Modifying a Fender Bassman head

Bob Tuttle · From: Republic, MO 65738

Does anyone have any suggestions for modifying and cleaning up the signal path in an early 70's Silverface Bassman? I played through one the other night and I loved the tone, but at club volume it was a little bit dirty or distorted.

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Most silverfaces (actually, which model? the 70w version, or the 50w version? there IS a difference!). already have the phase inverter optimised to stay clean, as well as the output. You COULD be overdriving the input stages, if so, try using input '2', which is slightly reduced gain, it MAy give you a bit more headroom, otherwise, you may need more efficient speakers etc to push more volume before overdrive. Basically, unless you are overloading a particular stage, the point where the amp starts to clip, you are pushing the wattage the amp is rated at. Another prospect is that the filter caps in the power supply are getting weak, therefore sagging a bit, and not delivering the power to support the transients...

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Forgot to add

You could also look into having the amp set up to use 6550 or KT88 power tubes, to give a bit more volume before the amp starts to clip... I'll have to check on the ratings to see if the transformers in a bassman can support it, but if I recall, I think JayDee Manness has his Fenders modified to take 6550's for the same reasons you stated...

Bob Tuttle · From: Republic, MO 65738

Gino,....This is the 50 watt version. I considered the possibility that I might be overdriving the input. I was using a Hilton pedal and a Lawrence 912 pickup. That's a pretty hot signal. I was using two speaker cabinets that I use with my rack system, so I don't think it was a problem with the speakers. The single notes were clean, but when I played a chord, especially in the lower register, it started getting kind of fuzzy. I was trying the amp out to see if would work for steel on some of the smaller gigs, and if I could clean it up just a little bit I think it would sound great. It's very possible that it might need a cap job and some new tubes.

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

A cap job and new tubes would help, i'm sure, but to be honest, with the high output pickup+ the hilton (active) pedal, first just try plugging into input 2 (not to be confused with the other channel

... the reduced gain fo the second input will overcome some of the overdrive problems you are running into. i know when i played my les paul into my vibrolux, dropoping to the second input helped a LOT (I would overdrive the first stage of the amp regardless of the volume setting). Plus input 2 has an input impedance of 68k ohms, versus the input of well over 1meg of input 1, which I would say is a MUCH better match to the output impedance of the Hilton (not sure what Keith has it as, but most active devices have a fairly low impedance for driving cables etc).

Kerry Wood · From: Tucson, Arizona, USA

Bob, I haven't got any experience with your pickup, but you might consider replacing the first 12AX7 preamp tube of the channel you are using with a 5751. They are a little bit lower in gain, and sound fantastic. If you do this, get a NOS American one, the new Russian ones seem to be as high gain as a normal 12AX7.

Kerry

ajm · From: Los Angeles

It's true that plugging into input #2 would let you turn your guitar volume up more. But the overall volume that you hear is going to be limited by the amount of signal that actually makes it to the first stage of the amp. The resistor network on the #2 jack divides the guitar signal in half. You may think you're louder because you can turn your guitar volume up more, but it could be misleading.

I think the suggestion about swapping preamp tubes is a good one for starters. Go and get a 12AX7, 12AT7, 12AY7 and a 12AU7. Experiment with them.

I have a Tube Works Real Tube Overdrive pedal that came with a 12AX7. I read on Harmony Central where guys were swapping out the tubes, tried a 12AU7, and am much happier now.

------------------
Artie McEwan

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Actually, you WILL get more overall volume. The power limitation generally in the amp will be the power amp section, what most likely is happening, is that you are overdriving the first stage. Fender amps are designed with more gain than needed to push the power section into it's full power potential, a pedal steel puts out WAYYY more power than the Strat, Tele, Precision Bass etc that was the norm when the amps were designed, and as a result, the first stage is overdriven, that overdrive only accumulates faster as the second stage is hit harder (second stage is controlled by the volume knob/tone stack which reside smack in the middle of stages 1 and 2). If you decrease the gain going into the first stage by plugging into input 2, your first stage will be safe, as you turn up the volume knob, you can do so until it starts to hit the 3rd stage, which gets amplified more etc... if you hear any overdrive at this point, you are most likely overdriving the phase inverter, which at that point, you'll have more than enough signal to start to overdrive the power tubes.

Ken Fox · From: Nashville GA USA

The preamp section of most Blackface amps are pretty much identical. Looking at the steel channel of the newer Vibrasonic you will see an intersting mod. The first cathode of the 12AX7 does not have a 25ufd bypass cap! This reduce gain of the first gain stage.
Also go through the signal path and replace the tone caps with Sprague Orange Drops and Silver Micas (for the 150 and 250 pf caps). This will make a significant difference in the tone quality of the amp. Having done this to several amps, I could really hear more fidelity and string seperation. Good quality preamp tubes and yes power tubes, too! I have really noticed that the sound of the JJ/Telsa 6L6GC tubes is more to my liking that the Sovtek 6L6GC/5881 tubes. They also are a 30 watt tube, as to the 22 or so watts of a Sovtek.

[This message was edited by Ken Fox on 16 March 2003 at 12:13 PM.]

Bob Tuttle · From: Republic, MO 65738

Thanks guys for your replies. That will give me some things to try. I had forgotten how good these old tube amps sound. I sure would like to get it cleaned up so it has a little more headroom.

Dave Van Allen · From: Doylestown, PA , US , Earth

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Turn down the Hilton pedal...they're usually set pretty hot from the factory. If that doesn't do it, take the amp to a tech. Don't go replacing parts (tubes, caps, etc.) if you don't have to! An O-scope will pinpoint exactly where any distortion is comming from.

Kerry Wood · From: Tucson, Arizona, USA

Hey Bob, any luck yet with this one?

Kerry

Bill Hatcher · From: Atlanta Ga. USA

I would personally not even go through the trouble/expense of trying to get the Bassman sounding the way you want it to sound with a steel guitar. It just won't give you the dynamic range that you need with steel. The attack of the guitar will feel "soft" and not fast enough due the the distortion in the amp, ESPECIALLY since you mentioned you were playing at "club volume". This is a cool sound and feel for the underarm guitar but not for steel unless you are mostly playing rock or blues.

Find you a nice Showman/Dual Showman. Much bigger sound and plenty of headroom to let you open your steel up and hit the preamp section harder without breaking up so much.
A very efficient speaker such as a JBL or Altec or EV will help even more.

Just my opinion affected by lot's of solder fumes. Luck to you.

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