Thanks for the video! I just got in an AC30C2 from a customer, the complaint is it crackles and occasionally cuts out. I just ran it through its paces and it does have some noise issues - especially in the input jacks and volume pots. This will be the first Vox I have worked on, so your video has already been a huge help.
Fixed bias is called that because once the bias is set it does not change as a signal goes through the tube - the bias stays constant. Where as a cathode biased amp, the bias slightly fluctuates based on the signal going through the tube. Their names are derived for how they operate, not how the bias is set. It is confusing though. I really enjoy your videos. Thanks for doing these.
Yes that's true but the 'take home' fo 99% of people interested in this is that fixed bias is adjustable (weird, I know, but it means it 'fixed' once you've adjusted it!) and cathode bias is 'automatic' - i.e. you don't have to adjust it as it dynamically alters its bias according to conditions. You CAN adjust cathode bias if you really want to, by changing the cathode resistor, but this is not normally done. So you don't need to get a cathode bias amp biased, you do need to get a foxed bias amp biased. If I were starting with a clean sheet I'd call one 'Adjustable bias' and the other 'automatic bias' - but I'm not! I know you know all this, this is for other readers.
carefull, this is not a vox ac30C2, but the earlier model AC30CC2 (Custom Classic). well known for its destructive way of wiring the standby switch for the rectifier tube. Don't use the standby switch on this model! or even better, when open, modify it to jumper the standby switch or relocate it after the reservoir cap. just do a google search and read up on this. although i agree that the design of this amp 'could be better' ;), after modifications is can sound like a very good ac30.
You're quite right Dieter, a few months ago I had problems with my ac30cc2 & while the tech had it I mentioned the rectifier problem, as it happened he knew about it & fixed the amp so I can now use the standby. Other than that I love it!
Dieter is absolutely correct. Don’t ever use standby on a stock AC30CC2. Also check out the Lyle Caldwell mods, especially the effects loop fix if you ever plan to use the effects loop.
@@stuartukguitarampguy5830 I have had several recto valves in these amps with internal shorts, not nice (AC reaching filter caps) I add series 1n4007s in the rectifier anode leads as a matter of course. I also place a high value resistor across the standby switch to reduce inrush current when operated.
The rectifier valves on the AC30 CC2 is a major major weakness. Never ever use the standby switch. I've blown numerous rectifiers on mine and every time it happens when the amp is switched from standby to on. It also blows the main fuse at the same time. I have now (just today actually) bypassed the standby switch on mine. Other than that it's been solid since 2008.
I got mine in 2008 and it's never been a problem. Fortunately the power valves radiate their heat directly up and out of the vents on the top of the amp. The PCB is totally unmarked on mine and looks like new. They're not bad amps. It is a pain to change the valves though. And never use the standby switch, whatever you do. I've bypassed mine!
I had a problem with my Marshall built AC30 that even a guitar tech who specialised in valve amps could not fix, for which he charged me £180, so over $200. The top boost channel would often fade out gradually after a few minutes. This had been happening for months and was driving me mad. Having obtained the circuit diagram-as one who knows nothing at all about electronics- I realised that the splitter governed both normal and top boost channels.but Normal was working fine. I asked an electronics expert I knew about whether half of the splitter could work whist the other half was faulty and he said yes. So I changed that and have now had any further trouble. Maybe a bit of applied common sense helps in these situations. Now I have some horrible loud whooshing and popping noises, so thank you, and I shall suspect the output valves first. Well it is an AC30 after all.
Hi Nigel Very poor of that tech to charge you anything at all, let alone $200. That would have been a relatively easy fault for a tech to find. Well done for sorting it.
I didn't know bad power tubes can cause so much crackling like that. What do you figure is wrong with the tubes when they are working but totally crackling like that? Would that still just make them microphonic?
I think Dick Denney would have something to say if he could see this design. I'm hoping my AC15 isn't as awkward to work on if it ever develops a fault. It looks almost as bad as the Fender bassbreaker you showed us. Apart from the obvious sound benefits, one of the attractions of valve amps is you expect to be able to repair them rather than having to replace them.
@@chrisdee5032 Really? All the JMI AC15's I've seen have been the same square shape as the current "hand wired" replica models, so couldn't possibly have the same chassis. I didn't know the modern shape was around then - live and learn eh? 😊
@@davidbarlow431 Yes the original AC30s had the same layout as this amp. Definitely a pain if you need to replace a valve quickly lol. The AC15 was always different I think. The newest AC30 has the valves hanging down at the back so a bit more user friendly lol.
Curious if you used any contact cleaner on the sockets and jacks. As well as check the socket pin clamps. Many times that will solve "mechanical/crackle" issues as well. Especially in the FX loop as you mentioned. I typically do that first on any amp I look so I can check those off the list before diving deeper. I also hate the preamp sockets mounted straight the pcb. If the they are easy to access I always reflow them as well. However, as you also mentioned, the horrible design of this amp would require you to completely dismantle it to get to them.
Great input thanks. Yes I would normally Deoxit the socket and check pin tensioning. But for some reason I didn't on this amp. That Valve just seemed SO sensitive and microphonic that I went straight for the valve. It needed a new set anyway, so nothing lost. I think I missed a trick there though and should have just tried Deoxit to see.
DeOxit and other such contact cleaners are okay use on for pots, jacks and most switches, but inappropriate for use on tube sockets because of the residue left behind. I know, I know, most techs spray it on tube sockets regardless, but you *shouldn't* ---- the residue will collect dust and can create conductive paths between the pins, which might even arc, especially in humid weather. The possibility also exists that the cleaner could soak into the PCB and help make it conductive (PCB materials aren't always as good an insulator as you might think, especially for high voltage, high impedance tube circuits ---- one series of Marshall amps had a runaway bias issue caused by conduction through the PCB). Use pure alcohol on tube sockets, not contact cleaner. You could use skinny nylon paintgun-nozzle brushes (or perhaps even a mascara brush) in conjunction with the alcohol, although this may not remove years of oxidation buildup; my favorite cleaning technique is to use a segment of wound guitar string, .036 to .039 diameter, worked back and forth in the socket contacts like a miniature round file or rasp, followed by a flushing with alcohol, and retensioning of the socket contacts. PS, if you ever try to resolder socket contacts that have contact cleaner residue on/in them, you'll find that they don't accept solder well without a thorough degreasing first. The only RU-vid tech I've seen who agrees with me on this issue is Psionic Audio in Memphis. His channel is definitely worth watching ---- he does a fair bit of re-engineering to fix design flaws on the more modern or reissue amplifiers with PCB'S.
@@goodun2974 Good point about the DeOxit on the tube sockets and the guitar string is a good tip as well. I bought a Blackstar Artist 30 for cheap (less than the cost of the 2 Celestons in it) b/c of an intermittent/dead 2nd channel. The pcb looked clean but felt like a fly trap. Probably from smoke or humid environment. I cleaned the pots, jacks etc, including the tube sockets with the DeOxit and then cleaned the entire pcb, and the tube sockets again with Alcohol. Then reflowed all of the tube sockets and reset the pin clamps. Problem solved. This method doesn't necessarily point out the specific issue but on modern pcb (trace board) amps I consider it part of the general servicing and preventative maintenance. I should point out that I typically only work on my own gear and am basically a newb to High Voltage circuits so this may not be a cost effective approach for a tech on the clock.
@@goodun2974 USeful thanks. Strangely enough I hardly ever just squirt Deoxit onto the socket like that. I use a wooden coktail stick soaked with contact cleaner, and work that round and round in the pin holes. That works very well.
@@jbolt247 , I was an electronics bench tech for 15 years, and an audio system installer and telecom tech for decades beforehand ---- and the basic cleaning and tuneup you describe is the minimum I would consider doing for a customer. If they can't afford 1.5 to 2 hours of bench time, I'd just say no to the repair. You cannot consider an amp to have been "serviced" without inspecting the soldering and reflowing the known trouble spots like the jacks and pots ---- especially now that lead-free solder is the defacto manufacturing standard. Terrible stuff, and extremely failure prone.
Hi Stuart, I'm kind of a newbee on amps. I bought a new Vox AC30C2X (Made in China). I want to ask you if it is normal that the amp humms when i don't dampen the strings ??
I assume the rectifier tube is a GZ34, aka 5AR4..... and you've never seen one fail?! Really?! I certainly have !! You must lead a charmed life! Some brands of reissue GZ34/ 5AR4's are known for arcing or shorting out; but I've seen original Mullard GZ34's arc-over as well. The design of the closely-spaced, inpendently-heated cathode elements that gives the tube it's desirable qualities---- minimal voltage drop (compared to a 5U4 or 5Y3) and slow warmup time, therefore gentler on the tubes and filter caps ---- also makes the 5AR4/GZ34 more prone to arcs and shorts if the insulation between heater and cathode breaks down. By the way, although you probably don't run into them much on your side of the pond, a useful rectifier tube is the 5V4, which is independently heated (like the GZ34), and has a similarly slow warmup time, and not as much voltage drop as a 5Y3. If one is working on say a late 40s to early 50's low-power amp that runs the tubes at conservatively low voltages, you can sometimes reduce the sag and compression and improve the dynamic response of the amp (assuming the caps are healthy and rated for the extra voltage) by substituting a 5V4 for the 5Y3 ---- it'll likely increase the B+ by 30 volts or so, without drawing any additional 5- volt heater current. Occasionally, you can replace a 5U4 with a 5V4, depending on the circuits' original working voltages---- but alas, you probably can't replace a 5AR4/GZ34 with a 5V4, certainly not in this Vox with 4 EL84 outputs.
Only if desparate and as a last resort! you can fix 99% of problems without. I'm fascinated to see that the other amp repair techs who make similar videos virtually never use a scope either! The most times I use a scope is just to check the power output across an 8 ohm load. For those interested, take half the peak-peak sinewave voltage, multiply by 0.707, square the result and divide by the load (e.g. 8 if 8 ohms)
@@mikejamieson419 This could be a few things. Eliminate the valves if possible. You can grab each one formly using a tea towel, then play a chord, if still crackles, it's not that valve. Move to the next. If not valves it could be a dry joint inside - the loud sound causes a component to physically rattle, making and breaking the connection. If you feel competent, take the back off and tap around with a suitably insulated implement. See if you can get it to crackle just by tapping. Do you mean Fender Deluxe btw?
Waste time waiting minutes for the valves to cool down? Nooo - get yourself some thick cloth gloves and whip those bad boys out immediately! Must have saved me hours swapping 6550s on our SVTs :)
Ridiculous design? I have a better word than ridiculous. Amps sure aren’t what they used to be, are they? Whoever designed the PCB in guitar amps should be flogged. Sorry. I’m in a mood today as well.
Yes it's hard to believe it passed all the design stages and was signed off by senior pople and so on. "Yes, that's great, you're good to go on producing 10,000 of those... Well done guys."
@@lpoolck17 Most amps which are pre-pcb are fairly good. That's because they are hand wired by skilled wiring people. The pcb offered the chance to dramatically slash construction costs and it was irrisistable to manufacturers (and customers of course as they always want 'cheaper'). Pcbs are prone to dry joints, cracking tracks and so on, particularly in the high temperature high current and voltage of a valve amp. This leads to reliability problems. It's also poor design (e.g. 5W power resistors soldered direct to the board. "What? They got hot and burned the board and caused a dry joint under? Sheesh - we didn't see THAT coming boys...." I often think that any new generation of designers have literally no contact or input from the previous generation. It's like they retire a whole cadre of old designers and replace them same day with designers fresh out of college. They just keep making the same silly handful of newbie designer mistakes.
