Primary Z early Portaflexes

[Chris / CMW amps]

Hi all

Due to some requests I need to know a little more about the early, cathode biased Ampeg Portaflexes;

- primary impedance OT
- B+ voltage (440V on a '64 according to the schematic, each 6L6GC biased at > 28 Watts...)
- treble-control, 4M on the early schematics, 1M on the later ones. Where to get 4M pots?
- pre or post 2nd stage EQ?

Any experience, story, etc. is welcome. I've only seen and serviced SVTs and V4Bs over here in the Netherlands.

Many tia!!!

[Rob Mercure]

Hey Chris,

From the "4/69" factory B-15N service manual:

OT-214, 6.5K primary, 8, 16 ohm secondary, 290 ohms pri. DCR, 1, 1,4 ohm secondary, stock # 8950-012

6L6GC outputs: Ep+450VDC, Eg2+445VDC, Eg1-50VDC (fixed bias model, best I've got)

This one has the EQ after the 2nd stage - all 6SL7 signal level tubes.

And I've got one of these myself and it sounds really great - wonderful bass from the 15" JBL and actually nice, in a boomy way, with my '60 Strat. I'm not sure that cathode bias would improve this but who knows?

PI is an "anode follower" driven from unequal divider resistors.

And despite what you may have read elsewhere the Ampeg, of Linden, NJ, rated these beasts at "70W peak, 30W RMS."

Rob

[Chris / CMW amps]

Hey Rob

Many thanks, it does certainly give me some more info about these bass-classics.

Greetings from the (now) sunny Netherlands!