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

[J M Fahey]

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

They werenīt, period.
To think non tech Musicians messing with amp guts would be ludicrous.

Amp was assembled and tested at the Factory, on a workbench where that would not have been an issue and premp tube failing would have been a non issue, once they were tested at the Factory.

IF one still failed, a very unlikely issue, it would be replaced by a Tech who would discover the failure when brought the amp for repair.

That they probably had a ton of "short" shields would have been an extra nail in the coffin.

FWIW I still repair *old* Fahey amps from the 80īs, many of which have fried egg shaped TO3 2N3055 or "Plastic TO3" 2N3055, TIP 35 and early TIP142/147 , again with an egg shaped end,

and all modern replacements are plastic rectangular shaped and somewhat larger TO218 or TO247, sometimes they fit very tightly on heat sinks

Itīs not fair complaining about a designer not having a crystal ball and predicting what will be used to maintain their stuff 30 to 60 years later

[dchang0]

Re: 30 to 60 years later--well, I'm wondering:

1) Was this an issue at the time (in 1961 through 1965)?
2) If it was an issue, did JMI choose not to do anything about it?

It would have affected service techs of their time, not just service techs 30-60 years later.

I do get what you say that the factory probably expected V5 to never be replaced. Even so, the assembly line probably didn't like having to move the choke to install the tube shield (assuming preamp tubes and shields were installed last--they might have been installed before the two chassis were put together). If they did have the choke blocking V5, why did they not adjust the spacing of the tube sockets to move V5 a bit to the left and/or move the choke a bit to the right? Easy fix and good for everyone including the assembly line and service techs then and into the future.

Also,

3) If it weren't an issue, what was the configuration (that I could hopefully copy today)?

[Tom Phillips]

...If it weren't an issue, what was the configuration (that I could hopefully copy today)?

Probably use the exact original production parts (Main chassis AND Pre-amp chassis AND Tube socket AND tube AND tube shield AND choke) You currently have none of those and therefore probably have a stack up of small differences that cause the problem in your build.

[dchang0]

I'm going to ask owners of original 1960s AC-15s on vintage amp forums to indicate if they have the problem and hopefully measure things if they can.

No word back from Doctor Vox yet. If I can contact Jim Elyea, I will...

[dchang0]

It occurred to me that there is another way to get to the tube shield of V5 after the amp has been fully assembled.

If one removes the four screws tying the feet of the transformers to the upper chassis and loosens the three lower screws that hold the upper chassis to the lower chassis, it is possible to tilt the upper chassis back a bit so that the tube shield of V5 points up, over the top of the fat windings of the choke. We don't have to clear the top of the frame of the choke, just the top of the windings.

This avoids having to remove the lower chassis from the slider board (if the three screws joining the two chassis have Keps nuts or locking star washers, which can be tightened without removing the slider board by pulling out on the screw while it is being tightened). All of these screws and nuts are accessible without having to remove the slider board. And the oblong holes punched in the upper chassis for the transformer feet would have allowed for tilting.

These actual steps are not possible on my build because the modern cage nuts are large enough to prevent tilting the upper chassis back very much, but the older cage nuts of the 60s amps may have allowed it. Really early chassis did not have cage nuts at all, so this would have been even easier.

So, the service bulletin would say:

1) Remove back cover and slide out slider board no more than half way. Discharge capacitors using bleeder probe and observe all safety procedures.

2) Remove upper transformer screws (x4) and nuts and set aside. (Circled in red in attached photo.)

3) Loosen lower chassis screws (x3) without removing them completely. (Red arrows in attached photo.)

4) Tilt back upper chassis without removing it. Be careful not to sever any wires.

5) Replace V5 tube shield and tube.

6) Tighten lower chassis screws, starting with center one and using the tilted chassis to keep the unreachable Keps nut engaged while turning the screw.

7) Replace upper transformer screws and nuts, making sure grounding tabs are tightly secured.

8) Slide chassis back into amp and replace back cover.



This procedure would be acceptable to me as a field service tech. Not fun, but at least you don't need a counter top or workbench, and the most lethal voltages are hidden by the slider board.

From the photos of old AC-15s, it does not look like the transformer screws are long enough to leave them attached while tilting back the upper chassis, but it may have been possible if the nuts were hanging on by just one turn.

[dchang0]

Speaking of service bulletins, does anyone have a copy of this out-of-print book:



"A Service Engineer's Guide to the Vox AC30 Valve Amplifier: Includes the AC15, AC10, and AC4 Amplifiers" by Stephen Grosvenor

It may be possible that this book talks about the choke blocking the V5 tube shield issue...

[J M Fahey]

Looking again at pictures posted here:
1) the tube itself is well away from the choke, no problem there
2) the tube shields are way longer than the tube, in part because they house a rather large spring, ... which isnīt really necessary, since the amp works happily for ages without any kind of shield
3) the main usefulness of a shield is .... well .... to shield.
Not even the long one shields 100% since it has that hole on top; even if that hole were larger, as in: same diameter as shield, that would change shielding very little, if at all, the main effect comes from enclosing sensitive tube parts head to feet inside a grounded metal tube.
4) so I suggest you fire up your trusty Dremel with a cutting wheel and cut shield top level with tube top ... I bet it will work well and be easy to insert/remove.
It will be flimsier, but I bet it can still be handled, just donīt be heavy handed. Remember to deburr the edge.
5) to make it better, you might cut it 5 mm longer than tube glass and bend top edge inwards, not unlike a shotgun shell crimp:


wonder why they labelled this picture: "the good, the bad and the ugly"

[dchang0]

Thanks, JM--

Actually, I was planning on cutting the tube shields 1/4in shorter from the bottom. It should be possible to re-cut the J-shaped slots with very small tin snips and clean them up with needle files and re-bend the little "arches" that the bayonet lugs go under with a pair of small pliers. But I wanted to find proof that JMI used shorter tube shields before doing this. I would cut all five tube shields shorter to keep the look uniform.

Otherwise, I was going to buy one of these NOS short 1.5in IERC tube shields, still widely available and cheap:

Tube Shield 9 Pin Miniature IERC TRN6-6015B NEW | eBay

It is exactly what you are thinking, but with no cutting necessary.



This is a last resort--I'm hoping to get confirmation from a Vox owner of the original part and then go find a set of five of those to try to get as close to period correct as possible. I see that one other clone builder (plexi over at vintageamps.com) uses more-accurate British NOS tube sockets and shields than I do and they look really nice, so what I may do is replace all my definitely incorrect tube sockets with the correct ones once I find out:

a) will they solve this choke-in-the-way problem (are they shorter or do they have a wider mouth)?
b) are they period-correct?
c) where do I buy them?

[Enzo]

I have a bag of shields like the ones at the botom of this image, they grip the tube, not the base.

[dchang0]

The Attachment 41194 link is broken. I assume you refer to the IERC tube shields and not the Eby/Cinch/Elco ones... Yeah, that is one reason I consider the IERC tube shields a last resort--they would be skinnier than the typical bayonet-style tube shields and thus might be harder to slip on over the tube at an angle, though the 1.5in ones are so short they will likely still succeed.