Saturday, March 20, 2010

Tweaking a Fender Bassman Reissue amp

On Saturday we spent about an hour at Mission Amps' shop tuning up AC Blue's Bassman reissue amp. The amp is late 80s reissue with lots of patina from being vigorously gigged, first as a guitar amp and for the last couple of years as AC's main gig rig for blues harp. It was sounding a little tired so he brought it to Mission Amps for a check-up, and I hung around and took notes and photos.

First, Bruce Collins -- the owner of Mission Amps -- re-wired the rectifier socket so it would accept a tube, and replaced the solid state rectifier with a new old stock 5U4GB tube. Next, he found that the 6L6 power tubes were mismatched an one was failing. Bruce replaced them with a matched pair of new Sovtek 6L6 tubes, and biased them at 15 watts each. The amp produced 48 watts of power when bench tested.

The improvement in tone was impressive. I tried it with both AC's EV RE-10 mic and my Front & Center crystal bullet mic, and the Bassman just ripped. Lots of texture, lots of beef. The tube rectifier gave it a nice touch of sag. It's now a very good-sounding harp amp.

The last task was a line out. Bruce removed the "Ground" toggle switch (which did nothing) and replaced it with a 1/4 inch phone jack. He stepped down the speaker tap with two resistors, 270K and 27K, shunted to ground. The unbalanced line out delivered about 700 millivolts with the volume on 4, when the amp was just barely tipping into distortion. When we plugged it into Bruce's Peavey PA the Bassman sounded big and bad-ass. AC is a happy harp guy.

NOTE: I played AC's EV RE-10 mic for several minutes and really liked it. It has a big sound and a deep tone. The mic itself is a bit too small for me to hold comfortably when cupping the harp, but the sound was excellent, very similar to my F&C crystal bullet mic. I can see why some players are so devoted to the RE-10.

New 5U4 rectifier and 6L6 power tubes:
The line out jack where the "Ground" toggle switch used to be:

The line out originates at the speaker tap:
The most important instrument at Mission Amps: Bruce Collins ears:


Paul "Kingley" Routledge said...

Rick, my first comment is why are there no sound clips of the modded Bassman? C'mon throw a dog a bone!

Interesting that Bruce (I presume it was him?) decided to use a 5U4GB over a 5AR4/GZ34. The 5U4GB does give more sag so I'm told, but apparently also shortens the life of other components as well? Is the difference in sag to a 5AR4/GZ34 really that great?

Really good thinking on the line out idea. Very useful and a logical place to put the jack socket.

I hear you on the Ev RE10. I have one and it has a fantastic sound, really great mic. They are just too small for me too cup comfortably (like most stick mics) even though I only have smallish hands. I much prefer a bullet mic for comfort.

I'm using a '65 Princeton Reverb Reissue which is in my opinion a killer little harp amp as my main gigging amp nowadays. I just mic it up for bigger rooms. Although I do keep hankering after a Bassman just for those occasional outdoor gigs.

Rick Davis said...

Hey Paul-

Sorry for no sound clip. I just neglected to bring a recorder. That was dumb...

I'll ask Bruce why he chose the 5U4 rectifier. We tried a GZ34 first and everybody agreed the 5U4 sounded better. Just a touch more texture.

I no longer hanker after big amps. That could be a function of age and earlier abuse of my body from athletics. I have zero interest in lugging a 50+ pound amp around. I've become a great fan of smaller amps -- like your RI Princeton Reverb -- and lines out to the PA.

Bruce Mission Amps said...

The reason for using the 5U4GB was simple,
... it just sounded a little better.
With the NOS 5U4GB, the amp was still making close to 48 watts right at the onset of visible clipping on the scope.
When Ricky and AC brought it over, I tested the amp before doing anything to it and it seemed a little bit anemic to me, making about 26 watts.
One soft tube and one going bad tube, both of which were off by about 12mA-14mA in idle current so not a great match.
AC thought it was still sounding pretty good and I don't think he realized it was low on power.
With the new power tubes and biased at 17 watts each, it did make 56 watts with the GZ34 rectifier but also had a sharper attack tone to it that I thought was just a little too hard/firm for a dedicated harp amp.
The 5U4GB slightly softened the attack and gave the amp some breath.
I always think of the harp amp as being one of three pieces of a "vocal instrument" and it also needs to breathe a bit.
A real stiff amp hinders that vocal quality and in this amp, the GZ34 took some of that away so I stuck with the 5U4GB and a little lighter idle current on the tubes, setting them right at 15 watts each and let AC and Rick decide.

Big Ernie Fuller said...

Hi Rick,

I also own a reissue Bassman (a 1990 model) & have had a love/hate relationship with the thing since I got it. I've tried the 5u4gb recto in mine & agree the increase in sag warms the tone a bit. But I still find the sound to be a bit "tinny" most times. Does Bruce have any ideas why that is & what can be done about it? Maybe some nice Weber speakers......?

I'm with you on the big amp thing. In my old age, lugging around a
Bassman can wear you out!

ijones7 said...

I know this has been discussed a lot and that this is an old post, but I'm yet to find a true answer! The 5U4GB filament draws 3A, which is higher than the 1.9A of the 5AR4. Time and time again people warn to be careful because the power transformer on the bassman RI or LTD may not be able to handle the increase and this could damage the amp. No one that I can find has truly answered (through measuring) whether the PT can handle the 3A of the 5U4GB. So my question is, can the PT handle the increase from 1.9A to 3A safely, or does this risk causing damage? Are there any numbers from the PT that confirm the previous answer one way or another?