Gallien Krueger 700RB Turn on "thump"

[Tech. Tom]

Originally this amp was not going out of standby. I replaced several parts and now it turns on and seems to be ok other
than the turn on thump. I saw a service bulletin someone posted regarding a turn off thump and replaced R28 with a diode and C14
with a 470/25V cap.

I wasn't thinking at the time that the capacitor has about 85V on it so when I turned the amp on R17(10 ohms) smoked and C14
got very hot and bulged. I replaced R17 and replaced C14 with a 47/100V cap., but I still have the thump.

Any help would be appreciated.

Tom

[Jazz P Bass]

What do you still have?
Turn On & Turn Off thump?

The mod is supposed to help with the turn off only.

[Tech. Tom]

The turn off thump is acceptable, but I have the original value capacitor back in the set 47/100V. I couldn't try the set with the modification in the bulletin
because the resistor kept burning up. Why would they tell you to install a capacitor rated at 25 volts when there is 85 volts in the circuit? Maybe that was a typo
and it should by 470/100V, but that would not physically fit on the board. I guess I could mount it separately and run wires to the board.
Anyhow , do you know
of a fix for the turn on thump? I thought of installing a switch on the speaker cabinet so that after the amp was up and running you could turn on the speakers.

[Mick Bailey]

First check there isn't DC on the output after the initial thump. Probably one of the supply rails is coming up quicker than the other before the amp centers on zero, but you need to make sure that this is the case and there isn't a residual offset.

Sometimes a thermistor on the mains input can help to tame the inrush a little. Not guaranteed, and sometimes it can make the thump worse by prolonging the DC imbalance. There are various commercial soft-start PCBs (and plenty of DIY schematics) to operate a relay after a predetermined time from power-up.

[Jazz P Bass]

Can you kindly post the schematic that you are working from?

It is always a good idea when stating a certain component, to declare where it is on the schematic.
Such as the coordinates & page number.

The schematic that I have shows that R28 & C14 are a time delay on the signal jfet Q1.
(Coordinate 2/B on the preamp schematic)
So there can only be +15Vdc on that cap.

The power amp schematic shows R17 as a 150 ohm 1/4 watt resistor. (D/6)
So maybe we should all get on the same page, so to speak.
You may have a revision that is not shown. (D & F).
Maybe look for a circuit board number. That will help.


Keep in mind that the orientation of the 1N4148 diode is critical.

[Tech. Tom]

I will attempt to attach the schematic I am working from.
Thanks for your time.

[MarkusBass]

The modification applies to the preamp. It seems that you are trying to apply it in the power amp (which is not the same) . On C14 in the preamp there is only 15V.

Mark

[Jazz P Bass]

As Markus pointed out, the 'Turn Off Thump' mod applies to the preamp of the GK 700RB.

It does not apply to the 700RBII.

Which amp do you have?

[Tech. Tom]

Ok thanks Mark. I will restore the circuit I changed back to original then do the modification. Why they would use the same location number for 2 different parts in the same amp is beyond me, even though one is in the power amp and the other in the preamp.
Tom

[oc disorder]

Here's the G&K turn off thump bulletin

[Tech. Tom]

Thanks for that OC. The bulletin I had didn't include the schematic. If it did I wouldn't have made such a stupid mistake.
Oh well live and learn.
Tom.

[Jazz P Bass]

As you have not said which amp you have, I did a little digging.

Being a newer amplifier, the GK 700RBII incorporates the 700RB 'mod'.

Your confusion arises from the fact that you may have used the incorrect schematic for the amplifier that you have.

[MarkusBass]

Ok thanks Mark. I will restore the circuit I changed back to original then do the modification. Why they would use the same location number for 2 different parts in the same amp is beyond me, even though one is in the power amp and the other in the preamp.
Tom

It would be nice to have unique symbols for every part in an amp but there is a simple reason why it is not. Manufacturers often use different power amps with the same preamp. It is also a case with power supplies. This would force them to keep unique symbols across every module in every amp, which is too difficult.

Mark

[Tech. Tom]

Thanks for the info. Mark.

[Tech. Tom]

Hi Jazz:
All I originally had was the power amp schematic, and that was correct. I am impressed with all of the knowledgeable people on this forum and your sharing of schematics and bulletins.
Thanks.
Tom