Unwanted/early distortion in Fender Bandmaster Reverb

[Farkus]

Hey fellas,

Just like the title says, I am getting some unwanted early distortion/breakup on my fender silver face amp. I have tried multiple guitars, multiple speaker cables, multiple guitar cables, multiple cabinets as well as changing every tube (preamp/power amp/rectifier) and the problem is reproduced every time. The breakup sound appears to be happening mostly in the bass frequencies. It almost sounds like there's a distortion or overdrive pedal on with a low setting. I have taken the amplifier to a tech, he is not sure what is happening. He did change out the filter caps since they had not been changed since the amp was built in around 1968, but the problem did not improve.

I thought I would see if any of the gurus here have suggestions. If I can't figure this out I'm going to find myself another technician.

Thanks in advance.

[jeremy]

Re: Unwanted/early distortion in Fender Bandmaster Reverb

how early? with what eq settings? different speaker cab doesnt change the result? filter caps need to be replaced but other caps can also die. putting it on a scope will tell ya where things are getting weird

[JamesPaul]

Re: Unwanted/early distortion in Fender Bandmaster Reverb

Very thorough troubleshooting on Farkus' part. Most techs would catch the obvious causes (and that is not a negative comment on this tech).

I agree with jeremy that you are probably looking at applying a signal and tracking down the distortion with a scope. I understand how that can get expensive as I have spent many hours with a scope chasing a few issues.

This one appears to be quite challenging.

[Farkus]

Re: Unwanted/early distortion in Fender Bandmaster Reverb

Thanks for the reply guys. Really appreciate it.

Jeremy, in response to your questions, I have tried it on three different speaker cabinet with different speaker wires. Your question about the EQ is interesting, because I have noticed that the bass control on the Reverb channel plays a large role. If I back it down to one or so it is less distorted (even if the "volume" control is not changed), but is of course painfully bright. It also loses a lot of volume when the bass control is reduced (again, even if the "volume" control is not manipulated). If the bass is on four or five, it starts breakup at two on the "volume" knob.

And both of you are likely correct about the oscilloscope. I don't own one, I was just wondering if we could pare down the problem a little bit to give my tech some ideas. Maybe someone else has experienced this in the past. I'm not saying this tech is bad in any way, he got one of my Marshalls really working better than ever. This is just a weird problem. I would be tempted to sell it "as is" but it was my uncle's so I'm not sparing expense even though it could get expensive. LOL.

[NegativeEase]

Re: Unwanted/early distortion in Fender Bandmaster Reverb

If the problem hasn't been isolated from the Reverb yet, my guess (if the pre and tubes are proven clean) would be oxidized reverb tank connections or oxidation, dirt, crap on the tank coils themselves.

Old Fenders with the Reverb Tanks on the bottom are susceptible to moisture in the bottom where the tanks sit and if they sit on the ground for any real length of time and it make a nasty gritty low distortion noise sometimes.