reverb too much sensitive

[Colas LeGrippa]

Good morning ,

Just finished modding a Fender super reverb silverface to AB763 specs. All voltages now are pretty good. Some stupid tech have changed two of the power rail resistors to different values. Two things though, first: I had to replace the bias pot ground resistor from 27K to 4.7 K to get the proper negative voltage, as indicated on the schem. : -52V. I can't see how Fender could get -52V with that arrangement. Second: before the mod the reverb was hyper-sensitive, couldn't adjust the reverb pot over 6-7 without having a howling feedback ( it makes me think of a name of a blues band: ''Howlin' feedback'' ). Ok serious now, I have swapped the reverb tubes for new ones, checked everything in the reverb circuit, swapped the reverb tank for a Peavey one, same thing. Over one month on the same issue..I can't sell the beast as is.

Thanks for suggestion.

Colas

[jjasilli]

Are you sure all circuit components & tubes are the correct value?  Anyway the simple solution may be to reduce gain in the reverb recovery stage.  Try disconnecting the cathode bypass cap. 

Also this stage's plate voltage is all over the place from 200 - 270 volts on different models of this amp.  Maybe use a resistor in the B+ rail to drop plate voltage on this stage, maybe as low as 150V.

You could also try reducing the effective gain in the driver stage by attenuating incoming signal strength, by reducing the value of that grid leak resistor from 1M to 500K or 220K.  (The recovery stage grid leak resistor should already be only 220K which is essentially the min value.)

[alerich]

While you are in there modding anyway an easy mod to make the reverb more usable is changing the reverb control pot from 100K linear taper to 100K audio taper. It allows the reverb to increase gradually instead of the virtual "on/off" condition you get with the stock linear pot. I like Fender's stock reverb but I loved the reverb on my old Super Reverb after that mod.

[HotBluePlates]

The tank is inside a tolex bag, lightly retained in place with the plastic strap and screw, with a piece of corrugated cardboard over the open side?

The bag, carboard and light pressure from the retaining strap help minimize microphonics from the tank due to speaker vibration.

Other than that, as Alerich said, the linear taper on the stock circuit gives almost too much reverb too fast; I never turned the Reverb knob on my old '67 Super Reverb above ~4, and I really liked a lot of reverb.

I also noticed a lot of hiss in my '67 coming from the reverb circuit, and always suspected the driver tube was fed too much supply voltage. I didn't experiment with moving that tube to a different supply node because I wanted to keep a collectible amp stock. I later sold the amp for more than I paid for it.

[Colas LeGrippa]

Thanx