ALL TUBES ALL THE TIME

soundmasterg · Post by **soundmasterg** » Sun Jun 05, 2005 6:51 pm

Kent, before you just go and plug the 5Y3 and 12ax7's in, you may want to have your amp checked out and make sure it wasn't modified for the tubes that were in there instead of what was supposed to be stock. Read Bill Yantz's post above about what was done to his and you will get the idea. If someone did that to yours, and you put the 5Y3 in when it needs a GZ34, then it could cause some problems!

Those Groove Tubes 12ax7M's that came in your amp are very nice tubes and are new enough that I am sure they don't need to be replaced. So you really only need 3 more 12ax7's at the most. If you plan to buy the EI 12ax7's, you may have problems with microphonics. I haven't got any EI 12ax7's from AES (Antique Electronics Supply) before, so I don't know if they screen them very well or not. I know people like LordValve and Jim McShane do, so if you are set on the EI 12ax7's, then get them from someone like them who screen the tubes for microphonics. It may cost a little more per tube, but its worth it.

I haven't heard of any microphonic problems with the EI EL84's so get those from AES if you want. You may consider getting the new JJ ECC803S which AES sells. It is a low noise 12ax7 type they just came out with. I've heard a couple in use, and an amp repair friend of mine has now switched over to these completely. I'm planning on getting some myself soon to try out.

Full wave rectifier means that the rectifier acts on the full sine wave when it functions. Remember from trigonomety where they are talking about sine waves? You may have seen one on a scope too? A half wave rectifier only works with half of that wave, so the resulting DC the tubes are seeing is more choppy and has more line noise in it, which makes your amp hum more. The amp also puts out a little more power when using a full wave rather than half wave rectifier. A Full wave rectifier works on the whole sine wave. You don't have to look for one specifically because whatever tube type you are looking for, whether it is a 5Y3 or GZ34, are made to a certain spec. So if a GZ34 is a full wave rectifier, then all of them are. It doesn't mean it has to be hooked up that way in use, but since the tube socket in your amp is already wired up as a full wave rectifier, then you are good to go once you get the correct tube to put into the socket. Just make sure to have the amp checked to see if it was modified before you got it to use a GZ34 and those other preamp tubes that were in there or not. Or else you could cause damage.

rictified · Post by **rictified** » Sun Jun 05, 2005 8:42 pm

Yeah I like JJ tubes I've had good luck with them but they are far better places to buy your tubes than from Lord Valve, that's all I'll say about that.

kevin · Post by **kevin** » Mon Jun 06, 2005 5:24 am

I just received word from Mitch Colby at Vox that we both recalled correctly -- the AC15-TBR preamp section was designed to have 12AX7 tubes in all positions.

rictified · Post by **rictified** » Mon Jun 06, 2005 6:38 am

I would still take Greg's advice especially for the rectifier tube, you could have some nice sparks or worse if it has been rewired.

brammy · Post by **brammy** » Mon Jun 06, 2005 10:10 am

yes, I can see that.... I'm going to go with a GZ34 recifier. Thats what was in there before.

One interesting thing... especially now that Kevin has confirmed that the AC15 preamp section was designed to have 12AX7 tubes in all positions.
The reverb on this amp has not worked (although the tremelo works great). Even when I brought it to the tech he couldn't get it to work. I'm wondering if it could be a simple case of the reverb preamp tube being way too weak... if you remember I pulled out 3 12AX7's but also two others which were much weaker. If that turns out to be the case, by amp tech guy deserves this year's boobie prize.

rictified · Post by **rictified** » Mon Jun 06, 2005 10:38 am

I meant have the wiring checked by a tech before you touch it.

brammy · Post by **brammy** » Mon Jun 06, 2005 11:19 am

Yea... I know. But it was gone over by a tech a few months ago. He was the one who put in those two weak preamp tubes. He also used a GZ34 recifier.... so I'm thinking that I'm pretty safe using the same now (?). The real danger would be to put in the 5Y3 without a larger fuse... am I right about that?

I'm trying to get this amp working before Friday. I dont want to do anything that will blow it up, but I'm thinking that simply replacing all the tubes with identical ones (except for the two weaker preamp tubes he had in there) will be safe.

Or am I totally flat-out pigheadedly wrong? (hard to believe, I know, but it's happened a few times before)

soundmasterg · Post by **soundmasterg** » Mon Jun 06, 2005 4:37 pm

You would have to read up at that post about it, but I think a larger fuse was put in when the GZ34 was in use rather than the other way around. If you use the GZ34, the plate voltages are higher and to make the amp run correctly, in addition to the bigger fuse, you may have to have the cathode bias resistor resized. A good tech could do it for you. Theres not any particular set value that I coudl say to use as it all depends on the tubes and the circuit, but any tech could substitue in resistors untilt eh correct value is reached to get around 70% of max dissipatino for the tubes.

If it was working ok before, then yes, you coudl replace the tubes with new ones and it would work. But to get it wokring as best as it can, you should have it checked out by someone knowledgeable to make sure it is optimized for teh GZ34 instead of the 5y3, and also to make sure that the preamp wasn't modified to use those other tubes instead of the correct 12ax7's.

brammy · Post by **brammy** » Mon Jun 06, 2005 4:55 pm

yea, I guess you're right. I'll take it to a tech, but not the same tech as before. I know someone else who is good but he takes a long time.

Man... is it just me or is the VOX AC series more tempremental than other tube amps? I dunno... its probably just the nature of living in Tubeville.

But in the end its worth it. No other amp I've played sounds as good as that VOX AC-15 when its healthy. thanks for the heads-up.

soundmasterg · Post by **soundmasterg** » Mon Jun 06, 2005 8:17 pm

If you were local I could look at it for you, or there are some other good local techs. I'm in the Portland Oregon area. But I'm guessing you're somewhere else so that won't work. If you find anyone worth their salt, you should get it checked out well enough. Just make sure to tell them that you suspect the amp may have been modified for a different rectifier tube and different preamp tubes and you want them to check it out and make sure before you plug in new tubes.

The old AC series from the 60's were tempermental if you didn't know what you were dealing with. A lot of techs in the US are familiar with Fender's and know how to work on them, and they try to apply the same ideas to a Vox. But a Vox is a different amp and has it's own set of issues just like a Fender has it's own set of issues. If a tech keeps that in mind and approaches it for what it is, and goes from there, then its all relative and most amp brands are as reliable as any other. The newer AC series like your AC15 is fine and not at all like the older stuff. Tube amps are more finicky than transistor stuff, but they are also more forgiving of a lot of things that would kill a transistor amp, and they also sound light years better. They're easier to work on too.

brammy · Post by **brammy** » Mon Jun 06, 2005 8:46 pm

Thanks for the offer Greg but I'm out in HI. Its nice out here but sometimes its a bit limited... like in this case where a good amp tech can be hard to find. In general the rock'n'roll scene here is a small fraction of a place like Portland. On the other hand I sometimes think that that works to my little band's advantage... less venues but also less competition. I dunno, cuts both ways I guess. For years here the big thing has been "Jawaiian".... sort of an island style sicky sweet "I love my girl on the beach under the swaying palm trees" kind of pap set to a vague reggae rhythm often with a ukelele prominent. One or two songs can be ok since it relies heavily on good harmony and occasionally it can be good. But it has NO GUTS and when a tight little old time rock'n'roll / fast blues band comes along it actually seems (to some) like something "new". We're a good band, but not great... but we do get a certain positive reaction that is borne out of relief that at least its not the same old Jawaiian ********.

I'll be sure to get it professionally worked on. The sound just cant be beat so its worth it. thanks again.

bill_yantz · Post by **bill_yantz** » Tue Jun 07, 2005 2:27 am

Kent, all good advice from the bros here.
Note: It appears you have been using the amp all along with the GZ34 mod previously appplied. If so, then no additional work on it is necessary. Aside from replacing the original rec tube with the GZ34, the only other thing needed is to upgrade the power fuse from 1amp to 2amp as previously mentioned. It won't operate with a 1 amp fuse; it will blow each time you replace it and turn it on. So, I think your OK to change the GZ34, if needed, without fear you will damage something or lose sound quality. BTW, this was covered 4 years ago on VT and Don Butler chimed in and set us all straight. I have learned that if Don says it, you can bank on it.

Once I made the mod, it worked flawlessly. In my estimation, it expands the headroom by about 10-15% before it starts to breakup. If you like to get cleaner sounds at a slightly higher volume, you've got it. I ran mine with the master volume all the way up and adjusted the volume/gain from zero to whatever volume needed. This is where I got the headroom.

Got to say it: I am not an AC15 expert, but I got the information, like you are now. It worked and I learned a lot in the process. That's why I come here. Great Ric friends helping and sharing their knowledge and experience that's appreciated by all. A good example is the restoration thread with Paul, Dale, Larry, John and the others, and anytime Peter or John H. chime in - cool!

brammy · Post by **brammy** » Tue Jun 07, 2005 9:35 am

yup!.... for an electronics dummy like me, simply going through this process is great.

Yea, it was working fine for a few months with the GZ34 so I figure I'm safe to simply replace it (even though the old rectifier probably is just fine). I could be that I'll put in the new tubes and there still still be a problem. When the first tech worked on it he said he was having problems getting the reverb to work. Since then I've been running through a HOLY GRAIL reverb box which sounds great. Putting in all the same might bring the reverb back... or it could cause problems if the tech did something fancy with the reverb circuit.

I'll be sure to let you all know. I ordered all JJ tubes and they should get to me on Thursday. I'll cross my fingers and put them in. If there are any problems at all I will bring it to a new tech.

soundmasterg · Post by **soundmasterg** » Tue Jun 07, 2005 10:46 am

JJ tubes are reliable so they should work fine. Some others like the EI's sound better, but are not as reliable. Did you get any of the new JJ ECC803's, or did you stick with their ECC83/12ax7 type?

brammy · Post by **brammy** » Tue Jun 07, 2005 11:22 am

I got the ECC83/12ax7 type ..... EI's sound better in what way?

My main thing is to get the right tubes and to get the bugger working well. After that I wont mind spending a few bucks to experiment with different brands of tubes to tweek the sound.