Ampeg Squareback wiring question

[sleepy_monkey]

I picked up an Ampeg squareback last month and just now getting around to messing with it. It has the old school connection and I want to replace it with 1/4” input.

Is it as simple as taking these two wires off and soldering onto a 1/4”? I looked for the info and couldn’t find it. Just want to make sure I’m not missing something.

 

[agedhorse]

Not if you plan on using it with the matching head.

Iirc, there are 2 additional connections that are jumpered on that 4 pin jack.

 

[Wasnex]

I picked up an Ampeg squareback last month and just now getting around to messing with it. It has the old school connection and I want to replace it with 1/4” input.

Is it as simple as taking these two wires off and soldering onto a 1/4”? I looked for the info and couldn’t find it. Just want to make sure I’m not missing something.

Here is how the 4 pin connector was wired:

• Pin 1 should be ground (-) and pin 4 should be signal (+)
• Pins 2 and 3 were shorted together on the speaker side. The short served as an interlock for the matching amp's power supply. Essentially the amp was kept in standby unless the interlock was in place.

This is shown on the 70 schematic.

If you need convert the main speaker out on your amp to 1/4", you need to add a jumper to the power supply (connect X and Y).

Here is the same part of the power supply from the 74 schematic, sans interlock.

When Ampeg converted to 1/4" on both outputs, they used a shorting plug on the main speaker out (J4).

The shorting jack is to protect the amp if it's set to operate with no speaker connected.

 

[sleepy_monkey]

Not if you plan on using it with the matching head.

Iirc, there are 2 additional connections that are jumpered on that 4 pin jack.

I dropped the head off with a tech today to see if it is worth getting fixed.

 

[sleepy_monkey]

Here is how the 4 pin connector was wired:

• Pin 1 should be ground (-) and pin 4 should be signal (+)
• Pins 2 and 3 were shorted together on the speaker side. The short served as an interlock for the matching amp's power supply. Essentially the amp was kept in standby unless the interlock was in place.

This is shown on the 70 schematic.
View attachment 7257356


If you need convert the main speaker out on your amp to 1/4", you need to add a jumper to the power supply (connect X and Y).
View attachment 7257358


Here is the same part of the power supply from the 74 schematic, sans interlock.
View attachment 7257374

When Ampeg converted to 1/4" on both outputs, they used a shorting plug on the main speaker out (J4).
View attachment 7257357
The shorting jack is to protect the amp if it's set to operate with no speaker connected.

Thank you. Great info.

Probably easier just to use the cable from Fliptops I guess.

4 Pin Female to 1/4" Plug Cable for Portaflex Amps, V4, SVT - Product Details

www.fliptops.net

 

[Wasnex]

Thank you. Great info.

Probably easier just to use the cable from Fliptops I guess.

4 Pin Female to 1/4" Plug Cable for Portaflex Amps, V4, SVT - Product Details

www.fliptops.net

AFAIK,your head does not require the interlock in the cab; so I think it would be better to convert the cab to 1/4".

I copied this from you post here: Ampeg SVT (old school) amp help - 60s/70s

This amp has two 1/4" outputs. The one one on the left should be the main Spkr out and the one on the right should be the EXT out.

To run one 4 ohm cab, use the main Spkr out. To run two 4 ohm cabs, use both. I suggest labeling them so you don't forget which is which.

 

[sleepy_monkey]

AFAIK,your head does not require the interlock in the cab; so I think it would be better to convert the cab to 1/4".

I copied this from you post here: Ampeg SVT (old school) amp help - 60s/70s
View attachment 7257409

This amp has two 1/4" outputs. The one one on the left should be the main Spkr out and the one on the right should be the EXT out.

To run one 4 ohm cab, use the main Spkr out. To run two 4 ohm cabs, use both. I suggest labeling them so you don't forget which is which.

Thanks. I dropped the amp off at a tech today. Should find out in a few days if it is worth getting up and running.

 

[agedhorse]

If the head and cabinet were used together, they both should have the 4 pin connector unless I am missing something here.

A truly qualified tech shouldn't have a problem servicing it properly, may not be an inexpensive repair but most of the parts are still available.

 

[Bassamatic]

Just as an aside - the XLR 4 pin connector is probably a better speaker connector than a 1/4". The contact area for a 1/4" plug to socket is so small! Yeah, I know they work but they have always made me uncomfortable running a lot of power through 1-2 square mm of contact area. This is a strong reason behind the development of the Speakon connector.

 

[coreyfyfe]

If the head and cabinet were used together, they both should have the 4 pin connector unless I am missing something here.

A truly qualified tech shouldn't have a problem servicing it properly, may not be an inexpensive repair but most of the parts are still available.

That amp is too “new” to have come with the 4 pin connector. The connectors were removed in favor of two 1/4” jacks as early as 1972 or 73, it’s off the 72-dated schematics. The flatback cabs were phased out by 74 and the amp in the pictures is at least a 75.

If they were used together I would assume the original owner had been using one of the conversion plugs.

 

[agedhorse]

That amp is too “new” to have come with the 4 pin connector. The connectors were removed in favor of two 1/4” jacks as early as 1972 or 73, it’s off the 72-dated schematics. The flatback cabs were phased out by 74 and the amp in the pictures is at least a 75.

If they were used together I would assume the original owner had been using one of the conversion plugs.

Sorry, I was under the impression that some of these amps continued to use the 4 pin jack with primary interlock into the mid 1970's. The AC primary interlock created a regulatory problem for Ampeg once they migrated the safety testing to a UL contract.

Bill Hughes was proud of his interlock solution, though it may not have been his ideal alone. It did insure that the cabinet was connected since the interlock jumper was contained within the cabinet jack. It was pretty much 100% effective against an unloaded speaker out condition when used with the matching Ampeg cabinets. Other early Ampeg amps also used this interlock system, I remember seeing this on an old amp in his office at Fender when I was visiting (and being grilled, I passed the grilling and remained employed), I hadn't actually seen one in the wild before, so he told me the story (some of which perhaps deserves a grain of salt, but he was a very sharp old school engineer).

 

[sleepy_monkey]

Got it, I was a little confused by that as well. So as long as I get the 4 pin to 1/4” cable I can run whatever amp (within reason) I want, correct?

 

[coreyfyfe]

Sorry, I was under the impression that some of these amps continued to use the 4 pin jack with primary interlock into the mid 1970's. The AC primary interlock created a regulatory problem for Ampeg once they migrated the safety testing to a UL contract.

Bill Hughes was proud of his interlock solution, though it may not have been his ideal alone. It did insure that the cabinet was connected since the interlock jumper was contained within the cabinet jack. It was pretty much 100% effective against an unloaded speaker out condition when used with the matching Ampeg cabinets. Other early Ampeg amps also used this interlock system, I remember seeing this on an old amp in his office at Fender when I was visiting (and being grilled, I passed the grilling and remained employed), I hadn't actually seen one in the wild before, so he told me the story (some of which perhaps deserves a grain of salt, but he was a very sharp old school engineer).

Most of the B series combos amps had the 4 pin cables including this 60s and that carried over into some of the early 70s heads including the B25.

 

[Wasnex]

Got it, I was a little confused by that as well. So as long as I get the 4 pin to 1/4” cable I can run whatever amp (within reason) I want, correct?

Yes.

The shortcoming is if the adapter cable is forgot our damaged. 1/4" to 1/4" speaker cords are much more common....more likely a spare will be readily available.

 

[sleepy_monkey]

Here is how the 4 pin connector was wired:

• Pin 1 should be ground (-) and pin 4 should be signal (+)
• Pins 2 and 3 were shorted together on the speaker side. The short served as an interlock for the matching amp's power supply. Essentially the amp was kept in standby unless the interlock was in place.

This is shown on the 70 schematic.
View attachment 7257356


If you need convert the main speaker out on your amp to 1/4", you need to add a jumper to the power supply (connect X and Y).
View attachment 7257358


Here is the same part of the power supply from the 74 schematic, sans interlock.
View attachment 7257374

When Ampeg converted to 1/4" on both outputs, they used a shorting plug on the main speaker out (J4).
View attachment 7257357
The shorting jack is to protect the amp if it's set to operate with no speaker connected.

AFAIK,your head does not require the interlock in the cab; so I think it would be better to convert the cab to 1/4".

I copied this from you post here: Ampeg SVT (old school) amp help - 60s/70s
View attachment 7257409

This amp has two 1/4" outputs. The one one on the left should be the main Spkr out and the one on the right should be the EXT out.

To run one 4 ohm cab, use the main Spkr out. To run two 4 ohm cabs, use both. I suggest labeling them so you don't forget which is which.

My project is moving slower than I’d like. So revisiting this…since my SVT has 1/4” out…and I want to wire up a 1/4” jack on the cabinet instead of using the 4-pin. Do I need to rewire any speakers or anything or is it as simple as one of the wires is - and one is + and move those to the 1/4”? Any idea which one is Pin 1 and Pin 4 on the 4pin?

 

[agedhorse]

The PIN numbers are on the front side of the jack.

The interlock contacts have a wire jumper installed that doesn’t go to the speakers.

 

[Wasnex]

is it as simple as one of the wires is - and one is + and move those to the 1/4”?

I am about 99.9999% sure this is the answer.

 

[sleepy_monkey]

The PIN numbers are on the front side of the jack.

The interlock contacts have a wire jumper installed that doesn’t go to the speakers.

I am about 99.9999% sure this is the answer.

Thanks fellas. So Pin 1 is the green wire and Pin 4 is the black. So if you are looking from the outside of the cabinet the pins are in order…1, 2, 3, 4….and reavers from the speaker wire connection side.

All speaker connections are marked with a green dot next to the connectors.

So based on the guidance above the green wire is ground and the black is signal. No need to mess with the jumper/interlock stuff once converted to 1/4” I assume.

 

[agedhorse]

Correct, the interlock only applies to the amps that use this style of interlocking connection.

 

[beans-on-toast]

All speaker connections are marked with a green dot next to the connectors.

Color’s aside, the standard is, all the green dot terminals are connected together and the wire goes to ¼” jack tip. The other speaker terminals are connected together and go to ¼” jack sleeve.

It would be a good idea to test the polarity of the connected speakers. Do a 9V battery test. Plug a speaker cable into the 1/4” jack. On the other end of the cable, connect battery [-] to the plug’s sleeve. Momentarily tap the ¼” plug tip to battery [+]. All the speaker cones should move outward. If any speakers cones move inward, reverse the those speaker connections.