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

[Tom Phillips]

Ha! Forgot about that.
Another idea is to elongate the choke mounting holes both in the choke flanges and in the chassis. You might gain enough space and still not be able to see the mounting hole elongation after the mounting screws are back in place.

[dchang0]

Good idea! I could elongate the chassis holes quite a bit and the flange would totally hide all the filing.

[dchang0]

More confirmation that the shorter tube shields in the "hodgepodge" gut shot are 45mm:

9 Pin Ceramic Tube Socket with A 46mm Shield | eBay

These are Russian milspec and are 46mm. This appears to be the shorter tube shield sitting low on the middle socket.

I will order one of these and try it out.

---

EDIT: Never mind. The 46mm measurement is for the entire height including base. The top tube shield is 38mm. I am looking at a different model that appears to be of the same design/make that should be slightly taller. So much for confirmation...

It's not all bad news, though. The seller did confirm that the inner diameter of the tube shield is 26mm, which IS wider than the typical 24mm for noval sockets. This confirms what we can see visually, that this Russian military tube shield is wider than the other ones.

[dchang0]

Well, I took off the choke's screws today and found out that egging out the holes is not going to work at all, at least not for the standard 50mm-tall tube shields.

The choke would have to be moved quite far away to get it to fit--see photos. The choke is moved outward by 3/8in but the 50mm shield still doesn't clear!



Chuck was probably right all along--JMI probably simply dumped this burden on the service techs.

Still, a wider 1.75in tube shield combined with a skinnier choke and slight differences in the relative position of the choke and tube socket could have made it possible to do without removing/moving the choke. For instance, if the V5 tube socket were moved slightly to the left in the photo, it probably would work out. (Based on some gut shots of vintage AC-15s, V5 did appear to be separated slightly from the other four tubes--in other words, the sockets were not evenly spaced.)

In my case, I am screwed. I have a fat choke and no intentions to buy a skinnier one. I have holes predrilled in the chassis that can't be relocated without being visible. And 45mm/1.75in tube shields are now very rare.

So, it looks like g1's suggestion of using a captive nut for the choke is the best solution to make this amp more field-serviceable. In my case, because installation of a PEM nut appears too troublesome, I'll just buy a short 1.5in IERC open-top tube shield and call it a day.

Thanks again to everyone for their help!

[dchang0]

While searching high and low for a 45mm/1.75in 9-pin tube shield (just measuring the top, not including the base), I have arrived at the conclusion that they don't exist (or were very rare).

The shortest tube shield I can find for 9-pin tubes is 38mm/1.5in (nominally a 40mm or 1.5in tube shield), for the "stubby" 9-pin tubes like the 5670. One example is the IERC TR6-6015 series.

The next shortest tube shield I can find is the 48mm/1-7/8in (nominally a 50mm or 2.0in tube shield), for the "40mm" 9-pin tubes like the 12AX7. One example is the IERC TR6-6020 series.

Then there are 55mm, 60mm tube shields and longer, up to 3in, usually in 5mm or half-inch increments, depending on whether using the metric or imperial systems. I cannot find any 9-pin tube shields longer than 3in, so this appears to be the upper bound.

I don't think anyone made a tube shield for 40mm tubes like the 12AX7 shorter than 48mm, at least not in large numbers. This makes sense since there must be room for the spring to contract within the shield top.

So! My final conclusion is that Chuck was right--the problem of installing the tube shield for V5 was left upon service techs, and based on my real-world tests, I think the techs were expected to remove the choke's screw(s) and swivel at least one leg of the choke out.

I have updated the photo of the uneven tube shields to match what I believe are the measurements. (The Russian tube shield was confirmed by current-day suppliers on ebay to measure 48mm, which, combined with their shorter seated height, makes the middle tube shield appear to be as 4 to 5mm shorter than the 50mm tube shields.)

[Chuck H]

I'm not going to try to re-find and link, but I did read a couple of VOX oriented forum posts where this phenomenon is mentioned. I couldn't actually find any reports that back up the problem, but I did find posts where it's said that the actual VOX amps do allow for removal of the tube shield and tube, but just barely. Looking at many photo's I do think there must be some original VOX amps with the problem. Most look to either have a slightly different configuration. Choke thinner, shorter, both or moved a little and/or the tube sockets are slightly closer to the front panel. I'll speculate that the repro choke you have must be a clone of some model with the chassis that had the tubes a little closer to the control panel. They may have even been sold by the same vendor as compatible and stock for that amp model even though that may not be strictly the case.

[dchang0]

Thanks, Chuck. Yeah, at least one Vox historian/fanatic has to know about this, especially if it resulted in an engineering change.

Re: closer to the front panel--that's harder to pull off than left/right movement of V5, because of the fixed width of the tagstrip boards. In my build, the uppermost tagstrip board is jammed right up against the top edge of the chassis (almost to shorting out all the tags). I have seen some photos where V5 is strangely/non-uniformly spaced left-to-right with the other four tubes--if JMI consistently mounted V5 closer to the output transformer by about 1/4in, it could have solved this problem easily. But moving V5 1/4in to the left would be very strange looking without adjusting the spacing of the other four tubes...

I wonder what Jim Elyea knows about this...

Re: choke height. Yeah, if there was an engineering change at one point, it is likely that the choke I have mismatches the chassis I have: choke is pre-engineering-change and chassis is post-change or choke is post-engineering-change and chassis is pre-change OR both chassis and choke are pre-engineering-change. If I had to guess, I'd say both are pre-engineering change, that I built a honest replica of a too-early 60s-era AC-15.

Meanwhile I will google around too... I just sent an email to VOX Showroom's "Doctor Vox" to ask about this. I'll also post this question on some vintage amp forum(s).

[Tom Phillips]

How does the amp sound if the tube shield is simply not installed?
Any audible background noise difference?
Some people even believe that removing a tube shield improves the tone.

[dchang0]

I cannot hear any difference with the shield removed or installed and am currently leaving it removed, although someone with sharper ears could probably tell. Part of the problem is that by the time I have removed or reinstalled the shield, I can't remember the exact level of white noise that I had just heard. (I need an objective measurement of the amount of noise, before and after or some other form of A|B testing, like a footswitch that instantly takes the shield off or puts it on, mid-chord.)

What I will probably do is leave the shield off and install o-ring dampers (that I don't really need on V5) to convince myself that I really didn't want to install the shield in the first place, LOL. I am forced to put tube dampers on the EF86 anyway, so there will be at least one tube shield missing anyway.

As to why this bothers me--there is definitely more white noise in this amp when dimed than I would like, and it is coming from somewhere in front of the volume pots. I'm just trying to do everything within reason to minimize the sum total of noise coming from the preamp.

As you can probably tell, a large part of this is also historical curiosity. Would Dick Denney allow amps to be built like this, when he clearly went to a lot of trouble to design the chassis?

[Enzo]

Oh lord, my eyes say "Dick Cheney" there at first...

Sharper ears don't matter, your amp has to please your ears. Tube shields are there to shield out hum. I doubt the white noise would be affected anyway.

Is this tube also microphonic? If not, why add the O-rings?

[dchang0]

Cheney! That would be one massive autocorrect error, LOL!

Re: pleasing my ears: There are basically only two "problems" left with this build.

1) a bit more white noise than I'd like (if white noise is even the correct term). This may be totally normal levels of white noise for a build such as this--I may simply be nit-picking with unrealistic expectations. There is no problem with hum on this amp (any hum I can hear comes from one of my guitars that apparently has poor shielding in the control cavity, but once I unplug the cable, any and all hum goes away).

If tube shields don't cut down on white noise, what, if anything can be done to reduce white noise? I do know from experience that carbon comp resistors introduce quite a bit of white noise and went with metal film resistors for this build.

2) One or more tubes (other than the already-tamed EF86) rattles/rustles at certain notes (open A through F on the D string). I have not figured out which tube(s) it is yet. I've got more o-rings on order and will put them on each tube until the problem goes away. The pencil tapping test doesn't make it obvious which tube(s) it is. It may or may not be V5 (probably not).

I put "problems" in double quotes because really, at stage volume, other people won't notice or care about these issues.

[g1]

The pencil tapping test doesn't make it obvious which tube(s) it is. It may or may not be V5 (probably not).

Sometimes a 'reverse' tapping test will help. Hold the tubes one at a time with a rag. Tap the chassis.

[dchang0]

I learn something new every time I talk with you guys. I'll try this reverse tapping test today--thanks!

UPDATE: The reverse tapping did not identify the problematic tube(s). However, while handling the power tubes with the rag, they seemed to be more sensitive to contact than any of the preamp tubes (minus the EF86 which was by far the most sensitive to touch before I wrapped it with 3 layers of heat shrink).

[Chuck H]

Try removing the preamp tubes one at a time, starting with the first tube in the signal chain, and tapping the chassis. Ta Da!

White noise can be (usually is) thermal hiss. Some noisy preamp tubes can mimic it. And there are plenty of noisy tubes around. A loose ground, cold joint, miswire or circuit failed open can sometimes cause hiss without much other obvious difference in performance depending on the specifics. It's not a particularly high gain amp. I'd be suspecting the preamp tubes.

If you pay attention to hiss while removing tubes to isolate microphonics you may identify a hissy tube too.

[dchang0]

Thanks, Chuck! I'll try what you suggested.