18 May 2013

Amps: how to re-tube an amp


Note: this blog entry was already published in my old Spanish version of this blog, around April / 2011. Just revisiting it here, for it's possible interest.

If you are already done to tube swapping, you can completely forget this blog entry, which was written as a support for people that is still afraid of changing its first tube.

Some people seems not happy with the actual tone of their amp and, it's considering to try some tubes but, it's afraid about to swap tubes. It's really difficult to swap tubes?.
Really not. Can be a bit more tricky in some amps than other but, all that depends on how tubes were arranged in the amp and how easy is to access them and how much room do you have to move your hands around.

As I've already reasoned in a previous entry, called The Cost of the Cup of Tone, last tone generator is the one that really creates your tone so, consider first a change of speaker, if the overall tone doesn't satisfies you.

If you just need to polish some aspects of your current amp tone, a tube swapping can be of help. Amp is the tone generator before the speaker and, usually new tubes will cost less than a new speaker.

In previous entries (browse Page Amps, on the upper bar), we talked about how tubes work and, what they do in every position, as well as the gain cascade and, even tube types and how they sound.

We also clarified that no change of tube or speaker will dramatically change the core of your amp. In the same way a Les Paul will never sound the same as a Stratocaster or Telecaster, a classic gain amp will never sound as a modern hi gain amp. Tubes will not do that for you. Period. If this is what you are expecting, you need a new amp.

The right set of tubes for our own needs, can help to get exactly what we like more from our amp, at the levels we want to use it and, with our rest of rig.

Handle your tubes with care

Usually around the area were tubes are placed, there is no other component that can shock you. Usually, rest of components are hidden under the tubes and, it's safe to swap tubes.

Anyway, be sure to give sometime to tubes to go cold, after you switch off (and unplug) the amp.
While working, tubes reach really hot temperatures and can burn your fingers but, you can also damage them moving the tube while their internal components are still hot.
Also, be sure you don't accidentally touch any transformer that can be eventually close the tubes, because they go also really hot.

It seems that our finger's fat can damage tubes also, in similar way it happens with halogen bulbs (maybe because it creates areas where the temperature varies respect of the rest of the flash) so, it's a good idea to use some latex gloves, any paper, any drape, anything that avoids direct contact of your hands with the tube while allowing you to work comfortably.

Did I already said it?. Handle tube only when the are cold!. Be patient, mate. Wait until they go cold.

How to remove old ones and how to insert new tubes

Pre-amp 12AX7 tubes and family

Pre-amp tubes usually are, or inserted directly in their socket or, inserted and retained with something that pushes them against the socket (as some springs, brides, etc.).
They can be also inserted in their sockets and covered with a metallic cylinder. This cylinder is there to create a shield that will throw to ground any potential RF or EMI noise. This is often seen just for the very first pre-amp tube (V1) but, there are amp with more or all pre-amp tubes shielded.

If the tube is retained with the help of springs, we should first gently remove pulling up the strings to access the tube.
If what we have is that shielding cylinder, we just need to push the cylinder a bit down and, give it a quarter of turn, to pull it up and then, to discover the tube.

Once we see the tube naked, inserted in its socket, we start gently pulling up the tube while we move it in different directions or circularly. Just be gentle enough to avoid to bend the pins that were inserted in the socket.

Most of pre-amp tubes are of type 12AX7 and family and, all them have a clear gap between the first and last pin. To insert the new tube, you will just need to align that gap with same gap in the socket.
We will insert the tube also gently, moving it circularly or with small movements in several direction, until we see clearly all its pins perfectly inserted and that the bottom of the bottle lays on the socket.
Don't go really rude pushing hard the tube, you can break the socket or the tube flash. You need to use some force, indeed, but not so much.

Once in its socket, we can then to mount again the retaining springs or the metallic cylinder and, we are done.
You will do it with some religious feel the very first time but, after a couple or three tubes, you will do it with complete confidence.

Potencia EL84

EL84 tubes are very similar to 12AX7 pre-amp tubes but, bottles are wider and higher. They also have same number of pins and arranged in same way as 12AX7s. So you need to align that gap between the first and the last pin and proceed as explained for 12AX7 tubes.
Usually, amps that use EL84 don't require to re-bias your tubes. But, be sure your power amp was designed as cathode-biased (don't require re-biasing tubes).

Other Power tubes

Most of other power tubes (and a few of pre-amp tubes) have an octal (8 pins) socket and, they are aligned in a very different way than 12AX7s or EL84s.
But, in the center there is a circular hole, with some small square carving that serves as a guide for the central post in the tube, that also presents a circular shape with an small rectangle in a side, you will see how to easily align the tube post with the socket hole.
Those tubes can have also a different kind of retainer but, you will easily see how to move the retainer to free the tube.

Just be sure to correctly align the post with the hole, otherwise, you are in risk to damage your amp.
Usually amps loading EL34, KT66, KT77, KT88, 5881, 6L6 and 6V6GT tubes need to re-bias your tubes after you mounted new ones.

Always read your amp' specifications and be sure if you need to re-bias your new power tubes.

Important note
In case that your power tubes are in good condition, there is no real need to swap them.
The bigger impact in tone will be produced by swapping pre-amp tubes and, more specifically V1 (first tube) and PI (Phase Inverter or Driver).
Therefore, if you still don't know if your power tubes need to be re-biased, you can still change all your pre-amp tubes and check the changes in tone.
Remember: no pre-amp tube needs to be re-biased, never!. So, this is the easiest way.

Re-tubing example: Koch Studiotone Combo

Well, this one was my very first tube amp and, when I bought it, I had no idea about tubes (neither about amps, to be honest).

As it came stock, it sounded to me a bit cold and sterile, except under high gain settings.
I've early feel in the need of enhance its sound and, I bought several tube types, following some written tips available in TAD's page.

Without understanding the design of such and amp, and without knowing the different tonal characteristics of each specific tube, I was blinded swapping a tube here then there, with not great success. I've was even able to make it to sound worst.

With the time and, while I was saving some money to buy a couple more of tubes, from time to time, I was able to stock a couple or three of same tube types and, I was trying them in all positions until reaching a compromised solution that lasted for a long while.
With more stocked tubes and a better knowledge about how an amp works, which are the tasks of each position and how they affect to the the sound and, how each tube type sounds. I was able to fine tuning my first compromised solution.

As anything, to have success with a certain set of tubes can take a while but, the good thing is that once you've swapped a couple of tubes, you will loose your fear and, you will love to experience tube changes, that will help you to better understand your amp.


Sometimes, an image can say more than 100 words so, I've prepared some video showing you how to swap tubes in this specific amp.
Video starts with a demo about how to substitute the pre-amp tubes in a Koch Studiotone Combo, with the security measures and handling recommendations already described above.
In the second part, I am testing the sound finally achieved, after swapping those tubes.

I am now around the 90% of the tone I was looking for this amp. As soon as I can swap those power tubes with some Genalec or Mullard reissues, I will be at 100%.

No comments:

Post a Comment

Please, feel free to add your comments.