Changing valves

Discussion in 'Amps and Cabs' started by shuffle, Oct 2, 2005.


  1. shuffle

    shuffle Member

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    Hi there -

    My Power up valves need replacing. I have bought some new ones and decided i will give it ago myself. Dont know if my amp needs biasing but i'll soon find out.

    Anyway i was wondering if someone can give me a fairly detailed description on how to change my valves. This is my first time.

    Firstly - To take out my previous ones (some one said to me just pull them straight out - i tried and they seem pretty stuck)

    Secondly - saftey procedures for putting in the new valves - is it true that i should not touch them with my skin at all???

    Im urgently trying to fix this as i wanna play!!

    Thanks!
     
  2. rastaman

    rastaman Member

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    -First, find out if amp needs to be biased for new tubes, some do and some don't.

    -If bias=yes, take it to a tech if you are the LEAST bit unsure.

    -If Bias=no, power down amp and unplug.

    -Grab tube base and gently wiggle/rock side to side while pulling up. Take a look at the socket tab or spacing between the pins to make reinstallation a more accurate task. Sometimes it's hard for me to see the pin sockets so I get an idea while removing them.

    -Install new tubes gently, I never force tubes. Sometimes I double check and triple check the orientation before I push them down.

    -I've touched them w/ my skin a million times, just like incadescent light bulbs, it's not a quartz bulb. You can use cotten gloves for safety however.

    -Before I turn on the power I double check that the amp is on standby, then position myself where I can see the new tubes but I'm not right in front of them. Then I turn the power on and watch them for a couple minutes then throw the standby off and observe for a couple more seconds and then JAM! :dude
     
  3. shuffle

    shuffle Member

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    Hey mate thanks for your help!

    I went and changed them over now - i turned it on standby then powered up.

    Everything looked normal so after a few minutes i started playing. The amp sounds exactly the same to my ears. IF MY AMP needed biasing - would there be a difference in the sound that is VERY noticable???

    Thanks! (im just scared whether im going to kill my new valves in a matter of months)
     
  4. rastaman

    rastaman Member

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    You're welcome. As I don't know all that much about amps, I do know that the amp techs around ALWAYS stress biasing w/ new tubes. I don't really understand why a fixed resistor in a bias circuit would change really, but it's very possible for a pot to "drift"/move from vibration and such. Also, some amps do not require biasing when tubes are changed. What kind of amp is it??

    You would do better to talk to John Phillips or James Peters, D. Roccaforte etc. etc.
     
  5. shuffle

    shuffle Member

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    Its a hughes and kettner duotone 50W combo.

    I love it~ I cant find anything on the web about biasing...

    Thanks for your help - who are hose other people and how do i reach them???

    Cheers!~
     
  6. stratopastor

    stratopastor Member

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    hi Shuffle

    John Phillips is a UK-based guitar tech. I have managed to contact him in the past by posting here with 'paging John Phillips' in the title.

    Does the amp need rebiasing? 3 answers...

    (1) read the manual. I see this amp sells for over 2000 dollars in the US. H+K should tell you!

    (2) technical answer is, if it's cathode-biased, no, cos it's self-adjusting; if it's 'fixed bias', yes. Is there a circuit diagram on the web? Let me know and I'll have a look for you

    (3) bonehead answer which works a lot of the time... let the amp warm up but don't play it. Look at the power valves glowing in a semi-darkened room. Note which bits are glowing. Now play it at gig level for several minutes and, while still playing, look at the power valves. If any of the flat-metal parts of the valve are starting to glow, stop playing and get the amp adjusted before playing loud again. Otherwise (assuming the amp sounds OK) you're probably OK. NB this is obviously a rough-cut method and no valve supplier will warranty their valves if this is the only rebiaising check you do!

    best wishes from Berkshire

    SP
     
  7. John Phillips

    John Phillips Member

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    Thanks for the recommendation... although I'm trying to avoid the 'paging John Phillips' threads... ;)

    There are other knowledgeable folks here too!

    The main reason for rebiasing is not that a fixed resistor or trimmer will have changed its value, but that not all tubes are made the same and there can be significant performance differences even with the same brand and type - so when run at the same bias voltage, will actually operate at considerably different current, which is the more critical part. This is particularly true of modern-production tubes which aren't made to as tight tolerances as the old ones were.

    There are several ways around this...

    Some amps (like Mesa) purposely do not have a bias adjuster. They rely on you using new tubes that are close to the same spec as the originals. If you know what you're doing, this actually means quite a lot of them, but if not, it's best to either stick to the maker's own-labeled tubes (which have been tested for their amps) or buy some from a reputable seller who will select them to run in that type of amp.

    Some amps do not need biasing at all, because they are 'cathode' or 'self' biased - this means that the bias voltage is generated by the tube itself and so is to a large extent self-compensating. Why aren't all amps like that? Because it produces different tonal characteristics and slightly lower efficiency. In any case, even in a self-biased circuit not all tubes will always run correctly if they're at the outer ends of the tolerance range.

    The third way is to have a bias adjuster in the amp - it's often inside where inexperienced/unqualified owners may be afraid to look, and it really is dangerous to do so unless you know what you're doing. You can get various devices to plug into the tube sockets to measure the current, but you still have to get inside the amp to actually adjust it if it needs - though there's no harm in checking the current from the outside, since most of the time it will probably be in the correct range anyway (there is a range, not one right value). A slightly confusing piece of terminology is that these are still fixed bias amps even though they are adjustable! The bias is fixed by the control, as opposed to being 'self adjusting'.

    I'm not 100% sure (I'm not familiar with that amp) but I would guess that it's a fixed-bias amp - self-biased amps of 50W or greater are rare. If you put the new tubes in and the amp seems to run fine, there's probably no need to do anything else. One check you can do is to run the amp in a dark room and look at the tubes - the slightest trace of a dull red glow on the outsides of the grey box structures (the plates) inside the tube - not the orange glow inside the plates - and the tubes are running far too hot and won't last long.

    If you replaced the old tubes with new ones of the same type, there's a good chance that you don't need to do anything. It may sound different - if it sounds clearer, brighter and louder that's just the difference between old and new tubes. If it sounds 'grainy' or harsh at lower volume (especially on a clean sound), the tubes are biased too cold and the amp will need adjusting; if it sounds mushy or too thick and overcompressed, it may be running too hot even if the tubes aren't glowing red.

    If you're at all worried, a trip to a tech might be a good idea, or you could buy yourself a bias tester - probably more expensive, although you obviously get to use it more than once. But if everything looks and sounds normal, I wouldn't worry too much really.

    NB - this is just my opinion. Some people are going to disagree and insist that you must bias an amp when fitting new tubes. It's certainly a good idea to check, if you have the equipment, but of the amps I've worked on which are non-adjustable, I've never had to adjust a single one that I can remember if the manufacturer's intended tube type is used.
     
  8. stratopastor

    stratopastor Member

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    John - sorry about the paging thing... best wishes
    SP
     
  9. rastaman

    rastaman Member

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    Thanks for clearing things up for me, great information! Thank you!
     
  10. shuffle

    shuffle Member

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    Thanks JohnPhillips -

    Ill go home today and have a closer look at the valves in a darker room. It seems to sound EXACTLY the same as before - i cannot tell any change in the tone or sound at all.

    Hopefully this is a good sign. Ill let you know how it goes.

    I forgot to mention - One of my new tubes doesnt have 8 Little metal things. One was missing - Does this matter? I put it in anyway and seems fine. (By little metal things i mean at the base of the tube - those things which get stuck into your amp) One tube only had 7.

    Many thanks.
     
  11. John Phillips

    John Phillips Member

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    What it actually means is most likely that you didn't need to change the tubes at all. Don't worry - no money wasted, since you now have a spare pair of tubes, which is a most useful thing... you didn't throw out the old ones I hope :).
    That's surprising if they're from the same manufacturer, but not impossible. It shouldn't affect the operation since the chances are that it's a pin which is unused normally - EL34s have one like this, 6L6s have two. If it was any of the other ones the tube wouldn't work and the amp would most likely sound very bad.
     
  12. shuffle

    shuffle Member

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    Nah i definately know that i needed new tubes - my amp was making SOO much noise by itself and i could smell burning. AND when ever i used my dirty channel it would just screech sometimes. heh definatel needed them. Crackling sound sometimes too. Im pretty sure this is about every possible sign of dieing tubes.

    By saying it sounded then same was that it sounds the same as when my amp worked properly.

    Umm whats funny is that - in my 'matched' pair of tubes, only one of the two was missing that metal thing (im 99% sure anyway) and it looked as though it had been snapped because there was a hole in the bit where it should have been.

    Anyway it sounds fine so hopefully this means its ok and doesnt need biasing cos i cant afford a tech.

    Thanks heaps!~
     

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