The comon problem people get with Vox AC30's! I have it sussed!

kranog · 11-08-2008 03:58PM #1

Howdy folks...
I've read on a few occasions that people are hating the reissue AC30's as they are very volatile and tend to just stop working for what seems to be no reason at all!

Well...this happened to me recently....I brought it to Martin Nolan who took a look and fixed it permanently for 60 blips....here's the deal!

Vintage Vox AC30's came loaded with some kind of non conductive Rectifier...these caused nothing but problems from the very start...they were trouble in the 50's and they are still trouble....the reissue's used these in the spirit of keeping them true to the vintage model.
Martin replaced the broken rectifier with a brand new Self Conducting rectifier which has sorted the problem and does not affect the tone in any way and is very unlikely to cause this same problem again.

The man's a gent!

fish-head · 11-08-2008 04:03PM

Who's Martin Nolan?

Voodu Child · 11-08-2008 04:11PM

Good stuff, good to hear you got your amp sorted.

Amp manufacturers are funny like that, they'll often 'faithfully' reissue something that wasn't a great design at the time. I read an interview with Roger Mayer where he was saying similar things about Hendrix's amps and the Marshall re-issues...

I suppose if they do try and change things for the better, they'd have the vintage die-hards complaining that they weren't 'as good' as the originals. Kind of 'damned if you do, damned if you dont' type situation:pac:

Paolo_M · 11-08-2008 04:46PM

My understanding on the AC30s was that in an attempt to "improve" the original design Vox decided to put in a stand-by and there-in lies all the current problems. The GZ34 rectifier naturally provides a "slow start" on the B+ line as it's own heater has to heat up first. This delay allows the power and preamp valve cathodes to heat up properly before full B+ appears across them. The older (60s) Ac30 amps didn't have a stand-by as one is not required in this design. In the new amps after the power is on for the usual 1-5mins, flicking the stand-by causes excessive stress on the rectifier as the filter caps try to charge all of a sudden causing a sudden current spike leading to all the failures that have been reported. A simple work around for this problem for any AC30 reissue owners, strange as it seems, is not to use the stand-by, again it is not neaded with a GZ34 rectifier valve. Simply leave the stand-by perminantly in the on position and use the power switch as normal. The older AC30s experienced now legendary repliability problems alright but for a whole different set of reasons.:pac:

ball ox · 11-08-2008 05:43PM

Paolo_M wrote: »

My understanding on the AC30s was that in an attempt to "improve" the original design Vox decided to put in a stand-by and there-in lies all the current problems. The GZ34 rectifier naturally provides a "slow start" on the B+ line as it's own heater has to heat up first. This delay allows the power and preamp valve cathodes to heat up properly before full B+ appears across them. The older (60s) Ac30 amps didn't have a stand-by as one is not required in this design. In the new amps after the power is on for the usual 1-5mins, flicking the stand-by causes excessive stress on the rectifier as the filter caps try to charge all of a sudden causing a sudden current spike leading to all the failures that have been reported. A simple work around for this problem for any AC30 reissue owners, strange as it seems, is not to use the stand-by, again it is not neaded with a GZ34 rectifier valve. Simply leave the stand-by perminantly in the on position and use the power switch as normal. The older AC30s experienced now legendary repliability problems alright but for a whole different set of reasons.:pac:

Can you back this up at all? I'm not doubting you or anything, just wanna be 100% certain that its a good idea to stop using my stand by switch. Is it the same when turning it off?

Paolo_M · 11-08-2008 07:31PM

ball ox wrote: »

Can you back this up at all? I'm not doubting you or anything, just wanna be 100% certain that its a good idea to stop using my stand by switch. Is it the same when turning it off?

The GZ34 rectifier valve (see attached datasheet) has an indirectly heated cathode. It slowley begins to conduct current as it warms up. This means that the pre/power valves do not get full power until the GZ34 valve has warmed up. The GZ34 takes the same amount of time to warm up as any pre/power valves and therefore provides a (better than IMO) stand-by type function without the need for a stand-by switch. This is universally true of any amp using the GZ34 rectifier or any other indirectly heated rectifier valve.
The stand-by switch is therefore redundant in a GZ34 rectified amp.

http://vintageamps.com/plexiboard/viewtopic.php?t=75667&highlight=stand
This article highlighted for me the problems with the new AC30CC which uses the GZ34. It propmted me to go off and do more reading, look at the schematics and hit the books and this guy is definitley correct in what he is saying. Lyle is extremely well respected and even has the distinction of receiving a law suit package from Vox, pretty impressive!!:pac:
I think it's this article where one tech even does the "simply remove the switch from the circuit" mod and let the user go ahead using the switch as normal. The theory being that the use of a standby is so engrained ini valve amp users that people can't quite comprehend not using it. I think this is a bit cynical and don't agree but it does put across the point.
Stand-bys have only really become essential since the use of semi conductor rectifiers as they spit out full power instantaniously which is generally considered bad news for valves.
I've also read other articles that say that the standbys in diode rectified valve amps are also redundant as "cathode stripping" (what a standby is preventing) only really applies with plate voltages over 1000Vdc. At anything less than that the valve with have long died before it becomes an issue. Typical plates voltages in valve amps are between 300-580Vdc. I'm inclined to continue using it in these amps though aa why would you give it another possible reason to fail?

I could post the schematic for the Vox AC30CC if you're interested and I can point what I'm on about, but I'm not sure if the mods will allow it? Feyla?

Paolo_M · 11-08-2008 07:35PM

Paolo_M wrote: »

The GZ34 rectifier valve (see attached datasheet) has an indirectly heated cathode. It slowley begins to conduct current as it warms up. This means that the pre/power valves do not get full power until the GZ34 valve has warmed up. The GZ34 takes the same amount of time to warm up as any pre/power valves and therefore provides a (better than IMO) stand-by type function without the need for a stand-by switch. This is universally true of any amp using the GZ34 rectifier or any other indirectly heated rectifier valve.
The stand-by switch is therefore redundant in a GZ34 rectified amp.

http://vintageamps.com/plexiboard/viewtopic.php?t=75667&highlight=stand
This article highlighted for me the problems with the new AC30CC which uses the GZ34. It propmted me to go off and do more reading, look at the schematics and hit the books and this guy is definitley correct in what he is saying. Lyle is extremely well respected and even has the distinction of receiving a law suit package from Vox, pretty impressive!!:pac:
I think it's this article where one tech even does the "simply remove the switch from the circuit" mod and let the user go ahead using the switch as normal. The theory being that the use of a standby is so engrained ini valve amp users that people can't quite comprehend not using it. I think this is a bit cynical and don't agree but it does put across the point.
Stand-bys have only really become essential since the use of semi conductor rectifiers as they spit out full power instantaniously which is generally considered bad news for valves.
I've also read other articles that say that the standbys in diode rectified valve amps are also redundant as "cathode stripping" (what a standby is preventing) only really applies with plate voltages over 1000Vdc. At anything less than that the valve with have long died before it becomes an issue. Typical plates voltages in valve amps are between 300-580Vdc. I'm inclined to continue using it in these amps though aa why would you give it another possible reason to fail?

I could post the schematic for the Vox AC30CC if you're interested and I can point what I'm on about, but I'm not sure if the mods will allow it? Feyla?

Damn, after all that I forgot the data sheet. :mad:
http://www.drtube.com/tubedata.htm#Rectifiers

feylya · 11-08-2008 07:58PM

Don't see any problem with posting the schematic. Should be readily available with a google search.

BTW, there's an edit function on your posts for adding in anything you forgot

Eoin Madsen · 12-08-2008 10:04AM

ball ox wrote: »

Can you back this up at all? I'm not doubting you or anything, just wanna be 100% certain that its a good idea to stop using my stand by switch. Is it the same when turning it off?

I would second it anyway, I've read similar things in various places. Old valve designs predating diodes tend not to have standby switches at all.

Rectifiers do affect amp tone, as should any component in the power supply. 5AR4 (aka GZ34) is the traditional rectifier valve for an AC30, so it's not surprising that they're still used. (The Rose Morris AC30 of the 80s used a solidstate rectifier. Everybody hated them.) Tbh, all you should really need to do is replace the cheap stock 5AR4 with a better-made 5AR4. It's a plug and play operation that shouldn't cost more than the price of a rectifier valve.

Paolo_M wrote:

I've also read other articles that say that the standbys in diode rectified valve amps are also redundant as "cathode stripping" (what a standby is preventing) only really applies with plate voltages over 1000Vdc. At anything less than that the valve with have long died before it becomes an issue. Typical plates voltages in valve amps are between 300-580Vdc.

Hmm, I would be inclined not to entirely trust said articles, given that even an original Mullard EL34 could only take at most 700Vdc on the plate before melting down. I've never heard of a plate voltage any higher in an amp, except when there's something terribly wrong. :pac:

Paolo_M · 12-08-2008 11:58AM

Eoin Madsen wrote: »

Hmm, I would be inclined not to entirely trust said articles, given that even an original Mullard EL34 could only take at most 700Vdc on the plate before melting down. I've never heard of a plate voltage any higher in an amp, except when there's something terribly wrong. :pac:

I think the point the guy was making in his article was that in his opinion cathode stripping is only a problem in larger valves used for transmitters that use over the 1,000Vdc and that typical valves used in guitar amps (such as the EL34, 6L6, EL84 etc.etc.) will have died from general usage long before cathode stripping would have caused a problem in them. I'll be sticking to the stand-by in this case though. :pac:
On the quality of GZ34; the guy who wrote the aticle on Vintage Amps (and from what I've read myself he's right) stated that it's the sudden current draw from charging filter caps that kill the rectifier. The stand-by is located after the valve (on the plate) but before the filter caps so that while you've the stand-by switch open the GZ34 cathode is heating up but the filter caps have zero charge as the connection is open. When you then turn the stand-by on the circuit is suddenly closed and the filter caps try to draw current suddenly (as they do!!) producing a large current spike which settles back when they charge fully. This would kill even a military grade GZ34 valve eventually. You can see this is the power amp schematic attached. I suppose you could re-wire the stand-by to come after the fliter caps...

Eoin Madsen · 12-08-2008 01:45PM

Paolo_M wrote: »

I think the point the guy was making in his article was that in his opinion cathode stripping is only a problem in larger valves used for transmitters that use over the 1,000Vdc and that typical valves used in guitar amps (such as the EL34, 6L6, EL84 etc.etc.) will have died from general usage long before cathode stripping would have caused a problem in them. I'll be sticking to the stand-by in this case though. :pac:

Interesting. Got a link to the article? It would still strike me as odd that a redundant feature would have been added to pretty much every amp by every company if it wasn't deemed necessary by the engineers. Cost cutting generally wins out on these things. :pac: Think I'll stick with the standby for now as well.

Paolo_M wrote: »

On the quality of GZ34; the guy who wrote the aticle on Vintage Amps (and from what I've read myself he's right) stated that it's the sudden current draw from charging filter caps that kill the rectifier. The stand-by is located after the valve (on the plate) but before the filter caps so that while you've the stand-by switch open the GZ34 cathode is heating up but the filter caps have zero charge as the connection is open. When you then turn the stand-by on the circuit is suddenly closed and the filter caps try to draw current suddenly (as they do!!) producing a large current spike which settles back when they charge fully. This would kill even a military grade GZ34 valve eventually. You can see this is the power amp schematic attached. I suppose you could re-wire the stand-by to come after the fliter caps...

I dunno, a lot of AC30s have run fine with 5AR4s and standby switches for a long time. If it's a more prevalent problem with the new issues of the amp it would suggest that the quality of the components or some new variation in the circuit has exacerbated the issue. Using a valve will kill it eventually.

The standby in that schematic is between the AC high voltage on the transformer secondary and the rectifer valve plate - the heater for the valve is fed through the cathode so that the valve can be heated even when the plate voltage is killed while the amp is in standby. I haven't seen an original JMI or the new CC first hand, but this is the same as a (90s) Korg AC30.

I have seen one or two amp schematics in general with the standby placed on the B+ but I've never actually seen it in a real circuit (it's in the original Marshall 100w SL schematic, but I haven't seen one that was really wired that way). Switching a high wattage DC is much less safe because it can cause arcing - you need a much higher rated switch to do it (safely). Plus, the caps wouldn't discharge if you left the amp in standby when you turned it off.

I'm kinda curious to know what this "self-conductive" / "non-conductive rectifier" jazz is all about. I've never heard those terms used before, and on face value they appear to make absolutely no sense whatsover. But I'm no expert, obviously.

Paolo_M · 12-08-2008 02:31PM

Eoin Madsen wrote: »

Interesting. Got a link to the article? It would still strike me as odd that a redundant feature would have been added to pretty much every amp by every company if it wasn't deemed necessary by the engineers. Cost cutting generally wins out on these things. :pac: Think I'll stick with the standby for now as well.

I dunno, a lot of AC30s have run fine with 5AR4s and standby switches for a long time. If it's a more prevalent problem with the new issues of the amp it would suggest that the quality of the components or some new variation in the circuit has exacerbated the issue. Using a valve will kill it eventually.

The standby in that schematic is between the AC high voltage on the transformer secondary and the rectifer valve plate - the heater for the valve is fed through the cathode so that the valve can be heated even when the plate voltage is killed while the amp is in standby. I haven't seen an original JMI or the new CC first hand, but this is the same as a (90s) Korg AC30.

I have seen one or two amp schematics in general with the standby placed on the B+ but I've never actually seen it in a real circuit (it's in the original Marshall 100w SL schematic, but I haven't seen one that was really wired that way). Switching a high wattage DC is much less safe because it can cause arcing - you need a much higher rated switch to do it (safely). Plus, the caps wouldn't discharge if you left the amp in standby when you turned it off.

I'm kinda curious to know what this "self-conductive" / "non-conductive rectifier" jazz is all about. I've never heard those terms used before, and on face value they appear to make absolutely no sense whatsover. But I'm no expert, obviously.

The key difference between the new ones and the old ones (see attached from the JMI era) is that the JMI era didn't have the stand-by, this is why there was much fewer problems with the retifier circuit on these amps. The schemtic I showed was for '93 which is the Korg era, well spotted

and is the type (rectifier circuit anyway) currently still in production AFAIK.
I'm working on a '65 AC50 ATM which has a solid state rectifier and it has no stand-by either, it uses a brimistor which simulates the slow start of the indirectly heated type rectifier valve. You can't get them (brimistors) any more but luckily the one in this amp one still works.:D.

I'm too am interested to hear more about a "non-condutive" and "self-conductive" rectifier...

Edit: Hang on does he mean the original were "non semi conductor" ie. (valve rectified) and that his mod was to make it semi conductor ie. diode bridge? If so..........well I'm saying nothing. Eoin?

Eoin Madsen · 12-08-2008 03:11PM

I didn't realise that none of the JMIs ever had a standby. Makes sense I guess!

You can get a solidstate rectifier drop-in replacement (with emulated sag and voltage drop) from Groove Tubes and also Weber (the Copper Cap). They fit right into the valve socket. I don't think they do the slow start though.

I know the Rose Morris AC30s back in the 80s used a solidstate rectifier. They are kinda renowned as being the crappest sounding AC30s ever made though.

Doogle69 · 16-10-2009 01:29PM

I've been looking at this thread with some interest as the standby light on my ac30 doesnt work. Just before it stopped working the amp started making loud rattling noises from inside it. I'm not going to start messing about with it, so I was wondering does anyone know of someone who can service amps. I dont mean the average teenager you find in music shops who has a million and one differents opinions on what to do (sorry for being ageist!) but someone with proven experience and a good reputation?

eoin5 · 16-10-2009 02:50PM

Whats wrong with wiring the standby to work as an input killswitch? No sudden spikes or whatever.

tradman · 28-11-2009 03:00PM

My ac30 recently went on the blink. The main fuse kept blowing on me. The red power light would go on but once i flipped the standby to on it would blow. Brought it to Martin Nolan in Beaumount who sorted it.

Its a common problem in the ac30's. It was a gammy rectifier so he replaced it and while he was at it he replaced the ****ty chinese EL84's for some JJ's. 160 notes in total. (60 for the rectifier, 100 for the four EL84's)

Nice guy - been doing it his whole life 086 8931034

Eoin Madsen · 28-11-2009 06:17PM

Ah, I remember this one. Myself and Paulo were on fire. :pac:

eoin5, what sudden spikes are you worried about exactly? The problem with wiring the standby as an input killswitch is that when you have the kill turned off, you've got an open conductor to your input stretching right across the inside of the amp.

The input jacks will automatically ground the signal lug when the jack is unplugged anyway.

Eoin Madsen · 28-11-2009 06:22PM

Paying 60 euros for a new 5AR4 is madness, btw. Ditto for the JJ EL84s. You can get a quad for about $40, and they don't cost much to ship.

eoin5 · 28-11-2009 07:45PM

Eoin Madsen wrote: »

Ah, I remember this one. Myself and Paulo were on fire. :pac:

eoin5, what sudden spikes are you worried about exactly? The problem with wiring the standby as an input killswitch is that when you have the kill turned off, you've got an open conductor to your input stretching right across the inside of the amp.

The input jacks will automatically ground the signal lug when the jack is unplugged anyway.

I dont pretend to understand about all of this but just going on paulos post:

...flicking the stand-by causes excessive stress on the rectifier as the filter caps try to charge all of a sudden causing a sudden current spike leading to all the failures that have been reported.

I thought instead you could just wire it as a killswitch so you'd avoid this problem and you wouldn't need to adjust anything when you're leaving down the guitar. I see what you mean about it being a big unnecessary conductor in the amp though.

Eoin Madsen · 29-11-2009 02:20AM

Ah, I thought you might be worrying about something else. Yeah, were it not for the standby being on the other side of the amp, a killswitch would work well as an alternative use for it. I've found layout spoils a lot of otherwise good ideas. :pac:

You could rewire it to trigger a killswitch relay over by the inputs. The problem, as always, is powering the relay.

eoin5 · 30-11-2009 02:38AM

Eoin Madsen wrote: »

Ah, I thought you might be worrying about something else. Yeah, were it not for the standby being on the other side of the amp, a killswitch would work well as an alternative use for it. I've found layout spoils a lot of otherwise good ideas. :pac:

You could rewire it to trigger a killswitch relay over by the inputs. The problem, as always, is powering the relay.

If you were crazy enough you could make a manual isolated switch for it. Theyrd be some good head-scratching from the next service guy

The comon problem people get with Vox AC30's! I have it sussed!

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