Gibson GA-15 RVT Feedback / Howling

[Drewline]

Hi all,

I'm wondering if anyone has encountered a similar problem: I have a Gibson GA-15 RVT which produces a howling feedback with or without a signal. It seems to be caused by the proximity of the reverb tank to the speaker which was changed at some point to a Jensen Mod. Re orienting the reverb tank makes no difference. Removing the tank & setting it directly behind the amp stops the problem. Changing the tank makes no difference.

Thanks,

Drewline

[Jazz P Bass]

What are the specs of the Mod tank?
Part#?

It may be incorrect for the amp circuit.

[Drewline]

Sorry I wasn't more clear.

The tank is original including cables & bag. The tank is marked 6199. The speaker was replaced with a Jensen mod.

[52 Bill]

Sorry I wasn't more clear.

The tank is original including cables & bag. The tank is marked 6199. The speaker was replaced with a Jensen mod.

Is the problem acoustic or electronic?

Does covering the tank with a towel or a blanket stop it? How about grounding the return plug? Check for a microphonic return tube?

A lot of Gibson amps used a control on the input of the drive circuit and left the return full on. It may be oscillating when the return cable comes too close to the speaker.

[g1]

If the speaker magnet is much larger than the original, I think you are stuck. You will have to find another way to maintain the original distance from tank to magnet. Either a different speaker or a larger cab.

[Drewline]

I'm pretty sure it is electronic. Covering the tank or putting it in its bag didn't change anything. Neither did longer different cables. The only thing that stopped the oscillating was physically moving the tank.

[The Dude]

I recently had the same problem. It was cured by gluing the transducers in the tank to the laminated steel they surround. The speaker magnet was making them vibrate. Check out this thread.

http://music-electronics-forum.com/t38558/

[Drewline]

Thanks, I'll try this tomorrow morning.

[Drewline]

It turrned out that it was the speaker causing the problem. My work around was to reduce the gain of the reverb recovery by replacing R26 (220K) with a 47K & adding a 82K between C13 & V1B pin 8. Still had plenty of reverb.