AC30 Comparisons - TB/TBX Hum Fix
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- เผยแพร่เมื่อ 10 ก.ย. 2024
- How a tech can fix the ground loop, heater noise, and background noise on the Vox AC30 TB/TBX.
Note: these amps have all been modded to the actual early '60s JMI circuit with the mistakes of each model corrected. Separate videos on each model will be uploaded soon that get into the strengths and weaknesses of each.
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I stumbled on your channel a few days ago and have to tell you what a remarkable find it is. While I have none of the specific amps you work on - there is of course a great deal of commonality in everything tube. I very much enjoy your style of honest presentation, content and approach. Just wanted to drop a bit of encouragement your way. I work on lots of things myself and can only imaging how difficult it must be to attempt to record while you’ve got you hands in something. This Vox part is fascinating (watched the 2nd part first). With as many people who’ve bought those - I can’t imagine why there’s not a hoard of viewers over here
I did everything suggested here except for changing the power transformer. Amp is useable now and my client is happy. Thanks again! I did do one more thing. I added a hum balance pot from the heater AC points to the cathode. When I had all three channels turned down it had a large amount of buzz. If I turned the normal channel up the buzz was greatly reduced. Once I added the hum balance I turned all three channels down and adjusted that. Killed that buzz entirely. Just thought I'd share this.
Yeah, it’s weird how some of them are fine with just a fixed ACT but others benefit from a humdinger too.
Glad it did the trick!
Hey Coilhead, where are you located?
@@fallingmanjutsu I’m in Kingston NY
@@coilhead what is the best way to reach you? Thanks for your reply
@@coilhead I could use your help with my 6TB, if you offer the service, I’m also located in NY. Lyle is way too far from me.
Good god, thank you!!!! I just re-capped two original ones from 1963 and they work gloriously. Then I put up one of these and I was stumped as to why it was so noisy and full of hum. Time to get to work! :)
Thank you so much for this information, I am watching this across the world from Sri Lanka, and I have a TB/TBX
The fix works. Both the AC 30’s were worked on and yes the 2 suggestions you made works perfectly. Thx
this video is gold ! thank you for sharing your knowledge and expertise
Absolutely love these videos! Can't wait for your AC30 Custom video! I prefer my C2 over my 1964 AC30TB. My C2 is my favorite amp, period, and I've owned some fancy Voxy boutique amps along with a TBX, '92 AC30TBR, a CC, and before these, a pile of vintage Fenders. Your comments regarding the CC highlight exactly why I no longer own one.
I am strongly considering investing in an AC30CH and sending it to you to modify just to hear the difference while still having my favorite C2 as reference. I am also strongly considering sending you my '64 AC30TB but should probably get a head cabinet for the '64 before shipping as I wouldn't want to put the '64 combo cab through the stress of shipping. Anyway, these videos are intriguing and fascinating! Thank you!
I’ll buy your ‘64 😄
On the rectifier, the red wire should be moved to pin 8. As it is, on pin 2, the rectifier's filament carries the HT current. This is a production error that puts unnecessary stress on the rectifier and may introduce noise.
You're absolutely right. I didn't catch that because I'm so used to that connection always being on 8. I'll look more carefully in the future. Another TBX just arrived, will check that. Thanks.
Thank you for your videos, glad I could contribute something.
You're doing the work of a saint, my friend.
Nice work! The pre-supposed "advantage" of using printed circuit boards is supposed to be that you can lay out the circuit to minimise interactions, especially if the board is double sided and you can have a heavy ground plane on it, and then the layout is "fixed in stone" and will predictably behave in the same way no matter how many of them you build or who does the actual assembly. The problem that arises is, a poorly thought-out design is also niw fixed in stone, making it difficult to track down the source of the problems and remedy them by modifying the board and changing the circuit layout. Some of these types of mistakes such as running the ground trace underneath the oscillator components are likely due to engineer attrition where the older, more experienced designers and engineers retire or pass away and the "new guys" predictably make the same mistakes that the old guys found out about the hard way and learned not to make decades beforehand.
There's also the strong possibility that the engineers design the circuit but then the bean counters....oops, sorry, the accountants hand it off to some company in China to design the circuit board layout and figure out how to fit all these parts onto circuit board of a given size ---- and at a given price point.
Except this was made in England at Marshall
This is a very useful video and clearly explained, I like the style. I have just applied most of these mods on a TBX from 1995 and the effect is that 45mV of noise was reduced to 19mV or in laymans terms from HISSSS to hisss. I am also going to add screening foil onto the rear panel. One concern I did have on the mod description is that I think I heard artificial centre tap (2x100R) and elevate the heaters. I think it should be one, or the other, but not both. Since the amp I have UK spec does have a centre tap (grounded) I have elevated to the cathodes ~ 10v. I didn't notice or measure any significant improvement with that but it's there.
Thanks. Doesn't matter if you use physical or artificial CT when elevating the heaters. But artificial can give a better result (the transformer winds aren't as matched as a resistor) and offer a fusing function should something really bad happen. Instead of needing a new PT you need two new 1/2W resistors.
It was great to watch this (again). I would love it if you could share your thoughts on the Vox AC15 50th anniversary. I know you once said that you are not a big fan, but I wonder if there are “simple” improvements possible in a manner similar to these TBXs?
Thank you very much for sharing this information i'm waiting for a clean looking 1999 6TBX to arrive and i would like to have a tech check it out and I will share this video. much appreciated!
You're very welcome! And please do share!
Great video! Gonna have my tech do this to my early 00s TBX eventually. He had some ideas previously about noise reduction but this appears to be more comprehensive.
Info is greatly appreciated . Thank you .
I tried to get someone to this kinda of work, they where a respected tech but they messed it up! Would be great for your channel. Maybe I should send it to and make a video on it.
Great info, Lyle.
Thank you for your advice. I try this mod.
Awesome.
I have the same revision board as you, it's marked AC30-60-00 (94V-0). Yours has resistors at R72,74,76,78 where mine has jumpers. What gives?
Earlier amps had the grid stoppers on the board, later amps moved them to the tube sockets and put jumpers on the board locations.
I have this problem bad. I would do it myself but tech service is limited and the only issue really is cutting the trace. I heard your advice and recommendations and I want to scrape and slice with a razor since literally that’s only real thing I’d need help with.
What’s the risk here with me doing that? As I understand it, If I have both green wires present, I need to cut the outer small trace; one wire present first needs the other wire and then both traces cut. Where is the danger for me here? Thanks - spectacular videos and assistance/ info on the AC30 tbx😊 and Concert II, BTW.
Amazing info, sir!
Great video!
I’m currently performing all this on a clients amp. He already had it in for a different issue but now it’s the loops. I’m having a hard time deciding what to charge. $250 seems like a damn good deal for him. Would you mind sharing your cost for this operation?
That's a 3-4 hour job. So $240-320 plus parts.
@@PsionicAudio thank you for the response and for taking to time to point out all the issues with this era AC30.
Thanks so much for this! How’s the stand by switch for this model? Should it be used or disregarded?
Never needed but it does no harm.
@@PsionicAudio thanks so much! I have a 94 AC30 6 TB in red. I want to have the hum mods done, although I worried the tone will change. Do the mods change the tone?
The Volume pot Ground Loop was fixed in late 1994
But in fixing that loop they created another.
this video is gold! thank you for sharing your knowledge and expertise