How did JMI expect Vox AC15 owners to install V5 tube shield with choke in the way?

[dchang0]

Here's an interesting and curious problem--on a JMI 60s-era Vox AC-15, how were owners expected to install the V5 tube shield with the choke in the way?

See the photos of my build of the 60s-era AC-15.



Also, see the photo of an original 60s-era AC-15 at North Coast Music:

http://voxshowroom.com/ct/amp/uk/ac1...awn_hood_8.jpg

It is pretty obvious the choke was in the way on that amp too.

Did owners loosen the choke screws? Probably not, as that would require taking off the slider board to tighten them back up.

I figured out a trick to put the tube inside the shield, then position the shield with tube over the socket, then use a plastic bushing to push the tube into the socket, but this only works for installation. Removing the tube becomes a nightmare--I had to find a thin plastic rod that could fit through the grounding hole in the bottom of the ceramic socket to push the tube out of the socket while the shield's spring was working against me.

So is there some trick to this?

Note that on my amp, the tube socket is 1/16" away from the chassis due to the silicone o-ring for vibration dampening. This does not matter to this question, as the problem with the tube shield still occurs without the o-ring and the socket attached directly to the chassis. In fact, with the o-ring suspension mount, I can tilt the socket a bit more, but not enough to slip the shield over the tube.

[Chuck H]

I can see by the photos that what you say seems accurate. I wanted to acknowledge that first. Your build looks to be the same as the original, so the phenomenon would seem real.

So my guess is that VOX simply didn't expect users to deal with this. Good quality preamp tubes can and should last for a few years. In that era it probably wasn't anticipated that nerds like you and I would want to experiment with different tube brands and even types and they certainly didn't anticipate the day when you'd need to try five fricken tubes in that slot to find a good, quiet one . They probably didn't even expect users of the day to do any service to the amp either, such as recognizing an issue that could be fixed by replacing a tube. To that end they reasoned that only service techs would grumble about this problem and that shouldn't affect their customers. So the amp was layed out as they saw fit for their own reasons (Balance? Ease of manufacture? Stability?). This just seems like the obvious answer. What other reason could there be?

[rjb]

So the amp was layed out as they saw fit for their own reasons (Balance? Ease of manufacture? Stability?). This just seems like the obvious answer. What other reason could there be?

Here's another possible reason: Somebody screwed up.

[Chuck H]

Here's another possible reason: Somebody screwed up.

Well, sure! Clearly. But you don't produce a thousand amps without catching on and changing it if you think it's important. It must have been considered a minor issue.

[dchang0]

Thanks, Chuck. Your reasoning is sound. It's a boring and uber-reliable 12AX7 in that socket. I don't think I have replaced a 12AX7 for failure ever in 20+ years after the initial tube rolling to find a good one.

It would be funny if some Vox historian found a factory service bulletin directed at service techs saying, "loosen screws and remove choke in order to remove V5 tube shield or replace V5 tube--stop whining, just suck it up."

One idea: perhaps the tube shields used to be some other kind/design where this was not a problem, and then over time, they switched tube shield designs to save money, introducing the problem...

For instance, if the tube shield were a clamshell design, it could be installed with the choke in the way, etc... The photos I have found, though, of vintage AC-15s all seem to use the typical twist-lock tube shield, so this idea is not likely to have been true.

Here's another photo of a different vintage chassis clearly showing the twist-lock tube shield:

http://www.voxshowroom.com/ct/amp/uk.../chassis_2.jpg

Or maybe chokes back in the day were shorter/skinnier and so didn't get in the way, but the photo clearly shows that it does.

[Tom Phillips]

...maybe chokes back in the day were shorter/skinnier and so didn't get in the way...

The original design has an issue for sure. One of the hazards of trying to exactly reproduce an old design is that the bad gets reproduced with the good. However, slight differences between your build and the original may have made the difference in the shield being difficult to remove vs. impossible. Also note that there are several factors that contribute to the tolerance stack up.
1) The diameter of the original tube was smaller than those made today.
2) Choke size variations.
3) Tube socket height.
4) The other things you mentioned.

[Justin Thomas]

I think the first 1,000 Jeep Libertys came with an oil filter that was completely inaccessible without removing the engine... but nobody figured it out ill they all got 5,000 miles on them... so "failure to foresee a problem of routine maintenance" is not unprecedented...

Not arguing with you, Chuck; I kind of agree - no problems were ever foreseen. Just saying, "it's happened in other fields..."

Out of curiosity, what is that tube's job in the amp? If a PI or other "function" tube, yes, things like microphonics, noise, etc. don't require a critical select tube...

Justin

[Chuck H]

Good call. It's entirely possible that small differences in more than one of the mentioned factors allowed the design to "just work" for VOX Change anything and you're hosed.

[g1]

Was there ever a 12AX7 'stubby'?
From your description, the choke in the original uses nuts that are not captive and would fall off?
I would suggest using a captive or pem nut so you can loosen off the choke enough to tilt it and then be able to tighten it up again with no further disassembly.

[dchang0]

Good points, Tom.

If the 12AX7 in the V5 socket were 1/4in shorter and 1/8in skinnier in diameter and the tube shield were 1/8in wider in diameter and 1/8in-1/4in shorter, it would be quite easy to tilt the shield and slip it over the tube (already installed in the socket). It does visually appear that the older tube shields were wider than the new Chinese-made ones and that the bayonet lug fit was looser.

Also, this photo of a 60s-era AC-15 recently sold on Reverb.com shows some tube shields shorter than others (although I doubt JMI shipped it like this).



If I could buy a 9-pin bayonet-style tube shield that is about 1/4in shorter than the typical length, it would probably work.

Justin: V5 is the first preamp tube for the Vibrato/Tremolo channel. Out of all the 12A?7 tubes, it is the one most likely to be changed when tube rolling/sniffing. It would've been nice if JMI had switched it with V7 (immediately next to it), which is for the oscillator.


One other thing I noticed: JMI could have solved this by moving the choke 3/8" away from V5, towards the inside front of the combo cabinet. It could be done. Right now, the fat paper-insulated choke lines up with the bent edge of the lower chassis, so there is the entire flat lip's width that it could be moved, so that it overhangs the flat lip.

[dchang0]

Was there ever a 12AX7 'stubby'?
From your description, the choke in the original uses nuts that are not captive and would fall off?
I would suggest using a captive or pem nut so you can loosen off the choke enough to tilt it and then be able to tighten it up again with no further disassembly.

Another great idea. From the gut shots I have seen, the choke was attached with non-captive nuts, but the photos were blurry.

I'd be very happy with a slightly-shorter tube shield combined with a stubby 12AX7--that is probably enough to solve my particular problem in the easiest/best way. I can't find any stubby 12AX7s via google, so I'll settle for a shorter tube shield. My current tube shields are 50mm tall. A 40mm tube shield would give me all the clearance I need. A 45mm tube shield might work but probably not.

I am considering buying one of these short shields made by IERC:

http://www.ebay.com/itm/Tube-Shield-..._qi=RTM2247627

It would not engage the bayonet lugs but would touch the tube shield base and extend the Faraday cage over most of the tube.

[dchang0]

Here's a photo of the same amp on Reverb.com with a "skinnier" or flattened choke. This is the only one I have seen like this, though--other gut shots show "fat" chokes.

Also, using the same amp's gut shots, I drew an arrow illustrating the design change JMI could have done to permanently solve the problem. I guess the only reason not to do it is to avoid some sort of magnetic interference with the choke's position relative to the other transformers.



I wonder if I had bought a nylon-bobbin-wound transformer like a Hammond or Heyboer, maybe it would've been skinnier enough that this would not have been a problem.

[dchang0]

Upon further research, a shorter tube shield height is the most plausible explanation.



The shields I have are 50mm tall. 45mm (1.75in) were probably widely available at the time. A 40mm shield would be a tight fit. I just measured a 12AX7, and it is 40mm tall minus the pins, so the little glass nipple could stick out the top a bit. Given this possibility, it is more likely the photo above has 45mm shields for the three shorter ones, and the taller ones are 50mm.

Thinking about it, one reason the tube shields in the vintage photos appear wider/fatter is that they might be 45mm or 40mm tall. Although, it is clear from the photo that the military shield in the middle is wider than the other four shields.

So the fix to my problem is to order a set of five short tube shields. All I have to do is check to make sure the diameter is correct for the bases I already have.

Belton still makes 40mm tall shields, apparently: http://www.thetubestore.com/lib/thet...on-shields.pdf

Finding one seems difficult.

I found some modern-production 46mm shields:

EDIT: these appear to be 40mm with a 6mm base, identical to the Belton 40mm tube shields.

10pcs 46mm Silver Aluminum 9 Pin Tube Shield for 6688 5670 6386 Etc | eBay

45mm may not work for me (5mm is not a lot of difference, while 10mm is), but it may work if I:

1) Remove the rubber o-ring suspension setup (gain 1/16in clearance but lose wiggle/tilt capability).
2) Turn choke around so that flatter side faces V5.
3) Push choke towards front edge of chassis, possibly egging out the screw holes on purpose.

Thanks to everyone for their help!

[Tom Phillips]

...it may work if I:...2) Turn choke around so that flatter side faces V5...

Yeah Man! Didn't know that was an option. Do that first and avoid the need for special parts. Especially special tube sizes.

BTW - I don't recall ever seeing a purposely built "stubby" 12AX7.

[dchang0]

I just checked the choke, and while it CAN be turned around, that places its leads on the side away from the hole drilled in the chassis expressly for the purpose of passing the leads.. the "fatter" side happens to be fatter because of the thick leads that go into the paper wrapping... Oh well, it was a good idea while it lasted. The best I could do with the choke is move it outward by about 1mm (without egging out any holes). I noticed that the holes drilled in the choke's feet are not centered--if they were, I would gain another 1mm. If I switch from 6-32 to skinnier 4-40 screws and nuts, I could probably gain another 1mm, but 4-40 probably aren't strong enough to hold the choke in place if the amp were dropped.

I also just removed the rubber o-ring shock mounting system and found that it made it WORSE to get the shield on even though I gained 1/16in clearance. Apparently the wiggle/tilt ability is a huge boon.

Also, based on measurements, the 45mm tube shield will not work on its own without a skinnier choke. The extra 5mm gained over a 50mm shield will still be necessary, but it is not enough on its own. A 40mm shield would work fine without a skinnier choke, from what I can tell. But 40mm is probably too short for a 12AX7 unless I cut the spring inside the shield. I highly doubt JMI did that to make theirs work. Most likely they used 45mm (1.75in) tube shields.

What I'll probably do is order vintage 45mm tube shields and egg out the holes in the thin feet of the choke. It may be just enough to get it all working.

---

I found a European source for the current-production Belton 40mm tube shields:

http://www.tubeampdoctor.com/en/shop...al_socket_4319

To add to the confusion, they call them 46mm tube shields, which may be 40mm for the shield top plus 6mm for the shield base. It is possible that the 46mm tube shields I linked to on ebay are actually 40mm + 6mm, which would mean that it is very easy to get the 40mm tube shields cheaply.

At any rate, if this is true, then what I really want is vintage 45mm or 1.75in tube shield tops (45mm does not include the base). No luck finding those so far...