Changin' Tubes

[piebaldpython***]

1) OK, how hard is to change tubes??
2) How often do you have to change them, given one hour per day of use??
3) If you have a tech do the tube change, what kind of cost is that??
4) What's an "average" price for a power tube? preamp tube?
5) Do most of you change your own tubes or go to a music store tech?

[duhvoodooman***]

1. Depends. The actual changing is pretty easy. It's getting at them that can be hard, depending upon the amp design. I can change tubes in my Blues Junior or Valve Junior in a matter of seconds. Not so for the single tube in my Vox AD30VT, which necessitates entirely removing the chassis from the cabinet, which is closed-back.
2. Strictly speaking, when they die, audibly start to change in sound, or get microphonic. From what I've read, for light to moderate users, the average tube should last about 3 years.
3. Couldn't tell you. I've always done it myself.
4. Depends on tube type, and whether you buy current production vs. "NOS" (new old stock), which can be VERY expensive. The most common preamp tube, by far, is the 12AX7 or ECC83. New production tubes run in the $10 range, plus or minus a buck or two. Power tubes vary pretty widely, but the most common types (EL84, EL34, 6V6, 6L6GC) typically run about $10 - $20, depending upon size/power output. Keep in mind that for most amps, you need to buy these in matched pairs or quads. These prices are for standard tubes, not ones that have been specially tested/sorted or contain upgraded features, e.g. gold pins.
5. See # 3.

[Spudman***]

I change my own tubes as well. So I just pay for the cost of good tubes and installation is free. It's not hard to do.
However, if you change any tube value from the original you will need to have the bias checked. This can be done yourself also, but if you are having second thoughts about doing your own tube change then have a tech do the bias.

[Bloozcat]

Keep in mind also, that unless your amp is cathode biased (self biasing), you can't just plug new power tubes in and play (or plug and pray as it's often called). Amps other than cathode biased, have to be re-biased by a tech when new tubes are installed.

So, if your amp runs 5881's, 6L6's, 6V6's, EL34's, KT66's, 7027A's, et al, the amp will have to be re-biased when new tubes go in. There are notable exceptions like the Epi Blues Custom 30 which uses 5881's and is self biasing, but that's the exception to the rule. Most EL84 powered amps are self biasing (with few exceptions).

One way to minimize the re-biasing each time you re-tube the power section, is to buy the tubes in matched quads (for two power tube amps), and then you'll have a spare set in the same mA range for later on. If your amp takes four power tubes, you'd buy eight matched (but that can be a bit expensive).

Pre-amp tubes don't require re-biasing when replacing.

[Radioboy950]

Having just gone through this, here's my experience.
You can change the preamp and output tubes yourself. But since bias needs adjusting on my amp, I supplied the new tubes and just had a reputable tech do it all for $50. Bias may not be an issue for you. A full re-tube could cost you a hundred bucks with good new production tubes, or much more of you start going after some of those prized vintage valves.

There is a great deal of experimentation with different tubes to find the tone you're looking for. But whatever you do, get your tubes from a reputable vendor (and there are several good ones out there) as they will work with you personally, make recommendations, and provide quality tested and graded product appropriate for your amp and your own personal prefs and playing styles.

Tubes are awesome. They are are addicting. I've gone through quite a few different configurations of 12AX7's in the preamp section to get it "just right" to my ears. Once you find it...man, it's sweet.

[oldguy]

This might help you get a handle on it. Bob Pletka tubing and biasing an 800 Marshall (mostly the biasing). Alot of amps are very similar to this, including the Epi SoCal 50w head.
This should give you an idea whether or not you'd want to attempt it.
Most techs recommend against it because if you don't know what you're doing, or are careless you can kill yourself. Many amphs carry enough residual voltage in the capacitors to kill you if you make contact. Even after the amp's shut off, unplugged, and cooled off. Other people have blown their fingertips off. So it's serious business.

http://www.eurotubes.com/euro-Generic-Bias.htm

[aeolian]

Changing tubes are generally not hard, just be aware that there are high voltages in the amp and be careful. Read up on it before you do it, and the amp should be off and unplugged before you change tubes. There is no need to change tubes unless one starts to go bad.

On my tube amp the tubes are easily accessible and I have changed them because the rectifier tube (on the right) started going bad and destroyed the power tubes (the 2 in the middle). The pictures showed the old tubes, I have JJs in there now.

[duhvoodooman***]

Keep in mind also, that unless your amp is cathode biased (self biasing), you can't just plug new power tubes in and play (or plug and pray as it's often called). Amps other than cathode biased, have to be re-biased by a tech when new tubes are installed.

That's an excellent point that needs to be considered! I've owned nothing but cathode-biased EL84 amps, so I've been spoiled by not having to worry about re-biasing.

[aeolian]

That's an excellent point that needs to be considered! I've owned nothing but cathode-biased EL84 amps, so I've been spoiled by not having to worry about re-biasing.

Just like to point out that my amp has 6V6 power amp section but is cathode-biased and in general does not need to be re-biased either.

[ted s]

Hmm, so, is a Fender BJ a self biasing amp ?

[aeolian]

I believe the BJ is a fix-biased amp. I found the following page from a person who mods BJs to make them bias-adjustable. I don't know the person, his claims, or whether his mod works or not. I just thought by reading it I learn something.

http://home.comcast.net/~machrone/bluesjunior.htm

[ted s]

:
I've read that before.. duh on me !

[tot_Ou_tard]

Just like to point out that my amp has 6V6 power amp section but is cathode-biased and in general does not need to be re-biased either.

Mine is also cathode-biased (wanna trade ).

[markb]

To answer the original question, I recently sold a 1970 Fender Vibrolux. I'd owned it since 1990 and changed the power tubes about three times in that period. I think I changed one or two preamp tubes when they went microphonic. The rectifier may well have been the original. I got the local amp tech to do it, no point taking chances with a 30-odd year old power supply. Total cost of maintenance over 17 years, about £200 including a replacement cable for the footswitch, a new jewel for the panel light and a cap replacement in the PSU. And that's using NOS power tubes too!

[duhvoodooman***]

Hmm, so, is a Fender BJ a self biasing amp ?

As I understand it, the Blues Junior is cathode biased, so you don't have to manually re-bias after changing power tubes. The issue is that the bias is set to a very "hot" level by Fender by design, so some owners like to modify the amp so that they can dial in a lower bias setting and drop the idle plate current level in the two EL84 power tubes. This drops the operating temperature of the tubes, thereby lengthening their life. It also increases clean headroom some and is reported to give a somewhat clearer, smoother tone. The downside is that the amp doesn't break up as early from the output stage, but since its more common to generate distortion with preamp gain or with pedals, this isn't a big deal for most folks.

The terms "fixed bias" and "cathode bias" are confusing, because they actually mean the opposite of what you'd think. I came across this quotation at the Aiken Amps website, which I found very helpul in understanding the difference:

"There are two main types of biasing: fixed biasing and cathode biasing. Fixed biasing does not mean the bias is not adjustable, in fact, it usually means the opposite. Cathode biasing is usually fixed, and not adjustable, and fixed biasing is usually adjustable with a small trimmer potentiometer, or "trimpot". It is no wonder the subject is confusing to people!"

That Bill Machrone bias adjustment mod adds one or two trimpots to the PCB (depending upon whether you want to be able to adjust the idle current to the two output tubes concurrently or separately) so that the bias is cooler and more in line with the recommended plate dissipation for EL84 tubes. You can also simply change a resistor on the Blues Jr. PCB and cool off the bias that way, though you don't get the "fine-tune" optimization control that a trimpot setup affords.