Tweaking tone in a '67 Fender Bandmaster

[mort]

ok, I've brought this amp back from the dead and it's coming along pretty nicely. It has a new rectifier, bias supply and filter section. I haven't done much with the rest of the circuit yet. Sounds pretty good overall, has some extra noise that increases with volume, and I notice that the highs sound pretty brassy. The low end is good and smooth and the overall volume is good, but it lacks shimmer in the high to mids.

Where would I even start on trying to bring a little more life into the highs on this amp? I have no idea. I've played the same Tele through alot of different old Fender amps so I'm very used to what kind of sounds I should get, but this is the first Bandmaster I have had... I suppose it's possible that it could be a difference in the amp but I don't really think so. I really feel like it should sing a little better than it does.

[Jazz P Bass]

What speaker(s) are you using?
Without trying different speakers you cannot really blame the amplifier.
Then again, you haven't really checked the rest of the amp out.

[mort]

That's true, I haven't tried different speakers yet. It's an EV SRO with the coffee can magnet cover on the back. I have used the same speaker through several other Fender amps and don't have the same effect, if that counts as isolating the speaker from being a problem.

I did replace two 25uF caps as a blind shot to see if there was any improvement. Maybe made a little improvement but not much. In this layout there are a pair of 25uF caps toward the right of the tag board and a pair toward the center. I replaced the pair toward the center(only working on the vibe channel right now, normal channel is dead due to a bad tube socket at V1)

http://www.cnjradio.net/fenderamps/b...763_layout.gif

[mort]

ok ok ok, I think I've found the heart of this issue. Just checked the neg bias voltage and it was at -38.5vdc. Layout shows -48v, and I just adjusted up to -44vdc and now it sounds more like it should on the high end.

So hears the new question then.. should I bias by sound or by a particular method?

[Jazz P Bass]

You can bias the amp any way you want, really.
-48 volts is a typical Fender setting.
They did not run them 'hot".(like say -38.5)
One thing to keep in mind is the amp wants to see a 4 ohm load.

[mort]

Ok, I tried a few different bias settings and it seems to sound best around -44.5. It has a little extra purrr, so to speak, and loses it if I go any higher or lower.

I do have an 8 ohm rated speaker hooked to it(6.1 ohm measured). I do like to use 8 ohm speakers so I always have the option to run a second speaker from the ext jack.

Thanks for helping out a greenhorn. If you ever have a question about A/C or heat please let me know