Rickresource Rickenbacker Forum

From Ted Weber's great site.

Flabbing, Farting Out
Context: This speaker is really flabby.
Description: Flabbing, or Farting Out, is when the magnetic circuit loses control of the voice coil at high volumes on low bass notes. Since, with a big composite of tones, the mids and uppers are more or less supported by the low frequencies moving the cone in and out, the farting out can be very annoying and completely disrupt the structure and texture of your overall tone. Big magnets with tight gaps usually assure good, tight control of the voice coil under all conditions. Sometimes the speaker gets the blame for a problem that is occuring in the output circuit of the amp. Blocking in the output circuit from overdriving the output tubes, an unstable output circuit, and 120hz ripple components from the power supply on the signal can cause flabbing that appears to be a speaker problem.

Gregg: This is the first time I have seen a description on line that speaks to the problem that I am experiencing with my Bassman head. I am at least encourage that someone else has experienced this problem in an amplifier before. The plot thickens.

I'd forgot about that info that Ted had up. It could very well be blocking distortion inside the amp. What year was the Bassman amp? Some of those had some wiring issues that can be corrected to get rid of blocking distortion. Other eras used poor quality coupling caps and some of these going bad could be causing the problem. It can also be solved by using smaller value coupling caps going into the power section if blocking distortion is in fact the problem, but this will change the sound and make the bottom end tighter. Blocking distortion usually occurs as you begin to overdrive certain stages in the amp and what is actually happening is that some of the electrons that are going from the cathode to the plate inside the tube instead go to the grid, which "Blocks" the incoming signal. Taken to an extreme, it can cause the signal to completely stop at a given stage if you trace it through your amp with a scope. Speaking of scopes, blocking distortion can usually be seen on an oscilloscope, so if you have access to one, you should be able to find out if that is the problem or not.

Greg

Greg: The Bassman head was manufactured in 1967.

A 1967 should be the AB165 circuit, but it could also be the AC568 circuit, as that era was changing a lot. Can you see which circuit it is by the sticker inside the head box? It should have the blue Mallory "molded" caps which are usually fine and are great sounding, but sometimes can go bad. If I had this amp, I would be trying to troubleshoot and to narrow down where the problem is occuring. I'd check the voltages on the tubes and compare with the schematics. I'd check the resistor values to see if they have drifted over the years if the voltages are off. I'd check the coupling caps to see if they are leaking DC if the voltages are off. I'd check the wiring layout and see if any grid leads could be shortened to head off oscillation issues. Since it only seems to happen when the amp is up loud and overdriving, it makes it harder to troubleshoot. A scope and signal generator and a dummy load would be quite handy, but since you probably don't have those, then it may be tech time. Did you try different tubes yet Peter?

Greg: It is the AB165 and I have swapped out tubes with the same results.

Using a new head eliminates the problems and in my view rules out the speakers. Using a new cabinet results in no improvement and in my view points to the head as being the culprit.

Of course, I can always keep the volume resonable and never hear it, but I am worried that what I hear at the increased volume is symptomatic of a problem that might deteriorate and come back to bite me!

Does it make any difference if you plug into one channel or the other Peter?

The next step is to try to narrow down where the problem is occuring.....so trying both channels at the same settings will help. If it does it on both, then it could be in the power amp. If it only does it in one or the other, then it can be in the preamp area, and whichever one it doesn't do it on, you can ignore. After that, voltages should be checked in the offending area and that result will lead you to caps, resistors, or something along those lines to iron out. Basically it requires some troubleshooting now.

Greg

Greg: The difficulty persists regardless of channel. It is related to increased volume and attack. Playing two notes aggressively is more problematic than a single note. This must be related to making peak demands on the amplifier. Again, a bumping, theumping pressured sort of sound that sounds as if the speakers are physical restricted from freedom of movement. Turn back on the volume or use a lighter touch and the effect and all seems well.

The fact that it is either channel means the problem is occuring somewhere at or after the phase inverter. When the problem occurs, does it sound like the amp is breathing? If so, that is a form of motorboating and is an oscillation. The amp needs to be troubleshot stage by stage with a scope and a meter, and a chopstick to poke at parts. Subbing known good parts in could also be the ticket too. It could be a bad cap in the power supply, or could be something as simple as a wire being out of place. We can try to troubleshoot by just talking about it, but you'd get a lot farther to have someone look at it at some point. If I wasn't so busy I'd offer Peter, but I'm buried at the moment. Theres lot of good techs around though that could probably figure it out.

Greg

I'm thinking an open power supply cap. Easy to check and either confirm or eliminate, but the fact it is clean unless pushed suggests the supply sagging.

The 3d aspect ratio of the old Fender bassman cabinets makes them most effecient in the mid range frequencies. They seem to work better for guitar than bass. The old fender bassman cabs were less than 12" deep on the outside and had less than 10" clear inside. The old formula they used to calculate the volume of the cabinet did not take into accoune a good minimum dimension for the depth of a bass cabinet. Modern bass cabinets are 14" or 18" or more deep and quite often ported as well to reproduce the low frequencies. I personally like a 14" deep cab because it has some of the mid growl of the old 60s cab but it is deep enough to have more low end.

I have been experimenting with bi-amping my sound. I run one line into my bass pod and get a distorted mid range sound. Then I run the second signal into my mxr direct bos and get a clean signal with boosted highs and lows.

Returning to the original subject, I finally got around to re attaching the loose insulation in my cabinet, and that solved the complaints I had with it's tone.

Probably why it was there in the first place! Glad you got it how you like it Don.