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09-29-2008, 05:12 PM
| #1 |
| Junior Member Join Date: Sep 2008
Posts: 11
| 1970s-1980s Vox AC30 problems
hiya! my first post, so be gentle  (although I have read your forum for a long time).I got my hands on an old vox ac30 some time ago, and i have some major problems with it. first off, something about the amp. i don't know which model it is. it's missing the back panel. it's a double speaker version with 2 12' celestion g12m speakers (with gold labels, made in england). tubes are: 4x el84 power tubes, 6 tubes in preamplification and vibrato/tremolo speed control (5x 12ax7 preamp and 1x 12au7 trem). has a diode rectifier. the board is pcb. i can't find any picture of an ac30 on the net that has the same layout of the controls. i also can't find any schematics that match it completely. this is the closest: http://www.freeinfosociety.com/elect...oxac301986.pdf mine hasn't got a standby switch or a footswitch (or a footswitch jack). the previous owner repaired it himself (very poorly). he's a friend of mine, so he told me what he fixed and what was wrong when he got it: *the tone control was completely disconnected, he fixed it *brilliance channel inputs were disconected *the tremolo/vibrato switch was broken *the mains switch was broken *some of the caps were broken (split in two!), some even missing (it's possible that he replaced them with wrong ones, because he followed the schematic, and i'm not sure if the schematic is fitting) *lots of old caps were changed. he changed those two-in-one 450v caps with 2 normal 450v caps *changed some of the tubes *rewound the speakers (he had a professional do it) now, the problems: 1)it blows power tubes since i got it, it blew two power tubes, both in the same socket. the tubes were destroyed by heat i think, both broke above the same pin, which is connected to output transformer. it even melted the plastic tube socket, so i changed it for a ceramic one. it didn't stop it from blowing the tube again. the tube in that socket shines much brighter than the others, too. when i remove that tube and the one in it's pair (put it on half power), the one from the other pair, that's close to the glowing one starts glowing really bright, almost blue, too. 2)the brilliance channel is screwed first off, it really doesn't like pedals in the lower input. the second thing is that, after a few minutes, it just stops playing. it comes back on for a few seconds when i move the brilliance pot or take out the cable from the input and put it back in, and then it stops playing again. in the upper output, it plays for a bit longer and then it stops. it also gives the speakers some awful low-end speaker fluttering distortion, when really turned up. it seems to have way too much gain, too 3)the vibrato/tremolo channel it has almost no or no effect. when the guitar is connected to vibrato/tremolo channel, the sound is okay, low gain, but it has no vibrato or tremolo, no matter what speed settings i choose, both for vibrato and tremolo. 4)cosmetics it's missing the vox sign, some of the knob covers are from a bassman, it's missing the handles. anyone know where to find these cheap in europe? 5)the back plate it's missing. anyone have a picture, so i can make one? possible causes: 1) 220/110 mains switch is bypassed, and the power transformer is hard wired to the fuse holder, so my friend may have soldered the wrong wire 2) he may have connected the speakers to the wrong impedance output of the output transformer 3) he didn't have the right schematics, so the condenser values of the replaced missing caps may be wrong 4) the vibrato/tremolo switch is replaced poorly pictures:       |
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09-29-2008, 05:16 PM
| #5 |
| Junior Member Join Date: Sep 2008
Posts: 11
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09-30-2008, 04:48 PM
| #6 |
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Posts: 11
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fixed the power amp problems. it works almost perfectly. the vibrato/trem problem persists, though. anybody knows how to fix it?
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09-30-2008, 05:12 PM
| #7 |
| Senior Member Join Date: May 2006
Posts: 610
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Step 1 - you need to confirm the correct mains TX tap. Try measuring the resistance between the common end and the various taps, that will tell you the sequence. How many taps are there, what is your line supply? Same with the output tx taps (best to disconnect the speakers), assuming your meter is good down to fractions of an ohm. Not a big deal for the moment. Vibrato - the footswitch cable was probably hardwired in place, you'll have to work out from the schematic where the wire was soldered in to the pcb. It won't work without that point being grounded. That schematic in the link isn't your exact amp. Yours has a choke, the schematic has 2 x 390ohm resistors feeding the screen grid node. If TAD doesn't have the handles then AES in USA is the best/only source I could find when I was looking awhile ago. North Coast Music in USA might have them but they're more expensive. Any of those suppliers also for knobs, logo etc. When you've got the right mains tap, power it up with no tubes and check voltages, especially on el84 sockets. Peter. |
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09-30-2008, 06:57 PM
| #8 |
| Junior Member Join Date: Sep 2008
Posts: 11
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the mains transformer tap is okay, it is wired for 220v, exactly as it should. the output transformer, on the other hand, wasn't. the speakers (16ohm total) were connected to the 8ohm output. i fixed it, and it helped the tubes a lot. it's working now, but one of the power tubes isn't. i'll check that footswitch thing
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10-01-2008, 06:53 AM
| #9 |
| Supporting Member Join Date: Jun 2008 Location: Italy
Posts: 993
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Hi Andrej, If the +HT voltage in your amp matches the ones on the schematic you provided, you' ll suffer from reliability problems in the output stage for many years to come....... On older AC30s, as you might already know, there was a tube rectifier ( GZ34/5AR4 ) and the +HT(+B) voltage was set at 320VDC. When Vox phased out the good ol' AlNiCo speakers, ( Cobalt price was skyrocketing in the late 60s, due to its strategic importance in the nuclear market, so Celestion offered a ceramic replacement at half the price ) the AC30s performance suffered big-time, as the new speakers were less efficient and needed more power to give a comparable volume, so Vox engineers ( Dick Denney had left the company between 67 and 68 and he's not the one to blame ) found a gimmick to raise output power ( and save even more money ) by replacing the rectifier tube with solid state rectifiers. This way, due to the SS rectifier' s lower differential resistance and higher efficiency, the +B voltage raised from 320VDC to 345-350 VDC. The output power increased from 36 to some 45 Watts, but at the cost of a greatly reduced reliability. It must be remembered that the output tubes' operating conditions on older Voxes were already out of the boundaries, as the quiescent plate power was set at 15 W ( 47 mAmps * 320VDC ) by means of a 51 Ohm cathode resistor ( EL84s/6BQ5 are rated at 12 W!!! ) setting the cathode about 10 VDC above GND. In models like yours, with a +B (or +HT, as you like ) of 345 VDC and a 47 Ohms cathode resistor your output tubes are simply bound to self-destruct themselves within a supposedly short period of time. A ( little ) improvement can be obtained using special quality tubes, like the old Philips E84Ls, with a max plate dissipation around 13,5 W, and built with exceptional care, so they can probably better tolerate the extra stress ( meaning that they will fail later ). My advice would be : 1 - Lower the +B voltage back to 320 VDC and change the cathode resistor back to 51 Ohm ( if you don' t find it simply parallel two 100 Ohm 3 W resistors ). This way your tubes' life will be easier, and modern tubes ( which are far from the quality tubes had in the old times ) will last longer. 2- If you really dig the way AC30s sound, find a couple of "blue" or "silver" AlNiCo speakers, the reissues are close to the originals and not too much expensive, or, if you can afford it, get a couple of originals in good conditions - it' s really worth it - and listen to the difference. Hope this helps Best regards Bob Last edited by Robert M. Martinelli; 10-01-2008 at 07:07 AM. |
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10-02-2008, 08:52 AM
| #10 |
| Junior Member Join Date: Sep 2008
Posts: 11
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you helped a lot! i'm changing the cathode resistor (i doubt i'll find 51 Ohm resistor, but i'll parallel 2 100Ohm resistors, and then maybe series them to a 1Ohm resistor) today/tomorrow, but how to change the +B? Dump some over a resistor to ground? diodes? some kind of voltage divider? sorry, haven't got much experience with amplifiers, transformers, high currents and stuff. i'm mostly a stompbox man.
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10-02-2008, 09:45 AM
| #11 |
| Supporting Member Join Date: Jun 2008 Location: Italy
Posts: 993
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Hi, well, a voltage divider is not needed. In the schematic you provided there' s a dropper resistor in series with the +HT already ( 22 Ohm 10 W ), you want the voltage to drop about 25 V more ( 7,25% of 345VDC to get back to 320 ) so you' ll have to change this 22 Ohm 10 W resistor to a some 23,5 Ohm resistor - this isn't a standard value, but you could either substitute the 22 Ohms 10W resistor with two 47 Ohm ( the increased voltage drop calls for 6-7 W each, 10 W each will make 'em run cooler ) resistors in parallel or add a 1.5 Ohm 10 W resistor in series between the existing resistor and the line going to the st-by switch. This second option is more flexible, because if the voltage drop is not what you need you'll simply need to move up or down to the next standard value ( which is 1.2 Ohms for less drop and 1.8 Ohms for more drop; you can also try 2.2 Ohms which will be dropping about 35VDC bringing your +HT around 310 VDC, so the tubes will probably whisper a heart-felt thanks  ) In any case, you' ll simply need to use Ohm's law to suit the voltage drop to your needs ( R=Desired voltage drop / Current flowing in Amps ) and to properly size the resistors' power rating ( Pd=Voltage drop across the resistor you're sizing * current in Amps flowing ). Hope this helps, should you need further advice let us know... Best regards Bob EDIT : missing "0" in the arithmetic, please multiply the advised resistors values by a factor of ten, Pd = 10 W. My apologies Last edited by Robert M. Martinelli; 10-03-2008 at 06:20 AM. |
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10-02-2008, 12:55 PM
| #12 |
| Senior Member Join Date: May 2006
Posts: 610
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Bob no disrespect but I think these's some bad arithmatic or assumptions with your calculations ie to drop that additional 35V across a 2.2ohm resistor would need a current of 16 amps! It's a awkward thing to calculate accurately because at the rectifier end of the sag resistor is pulsating dc, at the cap end is smooth dc. andrej, I would say if your B+ is high (? do we know that yet) then double the value of that 22ohm resistor ie put another in series, and go from there if fine tuning needed. On my ac30 I've put an individual 270ohm 3 watt resistor for each el84 cathode, so they're not linked. They're individually bypassed at the moment but I'm planned on trialling various unbypassed mods. Whatever, it sounds the same as commoned and I feel should be more reliable as each el84 can find it's own operating point. Peter. |
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10-02-2008, 01:37 PM
| #13 | |
| Supporting Member Join Date: Jun 2008 Location: Italy
Posts: 993
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I beg your pardon, where did I have my head..... this happens when you try to run multiple programs in the same brain at the same time.....I apologize with andrej too, as I wanted to help and not to mislead, in fact I think ( no longer sure of anything ) the advices I gave were good ones and I believe ( at least ) the formulas to be accurate...Hope this does not mean I need brain surgery EDIT : Original post edited - my apologies Last edited by Robert M. Martinelli; 10-03-2008 at 06:21 AM. | |
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10-03-2008, 01:13 AM
| #14 |
| Senior Member  Join Date: Sep 2006 Location: Newcastle (UK)
Posts: 360
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Just to respond the the OP's initial post that he can't find a schematic that matches this amp - the pictures indicate that the amp is indeed a grey-panel top boost AC30. These are supposedly quite rare with the top boost integrated on the control panel and not added on the back like earlier versions. The vib/trem metal panel switch isn't original as you may have guessed. There are (or were) schematics of the various AC30s on the Vox website. Bear in mind that the old AC30s were made in Bass, Normal and Treble versions, not the standardised model we now have. There are a handful of circuit variations between the B,N nd T versions such as the size of the coupling caps to let more/less bass through or to cut bass depending on the model. For instance, the bass version used 0.1uF coupling caps from the PI to the EL84s. The Normal version used 0.047uF and the Treble version used 0.01uF caps. My old grey panel was a treble version and had the 0.01uF caps. Additionally, the Bass version used a 1000pF cap from the first gain stage plate whereas the Normal and Treble versions used a 470pF cap. Hope that helps a little - I've got a copper panel AC30 of my own that need renovated. Been here for 5+ years now; I'll get round to it one day, ha ha
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10-03-2008, 06:56 AM
| #15 |
| Supporting Member Join Date: Jun 2008 Location: Italy
Posts: 993
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Hi again, from the pictures I would say the amp is a later version, probably from the early-to mid '70s - the circuit boards are not PTP, they' re made of Fiberglass instead of Bakelite, the styling lines on the panel are not embossed and the panel doesn't sport the "rotating voltage selector" anymore, the amp has a SS rectifier and the general styling and construction method is that of the '70s. "Gray panels" JMI top boosts were built from circa the late 1964 to the late 1967; at that point Tom Jennings left the company in discomfort, so from 1968 onwards the amps were labeled with "a Vox product" instead of "JMI" and, later, after another change of ownership, "Vox sound LTD". Hope this helps Best regards Bob P.S. Hi HTH! I think your Vox collection is FANTASTIC! I already knew you owned a "copper panel", now I learned you also own a "gray panel" - WOW! Congratulations! |
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10-03-2008, 01:54 PM
| #16 | |
| Senior Member Join Date: Sep 2006 Location: Newcastle (UK)
Posts: 360
| Quote:
* red-panel JMI ('63/'64) non top boost head (normal version), matching 2x12 cab loaded with original alnico blues. This still had all the original tubes too (Brimar/Mullard) and had never been touched inside. It's been sold for a good few years now though. That amp could nail the Whats The Frequency Kenneth tone and I think it was the closed back cab that did it, gave a tighter, more aggressive tone. * grey-panel JMI ('64+), non top boost combo (treble version). this was a fixer-upper. had the wrong transformers installed (Drake ones that actually were perfect for a 50w Marshall), no choke and the speakers were total rot too - oh, and it had been recovered, badly. When I started looking inside, it struck me that it was a minor miracle that the amp worked at all. The actual PTP board was ok, just dirty - I got to use pretty much 90% of the original components on the board. Since it was a total refurb, I decided to add an EF86 channel which absolutely screamed - I can see why those early AC30s are so sought after. The PT was a repro from Drake - got it direct from them, they said it was the last one they had as they were no longer making PTs for the AC30. The OT and Choke were OEI from Speedracer (anyone remember Joe ???) and were fantastic units. The cab was recovered by Matamp in the correct basketweave cloth and I got some diamond fretcloth from Vox (£80.00 for a piece of fretcloth was/is daylight robbery though). I found a pair of early 60s alnico blues in a second-hand bass guitar shop (a total dive actually), paid £90.00 (bargain and a half). With the whole thing assembled, recapped and fully checked over it was an absolutely cracking amp - beutifull tone that went from chimey cleans to really agro overdrive. Again, this one is long gone - was skint and it paid the rent  * last one I still have is the copper panel, non top boost, transformers leaking wax and most likely in need of replacement. Got the original alnico blues, but they need reconed. cab needs recovered, but I've got some NOS fawn to use up on it - will be a smart-looking amp once its done. This will be a keeper since I'm pairing my collection down. It will be my main amp along with the JTM45/100 Metroamp kit I've still got to build up, my homebrew AC15 head (top boost preamp with many tweaks/mods running into a pair of cathode biased 6V6s, very sweet amp using a custom Shinrock OT). Oh, and I've got the rack gear too which is perfect for home use - the Cream Machine and the Marshall 9001 preamp. Sorry for the threah hijack
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10-06-2008, 12:20 PM
| #17 |
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Re the measurements for the missing back panel, see http://www.geofex.com/Article_Folders/AC30/ac30dims.pdf I would measure mine but forgot tape measure when going to rehearsal rooms, then noticed RGKeen had already published a full set of dimensions - Peter. |
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10-15-2008, 10:43 AM
| #18 |
| Junior Member Join Date: Sep 2008
Posts: 11
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well, i haven't read this article for a while. the email notification doesn't work. i did the mods voxrules! suggested, only to find them wrong now... the amp works better, definitely, even with those. the 51k cathode resistance probably works. the tubes are much cooler, they glow much less, and tubes in pairs glow evenly. there's no crossover distortion. i think ill try using 270 Ohms on each el84 cathode, as pdf64 suggested. what do you guys think about that? i also happen to have some money, so i'm thinking of completely redesigning the pcb for output. the chassis also has two extra spaces for tube sockets, so what do you think about making a pcb for a tube rectifier version? i guess i can always go back to solid state, by using an ss rectifier in a tube socket package (like weber copper cap WZ34). i also plan on changing all the electrolytic caps, and getting some nice new AlNiCo speakers, probably Celestion. the tremolo channel is still broken, it's even worse now. It has almost no gain, and there is absolutely no effect. what could be the problem? broken cap? and, yes, i know that the vib/trem switch is not original, it was replaced by the previous owner, since the old one broke. and the speed control isn't a pot, as suggested in the schematic i gave you, but a sp4t switch with resistors. anyone know what to do? Thanks! Andrej |
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10-15-2008, 11:07 AM
| #19 |
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Posts: 610
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Andrej, what is your schematic? Have you measured your B+? If you have a sag resistor there's no point in a copper cap, just try different resistor values to get the result you want. Have you got a scope, assume you have as you think there's no crossover distortion (btw 51k means 51000, assume you meant 51R cathode resistor). If so, is the vibrato oscillator working? Peter. |
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10-16-2008, 12:26 PM
| #20 | |
| Supporting Member Join Date: Jun 2008 Location: Italy
Posts: 993
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01-23-2009, 10:50 PM
| #21 |
| Junior Member Join Date: Sep 2008
Posts: 11
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Revisiting the project, since the amp blew a tube a month ago. It destroyed the (same) socket, above the pin connected to the output transformer. The amp makes a loud crackling noise when run with the other pair of tubes only, which wasn't the case earlier. I decided to do a quality job fixing it. I'm going to replace the whole PCB which has power section resistors, diode rectifier and stuff like that with a quality, ptp made board, because the copper on the old pcb started separating from the board. Sticking with the diode rectifier. Pictures and schematics: 1) http://img230.imageshack.us/my.php?i...calo024jv6.jpg The power section PCB is the one below. That's the one I plan on replacing with a ptp board. 2) http://img207.imageshack.us/my.php?i...calo022po4.jpg The power section PCB again 3) http://img207.imageshack.us/my.php?i...calo007kp5.jpg Top panel 4) http://www.freeinfosociety.com/elect...oxac301986.pdf Schematic to the amp. Here are the fixes I'm planing to do: 1)Redo the PCB in ptp technique. I'm going to replace all of the old components on it. I know I can rely on shipping only when ordering from Banzai Musical Products (a great company from Germany, they are well supplied), and I don't know which turrets for the ptp board to order. Here is their turret list page http://www.banzaieffects.com/Terminals-c-670.html. Could you tell me which to order? I also don't know how to insert them into a board. How? What should the board be made of? What thickness? I'm getting new wire too. What gauge, material, should it be solid core, what kind of isolation? The wire is used connecting the power supply to the pcb and the tube sockets to the pcb. What wire should I use for interconnecting the turrets on the board? Should it be bare? Gauge? So, what are the final values to the resistors on this board? What wattage and composition? 2) Replace the power tube sockets. 3) Get some handles 4) Make the back pannel. What is on the back panel? Power coord connector, footswitch jack? Could somebody take a picture of the back panel from the outside and the inside. 5) Revise the speaker wiring. My speakers are just soldered to the wires. How should it be done? Any jacks? Anyone have a photo? 6) Fix the tremolo/vibrato. Gonna get the original rotary switch. But, first of all, I need to understand how it works. Could anyone explain? How does the oscillator work and how does it control the effect? The channel is really low-gain too. Almost inaudiable even when set to the highest level, so I bet it's broken. Anyone know the solution? 7) My amp doesn't have a standby switch. Which 3-pos switch do I need? How to wire it? I want to have an OFF-STDBY-ON switch, like on an Orange amp, because i don't have enough room for another switch (see the picture). Is this a big complication? 8) Maybe replace some caps. I wonder if I should replace the sp4t switch (with four resistors) of the vib/trem speed with a pot, as suggested in the schematic. 9) Replace the tubes. Which ones to get? I plan to use this amp for practices and gigs, no studio for now, and it's used a lot. 10) Replace the big dual 32nF/32nF can caps. Anyone has any other suggestions? EDIT: added the pics again Last edited by andrej; 01-23-2009 at 11:04 PM. |
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01-24-2009, 11:17 AM
| #22 |
| Senior Member Join Date: Sep 2006 Location: Newcastle (UK)
Posts: 360
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theres a PTP AC30 layout on Hoffman.com - it's not vintage correct, but it will work. do a google search, I'm sure there will be some project pages where people have made PTP AC30s with vintage-correct layouts. I dare say that Voxrules will drop in on this thread - he's your go-to guy for Vox's.
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