A: Congratulations for getting new tubes for your precious tube amp. Chances are either your stock tubes are getting aged, or you are about to experiment some different tubes hopefully to improve the system sound to the better. Either way, it is a fun and exciting experience.
Please follow the steps below to change your tubes:
Step 1.: Confirm that your amp is working fine.
Sometimes you come to the point to buy new tubes because the amp somehow isn’t working properly. The issue could be the tubes currently in the amp are getting too aged; it could also be that the amp needs a re-conditioning in case resistor / capacitor are out of spec from aging. This is a very technical process – if you are not sure and cannot make an educated assessment from experience, talk with a tube amp service technician.
Step 2: Remove old tubes from the amplifier.
At this point, you know that your amp is working fine except tubes to be changed. Power off the amp and let the tubes cool off for at least 5 minutes. Use a COTTON cloth to wrap around the tubes then gently rock / pull it one by one out of the socket. Power tubes (such as EL34 and KT88, plus driver tubes 6SN7 etc) has a guide pin in the center bottom of the tube, plus 8 other pins around the base. You should only gently rock the tubes in case excessive force will break the center guide pin. If the tubes haven’t been removed for a long time, it may feel ‘sticky’ and ‘tight’. Be patient and slowly to remove each one. Never use bare hand to remove tubes – you will leave oily (you may not see it) finger print on your tubes and that will shorten tube life. Also the tubes may still hot to touch – do NOT use bare hands. If not careful you may also break the glass enclosure of the tubes (this is very rare as you have to use lots of force to break the glass), having a cotton cloth around the tubes will protect your hand from accidentally pinched by broken glass. Sound scary yet? 🙂 These are all ‘what if’s’ and do not happen often. Just use good common sense and be gentle when handling tubes. If you find yourself nervous, take a deep breath before doing it. It’s just glass tubes 🙂
Smaller signal tubes such as 12AX7, 12AU7 and 12AT7 do not have center guide pins. Just gently rock and pull it out of sockets. It is extremely difficult to break small tube glass enclosure so you are safe to handle it.
Use COTTON cloth when touching tubes – synthetic materials can easily melt with heat and leave residual and a big mess on your precious tubes. This is not very fun to clean up but it’s doable. Use pure medical alcohol (available from drug store – look for 99.9% purity) dabbed on cotton pad (also available from drug store, usually at women cosmetics department for removing make-up) to wipe the residual completely off.
Step 3:
Put your stock tubes away if they are still good and have life left for future back up use. Wrap each tube separately so that the glass won’t clunk with each other in storage. Tubes are very fragile especially the internal structure. Treat them like eggs, except tubes are non-perishable and can last longer than us 🙂
Step 4: Get on the new toys 🙂
Take your new tubes out of the package carefully (same rules apply as you remove tubes – cotton cloth, over a SOFT surface in case you drop it – yes it does happen especially when you are nervous or overly excited, and handle it very gently).
Do a visual exam of the tubes – you should have done this when you receive your new tubes from courier or mailman. Damaged tubes should be replaced by the vendor, and I hope you are dealing with a reputable vendor who covers shipping damage, not a seek-n-hide or ‘hit-n-run’ type of vendor. If no visible damage, no loose parts floating inside the glass enclosure, chances are your tubes have survived shipping and handling by a large extent. Please note that some large power tubes do have a silver-looking coating inside to indicate if air leaked into the tube or not. When this coating turns white, the tube vacuum has been broken from handling, and it cannot be used and must be exchanged.
Hold the tube in your hand (wrap with a cotton cloth!), and GENTLY shake beside your ear. Minor tinkling is fine as there are many metal parts inside the tube, but if something sound very suspiciously loud leading you believe some structure has been damaged, you should contact your vendor to clarify and possibly seek remedy.
When tube passing both steps above, your chances of tubes survived shipping have increased to very positive.
Now install each tube to the corresponding socket. Many tubes have the same pin layout but it DOES NOT mean you can put them in any matching socket. You MUST dig out your amp manual if the tube type is not printed on the amp to make sure the right tube is going to the right socket. THIS IS VERY IMPORTANT OTHERWISE YOU WILL PERMANENTLY DAMAGE YOUR AMP.
Use the guide pin of the tube to install it properly – then gently press it down to allow a secure firm contact between tube pins and socket holes.
Repeat this with all tubes that you are replacing, until your amp is equipped with your new tubes. Congratulations.. you are almost done 🙂
Step 4: Bias your new tubes…
Some amps are of manual bias – i.e. you can adjust bias settings based on amp manufacturer’s suggestions. Always follow these ‘suggestions’ – treat it as ‘orders’, not ‘suggestions’ if you want your amp run at optimal condition and sound the best. You can set bias about 10% below the suggested settings to extend tube useful life, but you may lose a tiny bit of power / punch. It’s a trade off that you need to negotiate between your ears and your wallet and see which one wins 🙂
Now turn on your amp with new tubes all installed – turn the volume to ZERO. Keep a very close eye to any sudden loud noise (this usually indicate an open plate in tubes with heat – tube needs to be replaced if this happens), strong flash of light (if this happens on small signal tube such as 12AX7/ 12AU7/ 12AT7 etc. and flash disappear right away, it is fine. But flash usually is NOT a good sign for large power tubes such as EL34 / KT88 and 300B etc.). If you hear loud noise from speakers, turn off the amp immediately and switch tubes from left channel to right channel – if same noise moves to the opposite channel, you know you have a problematic tube. Contact your vendor to arrange replacement.
If all good quiet and no big flash, gradually turn up volume to hear if the amp is playing normal. Do not bother with critical listening – this is just to confirm new electronics parts are working as they are designed to function in a live circuit. Give it 2-3 minutes for tubes to warm up, then turn OFF your source signal, turn amp volume to zero, check your tube bias now. If too high, bring it down to close to the suggested bias value. If too low, bring it up to close to the suggested value. In most cases when replacing aged tubes with new ones, you will need to turn down bias value. It doesn’t have to be dead accurate at this step, as with warm-up, tubes will change and bias will drift too. This is not tube defects – it is just warming up process.
Leave the amp on for about 10-20 minutes and play music. I strongly suggest you NOT to leave the room at this time just in case some tubes go south from heating up. Tube damages from shipping could be invisible by eyes, but audible by ears when playing in a live circuit. Do not get overly frustrated if some mishaps happen – these are all parts of the ritual of playing with tube amps. With a positive attitude and good faith in your vendor providing your solid after-sale support, your experience with a problematic tube will become a valuable learning experience then you earn the bragging right to be a tube ‘guru’ among your tube amp owner friends 🙂 Take it EASY 🙂
After 20-30 mins, turn off source signal, turn volume to zero, check bias again and adjust. This time more accurate please. Repeat the steps above – check it in about 1 hr again and 2-3 hours after. During this time, you can do things around the house without locking yourself in the listening room for every minute. But keep your mind alert about the amp going on and check it often.
Once you are done biasing for the first day, when you turn on your amp next day after playing music for about 10 minutes, check bias again and adjust… then check in a few hours to adjust. New tubes tend to drift a bit but with burn-in they will settle down to fairly stable. Check your amp bias every week or so to maintain your amp in top notch working condition.
That’s all – sounds very complicated but it will come to you as 2nd nature in no time.
If I can do it (yes, I do bias my showroom amp :), so can all of you! It is a fun interactive experience with your music playback equipment – enjoy it and be intimate with it. Music will sound better to your ears after you have invested time, care and love to your equipment. The journey of creating playback system is as enjoyable as passive listening, especially when you have your mind in the right gear 🙂
Happy listening.
Rachel @ Grant Fidelity
Treasure is still a treasure 7 years after launched originally in Dec 2008. This is another addition to the series and quality carefully selected by Grant Fidelity for worldwide audiophiles and music lovers. Reviewer Mark Wheeler from TNT-Audio said:
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