Finally A/B'd My B15 and B18 and What A Surprise

[OldFenderPlayer]

I finally got around to comparing my '70s Ampeg B15N that I've had for a few years to the 1960s (I'm thinking 1966) B18N that I picked up just a few months ago. The B15 has a replacement Dukane speaker; the B18 is all original but retains only one factory tube, the V4 which is a Sylvania 7199. My amp tech said the V4 is weak (testing at 300/600, with 375 the acceptable minimum), but he didn't think the weak tube was impacting the amp's performance.

For the comparison both amps were sitting directly on the floor (no carts) and the tone controls were identical with treble at 9:00, bass at 3:00 and the volume trimmed so they were at the same level; on the B15 the tone-enhancing rocker switches were off.

I played four jazz standards (fingerstyle) and one U2 tune (with a heavy pick) using a passive Bill Nash '57 Precision with Fender heavy gauge flats and an active 1987 Musicman Stingray with Ernie Ball roundwounds (45-105 gauge).

I was really surprised by the results. As expected, both amps gave a warm, full classic bass tone. But the B15 was both louder and bassier than the B18. I don't know if this was due to the weak tube in the B18 or the difference in the cabinet design. The B15 also seemed to respond quicker to a note.

[JimmyM]

Don't know about the response thing, but all speakers respond at the same speed, so it must have been a little extra mid and high beaming that caused it.

As for the lows, I have not heard a B-18 in person, but I believe it. The 70's Thiele cab is a lot larger than previous B-15 cabs, and has a low end response that is to die for. I could see where the B-18, which has a larger box but a bigger speaker, could have less low end.

Now what's truly weird, though, is that the new Ampeg PF115he is a sealed cab the same size as the early B-15 cabs and actually goes lower than the double baffle cabs I have, which are ported. It's kind of like a Thiele on the bottom and a double baffle on top. I'm sure the speaker has a lot to do with that, but it blew me away, and reinforced my belief that you can't judge a cab by appearances alone.

[walterw]

the obvious next step is to swap heads and see which characteristics follow the amps or stay with the speakers.

[beans-on-toast]

Having a strong 7199 phase inverter in the B18 is important for this sort of test.

The signals from this tube drive the power tubes. With a weak output, the power tubes will not be driven to full power.

So the B18 is not being used to its full potential.