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!!!

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.

Excellent insight. Thank you Nigel.

Thanks mate

Thanks Joe. The parts should be here by the end of the week, then I can slot the amp back onto the bench.

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.

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.

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.

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.

As the Greek god of sports sandals once said, Just Do It.

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

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.

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.

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.