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My new toy - Ampeg B-18-N head

aborgman

aborgman

Jun 12, 2007
4,000
1,158
Ypsilanti, MI 48197
#1
Well, when I went to check it out I was expecting a '63 B-15-N... and it turns out to be (I think) a '65 B-18-N.

It's definitely a B-18-N, serial 001523 under the power cord - which from what I can tell makes it a 1965.

Missing the plexi and someone cut the original speaker cord off... but everything else is original, including all the tubes except the power tubes.

DSCF8375.jpg

DSCF8379.jpg


I finally got my B-series Ampeg... and for way less than the re-issue B-15 is going to go for :hyper:
 
#5
aborgman said:
Well, when I went to check it out I was expecting a '63 B-15-N... and it turns out to be (I think) a '65 B-18-N.

It's definitely a B-18-N, serial 001523 under the power cord - which from what I can tell makes it a 1965.

Missing the plexi and someone cut the original speaker cord off... but everything else is original, including all the tubes except the power tubes.

DSCF8375.jpg

DSCF8379.jpg


I finally got my B-series Ampeg... and for way less than the re-issue B-15 is going to go for :hyper:
Click to expand...

Nice,
It's got twice the power output of the B-15 and it was also used as a head for the shortlived B-15 ND. A lot of players cut that cord off because it had a four pin connector that put the amp in stand by mode when you disconnected it from the portaflex cabinet. Unfortunately, that connector was a little hard to find.

Ric

Ric
 
#7
that's actually a nice feature. when jess oliver restored my b-15n he reinstalled it and i think it's great. not very universal, no, but it works just like it's supposed to work. beats that wacky way of connecting the cab to the sb-12. ever see that? it connects through the fliptop latches.

not every idea can be a winner, i guess.
 
#12
JimmyM said:
that's actually a nice feature. when jess oliver restored my b-15n he reinstalled it and i think it's great. not very universal, no, but it works just like it's supposed to work. beats that wacky way of connecting the cab to the sb-12. ever see that? it connects through the fliptop latches.

not every idea can be a winner, i guess.
Click to expand...

One nice thing about the B-18 is the way the extension speaker jack is wired. It expects a 16ohm speaker to parallel with the built-in 16ohm speaker - so when you plug into the extension jack it connects to the 8ohm tap... even if you've got the old cable cut off.

This means you can actually just wire an 8ohm cab to the extension jack and have it be connected to the correct OPT tap - as long as the actual cab isn't also connected.
 
#17
So I got a new, safe, power cord put in - and got the ground switch of death disconnected.

It turns out the output jacks have been completely rewired - so it has the 16ohm tap to the extension speaker jack, and the 8ohm tap wired to the external amp jack.

So no preamp out, and the negative feedback loop is being fed off the 8ohm tap instead of the 16ohm tap (which appears to be wrong from the schematic)... but no big deal, it just means a little less negative feedback.
 
#18
So last night I rewired the output jacks and the NFB loop.

Connected the 8 ohm tap to the jack at the extension speaker hole (it's been replaced, no longer a shorting jack).

Connected the NFB loop feed to the 16ohm tap on the OPT (was attached to 8 ohm tap).

Connected the "ext. amp" jack back as it should be - to the connection point between the two 270k ohm mix resistors and pin 7 of the 7199 (control grid of pentode section).

Plugged a cabinet in - sounds great.

Hooked a power amp up to the ext. amp jack. Uh oh. Not so great. With the connection made there was a huge loss of volume from the cabinet (80%?) and the signal going to the power amp (Carvin DCM1000) was so low as to be barely able to drive the amp at all. Unplug the connection to the power amp and everything is back to normal.

Tried different cable between the B-18 and the power amp - no difference.

I'm pretty certain it's wired correctly - or at least, it is wired exactly as every schematic I've been able to find.

I'm going to open it back up and take another look when I get home this afternoon, and if I don't see anything I'll haul it into work and put it on a scope... but anyone have any ideas?
 
#19
The Ext Amp jack is a little strange by todays standards and was designed to chain two Portaflexes together. Rather than working like a standard line out (preamp signal going one direction to a power amp for example), it almost works like a two way conduit - so that whatever you are connecting it to may also affect the originating amp.
 
#20
stiles72 said:
The Ext Amp jack is a little strange by todays standards and was designed to chain two Portaflexes together. Rather than working like a standard line out (preamp signal going one direction to a power amp for example), it almost works like a two way conduit - so that whatever you are connecting it to may also affect the originating amp.
Click to expand...

No doubt - it's basically more like an insert point than a traditional line out... but I still wouldn't expect a power amp with a 20k ohm input impedance would load it enough to have much effect.
 

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