Early Bassman vs B-15 - circuit comparison

[LittleRicky]

Early Bassman amps (5A6 / 5B6) had octal preamp tubes, 15" speaker, Paraphase / Cathodyne PI's, cathode biased power tubes, tube rectifiers - all very similar to the early B-15's.

After the invention of the P-Bass in 1951, the 5A6 followed around 1952, the 5B6 around 1953, and it wasn't until 1960 that the B-15 came around.

So assuming Fender had caught up to Ampeg in it's cabinet design earlier, how would these circuits compare to the early B-15 with tube rectifiers and cathode bias?

I was recently comparing a 1966 B-15 to a Hot Rod Deluxe through a GK Neo 112 ii and the Hot Rod / GK sounded great next to it, and this got me looking into Bassman schematics, which the Hot Rod is basically based on, and that lead me to the older Bassman schematics.

Thanks.

 

[beans-on-toast]

There are many differences between the two designs. The B-15's EQ stage is based on the James tone circuit which stands apart from the Fender tone stage. In terms of the power supply, there are differences in the plate voltages and where the tubes are biased. The output transformers are different. This makes two very different performing and sounding amps. The B-15's power amp was adapted from a circuit in an RCA tube manual.

The '66 B-15 has a single baffle speaker cabinet which is different than the early double baffle cab. It also has a fixed bias which boosts the output wattage a bit over the cathode bias of the early models revisions.

The tweed Fender Deluxe is a great sounding guitar amp. The Fender amp designs were also based on tube application notes. That's how amps were designed in the early days. Cathodyne phase inverters were common in small radios. Leo Fender was a radio repair man. The Hot Rod Deluxe is an evolution that's juiced up and loaded with enhancements.

Both good sounding amps.

 

[Ric Vice]

My educated guess is that the input impediances are also different as well.

 

[Iristone]

Even the later Bassmen will not sound bad through proper bass cabs. Marshall Super Bass and Sunn Model T are both spinned off the 5F6A, while the Alembic F2b and Orange AD200 are based on Blackface Showman (Blackface includes a modified 5F6A tone circuit).