Next up: 1967 Vox AC30 tune-up

[Raoul Duke]

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:
- clean it up/tighten it up internally (I.e. grounds, mechanical connections on chassis)
- replace the filter caps (29 years old LCRs)
- replace any out of spec resistors
- sort out the mains wiring (voltage switch is looking sketchy, probably hardwire to 120v)
- check the power switch (feels spongy - like it’s going to fall apart)
- add any sensible safety/reliability upgrades that are appropriate
- hopefully eliminate some hiss and noise in the process
- build the simple version of Rob Robinette’s bucking box with a spare Hammond 166N10 I have on hand
- give the cabinet a structural going over

Hunting around for reference material and looking for any best practices for this amp. Definitely looks more challenging than the Fender at first glance, but I’m confident I’ll learn a bunch. Found quite a bit here and just reading through it all.

First round of questions:

- worth hunting down vintage resistors or are good quality carbon films appropriate?
- in the event the switch is actually falling apart, is a standard Carling SPST a good substitute (has a Bulgin DPST, which is rare)
- stock filter cap values? (currently doubled-up on all three)
- stock screen grid and grid stoppers values?

Thanks in advance for any advice or suggestions!

[Roe]

- in the event the switch is actually falling apart, is a standard Carling SPST a good substitute (has a Bulgin DPST, which is rare)
- stock filter cap values? (currently doubled-up on all three)
- stock screen grid and grid stoppers values?

Thanks in advance for any advice or suggestions!

Don't replace a DPST with a SPST.

Filtering in preamp should be 10uf except 32/33uf for cathode follower.

Screens works well with 33uf imho

For mains, you need at least 33uf, preferably more (I'm using 64uf in one amp)

[Raoul Duke]

Thanks Roe, I appreciate your advice!

Got the switch out and the rivets’ flanges have broken, so I see two choices: get some really small screws and rebuild or use the DPST version of the SPST Carling (which has the same bat type and the Bulgin dress nut actually threads on to). Leaning toward a new switch just because it’s not 58 years old with the corresponding thousands of “on-off” cycles. It’ll look identical from the outside.

Started measuring resistors and plan to leave anything within 10% in place. Just about through the top board and roughly 30% are wildly out of spec (25% or more). Got a half dozen dog bone resistors in there as well, so I’m trying to find a solid reference for reading those. I’m using a somewhat legible schematic from VoxAC30.org as well as Turret’s layout from a few years back as reference so I’m sure I’ll decode everything in time.

Speaking of Turret’s layout: can anyone confirm that the transformer wire colors are correct? They seem to match what I’m looking at - just a little hesitant to trust my judgement on this first try with an old Vox, lol.

[sluckey]

Started measuring resistors and plan to leave anything within 10% in place. Just about through the top board and roughly 30% are wildly out of spec (25% or more). Got a half dozen dog bone resistors in there as well, so I’m trying to find a solid reference for reading those.

Be aware that you cannot always measure resistors accurately in circuit. A resistor that measures LOWER than the expected value most likely has some other resistance in parallel (many times not so obvious on the schematic). Disconnect one leg to measure accurately. A resistor that measures HIGHER than the expected value is bad. No need to disconnect one end to measure accurately.

Post some hi-rez pics of the dog bones and we'll figure it out.

[Raoul Duke]

Thanks Steve, great point. I’ll definitely keep that in mind as I go for sure.

Here’s some of those resistors (6 in total):

[martin manning]

Those would be tubular ceramic capacitors. See at middle left of this snip showing the vib channel input.

[Raoul Duke]

Thanks Martin, always appreciated!

This amp is certainly a new experience in following a schematic. I was just starting to feel a little more confident with the Dumble and Fender stuff. This one is a challenge… but I’ll get it.

[pdf64]

There's a 99% likelihood any black Hunts Moldseal cap (eg 0.002uF 600V cap is the 2nd photo) is bad.

[Raoul Duke]

Yeah, it’s cracked. I’ve also got a red Hunts 500pf about 18 tags to the right of it that looks cracked as well. What’s the best subs for these? Modern polyester?

Part of my research is determining what to replace things with so that it replicates the sound and doesn’t look totally scavenged.
I’m reading that carbon comp resistors aren’t always guaranteed to measure within spec even when new and that NOS caps may already have started degrading based on storage conditions and such. I don’t mean to assign value based on vintage perception, but the likelihood of me selling this amp in the next couple years is pretty high - so I’m trying to keep as much period correct as possible unless there’s performance-based reasons to upgrade.

Probably overthinking this…

[sluckey]

Probably overthinking this…

Yep.

[Raoul Duke]

Is this considered poor workmanship or an acceptable work-around? I’m sort of asking “tongue-in-cheek”; but I’m seeing a lot of this even with factory paint on the solder joints - which leads me to believe that there might be a range of acceptability for these amps - especially as this is at the end of the JMI years.

This cap still measures .1uf, but I don’t have the means to measure ESR. Chuck it or run it?

[Raoul Duke]

Another odd thing I’ve discovered is that this amp has a heater center tap connected as well as an artificial center tap (2x200R 2w metal oxides attached off the pilot light to ground). Is this something unique to this amp or a previous tech who meant well? I imagine I need to decide which to keep. I tend to favor the ACT but I’m not sure if both are there for a reason. I’ll have to research this more.

Also measured every resistor in the amp and about half are well beyond 10% with maybe a half dozen right at the limit. 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”.

Making a parts list, cleaning what I can get at, tightening hardware, doing more research trying to figure it out. Slowly starting to understand the schematic little by little.

[martin manning]

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

[B Ingram]

Here’s some of those resistors (6 in total):

Those would be tubular ceramic capacitors.

... 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”.

As Martin said (again), those are not resistors, they are capacitors. Let's connect all the dots on this:

Below is one photo you provided of these tubular ceramic capacitors in your amp. (Apologies for the size, but I haven't cracked the code on re-sizing on this forum).
Left-to-right, this photo shows a 0.002µF (or 2000pF) capacitor, a 68kΩ resistor, a ceramic capacitor in-parallel with a 330kΩ (Orange-Orange-Yellow) resistor, a ceramic capacitor in-parallel with a 180kΩ (Brown-Gray-Yellow) resistor, and a 0.1µF cap.




Martin gave you a partial schematic as a way to say, "These ceramic capacitors show up in the Vib/Trem channel." Sadly, the original is hard to read, so let's look at a snippet from a legible schematic. Here, I have drawn a box around a couple of resistors & a couple of caps.




From the schematic, we see there should be a 500pF cap in-parallel with the 330kΩ resistor. There should also be a 750pF cap in-parallel with the 180kΩ resistor.
I noted the resistor values earlier using the color code, and now you have caps that are also marked with the color code.

The 330kΩ resistor has a cap across it with the colors Green-Black-Brown-Black. We see from the color code chart that's "5 - 0" and a multiplier of x10. If you also know that these small ceramic caps are all in the picofarad range, then this is a "500pF" cap. And we look at the schematic snippet to see that there's a 500pF cap across the 330kΩ resistor.

Repeat this for the 180kΩ resistor: it has a cap across it with the colors Red? (or Purple?), Black and Brown. The schematic snippet shows a 75pF cap across the 180kΩ resistor. We see from the color code chart that "7" is "Violet" so that strange reddish-purplish color must be Violet. But "Black - Brown" indicates a "0" and a multiplier of x10. The color code marks this as a "700pF" cap. Good chance the factory installed that because it was the available part-value, as it's less than 10% off from the value marked on the schematic.


Someone has drawn up layouts for some 60s AC30 models. If we check a snippet of that layout from the Vib/Trem channel area, we see this matches our observation from your amp:





The Part # in the layout above matches the (mostly illegible) numbers in the schematic Martin originally posted. The full layout is here but warning that it does NOT/NOT match your amp completely because it is for an AC30/6 Normal, and not for the Top Boost version. However, the layouts that guy has drawn may help orient you to the gobs of stuff you're looking at in an AC30.

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

More warnings apply there: The part-numbers for the components in this 1992-drawn schematic do not match the 1960 schematic. We could perhaps forgive that, because it doesn't matter as long as all the part-values are the same. But the parts-values are not the same, they are "as close as modern parts-values allow." This isn't "bad" but notice that C20 is 220pF (across a 180kΩ) where the original schematic shows 300pF. However, Vox did work to approximate the "700pF" cap we actually got in the vintage AC30 by paralleling a 470pF with a 220pF (for 690pF).

[sluckey]

Adding to what Martin said... I hope you have hi-rez pics or good notes that show where each of those caps goes.