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Witloofboer
Gold Member
Belgium
513 Posts |
 Posted - 03/06/2010 : 16:20:02
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I decided that I need new powertubes and I want to replace them myself. As there are as many theories out there on the net as there are techs, I would like to know that I'm using a good method so I won't damage my amp.
1) I've read (on Mesa/Boogies website) that I just have to measure the grid voltage & adjust the pot so that it matches the value written on the schematic. (-45V in my amp.)
2) I've seen a video in which a tech uses a scope & a sine wave generator to bias amps. It seems quite easy.
3) If one (or both) of these methods work, why are there bias tools available?
Any tips?
I know that tube amplifiers carry lethal voltages.
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Edited by - Witloofboer on 03/06/2010 16:21:07 |
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Brutalitarian Supremacy
Silver Member
USA
270 Posts |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 03/06/2010 : 19:14:05
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I like the gadget that zerskies bought from eurotubes ..just plug it in the valve base between the tube and the amp and bias away with a digital multimeter...
http://www.bossarea.com/forum/topic.asp?TOPIC_ID=9124&whichpage=1
I know it's not right but i've plugged in power tubes as replacements...i.e. .the same brand and type without any major hassles over the years...... |
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zerksies
Double Platinum Member
USA
3406 Posts |
Posted - 03/06/2010 : 22:03:11
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HI, sINCE I AM SO FRESH ON THIS SUBJECT I WILL THROW MY 2 CENTS IN. First i am really not liking Smallbear i just ordered something from them 2 weeks ago,it took over a week to ship to me from there place in brooklyn. I could drive to there place in an hour and get it, very sad.
OK now to the bias, there are many, many different ways to do it. You get the plate voltage, do a little math, figure out about 70% load for the tube and you get a relative close range for tube bias. To long and complicated for me.
The wave method most shops i have been too, seem to use this way.they get a close range with the bias. and try and get a signal on the screen to look a certain way, still way to complicated for me, more tools and stuff.
But for the DIY guitarist like me.I just get a set of matched tubes. Plop the meter in the first tubes spot. Set the bias in the range for my amp, It was anywhere from 34Ma to 44Ma for a fender twin reverb.But Bob at eurotubes recommended 38Ma to 40Ma. I set mine to 39Ma. I am stilling messing around with the setting.But so far i am liking 39Ma. Now if i got a set of unmatched tube. i would have to take anther reading on another tube and average out the bias readings on set might be a little hot the other a little cold.But there was not really any price difference for me for a matched set.But this method i don't have to take the amp apart i can play around with the setting a get the sound i am looking for |
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zerksies
Double Platinum Member
USA
3406 Posts |
Posted - 03/06/2010 : 22:05:21
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| Also what kind of amp are you wanting to bias |
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Witloofboer
Gold Member
Belgium
513 Posts |
Posted - 03/07/2010 : 10:09:55
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| My old Laney 100W (4xEL34). |
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zerksies
Double Platinum Member
USA
3406 Posts |
Posted - 03/07/2010 : 14:19:53
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| my way should do it, is it an adjustable bias amp |
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Witloofboer
Gold Member
Belgium
513 Posts |
Posted - 03/07/2010 : 15:36:51
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| Yes, it it. It's a fixed bias amp, so I have to rebias it. |
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zerksies
Double Platinum Member
USA
3406 Posts |
Posted - 03/07/2010 : 16:29:02
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| Fixed Bias amps you don't have to do a single thing too just put new tubes in.If you wanna change the bias you have to change a few resistors in the amp.that i would recommend taking to a shop and let them do. or i would have them put in a bias pot so that you can change the bias in the future. |
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Witloofboer
Gold Member
Belgium
513 Posts |
Posted - 03/07/2010 : 20:28:37
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| I might be wrong, but I always thought that fixed bias amps needed adjustment, as cathode biased amps are self-compensating. |
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Tube amp bias
