Make a SF Bassman louder!

[steel1953]

At least that's what my friend wants from his SF Bassman AC568. Just good, clean, Fender tone for his guitar. Only louder!
Any ideas?BTW, I've already rewired the bias supply and NF to BF specs.

[Satamax]

Huh, get rid of the NFB altogether. But if it's for clean, this might not be too clever. Sell the bassman and buy a dual showman or a bassman 100/135, or a twin.

[mac1amps]

When he says "louder" does he mean more gain? (i.e. break-up) or more "clean" headroom? If he's one of those people that actually uses a Bassman for Bass (Ugh!), he'd be MUCH better off with a Showman head (Showman's can make excellent Bass heads, depending on what kind of volume levels you're playing at. They're very CLEAN.). I can't understand why Leo named those "Bass"-man's (the LAST thing I'd want to plug into a Bassman would be a bass), whereas they sound gorgeous for guitar if your going for a nice "crunch" sound (I own FOUR Bassman heads I like their [guitar] sound so much. BTW, I'm only referring to the "Bassman Amp" models, NOT Bassman 50's, Bassman 100's or any of those series amps). Then again, if he's going for a "metal" level of break-up, he'd probably be best off getting some decent kind of O.D. pedal to "take it to the next level"

[Tom Phillips]

Using more efficient speakers is one way to make it "Louder". A speaker 3dB more efficient will increase source level by the same amout as doubling the output power. You can often find speakers +6dB or more efficient than the stock one.

[steel1953]

He means more headroom..........he gets the OD with pedals.

[Slobrain]

I once changed the output to use 2-6550 power tubes in a bassman 50 head and it sounded loud and clean. It retained the same tone but more headroom.

Of course you need to mod the output to accommodate these tubes and add an extra filament supply to handle the heater voltage.

Ever heard a Showman with 4-6550s ? LOUD man....... Good bass amp too....

Just a thought.

SLO

[steel1953]

I understand about the extra filament tranny....that's easily done. What do you mean, mod the output? Surely you don't mean a bigger OT?

I had a BF Showman head w/2 6550A's once. I can't say it had the same tone, but it was loud and clean with a nice bottom

[Slobrain]

I didn't change the OT, I just modded the output section like a marshall 50 watt output that uses 6550's. I used the same 50 watt bassman OT and it had a bit more headroom with the 6550's. definitally would be a good amp to use in a small club jazz setup with a fender jazz bass. Thats if your not into using a stand up bass ;)

Of course using a bigger OT will get you more headroom too, even just using the 6L6 tubes. A twin OT would work but it will change the impedence of the output.
I think it would then go to 8 ohm using the twin OT with just two power tubes but don't qoute me on that as I haven't done it in a long time.

SLO.....

[hippybilly]

I didn't change the OT, I just modded the output section like a marshall 50 watt output that uses 6550's. I used the same 50 watt bassman OT and it had a bit more headroom with the 6550's. definitally would be a good amp to use in a small club jazz setup with a fender jazz bass. Thats if your not into using a stand up bass ;)

Of course using a bigger OT will get you more headroom too, even just using the 6L6 tubes. A twin OT would work but it will change the impedence of the output.
I think it would then go to 8 ohm using the twin OT with just two power tubes but don't qoute me on that as I haven't done it in a long time.

SLO.....
What all did you have to do to use the 6550s in place of the 6L6s? I bought my Bassman 10 back when I thought a tube amp was a tube amp and tubes were good. Bought 4 of them and only one was worth owning, Ampeg V4B. I've got a Gibson Super Medalist that I don't know what to do with either. It's soooo clean sounding, but I guess it's suppose to be is what I was told. Could you tell me if these "clean" amps would make a good fiddle amp. I've got an older friend that's looking for a good fiddle amp and I have no idea what to tell him. Any pointers?

[AL30]

More headroom?

The NFB resistor in the AC568 is 47K. Lower it. The earlier AA864 circuit has an 820 ohm.

The Normal Channel on AC568 has an extra gain stage - the blackface AA864 does not have the extra gain stage on the Normal Channel - it goes straight to the PI and because of this (partly) is cleaner. That may lower some volume though.

AL

[Chuck H]

More headroom?The NFB resistor in the AC568 is 47K. Lower it. The earlier AA864 circuit has an 820 ohm.

Whoa there. Those are two different NFB circuits. The AC568 sends feedback to the input of the PI and the AA864 sends feedback to the tail with another resistor acting as a voltage divider.

If you want to raise amp gain in this circuit you must increase the value of the feedback resistor. Lowering the 47k feedback R in the AC568 to 820 ohms will reduce gain considerably and kill the tone. If your friend wants more headroom, I don't think changing the feedback R will help. Clean headroom is not the same exact thing as more gain.

Chuck

[AL30]

Whoops. Sorry I should've been more clear. Nice catch and thanks for the clarification.

While I'm back here... try some other pre-amp tubes. 12AT7 etc.

AL

[mac1amps]

Bassman's are known to have very "drivey" preamps (i.e. alot of gain), which is the main reason why they break-up as easily as they do. Comparing a Bassman to a [non-reverb] Bandmaster for instance (also a 2-6L6/push-pull network), Bandmasters appear to have alot more headroom (and alot less gain) to them, which is entirely due to the more moderate amount of [preamp] gain they have (IMO, the main reason why Bandmasters were never as popular as Bassman's). The later Bandmaster Reverbs, have an extra gain stage (part of the reverb network, but it affects the amp's overall gain, even if the reverb is not in use), which results in a more "drivey" response, but still not as much as the Bassman heads of the 60's..........My main point being, focus more on your preamp, not your power amp.

[Old Tele man]

...switch from 6L6GC's to 7581A's and rebias appropriately.

[loudthud]

The AC568 is the one with the big cathode resistors on the power tubes, they gotta go. The Bassman's with the inverting output stage are not known for their guitar tone. Seriously consider rewiring the amp to blackface specs (AA864?). What is being used for speakers? Make sure the load is 4 ohms. New tubes and a cap job should be considered.