Making Vox AC30 Amps Sound Better

[The New Steve H]

My church has two Vox AC30 amps. These are 30-watt tube amps, as I'm sure everyone here knows. The worship people know I've started building amps, and they have asked me for some help.

They say the amps don't break up until they're turned way up, and they would like to fix that. They have also said they want isolation cabinets, which--I suppose--is another way of attacking the same problem. Basically, the amps are too loud when they're turned up to the point where they sound good.

Any suggestions for improving the performance of these amps? I would guess the church is around 10,000 square feet, and the walls and ceiling are not very reflective. The music is pretty loud, but apparently not loud enough to make the amps happy.

Also: they complain that the rectifiers blow a lot. Would it be okay to put backup diodes in these amps? Seems like an easy fix.

[selloutrr]

churches are a difficult place to balance amps.

What you can do is going to depend on which model and year the amps are. If they are vintage tag board it's pretty easy to mod. if they are reissue it's going to be a bit more complicated.

The easiest fix for any amp would be a power soak, airbrake, power brake. insert it between the amp and the speakers. since it's a combo you might need to make an adaptor cable.. male on one side female on the other. make sure you match up the volume attenuator for the proper ohm load. this will allow you to turn the amp up and hold back the volume to bedroom level giving you the rich distortion these amps are known for without getting in over your head trying to modify them.

no need for an isolation cab unless you are trying to record and even then once you get the volume under control you will probably find it's not needed.

You can increase the diodes from 1amp to 3amp that will solve the problem with them blowing. you may have to get creative to make the thicker lead fit in the PCB holes.

I would check the power and make sure the source if wired correctly.

[selloutrr]

you can also point the amps in a different direction or use a pedal to push the input stage for more distortion.

[The New Steve H]

They told me these amps have rectifier tubes, not diodes. I wanted to add diodes so they wouldn't crap out during services.

What about different tubes? Can you put 6v6s in there and lower the output?

[The New Steve H]

Whoops. I forgot that these amps have EL84 tubes. So forget the 6V6 idea.

Our guitarists are already serious pedal addicts.

[selloutrr]

if they are blowing up rectifier tubes they are

1.) abusing the amps... ie moving them and dropping them down hard while still hot.

2.) the amp has a serious problem in the filter or power supply section.

It sound like the guitar player needs to figure out how to dial in his pedals and live with the amp at the volume they can "run" at. or get a pair of power soaks. I'm assuming the church has a P.A. system and the majority of this is a volume fight between I can't hear myself and turn down your killing the mix. turn the amps around or in toward the drummer. give the guitarist a bit more in his monitor and call it a day.

[pdf64]

These are loud amps; if power amp overdrive at lower sound levels is needed then an AC15 or 18watt marshall type amp would be more suitable.
Don't be tempted to remove any output tubes to lower power output. These amps are self / cathode biassed and require all 4 el84 tubes to be working in order to get sufficient bias; take any out and the others will run super hot.
I've read somewhere that some late 80s AC30s were made with the rectifier wired incorrectly, with the B+ taken from pin2 rather than pin 8, thereby sending all the B+ current through the rectifier heater. Not really a problem in normal running but at start up, the surge current charging up the caps can blow the heater. So it's worth checking for that.
Putting series diodes in line before the tube rectifier, as per geofex http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#ampmods is a good idea.
One easy way you could lower the power output is to fit a rectifier with greater voltage drop. I've run a US made 5Y3 in my AC30 and it makes a noticeable difference, though the 5Y3 is being run beyond it's max current rating in this scenario.
Other options are 5R4 (most may be too tall though) and 5V4 - in order of voltage increase over a 5Y3 towards a GZ34. Or a Weber copper cap http://www.webervst.com/ccap.html
Hope that helps - Pete.

[The New Steve H]

I think you're right about the amps being too big; that's the obvious conclusion. Usually, it's hard to get the church to spend money on things, but this time, they spent too much, and it would probably be impossible to convince them to sell what they have, take a loss, and buy smaller amps.

I don't know anything about pro audio, but I would think the smart move would be to use small amps and run them through the sound system. Strange thing about tube amps: they sound just fine when their output is amplified by solid state sound systems. Even though Howard Dumble says the electrons get bent or flattened on one side or something.

Anyway, we're stuck with what we have, so I wanted to do whatever I could.

I don't know why guitarists go so far overboard on wattage. I practice with my Firefly all the time, and I prefer it to my next smallest amp, which is a Vox AC4TV. I can turn it up in a small room and get a really nice sound. A guitarist at my church tried my smallest amp out (tiny 6021 output tube), and on an insensitive speaker, it sounded fine in the first row of the church. You would think they would give 15 watts a shot.

Actually, one of them bought his own amp, and it's a 20-watt Morgan. So I guess they are starting to see things the same way.

[Reeltarded]

It takes a 30 watt amp to keep up with a snare.

Use something to muffle the fronts of the amps. I use saddle blankets on Marshalls. Use baffles on stands behind the amps, and not right up to the back. They need to breathe, obviously.

Power soak the outputs 3db. Look around at all the modern choices. Hot plates even work for the first click or two. -6db and we are back down in 'can't compete with a snare' range.

Takes all kinds. My bedroom rig is a '70 Major.

[Gaz]

I just don't think electric guitar was meant to be played at church

[Firestorm]

It is possible to split the cathodes on the AC30 and bias each push-pull pair independently so you can run it with only 2 EL84s. You use two cathode resistors of double the stock value two bypass caps of half the stock value. Not all that hard even on the new PCB-based models.

[The New Steve H]

I just don't think electric guitar was meant to be played at church

Have you heard of Sister Rosetta Tharpe?

The cathode-splitting idea sounds interesting. I don't really see the church leadership inviting me to crack the amps open and start yanking components off the PCBs, but if there's a way to halve the wattage and get a better sound, it's good to know about it.

[212Mavguy]

I like the Weber MASS attenuator. I had mine made with a balanced line out, uses TRS to XLR cable that can go to the sound board, the 100 watt rated one is what those AC 30's need. The line out has a vol treb mid bass tone stack so the guitarist has some control over what is coming through the PA volume and tonewise. Spendy and worth every penny.

Peace.

[selloutrr]

It takes a 30 watt amp to keep up with a snare.

Use something to muffle the fronts of the amps. I use saddle blankets on Marshalls. Use baffles on stands behind the amps, and not right up to the back. They need to breathe, obviously.

Power soak the outputs 3db. Look around at all the modern choices. Hot plates even work for the first click or two. -6db and we are back down in 'can't compete with a snare' range.

Takes all kinds. My bedroom rig is a '70 Major.

put a plexi baffle around the drummer if he is to loud. mic him and put him in the PA if u need more.

its a loosing battle turn the amp up to get over the drummer.. the drummer hits harder.. you turn up.. over and over.. the crowd leaves, the vocals are lost in the wash. I can tell you know everyone needs to live with less volume and the guy behind the board needs to mix for the house.

[Gaz]

Oh don't bring Ms. Tharpe into this!