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Freebander
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linear amp tube replacment


hi guys has anybody replaced the tube or tubes in there linear amp if so is there a certain way to do this as iv never done it before and i am hearing different ways like never touch tube unless u have a cloth or gloves on ? and when new tube is instaled u have to wear it in i would be greatfull for any advise on this matter tnx 73s

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23/Jul/2011, 1:20 pm Link to this post Send Email to DAVE129   Send PM to DAVE129
 
madmark1 Profile
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Re: linear amp tube replacment


Yes its straight forward really, ive a few amps here, A FL-2100 ( 572B tubes ) and a linear amp uk 'hunter' (3-500 tubes).. both run around 2500 volts DC on the Anodes.

If youve Not changed the Tubes in a amp before... i would do it this way..

Unplug the amp from the mains and leave it over night for the main HV capacitors to discharge.

Remove the relevant cover off the amp, its normally the top cover, now with a screw driver short out the Anode cap to ground or chassis. the Anode is normally the top of the tube.

This isnt really the best way to discharge the caps, but it works ok.

Now very carefully pull of the anode cap if its a spring clip type of undo the fixing screw if its a 3-500 type tube. be carefull not to stress the Anode/s if you want to re-use the tubes.

Now gently wiggle the tubes free of there bases, the tubes should be tight but with a little patience should work free.

replace the tube with the new one, re-connect the Anode cap or caps if youve more than on tube, check to make sure all is as it should be and nothing is loose or shorted, replace the RF compartment cover and the main cover and plug the amp in, take note of the Idle current when on TX with no rf power going in from the radio ( basically ssb with no audio or mic gain turned off ) but keyed up.

the Idle current depends on what tube you have, you may have to look this up or refer to the handbook for this.

If the Idle current is ok apply a small amount of drive and continue to tune up increasing drive untill full power is seen from the amp.

Thats it, it takes longer to write it than it is to do really.

Mark

Last edited by madmark1, 23/Jul/2011, 10:02 pm


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23/Jul/2011, 9:59 pm Link to this post Send Email to madmark1   Send PM to madmark1 Blog
 
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Re: linear amp tube replacment


Is it worth delving into the realms of gassy valves, and the notion of disconnecting the anode cap and just applying heater current overnight?

No, I guessed not. emoticon

You'll be alright - go for it!

And it is OK to handle the valve with bare hands - just don't smear it in KFC grease, that's all.

---
Andy - FB107
23/Jul/2011, 10:11 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
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Freebander
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Re: linear amp tube replacment


hi guys no iv never done this before as iv bought all new amps but i got this homebrew [sign in to see URL] amp and it dosent need tube replacing as of yet but its always good to know these things i thankyou all im greatful for your advise 73s

  DAVE 29FB129
24/Jul/2011, 8:57 am Link to this post Send Email to DAVE129   Send PM to DAVE129
 
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Re: linear amp tube replacment


Gassy tubes, ive not had much luck with them really, although ive not had mant gassy tubes either.

There is various ways to remove the gas from the tubes or so were led to believe, if the tube isnt to gassy then you can get it back up and running, but if its a bad one then i just bin them.

The worst thing you can do is leave the tube in the un-used state as the vacuum deteriorates over time, this lets the air in and causes the gassing, I tend to swap the tubes for my amps every few months and run them for a day or so at full power, then i pull them and box them back up again.

As you've said Andy you can apply heater voltage and a lower HT voltge otherwise flash over may accur at full HT and leave the tube to stew, the cathodes ( gettering ) will absorb the gass . or use the tube but run it at reduced HT voltage for a time.

have a read of this as theres more to the good old vacuum tube than meets the eye.

[sign in to see URL]


Mark

Last edited by madmark1, 24/Jul/2011, 1:30 pm


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24/Jul/2011, 1:27 pm Link to this post Send Email to madmark1   Send PM to madmark1 Blog
 
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Re: linear amp tube replacment


Those Eimac things seem to be quite problematic, as the getter is on the anode and doesn't work well unless the anode gets to several hundred degrees in temperature. Hence, in storage they can get a bit gassy and as they are very high voltage devices they need a very hard vacuum to avoid flashing over.

The more ordinary valves have the getter deposited on the inside of the glass and here a bit of a stewing can restore a previously unusable valve. The 813s come into this category.

Talking of 813s, I'm looking for a 2 - 3KVA transformer capable of outputting about either 1800 or 900 volts for my little project. Any suggestions? (Polite, please!)

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Andy - FB107
24/Jul/2011, 5:19 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
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Re: linear amp tube replacment


I use the 3-500 ZG tube from Eimac in my amp. Its a very rugged tube although you do have to get it hot to make it work, its no good just ' tickling ' it , ive also a few spare 3-500 tubes for my other amp, It uses the 3-500 as well, but i use the 'Z' in that amp, The ZG puts out more power, driving the ZG with about 50 watts on 20 mtrs i get about 600 watts plus from it.

Mark

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24/Jul/2011, 9:24 pm Link to this post Send Email to madmark1   Send PM to madmark1 Blog
 


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