Ampeg B15N: Using the Ext Amp Out

[Robal]

I recently acquired a B15N, probably a 1966, in very good shape. The tone is great but, of course, it isn't all that loud.
For band use I have a much louder and cleaner rig: an Avalon U5 DI/Preamp into a Crest CA9 power amp driving a Bag End 4x10 cab. Is there an optimal way to use the Ext Amp Out from the B15 to drive the Crest/Bag End rig?
I find that when I plug into the Ext Amp Out the volume to the B15N's own cab is cut a lot. Still sounds great. Is this an ok way to operate the B15N?
Alternatively, should I pull the power tubes on the B15N or disconnect with B15N's speaker cab while using the Ext Amp Out?
The Ext Amp Out does not provide a very hot signal but I can turn up the CA9's gain so that the output to the Bag End is fairly substantial. Alternatively, I could run the Ext Amp Out through the Avalon U5 to add some gain before the signal hits the CA9.
I will try all these alternatives but I thought it would be good to ask here first in case (1) any of them may actually harm my gear; (2) others have already gone through this process and found a winning combination or a better alternative.
Thanks for your thoughts on this.

[JimmyM]

it's an ok way to do it but certainly not optimal imho, nor easy to set up. this is why svt's caught on

[BassmanPaul]

The amp out was designed to run a slave tube amp. Trying to drive a SS power amp will load down the circuit and cut volume in the B15. If you wish to use the output with SS devices you'll need some sort of a buffer with a high impedance input to avoid the loading issues.

[aborgman]

Originally Posted by BassmanPaul

The amp out was designed to run a slave tube amp. Trying to drive a SS power amp will load down the circuit and cut volume in the B15. If you wish to use the output with SS devices you'll need some sort of a buffer with a high impedance input to avoid the loading issues.

Exactamundo.

The B15 ext amp out/in expects to see a tube grid - impedance >1 megaohm.

Your standard power amp input impedance? About 20-30Kohm.

The last stage in the preamp section can't sink enough current to drive that load impedance.

Stick a DI between the ext. amp jack and your power amp and you'll be golden.

[Robal]

Originally Posted by aborgman

Exactamundo.

The B15 ext amp out/in expects to see a tube grid - impedance >1 megaohm.

Your standard power amp input impedance? About 20-30Kohm.

The last stage in the preamp section can't sink enough current to drive that load impedance.

Stick a DI between the ext. amp jack and your power amp and you'll be golden.

So, running my Avalon U5 between the Ext Speaker Out and the Power Amp will do the trick, as far as impedance matching? Or would a passive DI be better?

[aborgman]

Originally Posted by Robal

So, running my Avalon U5 between the Ext Speaker Out and the Power Amp will do the trick, as far as impedance matching?

Yep, it has a claimed 3Megaohm input impedance.

Originally Posted by Robal

Or would a passive DI be better?

Probably no different.

[NecroticImbecil]

Interested in this as well, though I've never tried it. More generally, could this be plugged into another amp's normal input, where you would plug in the bass?

[aborgman]

Originally Posted by NecroticImbecil

Interested in this as well, though I've never tried it. More generally, could this be plugged into another amp's normal input, where you would plug in the bass?

Yes.

Anything you can drive with a passive pickup bass, you'll be able to drive with the ext. amp connection of a B15/B18.

Anything that requires a preamp/buffer, you won't be able to drive.

[NecroticImbecil]

Awesome. Thank you very much!