13, 2007)

greco said:


> cdub66:
> 
> Did you see this?
> 
> ...


I sure did Dave, I think that would be biting off more than I can chew at this point :smile:


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## greco (Jul 15, 2007)

cdub66 said:


> I sure did Dave, I think that would be biting off more than I can chew at this point :smile:


Don't be too sure of that...you get that old soldering iron smokin' away, your multimeter at the ready and your handy Tube Amp Textbook next to you....who knows what you could build.....J/K...sort of

Whatever you decide to try...Good Luck with it and enjoy.:food-smiley-004:

Dave


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## Wild Bill (May 3, 2006)

nitehawk55 said:


> Get a cathode bias amp and you don't need to adjust bias . you simply buy and install a matched set when it comes time to replace your tubes:smile: . Lots of good amps out there with that feature , what wattage range you looking to get ?


Not a bad idea! Still, like everything there are tradeoffs. Cathode biased amps tend to have less power output, since they tend to be Class A designs that run the tubes hotter.

Also, cathode biased amps sound DIFFERENT! Looser, with more compression and lots more harmonics. Think Neil Young's tone in "Rockin' in the Free World".

Fixed bias (we call it that even when there's an adjustment trimmer) give more power and sound "tighter".

Which is better? It's all personal taste! Me, I LOVE the sound of cathode biased amps!

Then again, I've been listening to George Thorogood on 11 for years, so what can my ears tell anymore?:smile:

:food-smiley-004:


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## pattste (Dec 30, 2007)

I bought new tubes for my amp recently, so it had to be rebiased. I read a little bit on the subject. After reading that:

1) You need to buy tools to do the job
2) You need to know what you're doing or you could die

... I decided that it was wiser to pay a technician to do it, since it is essentially a one-time thing. I had bought matched tubes from a reliable source (thetubestore.com). The rating is written on the tube and on the box. That means the next time I can simply ask for the same rating and won't need to have my amp rebiased. The technician did a great job, plus he fixed the rattle that I was experiencing; several screws on the cabinet had to be tightened as they had come loose (probably as a result of not playing the amp loud enough  )


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## greco (Jul 15, 2007)

pattste said:


> I bought new tubes for my amp recently, so it had to be rebiased. I read a little bit on the subject. After reading that:
> 
> *1) You need to buy tools to do the job
> 2) You need to know what you're doing or you could die
> ...


Pattste

I agree with your logic and approach. However, I sort of derailed this thread (blame my curiousity and enthusiasm)onto a parallel track as the electronics side of biasing tubes (both the theory and practice) is interesting (for some of us) to know more about about.

My apologies to the OP....looking back over the thread, it might have seemed like a hijack.

Cheers

Dave


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## cdub66 (Dec 13, 2007)

greco said:


> Pattste
> 
> I agree with your logic and approach. However, I sort of derailed this thread (blame my curiousity and enthusiasm)onto a parallel track as the electronics side of biasing tubes (both the theory and practice) is interesting (for some of us) to know more about about.
> 
> ...


No problem Dave, I'm learning too!


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## nitehawk55 (Sep 19, 2007)

I like the sound of most cathode biased too Bill , just suits my taste best ( usually ) 
I will say that some amp designers are getting smart by making it easy and safe for any amp owner to set the bias just using a VOM meter . My Carr Slant 6V has 2 test points on the back of the amp coloured red and black and a trim pot you can reach pretty easy on the underside of the amp . The Slant 6V uses 4-6V6 tubes , 2 operate in fixed and the other 2 cathode biased . You simply set the meter to read milivolts and set the fixed tubes to 66 MV . I know some other amps have a simular system but it sure is sweet :smile:


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## Wild Bill (May 3, 2006)

nitehawk55 said:


> I like the sound of most cathode biased too Bill , just suits my taste best ( usually )
> I will say that some amp designers are getting smart by making it easy and safe for any amp owner to set the bias just using a VOM meter . My Carr Slant 6V has 2 test points on the back of the amp coloured red and black and a trim pot you can reach pretty easy on the underside of the amp . The Slant 6V uses 4-6V6 tubes , 2 operate in fixed and the other 2 cathode biased . You simply set the meter to read milivolts and set the fixed tubes to 66 MV . I know some other amps have a simular system but it sure is sweet :smile:


Yeah, a lot of the newer amps are bringing out the necessaries so you don't have to open up the amp. For casual use it's not a bad idea but myself I always open things up.

I do this for two reasons. One is that sometimes the bias test point is not reading current but actual bias voltage. I've seen this a number of times and can't believe someone would do something so useless! Who cares what the bias voltage is? What's important is how much idle current the tube draws, which will vary from tube to tube for any particular fixed voltage. That's WHY it's adjustable!

The other is that if I'm going to bias a customer's amp I think he deserves more for his or her money than just a quick and easy tweak. I like to open up the amp and take a quick visual inspection for anything obviously amiss. I've found parts broken off, stuff that obviously has been overheating, etc. I'll measure the voltages at the various tube plates to make sure they're in the ballpark.

I even found a brand new Bogner that had 4 screws that were supposed to hold down a small board rattling around loose inside! Usually gravity meant they just rolled on top of the bottom cover plate but if one bounced up and hit a high voltage point it could have gotten exciting!

I mean, even when you get an oil change you expect that while the guy is doing it he will take a look around to make sure your transmission isn't ready to fall out!

But hey, that's just me!:rockon2:

:food-smiley-004:


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## dwagar (Mar 6, 2006)

A couple of years ago I decided to try a pair of JJ E34Ls in my JCM800. And decided that would be a good time to learn how to bias my amp.

I have a lot of respect for electricity (read that fear) so I decided the simplest way to do this myself was to buy a bias meter. I bought one that measures plate voltage too.

I thought that, by now, I really should know how to do this.

1) the money I'd spend at a tech would instead be spent on a meter that I could use again.
2) most good techs aren't sitting at the counter reading a magazine just waiting for you to come in the door. You'll be leaving your amp there for a few days, a week, maybe longer.
3) and, what if I decided I didn't like the JJs? If I wanted to swap back to the original tubes, it'd be back to the tech, to leave it again and pay again.

I look at changing tubes and biasing as part of routine maintenance. If there are any other amp issues, it goes to the Pros.


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## nitehawk55 (Sep 19, 2007)

Funny thing is back in the day we never biased tubes.....slapped in a new set and wailed away !!


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## nitehawk55 (Sep 19, 2007)

Wild Bill said:


> Yeah, a lot of the newer amps are bringing out the necessaries so you don't have to open up the amp. For casual use it's not a bad idea but myself I always open things up.
> 
> I do this for two reasons. One is that sometimes the bias test point is not reading current but actual bias voltage. I've seen this a number of times and can't believe someone would do something so useless! Who cares what the bias voltage is? What's important is how much idle current the tube draws, which will vary from tube to tube for any particular fixed voltage. That's WHY it's adjustable!
> 
> ...


I see your point but I've yet to see anything rattle much in a Carr and although I could open up the amp and do a bias check that isn't for me nor most guitarists . 
When I first got the amp I had a Compu-Bias and checked idle current against the recommended 66mv setting with it and it was bang on . I'm running a quad of Bendix 5992's in it and it loves em ! sdsre


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## Wild Bill (May 3, 2006)

nitehawk55 said:


> Funny thing is back in the day we never biased tubes.....slapped in a new set and wailed away !!


In those days it wasn't as much of a problem! Tubes were made to much tighter tolerances and didn't vary so much. Besides, they were comparatively cheaper to buy. You got away with it because the tubes were not the same.

If you visit a tube vendor like thetubestore.com and were to see their inventory you would see all the output tubes wrapped in matched pairs and quads for their bias current rating for a given bias voltage. They put a number on the tube and the box. Those numbers are all over the map! There can be BIG differences!

The problem is not just having the biasing current misadjusted. There is a wide range. Too low and the tube just puts out much less power, with a thinner sound. Too high and the tubes can start to burn out early, sometimes REALLY early! Chances are good that the bias will be at a safe point, even if not being in the sweet spot for tone.

No, a bigger problem is having too great a mismatch between the output tubes! The power is supposed to be shared reasonably equally between them. If one is much hotter than others it will try to deliver more than its share and likely flog itself to an early death.

Sometimes the problem can be a bit more spooky. I had a Dr. Z amp in with a pair of EL34's that someone had stuffed in. They were Groove tubes so I guess he had felt he had bought a good quality pair. The amp was blowing fuses at odd times and had a constant low level hum. I checked out the usual suspects but nothing seemed out of order. Then I sat back and looked at those tubes. 

I called the owner up and asked where he had gotten them. He had bought them from some guy on Ebay. Not a tube store, just an "average joe". I checked the bias in BOTH tubes. BINGO!

These tubes were seriously mismatched! They must have slipped through the matching process at Groove Tubes since they were numbered the same. The problem in the typical push-pull tube amp is that the tubes have to be reasonably closely matched to cancel out hum and to prevent wild current swings that could damage the output transformer. A little bit of mismatch like a couple of ma. can sound more rock and roll. Getting all anal about matching is a hifi thing. These tubes were badly out of whack! One was drawing 42 ma. while the other drew only 26.

That's what was blowing the fuses and that's where the hum was coming from. A new set of JJ's and the amp sounded like new!

The world has changed! They don't make things like they used to! We used to be able to trust that there was no poison in our pet food and no lead on baby pacifiers!

Oh well, at least one thing hasn't changed. My playing! I still sound like ass!

:food-smiley-004:


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## Wild Bill (May 3, 2006)

nitehawk55 said:


> I see your point but I've yet to see anything rattle much in a Carr and although I could open up the amp and do a bias check that isn't for me nor most guitarists .
> When I first got the amp I had a Compu-Bias and checked idle current against the recommended 66mv setting with it and it was bang on . I'm running a quad of Bendix 5992's in it and it loves em ! sdsre


Hey, I'm not most guitarists! I'm a tech and make my living working on amps. Most amps DON"T come with problems! Most manufacturers have pretty good quality control. 

Still, stuff happens. I had a new Peavey with less than 10 hours on it that had an open screen resistor, drastically unbalancing the quad of output tubes. I even had a 1967 Super Reverb with a resistor wrapped on a tube connection pin but never soldered! Took 40 years before the connection became noisy and intermittent!

Nothing can be counted on to be perfect. That's why some things need to get fixed once in a while. It's just a big lottery and if you win you lose.

Remember Clint Eastwood asking "Do ya fell lucky? Well, do ya?"

:food-smiley-004:


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## greco (Jul 15, 2007)

Wild Bill said:


> I even had a 1967 Super Reverb with a resistor wrapped on a tube connection pin but never soldered! Took 40 years before the connection became noisy and intermittent! :food-smiley-004:


Isn't this always the way.......amazing. 

Some things hold on for years, while other things that seem rock solid fall apart in a few days.

Cheers

Dave


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## Ripper (Jul 1, 2006)

Yeah it is very true. However with alot of the manufacturing quality these days I think alot of the issues are going to show up sooner than later.


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