Talk tubes to me.

L

loudnaybor

New member
I've got a 20 year old Acoustic 100 watt amp head, the brand not the type. It's got the original tubes in it and it's starting to sound really bad. It has 4 6L6 tubes and 4 12AX7A tubes in it. It's gonna be several hundred dollars to replace them all, so I thought I'd see if anyone has any thoughts or tips on the subject.
Can the 12AX7As be replaced with 12AX7WAs? What brand have you had the best luck with? Any thoughts would be appreciated.
 
To tube or not to tube....

To save a few bucks, just have the 6L6's replaced for now. The preamp tubes should still be in decent shape- check thier color- they should glow orange NOT BLUE. If the tubes seem blue in color or if the amp hisses and pops it's time to change them. Buying tips: always buy tubes in matched sets and don't get the most exspensive tube money can buy. NO MATTER WHAT OTHER PEOPLE MAY SAY< GROOVE TUBES ARE NOT THE BEST choice for every player or every amp. Ask your dealer for Ruby Tubes or EI, they are a good quality tubes for a more than fair price or call a company called MOJO TONE. You should be able to find thier add in Vintage Guitar or check ou their web site http://www.mojotone.com, they can also answer question about substitutions and swaps. And remember just because another brand cost more in isn't always the best for what you want to use the amp for......
 
You could also try buying one of each (a 12ax7 and a 6L6) and replace the tubes in the amp one at a time until you find the one that is really bad.

Are you sure all the small ones are 12ax7's

One may be the reverb driver which is ... hmmm usually something else but I can't remember what.

I'm with Daddy-O though - it's much more likely to be a power amp tube (6L6) that's gone.

Are you sure the cost of replacing them all is '... several hundred dollars...' that seems like a lot.

You say it sounds really bad - how does it sound?

foo
 
Virtual.ray,
You're correct, there is another tube for the reverb in addition to the 4 12AX7s. It's a 12 something or other but I didn't list that one because I didn't believe it to be part of the problem.
I could do the swap thing but I don't think I'd be able to get tubes with close enough specs to the originals. They were made by GE and I'm pretty sure they don't make 'em anymore. I might be able to get one matched set though and swap one pair at a time.

Daddy-O,
Good suggestion about just doing the 6L6s. I haven't called Mojotone yet, but I will.

Foo,
The prices that I found without a whole lot of digging were around $300 to replace all 8. The 6L6s are pretty expensive.
The sound isn't really bad at low volumes when playing clean. When I turn up though, there's a noticible loss of power over what this amp did when it was new. If I start driving it to distortion the, sound is kind of broken, like a cheap distortion stomp box. Even clean, the louder I go the worse the sound gets. This amp didn't used to do that.
Since these tubes have seen 20 years of action, I figure they've done their job and deserve retirement.

I seriously considered just buying a new head and using this one as a back up. But, I dont think a new head would really sound any better than this one, if it was working right. It really is a nice amp.
 
Word of warning - DO NOT check the power tubes (6L6) one at a time and replace the bad one. Anytime you have a bad power tube, you should ALWAYS replace all of them. Not so with the preamp tubes (12A_7). They only need to be replaced if there's a problem, and you only need to replace the bad one. Otherwise, you should change the power tubes about once a year, depending on how much you use it, and how hard you drive it. I used to own an Acoustic 164 tube combo. I got great tones out of that amp, but it did eat up power tubes a lot. Try not to let your power tubes get to the point where they fail as there's a chance that it could take the output transformer with it (very expensive fix). That's why you change them periodically, to avoid tube failure. Also, I think that the price you were quoted may be a bit much. Shop around. Maybe try Elderly's Instruments at elderly.com. Good luck and let us now if you get that amp going again.
 
be careful

First thing is that output tubes need to be biased (between 36-42 volts I believe) in pairs.If you haven't done this before get expert supervision.There is enough juice in the filter caps to KILL you if you touch the wrong place.Note to tube amp guys.Upon shutdown of the amp after the gig or whatever turn off the main power switch and NOT the standby switch.This will drain the filter caps.
Secondly,6550s are the milspec equivalent to 6L6s with a more modern high gain sound.The Sovteks are excellant.Avoid the bulging tubes like Ruby (chinese pot-metal impurities).As to preamp tubes,try the 12AT7 for a slightly hotter front end.Blues guys might use the english EL84 EL34 combination (remember to re-bias everytime you change tubes).
regards
Tom
 
3 POINTS
1. 6L6'S will need 48 - 52 volts in most amps, higher plate votages will need more bias.
2. milspec tube for 6L6 is 5881.
3. 12AT7 has a gain of 60, 12AX7 has a gain of 100.
12AT7 can handle more current. You can use it in preamp, but the gain will be lower.

Thats it !
 
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