Line out issue - 1965 Ampeg B-18-N


2008-07-31 1:31 pm
Picked up a 1965 B-18-N a week back, and finally got around to opening it up yesterday.

Previous owners had

cut off the original speaker cord
wired the OPT 8 ohm tap to the "ext. amp" jack
wired the OPT 16 ohm tap to the "ext. speaker" jack (jack replaced with a non-shorting type)

So I:

Put a new, safe, grounded power cord put in - and got the ground switch of death disconnected.

Connected the 8 ohm tap to the "ext. speaker" jack

Connected the NFB loop feed to the 16ohm tap on the OPT (per schematic).

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?


2008-07-31 1:31 pm
The input impedance of the power amplifier might be too low for the circuit to drive.
You may need to add a buffer stage with a high input impedance between the Ext out jack and the input of the Carvin amplifier.

Yep... that appears to be the case.

Throwing the circuit into spice and simulating it shows that sticking a 20k load on the "ext. amp" node drops the level ~26dB.

Guess the line out is really only capable of driving a tube power amp...