Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Interstellareye on October 31, 2021, 04:00:09 am
-
Why did Vox wire this way?
I’m assembling an AC15 using 1960 schematic and I’m wondering if I should follow the schematic precisely in regard to this mains wiring, or if it matters one way or the other? Looking for advice.
-
My guess is that the draughtsman had a brain fart and no one corrected it.
Fusing the neutral is a mistake, if the British Standards of the day covered it, I’m sure they would have directed against doing so.
Consider that if something at high voltage on the primary side shorted to the chassis (earth), the fuse wouldn’t blow.
Note that the contemporary ac30 fused the live side https://el34world.com/charts/Schematics/files/Vox/Vox_ac301960.pdf
I can see a merit in putting the awful voltage selector in the neutral side, eg a stray guitar string could find its way into one of the unused socket holes, so minimising the voltage there would be beneficial.
But with a modern voltage selector that complies with modern standards, that benefit disappears.
-
this is from Doug's files
Vox_ac15.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Vox/Vox_ac15.pdf)
Vox_ac15_reissue.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Vox/Vox_ac15_reissue.pdf)
might clear up some things.
I would recommend hard-wire the input volts instead of a rotary switch unless you do a lot of international giggin
-
I’m certain it’s not a mistake. I’ve seen it drawn this way on several JMI Vox schematics, and it corresponds to the actual wiring Ive found in photos.
The schematic detail i posted is from the service engineers guide to the AC30. I believe it’s probably the most accurate schematic available.
As far as providing protection for the voltage switch via connection to neutral, this sounds like a good explanation, but if the Live is wired to 0v (as in the schem detail) wouldn’t the Neutral be the more dangerous side?
-
It's not a mistake. You could switch the line and neutral wires inside the amp and it will still work. Back in the '60s there was no standard concerning line and neutral inside an amp. Very few buildings even had polarized electrical wiring. But standards were coming to building wiring codes and today the NEC (National Electrical Code) specifies that the neutral wire must be carried throughout the building electrical system in an unbroken fashion, ie, neutral must not be broken by a fuse, switch, etc. Only the Line may be broken by a fuse, switch, etc. But NEC does not rule internal appliance wiring. However, more and more we see that these NEC codes are being implemented inside appliances even though they are not governed by NEC. That's a good thing IMO.
I suspect Vox wired it that way merely because it was more convenient to do so. If I were building that amp today in the USA, I would connect the neutral directly to one side of the PT, and put any fuses and switches in the Line side. No need for a double pole power switch but OK to use if you want.
-
I’m assembling an AC15
schematic detail i posted is from the service engineers guide to the AC30
typo?
-
typo?
I think He's just showing another example to point out that the mains circuit was not just a "mistake" on the AC-15.
Most all of the old Fender amps did the same... fuse on one side, switch on the other side. It's a convenience thing. Look at the layout for this amp. Notice that both PT primary wires connect to the mains in the same general location. No need for a splice or extra terminal lug just to connect the "neutral" to the PT primary. How convenient is that?
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_ab763_schem.pdf
-
Why did Vox wire this way?
My guess is that the draughtsman had a brain fart and no one corrected it.
Fusing the neutral is a mistake ...
I’m certain it’s not a mistake.
It's not a mistake.
I just looked inside my 1964 AC30. Grosvenor made a mistake in his drawing.
The Fuse is on one wire of the power cord, which then goes to the Power Switch, then a Red wire running to the power transformer.
The other wire runs to the other pole of the Power Switch, then to the center of the Voltage Select Switch.
We could discuss which wire goes to the fuse & which to the switch (in my case, the Red Neutral wire goes to the Fuse), but it really doesn't matter because someone has probably been inside the amp tinkering at some point. Case in point, my AC30 Super Twin head has the Live (Black) going to the Fuse, but that's because I wired it that way when I replaced the power cord to have a U.S.-style plug. I cannot be certain no previous tech did not "fix the error" by putting the fuse before the power switch in the amps I own.
Every vintage Vox I have that I've checked has the Fuse before the switch, and the fused wire running into the power transformer's common connection. The unfused wire runs to the selector switch (after the power switch).
I’m certain it’s not a mistake. I’ve seen it drawn this way on several JMI Vox schematics, and it corresponds to the actual wiring Ive found in photos.
Different schematics draw it different ways. This JMI AC30 schematic (https://el34world.com/charts/Schematics/files/Vox/Vox_ac301960.pdf) shows the most-correct wiring with the fuse in the Live side and before the switch. Other schematics show the fuse after the switch, or no fuse at all. And of course, UK plugs have a fuse built-in inside the plug, so...
-
It's not a mistake. You could switch the line and neutral wires inside the amp and it will still work. Back in the '60s there was no standard concerning line and neutral inside an amp. Very few buildings even had polarized electrical wiring. But standards were coming to building wiring codes ...
My view is it is a mistake, ie the designer / draughtsman / engineering director were negligent. Reason being that in the UK, by the late 50s, such standards were already in place and should have been implemented, both within electrical equipment and in building electrical installations.
-
The AC-10 does not even have a fuse.
-
https://el34world.com/charts/Schematics/files/Vox/Vox_ac10.pdf
Yikes!
I suppose the designer must have been thinking that the plug top fuse would be sufficient, and perhaps didn’t know any better.
-
The AC-10 does not even have a fuse.
Very true. My AC10 Twin has an internal line fuse that had been added later by a tech. However, the AC10 SRT (https://el34world.com/charts/Schematics/files/Vox/Vox_ac10_srt.pdf) does have a fuse.
Anyway, you're building an amp. Your best move is to build it using best-practice that's available to you now: fuse before switch, on the Hot wire. Probably add a HT fuse as well to protect your output transformer, just in case.
-
I just looked inside my 1964 AC30. Grosvenor made a mistake in his drawing.
The Fuse is on one wire of the power cord, which then goes to the Power Switch, then a Red wire running to the power transformer.
The other wire runs to the other pole of the Power Switch, then to the center of the Voltage Select Switch.
I believe this is indeed how Grosvenor has it depicted in the AC30 schematic.
We could discuss which wire goes to the fuse & which to the switch (in my case, the Red Neutral wire goes to the Fuse), but it really doesn't matter
In the UK the convention was Red- Live, Black- Neutral, Green- Earth
-
I’m assembling an AC15
schematic detail i posted is from the service engineers guide to the AC30
typo?
Sorry, perhaps I should have mentioned that although the text is called “Service Engineers Guide to the Vox AC30 Valve Amplifier” it includes documentation, schematics and layouts for all of the original Vox JMI valve amps including all of the AC30’s, AC15’s, AC10 and AC2/AC4. It’s a fantastic guide, and considered cannon by many (from what I’ve read anyway).
Most all of the other schematics I’ve seen online are redraws by Korg. They may be updated for current best practices in some cases, which is helpful in some ways but they may not always depict exactly what’s in the amps.
-
We could discuss which wire goes to the fuse & which to the switch (in my case, the Red Neutral wire goes to the Fuse), but it really doesn't matter
In the UK the convention was Red- Live, Black- Neutral, Green- Earth
Good to know!
This AC30 was shipped from the UK, and I had to change out the plug. So I wound up wiring the plug Red to Neutral, Black to Hot.
-
I want only add this info
Some VOX amps were build in Italy and in Italy all wall electrical sockets are reversible
so neutral and line can be reversed at the imput of the amp
well, that isn't exact, for a small period we had a wall electrical socket that wasn't reversible, the MAGIC electrical socket
(https://i.imgur.com/9qyel1D.jpg)
so neutral and line can be reversed at the imput of the amp
Is this important to understand the schematics ? I dont know
As far I can know the electrical plugs in England are not reversible and have a built-in fuse but I don't know when this standard came into effect
(https://i.imgur.com/wvWaAKR.jpg)
Franco
-
As far I can know the electrical plugs in England are not reversible and have a built-in fuse but I don't know when this standard came into effect
I have a ‘62/‘63 AC10 twin that came from the UK with original attached fused lead (and no other internal fuse).
-
If I were building that amp today in the USA, I would connect the neutral directly to one side of the PT, and put any fuses and switches in the Line side. No need for a double pole power switch but OK to use if you want.
I’m going to follow this suggestion. I appreciate all of the replies and advice from everyone. I feel good about it now.
I was inclined to believe there was a design principle motivating Vox’s line/neutral mains wiring. If anything, I think of Vox amps as being somewhat over engineered, not cutting corners (so to speak) for convenience. But whatever the reason was, it sounds like it’s not gonna effect the operation to wire it up in the conventional way.
-
… in Italy all wall electrical sockets are reversible
so neutral and line can be reversed at the imput of the amp
…
I think modern best practice may be to fuse then switch both sides, as either may be live (with respect to earth)?
As far I can know the electrical plugs in England are not reversible and have a built-in fuse but I don't know when this standard came into effect ….
The BS1363 plug seems to have been introduced in 1947! :icon_biggrin:
It’s been ubiquitous as far back as I can remember, back to the early 70s. Though I can just about recall noticing that houses of elderly relatives had the occasional old appliance / socket with round pins (odd kid :icon_biggrin: ).
https://en.m.wikipedia.org/wiki/AC_power_plugs_and_sockets:_British_and_related_types
-
....I think modern best practice may be to fuse then switch both sides, as either may be live (with respect to earth)?...
True; but FWIW in the USA we do not bother with "best practice". (And that's not because our now-polarized outlets are perfect; in fact they are often wrong.)
Never open the box while plugged-in. Is that too hard?
Never change the fuse while plugged-in. (Do as I say, not as I do.)
The BS plug in the UK goes to post-WWII rebuilding. The UK had been plagued with many different plugs, and in the rebuilding a particular goal was a 3KW electric fire (central heat was rare). And low-low cost for the millions of housing units needed. So high-Amp ring mains, the 13A plug, and fuses in the plugs. If the plug is fused for the type of cord used (they go down to 2A) there may be no need for any other fusing. (But users are clever, also no other land uses fused plugs, Britain desperately needed foreign sales, so eventually internal fuses became common.)
-
I was thinking that with unpolarised mains connectors, to ensure that the equipment fuse blew in response to the live circuit shorting to the chassis earth somehow, both sides of the mains current circuit would need to be fused.
-
The BS plug in the UK goes to post-WWII rebuilding. The UK had been plagued with many different plugs, and in the rebuilding a particular goal was a 3KW electric fire (central heat was rare). And low-low cost for the millions of housing units needed. So high-Amp ring mains, the 13A plug, and fuses in the plugs. If the plug is fused for the type of cord used (they go down to 2A) there may be no need for any other fusing. (But users are clever, also no other land uses fused plugs, Britain desperately needed foreign sales, so eventually internal fuses became common.)
As a trained historian I always appreciate reading the context behind this stuff.
-
...to ensure that the equipment fuse blew in response to the live circuit shorting to the chassis earth somehow,....
That's the job of the cellar fuse.
If that is wired wrong, move out!
-
…
Most all of the old Fender amps did the same... fuse on one side, switch on the other side. …
I think that view may be taking the design out of the context / environment in which the designer was operating. In the era and region in which Fender wired things that way, the mains connectors used weren’t polarised or earthed, hence it wasn’t conceptually feasible to choose to fuse live rather than neutral. And due to the lack of earthing, it would be pointless fusing both mains conductors.
Whereas in the UK, polarised, earthed connectors for wall outlets were well established.
From the above, the mains fusing / switching of the 5E3 seem a reasonable choice, whereas those of the AC15 don’t. Though they’re an improvement on the AC10 :icon_biggrin:
We’re only human, good engineers make occasional ‘non ideal’ design decisions. If they’re the most senior competent person, due to deference etc, they may go unchallenged.