Next up: 1967 Vox AC30 tune-up

[Raoul Duke]

The 6 ceramic tube resistors all seem to have failed - I pulled every one of them out, cleaned them off to include the leads, and measured them. Every one of them read “OL”.

That's good if they are capacitors! Where do you see ceramic resistors?

Here is a very legible schematic that might be of use: https://www.drtube.com/schematics/vox/a ... 2-iss5.pdf

Ooops! I got that wrong. Apologies guys; and understood on them being caps now.

Steve - yes, I have notes and pics. Learned that lesson the hard way - once, lol.

B Ingram - thank you for the detailed response. Tremendously helpful and very much appreciated! I definitely learned quite a bit there.

Martin - my mistake for not fully comprehending your original identification. It was a helpful pointer that I completely missed - somewhat embarrassingly now in hindsight. And thanks for the link, I’ll check it out.

What are your opinions on that .1uf Lemco? Would any of you trust it?
Also - I’m leaning toward modern CF resistors but am still open to CCs if it’s the “right thing” to do. Opinions?

Thanks again all!

[martin manning]

B. Ingram, do you have a link to that very clear vintage AC30 schematic? What year is it?

Mark, my thought with the reissue schematic was that it could serve as a guide, and you can see what is different in your example. Apparently there were many changes made along the way.

On that janky cap, I would replace it with a new film/foil or metalized film cap, like M150, Illinois, or if you don't like yellow or white, a brown Panasonic: https://www.mouser.com/ProductDetail/Pa ... BJlA%3D%3D

I vote CF resistors. Better than CC, and look kinda period correct.

[B Ingram]

B. Ingram, do you have a link to that very clear vintage AC30 schematic? What year is it?

Unfortunately, I used only a snippet of that one because it's not a good example either. It's the last schematic in this group, and appears to diagram a Top Boost model with JMI's add-on Reverb Module.

I didn't know about the 700pF-actual vs 750pF-schematic until writing my post. I'm unsure how many other discrepancies might be out there, and am considering checking my own vintage Vox amps to find out.

What are your opinions on that .1uf Lemco? Would any of you trust it?
Also - I’m leaning toward modern CF resistors but am still open to CCs if it’s the “right thing” to do. Opinions?

Hot on the heels of my Vibrolux Reverb tune-up; I’ve decided to dig into my ‘67 AC30.

The amp is a gray panel integrated Top Boost with late ‘66 dated transformers and pots dated to January of ‘67. It’s had some servicing but is mostly intact with Erie resistors and red Lemco caps. Definitely “player grade”, it’s fairly rough looking and was once owned by Cesar Diaz. Has silver Celestion Alnicos with the tag board terminals (vs on the speaker frame). It’s always sounded ok, but hasn’t really lived up to what I expected of it when I purchased it 5 years ago. Hoping to get a little “more” out of it.

What I’d like to do is: ...

Opinion Time:
What I heard was, "I want to do stuff to my amp, and I'm hoping that will make it sound better."

I read posts all the time where guys want to see if anything is "out of spec" as though "the spec" has some magic that makes the amp sound good. Well, we just saw a second ago that "the spec" sometimes ain't real, and a different-value was installed in the amp at the factory (as I strongly suspect the dog-bone ceramic resistors are factory-original). Now if something is marked "100kΩ" and actually measure "2MΩ" then that's probably not good. But I think the Erie resistors look cool, and I'd tend to leave them in place if possible. I'd only want to "fix" things that are broken in ways that definitely affect the amp's performance.

So about that Lemco cap: I'd check whether any of the caps fail to block DC Volts when the amp is on. I don't really care about their value unless the amp is very-obviously malfunctioning. I wouldn't be unsoldering a bunch of stuff to measure outside of the amp because this is the circuit where you'll very likely booger things, and put parts back in the wrong place. You'll want LOTS and LOTS and LOTS of very large, well-lit, high-resolution photos showing where everything was before you started.

Filter caps can be replaced "just because." That said, and contrary to accepted wisdom, I've measured the leakage current of filter caps I replaced in a 1964 AC30 with a Sencore LC75, and the 60 year old filter cap performed better than the modern replacement. But absent test gear to prove the old cap is serviceable, replacing filter caps is a safe bet.

I normally don't replace coupling caps unless they are shown to leak DC Volts. The best way to check that is to simply measure DC Voltages through the amp a verify you get the expected amount everywhere. "Volts where there should be no-Volts" or "very-wrong-Volts" might indicate leaking coupling caps.

Measuring DC Voltages should also indicate whether any resistors are way-wrong. Unless resistor are way-wrong, I would only replace them if they're objectionably noisy. It can be tricky to isolate & find the noisy-resistor. I've got 2 mid-60s AC30s with those Erie resistors, and I'm not changing any of them unless they're proven noise-sources.


All that said, a guy who has owned and worked on more AC30s than I'll likely ever see remarked that they all sound a little different, and you cannot make them anything but what they already are. Some are cleaner, some are aggressive, some are harmonically rich. So if you aren't floored with the amp's sound, it may be better to move it along & get a "different old AC30" whose sound you like straight away. I've got an AC30 Bass, AC30 Normal, and an AC30 Treble (that's been halfway-converted to a "Normal"), along with several of the smaller-model 60s Vox amps. I also had a 1964 Top Boost, but sold it last year because it was "just okay" compared to the other amps.

[Raoul Duke]

B Ingram,
I understand what you’re saying and appreciate your perspective and opinion for sure. This is the only old Vox I’ve ever owned after many decades of wanting one and it kind of fell into my lap a number of years ago without me thoroughly researching and testing it - so your opinions derived from experience is valuable info to me.

My main goal for this is to give it a thorough servicing WRT old filter caps, clearly failing components (cracked signal caps, toasty looking resistors etc.), mechanical integrity (had lots of loose fasteners, broken tag strip where the screen grids and grid stoppers are holding it together etc), clean-up old work that looks like failures waiting to happen, and a thorough cleaning to the extent possible without major disassembly. I thought that given its age and “while I was at it” - I’d measure components and replace as needed just to get it back to original values. It would be nice if it sounds better afterward, but I’m not expecting anything miraculous. My thought was that given the age and values of the filtering along with failing components - how could carefully servicing the amp not improve either sound or reliability - or hopefully both?

Secondary goal is to document as much of this amp as possible and send that info to the VoxAC30.org.uk repository. Really interesting site and full of fascinating info. Hopefully giving a little back for others who find these amps interesting.

I also understand that component aging and drifting directly affects whether you have one that sounds fantastic or just ok. I figured at worst - doing what I’m doing - I’d get back close to the baseline and see how I like the results.

Definitely taking notes, pictures, and drawings. I really don’t want to mess this thing up.

Martin, I’ve printed out both, started comparing them and notating differences. Once I have that done, I’ll start working may way through comparing it to this amp. As always, I appreciate your feedback!

Thanks again guys! I welcome all advice and opinions and appreciate the time anyone takes to weigh-in on any part of my projects. I’m in no rush and enjoy the learning process as much as the end result.

PS - I’ve been researching and hunting for an LC75 for a few months now. I can see how it would be incredibly useful in this hobby. Probably the next piece of test gear I buy when I find the right one.

[Raoul Duke]

UNCLE!!!

I’ve been researching why this amp would have both a grounded heater center tap and an artificial center tap and I have to say I’m frazzled. Turns out that a common “fix” for Vox heater circuits was to clip a ground wire in the pre-amp filament circuit and add the 100R resistors to ground at pins 4/5 and 9 at that tube. No mention of disconnecting the 6.3v CT.

I even looked it up on YouTube and saw an AC30 restoration where a very busy and skillful amp tech in Memphis (hint) applied the fix in a slightly different way (100Rs off a power tube, same grounds clipped in the pre-amp); no mention of disconnecting the CT.

WHY would this be? My gut tells me to disconnect the CT and run the ACT for the reasons most knowledgeable folks mention universally: better balance, added protection for the PT, often lower noise.

What’s the consensus?

[martin manning]

One ground only for the heater circuit. I guess I’d go with the 2x 100R; maybe the transformer’s heater winding isn’t particularly well balanced. Could be at either end, but I’d try at the preamp tube.

[pdf64]

UNCLE!!!

I’ve been researching why this amp would have both a grounded heater center tap and an artificial center tap and I have to say I’m frazzled. Turns out that a common “fix” for Vox heater circuits was to clip a ground wire in the pre-amp filament circuit and add the 100R resistors to ground at pins 4/5 and 9 at that tube. No mention of disconnecting the 6.3v CT.
...

Most vintage AC30 models didn't have a heater CT, so there'd be nothing to disconnect.
Heaters were wired single ended, one side connected to chassis common.

[Raoul Duke]

Understood Martin, that’s exactly what I was thinking. I’ve had real good luck with that method in everything I’ve built - so it’s becoming my “go to” for heaters.

PDF, even though it’s a slightly different circuit overall - my PT is wired exactly as this one to include wire colors (see pic). Am I incorrect in interpreting the green wire labeled “0v” between the two 6.3v wires as the heater CT? If so - then that answers the original question. I’d imagine that would be the HV CT then?

ac306normalrev7.pdf

Thanks guys!

Making slow progress; but learning a ton. For such an iconic amp - there’s no real definitive guidelines on how to get the most out of them. Everyone seems to have their variations on the fundamental principles, I.e. grounding schemes, filtering, tube bias etc. Lots to digest and all very interesting.

[pdf64]

PDF, even though it’s a slightly different circuit overall - my PT is wired exactly as this one to include wire colors (see pic). Am I incorrect in interpreting the green wire labeled “0v” between the two 6.3v wires as the heater CT? If so - then that answers the original question. I’d imagine that would be the HV CT then?
ac306normalrev7.pdf

The labelling '6.3 - 0 - 6.3' means the end to end voltage supplying the heaters would be 12.6V. Obviously we know that's not the case.
Kudos to the guy that draw those very nice layouts, but I recall that he's an enthusiast rather than a trained / competent person.

I seem to recall this has cropped up previously, and the heater CT may have been included as an aid for builders using an aftermarket main transformers.

The layout labelling certainly implies a CT for the 6.3V heater winding, but I wasn't aware any vintage Voxes actually had one, your's is the first I've heard of.

The Hammond AC30 replacement mains transformer has a heater CT, this shows how it should be labelled https://www.hammfg.com/files/parts/pdf/ ... 1697661948

[Raoul Duke]

Thanks for the info, makes sense now.

My PT doesn’t have a heater CT. I’ve since confirmed that this green wire is the HT CT.

Apologies for the confusion. Onward…

[Raoul Duke]

Ok,
I’ve replaced everything that was beyond tolerance, cleaned things, re-flowed some things, tightened a bunch of hardware etc. Took notes and pictures to make sure everything found its way back to where it belongs. Referenced the “Service Engineer’s Guide” quite a bit as well as a few videos and felt pretty good about bringing it up on the variac and limiter today.

Here’s what I got while on the limiter (40w bulb) at 115vAC:

Bulb lights while caps charge, then drops to apx 50% brightness
B+1: 101vdc
B+2: 98vdc
B+3: 95vdc
Heaters: 2.3vac
Rect. Htr: 1.8vac
Power tubes
Plates: 101vdc
Screens: 101vdc
Cathodes: .01vdc
AC at the fuse: 42vac

So my question is: Voltage being inverse to current, this seems way low to me. I’d expect there to be a short to track down, but the fuse didn’t blow and nothing smoked or smelled while it ran for a good 30 minutes. I double checked all the connections I touched, ensuring they align with my notes, pictures, and the guide and everything seems to be in order.

Never lit up an old AC30 after service before, so thought I’d stop and ask for some advice and/or suggestions while I do some searching on here and Google to try and understand what’s happening.

Thanks!

[martin manning]

40W is pretty low, so I’m not surprised the bulb looks about half bright. You could try a higher wattage bulb, but my feeling is you’re ok.

[Raoul Duke]

That’s what I was wondering about Martin, thanks for pointing that out.

I used that bulb on my first builds specifically because it offered more protection for those ODS’ I started with. I just wasn’t sure if that small bulb would have such a dramatic effect on this circuit. I seem to recall it was definitely brighter with the Tweedle and the Dual Lite.

I’ve since checked everything a third time and still can’t find anything out of place. I was super careful moving wires around, removing old parts, and didn’t remove any original wires from the boards. Everything I did was pretty superficial specifically so I didn’t disturb anything by “digging around”.

I may try the bigger bulb or I may go full wall voltage… Flip a coin I guess, lol?

Thanks again Martin!

[B Ingram]

... bringing it up on the variac and limiter today.

Here’s what I got while on the limiter (40w bulb) at 115vAC:

...
B+1: 101vdc
B+2: 98vdc
B+3: 95vdc
Heaters: 2.3vac
Rect. Htr: 1.8vac
Power tubes
Plates: 101vdc
Screens: 101vdc
Cathodes: .01vdc
AC at the fuse: 42vac

So my question is: Voltage being inverse to current, this seems way low to me. ...

Your voltage is low because the amp has a lightbulb in-series limiting current, and dropping voltage. Meaning, doing its job.

... Bulb lights while caps charge, then drops to apx 50% brightness ...

I may try the bigger bulb or I may go full wall voltage… Flip a coin I guess, lol?

Nothing is shorted, or the bulb would be full brightness.

Stop being scared. Just plug the amp in the wall & enjoy! I'm 100% confident the voltages will come up to normal value, and you won't have a short-circuit that could pop a fuse.

[Raoul Duke]

Thanks B,
I appreciate the feedback. I’m always a little over-cautious when trying something new I guess.
My thought was the bulb would have kept increasing in brightness if something was shorted, but the semi-brightness and voltages had me wondering; which is when I realized the smaller bulb may have been a factor. Not enough experience or knowledge on my end - but I enjoy the learning.

Update soon…

Thanks again!