Making Vox AC30 Amps Sound Better

[overtone]

woo, a lot of variables in this question.

As the amps are not your own, the non-intrusive options suggested above (like orientation, blankets, plexi, soaks, pedals or putting the things in the sacristy) would be the first avenue to explore. That should keep them busy a while.

But then, the "blowing rectifiers" really needs to be looked into anyway. Did I understand you correctly that both of the AC30s are doing that?

If you get trigger-happy and decide to go inside and start splitting the cathodes / individual bias resistors / pulling tubes etc. check back with us with your exact proposal because, although quite simple to do, you need to address the OT load ratio and merely doubling the existing bias resistor can roast the tubes quicker than you can say AC30. But hey, could they not audition an AC15 first?

I have a feeling in my waters that it would be wise for you just to tactfully advise of all the options, rather than get your hands inside the box, because you could end up being married to issues that are not of your making, like it or not!

Good luck,
best, tony
(edit: spelling of luck!)

[Masco]

If using the AC-30 CC, DO NOT USE THE STANDBY!!! Leave it in the "ON" position at all times. The in-rush current is killing the rectifier when switching the standby.
If you want earlier breakup, you can use a 5V4 rectifier instead of the 5AR4/GZ34 (still, leave the stby "on"). Other things to try for cooler operation/earlier breakup: Increase cathode R up to 82 ohms. Give the EL844 (JJ's 9W version of EL84) a try. Power scale or add a simple VVR to reduce output (this is by far the best way to induce breakup).
Output tubes will last WAY longer if you install a VVR to reduce output Watts. Attenuators will only heat things up and cause component failures and reduce EL84 life. 15W/half power operation(splitting cathode R's) doesn't reduce enough volume and certainly doesn't give earlier breakup IMO.
What I suggest to OP: Have someone install a simple single control VVR for the output EL84's. The Church Lady will be dancin' ala Dana Carvey!!!

[Andy Le Blanc]

interesting about the rectifiers going...

that would imply that the first capacitance the rectifier sees is too large
cant really see a mfg making that little boo boo, what style stand by is it?

increasing the cathode resistor also increases degenerative feedback
and also gives one the opportunity to select a more critical value of bypass

feed back without resorting to global feedback in the power side

[Cliff Schecht]

cant really see a mfg making that little boo boo, what style stand by is it?

I guess you haven't been inside of a B-52 100W head then. They shove 110uF right after a 5AR4 (max cap is 60 uF). I've repaired a few of these simply by replacing the first electrolytics with fresh ones, and the ones in there are usually only a few years old. While the rectifiers /might/ be able to take this, the el-cheapo caps they use can't seem to handle the ripple current and almost always fail. Before the cap change the amps are hummy and oscillate badly, after the cap change they work fine (using better quality electrolytics).

[Firestorm]

It all depends on which versions these are. The better reissues pretty much preserve the original circuit (mistakes and all) and have only 22uF at the main filter. Maybe these have been modded. Or maybe the rectifier tubes are crap (or not even GZ34s). I did see a fairly recent reissue where one of the EL84s had shorted heater to cathode, effectively taking the cathode resistor out of the circuit. But that wouldn't kill only the rectifier. Odd that it's happening to both amps. Mains power issues?

[chrisc]

What about JJ EL844's from euro tubes?
I know some have flamed them, but I have a friend who is using them in an Orange AD30 and they sound really good. They will cut a few DB's and are cheaper than a power soak or attenuator. This with a nice and thick blanket might do the trick.



Chris

[renshen1957]

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.

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.

Hi Steve,

Yes, I would recommend using diodes before the rectifiers, Dave Funk's book describes this as bomb proofing the rectifier tube. Depending on the source of new build 5AR4 rectifiers haven't been as durable as the NOS tubes. (And a NOS 5AR4/GZ34 ain't exactly cheap these days as a replacement option) If the tube blows one half, no one will know (or hear) the difference.

My first recommendation would be to replace the 5AR4 with a red bank/ bendix 6106 tube, it is an overbuilt 5Y3 (with a slow start up). These tubes aren't cheap, but are built like a brick outhouse and can handle four times the rated load without breaking a sweat. It has milled ceramic heater insulators, and beautiful construction. Although you will have to look for a deal on ebay, the tubes last a very long time.

If the Church will allow some work on the AC30s, and you want to prolong tube life, take Masco advice and check into Power Scaling, you can have loud sound at a any volume level. This would be the easier path, in my opinion. If you follow this option, a master volume would be add versatility as to what volume break up occurs. London Power does have a kit for cathode bias power scaling.

The VVR is simpler and cheaper knock off of PS, however I would recommend at least talking to Kevin O'Connor or check out the www.powerscaling.com community to communicate to the man who originated the concept and perfected and continues to improve the concept.

Changing the resistors to the screens to a higher value (from 100 ohm to 2,200 ohms) would help preserve the power tubes and provide a loud sound at a lower volume, too, however, you would have to compensate with a cathode capacitor in the preamp value change.

All the above is only relevent if the Church powers that be supports the worship groups' idea to have the problem solved.

Other than neutering the drummer, finding a bargain on some form of volume reduction technology, or replacing the current speakers with less efficient ones (which would change the tone) I would say there isn't a single solution (other than the powerscaling option), it will have to be a combination of different solutions

Best Regards,

Steve

PS I sincerely hope these are not Vox AC30 C(hinese) C(laptrap) (and I am married to a Chinese lady).

I wouldn't work on them for Love or Money, short of rebuilding them as old school style point to point 1960's style Vox's with a few improvements. I would say the same for any poorly executed PCB built to a price break amplifier, yes and that includes Fender.

Most of the stuff from the Big name manufacturers was designed to be cheap, cutting corners where ever (everywhere) on everything from component parts (e-caps, resistors, speakers, etc) to unfortunately PC boards.

It is very easy to lift a copper trace, unless you have the correct desoldering equipment and a good deal of experience.

A PCB if well designed, with thick copper traces and proper separation of power supplies and audio signals can be sonically as good as point to point. However, other than some early Ampeg's (before their founder sold out), so Selmer's from 1960, and more recently some boutique builders who use Military or Aerospace (NASA) standards, the rest of the MI makers aren't so inclined.

PPS I once sourced a 15 rank Pipe organ for less money ($1000 including the move and setup within 5 minutes down the road from the church).

The pastor, however purchased at many more times the expense a new and inadequate for the building or the congregation size electric organ, from a music store. The store was owned by a church member, who just happened to make regular donations every Sunday.

The Lord works in strange and mysterious ways his wonders to perform.

[renshen1957]

cant really see a mfg making that little boo boo, what style stand by is it?

I guess you haven't been inside of a B-52 100W head then. They shove 110uF right after a 5AR4 (max cap is 60 uF). I've repaired a few of these simply by replacing the first electrolytics with fresh ones, and the ones in there are usually only a few years old. While the rectifiers /might/ be able to take this, the el-cheapo caps they use can't seem to handle the ripple current and almost always fail. Before the cap change the amps are hummy and oscillate badly, after the cap change they work fine (using better quality electrolytics).

Hi Cliff,

Have you tried paralleling the electrolytics so the Rectifier "sees" only lower value? I found this to work quite well.

Best Regards,

Steve

[Masco]

Hi Cliff,

Have you tried paralleling the electrolytics so the Rectifier "sees" only lower value? I found this to work quite well.

Best Regards,

Steve

[Masco]

Sorry, double post.

[The New Steve H]

This is all good stuff.

They didn't say the rectifiers were blowing every time the amps were turned on; I think the phrase used was "all the time," which probably means a couple of times a year. Whatever the actual figure is, it's too much, when you can fix it for $3.

[The New Steve H]

I should add that the drummer is already in a plastic cage, although I don't know how that would help if the guitar amps are too loud.

[Masco]

I should add that the drummer is already in a plastic cage, although I don't know how that would help if the guitar amps are too loud.

"plastic cage - so easy to break" -Hendrix "Stone Free"

[xtian]

A PCB if well designed, with thick copper traces and proper separation of power supplies and audio signals can be sonically as good as point to point. However, other than some early Ampeg's (before their founder sold out), so Selmer's from 1960, and more recently some boutique builders who use Military or Aerospace (NASA) standards, the rest of the MI makers aren't so inclined.

Amps from Black Heart Engineering are PCB, and actually very nice. Heavy boards, thick traces, nice layouts. Very easy to mod, and inexpensive.

[ampdoc1]

I think your problem at church is one we've all had on stage. You get the guitar to the volume you can hear yourself, but the guy in the first row has bleeding ears!
You might try one of those "hot-spots" or whatever they are called. It's the pad that attaches to your grille cloth to block some of the volume directly in front of your amp.

I manufactured the Speaker Tweaker years ago that did about the same thing, but it muted the signal a little bit more I think, because of the tight vane spacing. No magic bullet, but you could hear an amp much better on stage at lower volumes without changing the tone. While I've never been very good at guitar, I found it easier to play with them because the sound level and tone were very even from almost any location in front of the amp.

I'm not sure but I might have a couple laying around you could try on one of your amps if you're interested.

ampdoc1