Changing tube questions

[Dana]

Hi,
I've been playing a Fender Prosonic for about 2 1/2 years now with the original tubes. No problems yet but as I gig 1-2 times a week I'm getting worried that the tubes might burn out some night on a gig.

My questions are:
When should I retube the amp, and how often? Again, I use the amp 2-3 times a week for gigs, band practice, etc.

Can I change the tubes myself? Do I have to have the amp rebiased everytime I change the tubes?

[Bluesbob]

By my estimation (based on your estimations) you have about 1500-2000 hrs. on your power tubes. You only need to be concerned with them as the preamp tubes should last much longer. Always re-bias after changing power tubes. I looked up the circuit on the Fender Amp Field Guide. The bias adjust is connected to the rectifier switch, so any measurements you might take at one setting might not be optimal at another. What I'd do if I suspected my amp wasn't playing at its best is buy some NOS power tubes, install them, set the bias and keep the old ones for spares. Listen to your amp and see if it sounds better with the new tubes. If your old tubes were the stock Fender tubes it'll probably sound better than when they were new. Shop around, you can still get good deals on NOS GE and Sylvania 6L6's. NOS RCA power tubes are out of my price range now, but they will last nearly forever. The Groove Tubes 6L6GTGE is supposed to be just like the old GE's. Have fun but be careful! The left-over voltage in the amp can knock you on your butt! (Or worse). If you don't feel comfortable around high voltage take it to a tech.

[Lance]

Yeah - digging around the guts of an amp can kill ya.

Instead, I got one of these bias probe gizmos from WeberVST...you plug your tube into it...the plug the gizmo into the amp. The one I got (cheap!) requires that you already have a multimeter (which I did). The other models have an LED readout to see the bias measurement.

http://www.webervst.com/bias.html

Anyway, it's very easy - no danger of getting killed (although watch your fingers on the hot tubes).

Someone on this board can tell you the recommended setting for your amp...but that's just a guideline. You can adjust a tad up/down to see if it sounds better with your amp.

[Dana]

Can I learn how to do it? Are there any instructions anywhere?

[Lance]

First of all...not all amps have a bias adjustment. Some have a fixed bias, some require a little surgery...again..I'm not the expert so I'll let them clarify this.

The instructions that came with the probe are here:

http://www.webervst.com/biasread.htm

On my amps, there's a little hole next to the transformer that has a recessed trim pot screw. I first plugged the probe into my multimeter (not necessary if you buy the one with the readout). I plugged the tube into the probe, plugged the probe into the amp...turned the amp on and let it warm up. I adjusted the bias trim screw until the multimeter read 35 millivolts. (your amp may have a slightly different recommended setting)

Then...turned everything off...switch to the other tube and checked if it was near that value. I think the idea is to get them in the right range (around 35 millivolts for my amp). You want to pick a setting that keeps them both in the right range. If one tube reads a very different value than the other, you probably want to get a better matched set of tubes.

Takes about 10 minutes and saves you a trip to the amp tech for tube changes.

I'm sure a real amp expert (ala Cookie) will be able to give more accurate instructions.

[Kevin Nybo]

I just went and took a few photos of how I bias my main amp. You don't need to go inside the amp to find any of these numbers. You can zoom in on any of the photos for a better look. As you can see I forgot to rotate the last 2 and they appear sideways.

http://www.villagephotos.com/pubbrow...elected=219008

Value 1-This probe's instructions said to measure the plate-to-cathode voltage. On this amp it's always the same as plate-to-ground measurement. I use an old set of meter leads and connect one to pin#3 and one to pin#8 of one of the power tubes(see pic). I then plug the tube in and power up(see pic). Using this rectifier and the 6L6 Svet shown, the plate voltage is 341. This is one of the numbers you will need to figure bias.

Value 2-After that reading is taken you start using the bias probe. Mine's a duel model I purchased on ebay for $57/new. SRS continually lists these on there. Knowing what I know now I would have built one myself and saved the money but didn't know how they were designed when I ordered it.

You can switch between the 2 tubes and see their mA(mV) reading. These are the numbers you hear everyone talking about usually, "Biased at 39", etc. FWIW-these are the only matched set I've seen that exactly match. Usually the numbers have a difference of around 3-4mA.

Value 3-The other number you will need is the max-plate-disipation of the power tube. For instance, the 6L6 Svetlana in the photo has a mpd of 30watts, while most 6L6's are at 20.5W, 6V6's at 13W, etc. You can find these values all over the web.

I have seen so many formulas I don't know which one to go by. Given the ones in these directions we get-

30watts(Svet 6L6) X 500= 15,000

15,000 -divided by- 341(plate voltage)= 43.9mA

Acording to these instructions, 43mA will put you in the "low side of the midpoint" range.

A common furmula is "max-plate-dissipation -divided by- plate voltage =X(variable)". Then take 70% of X variable. For instance-

30W(mpd) -divided by- 341(plate voltage)=.0879 or 88mA. 70% of 88 =61.6.

61.6 would be the highest mA setting I would want, in theory.



For those who noticed-
I realize these Svetlanas are a little too much for my low plate voltage. They just happened to be in there when I took the pictures. I've been waiting for another set of 6L6's to arrive this week for testing and left them in. I also have several other sets of 6L6 and 5881's.

[Lance]

OK....I do mine the simple way.

[MJ Harnish]

The WeberVST bias probe is a nice product. However, as Lance's instructions illustrate, users do run the risk of oversimplifying the process.

#1
You really should not be biasing your amp without knowing the plate voltage. The reason is that you need to be concerned about the power dissipated by the plates which is directly determined by the voltage on the plates and the bias current (Power = Voltage * Current).

Depending on your amp's plate voltages, your tubes could be running overbiased, underbiased, or perfectly fine at 35mA

#2
There is no magic bias current setting. The correct setting depends on the type of tube, the plate voltage on your particular amp (do NOT use the values on the schematic b/c they vary from amp to amp and even region to region depending on the exact voltage at your electrical outlet). The 35mA, 70% of max dissipation, roll a die and add it to 30, etc. methods all ignore the most critical issue, the player. You'll get a lot of opinions on what the bias current should be set at but this IMO is the best rule to follow:
The bias current should be set to the point that sounds best to YOU that is within the dissipation specs of the tubes

Hence, 35mA may sound good to you but someone else may like to run their tubes hotter. There is a trade-off here, colder biased tubes (less mA) will tend to sound more sterile, but they also last a lot longer. Hotter biased tubes (more mA) tend to sound warmer (up to a point), but their life can be significantly shorter (even seconds if you're over the plate dissipation of your tubes).

#3
After you set your bias, make sure you turn out the lights and have a close look at the plates of the tubes (the big, darkly colored flat element in the tube; not a good description I admit). If it's glowing RED you need to decrease the bias current, no matter what your math (Power = Voltage * Current) tells you. Tubes vary and you probably have a set that don't quite meet spec. :)



Lance's method isn't wrong; it's just only half the story.

[Ben Harmless]

Wow Kevin. That's one of the best descriptions of the process I've seen. The photos made things much clearer to me. I never quite understood exactly how I was supposed to connect my probes to the various pins of the tubes. I always pictured poking around on the other side of whatever the tube socket is mounted to with my meter probes just to get those measurements. I never thought about connecting leads to the pins of the tube itself before installing it.

But then, I'm a newbie when it comes to this stuff. Now I just need to get an adjustable bias mod on my amp...

[Lance]

Yes...my method is a gross oversimplification. But for a setting that's very subjective, starting in the ballpark of 35mA seems to work and is safe. I usually do tweak up somewhat but not to where the plates are glowing.

I guess another thing that should be mentioned at this point is Groove Tubes. (not that I use them...but just so the conversation is complete) GTs are rated with a numbering scheme (1 - 10). Once you've had a set in your amp and had it biased, you can just order the same value tube. So, if your amp was biased with a set of 4s, you can be reasonably confident that you can slap in a set of 4s the next time. The point was to make it easier the musician to retube without a trip to the amp tech. Conceptually, I think it's a great idea. However, you are limited to their tubes which are just tubes from other manufacturers that have been rated, labelled, and repackaged (for more $$$)

However, I've found that I have found better sounding tubes through recommendations on the web and doing my simplified bias adjustment. Currently, I'm very happy with a set of Visseaux 6V6s in my '65 DR. They were recommended by Lord Valve http://www.nebsnow.com/LordValve He's always had excellent suggestions for tubes.