Let's Repair: Vox AC30 TB/TBX. It Blows Fuses & Hums
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- Опубликовано: 31 июл 2024
- This Korg-era Vox AC30 blows fuses. There are several reasons why this can happen. Let's investigate several possibilities. We will also tidy up some of the ground loops that are known to be issues with this otherwise excellent Version of the AC30. The owner wants it to sound as close as possible to a JMI era AC30, so we will make a couple of minor changes to the circuit.
Index
00:00 Introduction
00:35 A 3-minute history of Vox amps
04:10 Korg Era Pros and Cons
04:50 Inspection of the Circuit
08:30 How to test PCB-mounted capacitors
8:54 Hum from the Heaters/Filaments
11:30 Reduce Ground loops
11:50 Why replace Screen Grid and Cathode resistors with different values
14:15 Why reduce the value of the Reservoir Filter Capacitor 
Видеоклипы
Excellent, Chris!
Thanks mate
Great to see you again m8. Can’t wait for the sequel.
Thanks Joe. The parts should be here by the end of the week, then I can slot the amp back onto the bench.
Hey Chris, when I saw the notification on my phone, I thought here comes my weekend treat..! Such a fab amp man and the history. I learn so much from you and of course a few other fab techs online. Your explanations and info are second to none... Why couldn't I have had a teacher like you way back in high school, I would have been so interested in any subject you'd have taught..Thanks for sharing your valuable knowledge and look forward to part two..Ed..UK..😄 ( PS: Hope your daughter is doing well. )
You always make my day, Ed. Thank you.
Hi Ed. Can you drop me an email on amptech@teammania.com.au? I have a question for you. Don't worry, I'm not going to ask you if your spare room is ready!!!
Looking forward to part two.
Me too! I am hoping that the only cause of fuse blowing was the floating cathode resistor. I haven't applied power to it since swapping those out. I'm still waiting for the caps to arrive.
Thanks for doing a great job on my vox chris. Its great to have it documented online as well. It sounds great through what ive tested so far. The teke, danelectro and gretch.
The difference i know is, no variable voltage the jmi had for different countries and went from 4 to 6 inputs and im not sure, but maybe the tremlo channel.
Unless you're touring overseas with a 35kG AC30, the variable voltage is a disaster waiting to happen. Most of the time, I will just set the voltage to 240V to protect the amp. Yes there were ac30/4 and AC30/6 with 4 or 6 inputs. Not many people use the Tremolo channel. The gain structure is different and it doesn't sound as good as the Brilliant Channel (my favourite) or the Normal Channel (maybe even more popular). Its a beauty.
The AC30/4 had an EF86 driven preamp as opposed to the AC30/6 which has ECC 83 preamp valves/tubes. The 30/4 was better and Hank Marvin had some criticism at that time when he swapped from the EF86 to ECC83 and the AC30/6. He had lost the original sound of the early records which were all either AC15 Ef86 driven or the 30/4.. However, the preamp valve swap was done because of the brittleness/ failure rate of the EF86 valves. I have an original Mullard EF86 driven AC15 add-on attached to the back of my Korg AC30 to give the original early 60s tone if I want it I have also been using Harma retro preamp and power tubes in the amp from Watford Valves in the UK to give a vintage tone.
Excellent insight. Thank you Nigel.
Cool video! Its interesting the discussion about filtration.... Brad's Guitar Garage actually just finished a real 60s JMI AC30 and noted that the filtration was also way to low at 16uf which lead to a lot of ghosting. I think he also ended up around 32uf which left enough of the character of the circuit, but definitely focused the tone a lot better.
Excellent. Thanks for that. I'll have to look at Brads video. Its sad that we are both in Sydney but have never physically met. Sydney is just so big now that it would be an hour just to get to his place. We've talked on the phone many times. He's a nice fella. I'm undecided on whether to go 22uF or 33uF for the reservoir cap. I have plenty of the 22s in stock but have ordered a few of the less common 33s.
Lads! The ghosting is part of the sound and normally I'd leave it as is, in that particular case the customer is a very experienced mix engineer (you hear songs he's worked on every day) and he specifically asked for a little more filtering to lessen the ghosting when playing higher up the neck.
GREMLINS!! No other explanation for that cathode resistor to be broken off at the hole! Seriously. How the heck did that happen? 😂
I have no idea, William. It was not a dry joint. The stub of the leg was still firmly soldered in place. It was like metal fatigue. Yep. Gremlins.
What effects on tone does the reducing of value of reservoir filter caps have? I also have an ac30 tb from 90s and wanted to make it sound a bit more "vintage" and closer to 60s ac30s.
My concern is that by just copying cap values off 60s specs, does it lead to a similar result? The amp design is such that there are other variations in the circuit design which leads the designs to choose the values they do. So maybe a higher value was chosen by Korg/marshall in the 90s in order to offset other changes in the circuit? And that by simply just changing values to 60s specs won't work as well in a fundamentally different design. Otherwise, why would the designers in the 90s arbitrarily choose higher filter cap values and not 60s spec, if what made the amp famous in the first place was its original 60s models? Wouldn't they want to give it a sweeter tone to sell more units?
This is what confuses me the most
I did a series of videos on Sag. It may be interesting for you to look over those. It answers some of your questions. You need to consider the era in which the amp was made and the style of music that was popular at that time.
One of these days I'll psych myself up to re-wire my AC15
As the Greek god of sports sandals once said, Just Do It.
By changing grid resistor valley from 100 to 1000 I get that it will cool down modern tubes, but what if someone wants to use NOS tubes? Would leaving 100 be ok? or 470 as a compromise? Is there a happy medium to offer user flexibility for both new and old tubes?
Excellent question! Sure. Good quality NOS tubes would be fine with 100R Screen Grid resistors. No harm done in using 470R so you can swap between modern production and NOS
How much difference in time does increasing screen grids make I’m
Curious. I’ve got the hwx newer model and want to bring it more to jmi spec
This is the paradox, Michael. The original JMIs had 100R screen grids, but beautiful British 60s Mullards tubes. Today we have good, but not Mullard good EL84s, so let’s protect them by increasing the screen grids. If you don’t want to go to 1K, can you at least put in 470R.
If tube life isn't a consideration, which is better for tone? @@theguitaramptech
@@user-yf3ns9bx6n I like the tone and reliability that cooling the screens can bring. But that's my taste.
Can u have to many twists so it causes a problem ?
Thanks for the question, Tom. There is the rule of diminishing returns and economics.
All wires carrying an AC current generate an electromagnetic field (EMF) which then induces a smaller but similar current in a nearby wire. This problem is worse when the nearby signal-carrying wire is running parallel to the power current wire.
To minimize the external electromagnetic field, the filament wire should be tightly twisted so the fields generated by each filament wire tend to cancel. You can’t have too many twists. I would have liked to put more twists in the filament wiring for the TBX, but it comes down to costs. I would have needed to rewire the filament with longer wire, but it would have contributed to a larger repair bill. I am hopeful that I may have increased the twists sufficiently to help lower the hum in this amp. I am happy that Korg elevated the filament wires above the PCB to minimise the hum problem. The amount of induced EMF decreases by the square of the distance from the source, ie if you can move the filament wires twice as far from the grid wired the induced EMF is a quarter as strong.
Good question.
AC 30 / 4 4inputs ../6 6 inputs ?
Yes. The first correct answer. I know of one other change. I think you would know this one, too. It's one of those slap the forehead "Oh, yeah" things that we know but forget.
@@theguitaramptech Hmmmmmmmm
As usual Chris....great clip and nice to learn about some history !!
To control the voltage sag of GZ34 instead of changing first filter capacitor value, I often use small ( 22-33 ohms ) series power resistor between GZ34 out and first B+ filter cap. It increases the life expectancy of both the rectifier tube and filter cap as well.
@@jutukka yes. That would definitely increase the sag effect while still keeping the better filtering effect of the 47uF. I’ll give it serious thought.
@@abijag100 thanks mate…and no, you can’t borrow it for your next gig to play bass through it.