VintageKustom.com: Repairing Kustom Amps » How can I stop reverb howling on K 100?

Re: How can I stop reverb howling on K 100? [message #25322 is a reply to message #25320]

Thu, 28 July 2016 10:19

lassoharp
Messages: 13
Registered: July 2016

Junior Member

The one thing that putting a steel plate over the open side of the tank does that cardboard will not do is shield the insides from electromagnetic radiation.

If doing this stopped the feedback, this suggests that radiation from the power transformer is likely the primary source of noise associated with the problem. Now, this is not surprising given that the PT is but 3-4 inches from the tank - BUT, I think there is an important distinction that needs to be made here:

Predominant Mechanical Coupling vs Predominant Electromagnetic coupling

The mechanical coupling portion of it is due to vibrations from the speaker cab being transmitted through the aluminum frame rails the tank is mounted on, and into the tank itself.

The electromagnetic coupling portion is mostly due to hum radiation from the PT being picked up by the tank (which is also a giant 'antenna; for such things) AND, another portion comes from the speaker which is sending out an amplified version of the low level hum that's >>always<< present in the signal regardless of circuit gain due to the fact that tank is only 3-4 inches from the PT.

Personally I think both types are happening with this head.

Relative circuit gain can be a problem if it becomes excessive, but the stock circuit is already maxing out the signal to noise ratio parameters to begin with so you don't have a lot of latitude there. For example, on my head version, when the reverb gain control was on "0" there was already quite a bit of wet reverb signal (sounded about like a typical Fender with rev turned to about 3 or 4) Now, I went in and did the mods that made it so the reverb level was zero when the knob was zero. But that didn't seem to affect the howling feedback point at all - before the mod the head would slowly begin howling with the rev control at zero - sometimes it would go faster than others but it was still hypersensitive and more less a disaster just waiting to be set off by footsteps, the bass player or drummer etc. LoL After the mod you could stop the feedback by turning the rev knob to zero . . .but of course you have no reverb then, and when you turn the knob up to get the same level of reverb you had before the mod the thing starts howling = no real effective difference in the point of oscillation. I also tried some other gain lowering mods as mentioned previously and they really didn't affect the feedback problem much at all. I think the original designers were doing the same thing that the designers of the old Pioneer stand alone reverb units were - getting the reverb circuit gain as high as possible so that the signal to noise ratio was also high enough that you never heard the PT hum pickup in your wet signal. In the original pre modded Kustom, if you already have pretty wet reverb with the knob on zero, you're probably not going to ever think about turning it anywhere past 12 oclock and thus never getting it high enough to hear the low level PT hum that's always there in the signal. Of course with the gain so high, it's more sensitive to potential runaway feedback.. . . .

. . . But the overall point here is that regardless of relative circuit gain - mechanical and electromagnetic coupling issues are best remedied through PHYSICAL DISTANCE, PHYSICAL MANIPULATION (like padding to absorb vibrations or orientation/position of head) and/or SHIELDING (which is an effective from of creating distance via isolation)

PHYSICAL DISTANCE WORKS - If I put the head on the floor or on a table, no feedback.

PHYSICAL ORIENTATION WORKS - If I turned the head sideways, meaning setting it up vertically on top of the cab it works

PHYSICAL MANIPULATION (padding) - Helps a little, but doesn't really solve the problem overall

PHYSICAL SHIELDING . . . .didn't go that far to try, but based on what vintagefan just said - why not? It makes really good sense.

One thing it helps to remember is that PT radiation is a 3 dimensional thing with varying peaks and nulls that interact with everything around it in complex ways. My first thought when first opening this head up and seeing how they'd mounted the tank was "What the heck were they thinking putting the tank so close to the PT?!" We all know its more likely to pickup hum, but to their credit they had selected a PT and placed it so that the tank was in a decent null zone. I sat there and moved the unmounted tank around inside the chassis and you could hear the hum soar to crazy high levels as it moved out of the null zone, which is what you'd expect with that close of distance in general. But being in a decent null zone at 3-4 inches is not as good as being 17 inches away from the PT, so they had to address that in the rev circuit by making wet gain as high as possible. I build and restore vintage tube mic pres for a living so am well familiar with dealing with PT hum pickup in transducer coils. It can be a tricky problem to solve. If vintagefan's solution proves to be consistent, it suggests that PT radiation going into the open side of the tank is a dominant factor in the feedback problem.

I realize some of you might be thinking that using steel is a bad idea because of the belief that steel conducts hum currents more than aluminum. With EMR sometimes it's not what you think. It may not make a difference. If steel works, it works. If Aluminum works, it works. Seems best and easiest just to try them and see what happens.

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