Jess Oliver's legacy - AMPEG PORTAFLEX CLUB part 9

[Seans]

Just got myself a 67 B15N, really sweet and pretty much as original ( not sure on the knobs), 6l6's bit tired so out came a lovely pair of Brimars, unfortunately they're just too big.
One mod that has been done is Point 'F' has been taken to ground, anyone come across this before.

[beans-on-toast]

Congratulations on the new amp Sean. Very nice. I like the Brimars but you're right about them being too close for comfort.

The mod that you mentioned is done to allow the amp to work without the speaker cabinet connected. In the amp, the two point F's are joined together. Point F has to be grounded for the amp to work. If it isn't, the power amp is disabled. This protects the amp in case it is turned on without the speaker cabinet connected. The 4-pin connector on the cabinet has pin-2 and pin-3 shorted. When you plug in the cable from the amp, this makes the connection between pin-2 and pin-3 at the amp cable end and completes the ground connection to point F. This allows the power amp to function.

Why would someone perform this mod? You might do it if you wanted to connect a different cab that didn't have a four pin connector. You can buy or make a cable that has a 1/4" plug at one end and a for pin connector at the other end. Normally you would simply short pin-2 and pin-3 in this cable so the amp wouldn't need to be modified.

They did something similar, but in a different way, in the early SVT's that also had the for pin connectors. In your SVT they wouldn't let the high voltage power supply turn on. It was wired in-line with the standby, acting like a second switch.

[JimmyM]

Sean, I thought you were already in but I didn't see your name on the list. If you already have a number, let me know, but if not:

#316 Seans

[ddbassGA]

Seans, nice find. Yes those are stock knobs. Late '66, early '67 they switched to these. My late '66 has same ones.

[Seans]

Originally Posted by beans-on-toast

Congratulations on the new amp Sean. Very nice. I like the Brimars but you're right about them being too close for comfort.

The mod that you mentioned is done to allow the amp to work without the speaker cabinet connected. In the amp, the two point F's are joined together. Point F has to be grounded for the amp to work. If it isn't, the power amp is disabled. This protects the amp in case it is turned on without the speaker cabinet connected. The 4-pin connector on the cabinet has pin-2 and pin-3 shorted. When you plug in the cable from the amp, this makes the connection between pin-2 and pin-3 at the amp cable end and completes the ground connection to point F. This allows the power amp to function.

Why would someone perform this mod? You might do it if you wanted to connect a different cab that didn't have a four pin connector. You can buy or make a cable that has a 1/4" plug at one end and a for pin connector at the other end. Normally you would simply short pin-2 and pin-3 in this cable so the amp wouldn't need to be modified.

They did something similar, but in a different way, in the early SVT's that also had the for pin connectors. In your SVT they wouldn't let the high voltage power supply turn on. It was wired in-line with the standby, acting like a second switch.

Thanks David, all makes perfect sense when explained logically, I seem to remember this 4 pin on the SVT giving me confusion, already having been converted to a 1/4 jack and not the long cable.

[Seans]

Originally Posted by JimmyM

Sean, I thought you were already in but I didn't see your name on the list. If you already have a number, let me know, but if not:

#316 Seans

Cheers, JimmyM, I hadn't got a number.

ddbassGA, seems a minefield of changes went on in this period hard to keep up.

[Black Sheep]

Nice, Seans! I am working on my own '67 now, had it a few months, and still blows the fuse when turned on. First thing was to sort out the power and ground switch (bad previous mods), using a grounded cord. (It can be taken back to orig if necessary, but I plan on using this amp, so I wanted it this way)
I also had to deal with the speaker connection mod like yours. Both sides of the 4 prong connection were gone, so for now I used a TRS jack, still allowing the unpluged protection, but also allowing use of a standard 1/4" speaker cable.
Next is to test the orig caps, and possibly re-cap. Not sure if a bad tube would blow a fuse immediately like this.

With S/N 0658XX I believe mine is a '67, but it has the round knobs with the silver faces.
(says B15-N on the shell, but B15-NF on the main circuit board)
Any know of a schematic specifically for a '67? I have found '68 B15-NF, and '67 B18-N, but that's as close as I've come.

[spacebassed]

The "F" just designates a circuit revision - they are all B15N's (except for the original B15 made from '60 into '61). The original B15N was made from '61 into '63, then they made a revision to the circuit (replacing the 5u4 tube rectifier with a solid state rec) in '63 the called these amps B15NA's, soon after they changed from the octal plug to a 4-pin xlr to connect the head to the cab - this was called the B15NB. Then in '64 they went back to a tube rec., but this time a 5ar4 - this was the B15NC. Then in '65 they switched from cathode bias to fixed bias, these were called B15NF's - which is what you have. In '68 there were ultra hi/ultra low rocker switches added, so you don't want that schematic. There were other little changes (changed values, etc.) along the way that I didn't note, but those are the main differences. Any schematic labeled B15NF prior to the added switches is what you want - I'm sure someone will post one (I know Mark at Vintage Blue is redrawing all of the B15 schematics).

[Black Sheep]

Thanks, space.
The '68 Piazza drawing is the closest I've found and I don't see additional switches on it that you mentioned. There may be some other values that are different.
(tried to get back on fliptops to look at caps, I think I crashed the site...)

[christw]

I'd be happy to snap some pics of my black tolex & piping '67's tray schematic in the next few days. No rocker switches here.

[beans-on-toast]

Originally Posted by Black Sheep

With S/N 0658XX I believe mine is a '67, but it has the round knobs with the silver faces.
(says B15-N on the shell, but B15-NF on the main circuit board)
Any know of a schematic specifically for a '67? I have found '68 B15-NF, and '67 B18-N, but that's as close as I've come.

Ampeg would only issue schematics if the revision would change. The schematic was almost the same from 65 through 67. In 67 the B-15N started off looking like the earlier models with the stove knobs and blue check vinyl. Later in the year they changed the look. In 68 there was a redesign of the amp where they added the ultra hi and lo tone switches.

Check out Mark's schematics at Vintage Blue. Below is a 1965 schematic. Click on the image for bigger versions. In 66 the treble pot on channel 1 was changed from 4 Meg ohms to 1M. As far as I know, the 1967 circuit remained the same as the 1966.

It would be interesting to verify that against your schematic Chris.


David

[Aussie Mark]

Another photo update from the build of my Mal O'Neill custom B15-100 ....

1964 and 1966 circuits, at the flick of a switch

[JimmyM]

That's no amp! That's a park bench!

[adambomb]

Oh my Lord

Jimmy you are hilarious

[christw]

Originally Posted by adambomb

Oh my Lord

Jimmy you are hilarious

+1 I busted out laughing at my bar when I read this.

[Seans]

Mines 065992, it's got the silver binding on the outer rim of the speaker cab, thinking a lot happened this year and also thinking it was a high production year plus lots of spares used up, have seen a site here (uk) where many have been for sale and serial numbers recorded inc pictures giving a very interesting resource of many 67's all seeming different.

edit found it, prices ?? too... andybaxterbass.com/guitars.php?cat=8

[beans-on-toast]

Interesting site. If you want one, it looks like a good place to check out.

Ampeg worked hard to make inroads in the UK but their amps were expensive so they must have been a hard sell. They had a reciprocal deal with Burns. They were big in England but the Burns US prices were sometimes higher than comparable Fender instruments. Given a choice of a Burns or a Fender bass, the choice was clear for most. Still, Burns instruments are highly collectable.

You used to see Burns copies in the 60's made by Teisco and other companies. The instruments that Sears offered were more expensive knockoffs than the ones at places like K-Mart. Still you could find a playable instrument for not too much money. The first one that I bought, a Fender copy with rocker switches, was $25. It doesn't seem like a lot but you could buy a used VW that needed work for $50.

[Duke21]

Originally Posted by JimmyM

That's no amp! That's a park bench!

I didn't know Ampeg made B15's in a park bench format!!

[Seans]

Originally Posted by beans-on-toast

Interesting site. If you want one, it looks like a good place to check out.

Ampeg worked hard to make inroads in the UK but their amps were expensive so they must have been a hard sell. They had a reciprocal deal with Burns. They were big in England but the Burns US prices were sometimes higher than comparable Fender instruments. Given a choice of a Burns or a Fender bass, the choice was clear for most. Still, Burns instruments are highly collectable.

You used to see Burns copies in the 60's made by Teisco and other companies. The instruments that Sears offered were more expensive knockoffs than the ones at places like K-Mart. Still you could find a playable instrument for not too much money. The first one that I bought, a Fender copy with rocker switches, was $25. It doesn't seem like a lot but you could buy a used VW that needed work for $50.

It was very helpful when researching mine, loads of pics especially, but really, does he get these prices, that's $2500 on average for what are basically good looking originals and perhaps not even re capped.

[Aussie Mark]

Originally Posted by Aussie Mark

Another photo update from the build of my Mal O'Neill custom B15-100 ....

1964 and 1966 circuits, at the flick of a switch

For the comedians who don't know the background of this build, here's the rest of the story ....

Another progress pic from the builder. Should be ready for load testing this week.