Ampeg B-15 speaker output problem

[stratslider]

I've got a 1967 Ampeg B-15 fliptop tube bass amp (used to belong to the CARS at their Boston studio, SyncroSound), that works fine and sounds great, but has a weird problem with it's speaker output. With the main (1-15") speaker cab plugged in, the amp will be on, but will NOT produce any sound until there is another speaker cab hooked up to the "extension speaker" 1/4" jack- at which time the amp works fine, and plays through both speaker cabs.
I've tried just putting a dummy plug into the "ext spkr" jack, and it doesn't work- it actually has to be connected to a working speaker load. I think this was a mod that someone at the studio did for some obscure reason, and I'm thinking that it will probably be a simple DIY fix (since the amp works finr besides this problem), but I have no idea how to do it.
Any ideas?

[Tom Phillips]

If the amp has been moded you will need to trace out the circuit and figure out what's wrong.

However, here is my best idea of what could be wrong.

First: Don't try to play through the amp if you are not hearing any background hum in the speaker. The output is running open circuit and you could damage the output transformer and/or output tubes.

The extension speaker jack in your amp has a shorting switch that selects the 8 Ohm tap when only the main speaker is plugged in. I suspect that this jack switch bar is not making contact. You said you tried to put in a dummy plug. Was it a shorted dummy plug? It nees to be in order to complete the circuit. Try that. If it works then I'd say that you need to re-tension the external speaker jack or replace it with a good quality (Switchcraft) new one.

This is the easiest thing to try first. Let us know if you need more help.

You can find schematics at Schematic Heaven.
There are some variations.
Try http://schematicheaven.com/ampegamps/b15n_portaflex.pdf
or
http://schematicheaven.com/ampegamps..._portaflex.pdf

Regards,
Tom

[stratslider]

That was the problem, thanks.

Hi Tom, thanks for the knowledge.
There was no mod on the B-15, just a loose (on the inside) ext spkr jack. As soon as I put in a dummy jack that was shorted out, the amp worked without an extension speaker cab hooked up.
I'll go inside the amp tommorrow and try re-tensioning the jack, and replace it if I have to.
I did know not to play with the amp fully on and speakers not working. However, would it hurt the amp to play through the one ampeg cab, with the shorted dummy plug in the extension speaker jack? From what you were saying, as long as that's shorted (same as not plugged in, if the jack were tensioned right?) the amp will put out the correct 8 ohms into the one 15" speaker?
Thanks again.

[Tom Phillips]

...would it hurt the amp to play through the one ampeg cab, with the shorted dummy plug in the extension speaker jack? From what you were saying, as long as that's shorted (same as not plugged in, if the jack were tensioned right?) the amp will put out the correct 8 ohms into the one 15" speaker?
Thanks again.

With the shorting plug in the external speaker jack the 16 Ohm tap will be selected so you will have a mismatch for your 8 Ohm speaker. However, I think it will be OK to play the amp that way for a while.

I'm glad it was something simple.

Regards,
Tom

[Tom Phillips]

One more piece of info.
If you have to replace the external speaker jack, note the insulating fiber washers that are used to mount it. The jack needs to be isolated from the chassis to work in that circuit.
Tom

[Tom Phillips]

Note my edit to message number 4 concerning the impedance mismatch.
I goofed up on my first response.
Time for some sleep.
Tom

[stratslider]

Well, it's good to know I can use the amp by just leaving in the shorted jack- or an extension cab, as I've been doing to use it up til now. But I'm going to try and fix it from the inside. Good to know about isolating the jack from the chassis.
Thanks again for all the info,
Jonathan Fixler
Waltham, MA