Ampeg ET-2 tremolo will not work! Why?

[guildguy]

I picked up a well loved Ampeg Super Twin Echo 2 that had spent some time sitting in recent years. It sounds amazing but it needs some TLC to be fully operational; the issues it has are:
-loud hum when the channels aren't bridged or when an instrument isn't plugged in
+filter cap cans have been replaced and this did not solve this issue

-reverb bleeds even when switch is off
+haven't been able to track this issue down as all components in reverb circuit are within spec

-absolutely can NOT get the tremolo to work
+after replacing all caps in the tremolo circuit that had drifted, still no luck. Went back and replaced all components in the tremolo circuit - still no luck. Swapped all factory Ampeg tubes out for new ones {even though they tested strong} - no luck.

When the tremolo is engaged I do get the momentary dip in volume but nothing else.

I've successfully repaired Ampeg tremolo before on two other Rockets I own and the circuit is nearly identical. What else can I check?

Thanks in advance.

[Justin Thomas]

I can only offer a suggestion as to the hum. Are the input jacks the shorting/switching type? If so, check to see that the shorting prong is not sprung. If not, I would let others advise (though I think the normal is to replace them with switching types).

Justin

[Leo_Gnardo]

If not, I would let others advise (though I think the normal is to replace them with switching types).

Ampegs of this period have non shorting jacks factory installed. So, if you turn up the volume with no instrument plugged in, you get hum. Your call to replace at least one input jack with a switching type say Switchcraft 12A. You can mimic Fender's input jack circuit, or opt for leaving it as-is.

[drewl]

Reverb bleed is vibration on pan and reverb pot letting some signal through.

The switch for reverb is on the send side, I usually move it to the return side so footswitch kills reverb completely.

[52 Bill]

When the filter cap cans were replaced was the original grounding scheme followed? Ampegs will hum from ground loops if the cans are not grounded correctly.

[Randall]

Is this the correct schematic?
https://tubeamplifierparts.com/schem..._schematic.gif

[guildguy]

Is this the correct schematic?
https://tubeamplifierparts.com/schem..._schematic.gif

Yes, this is the correct schematic, the one by Joe Piazza has a few minor discrepancies.

[guildguy]

When the filter cap cans were replaced was the original grounding scheme followed? Ampegs will hum from ground loops if the cans are not grounded correctly.

Yes, the original cap cans were in the amp and I swapped them wire for wire. The hum was equally bad after as before leaving me to believe this was not the issue. Ironically, I've found in the handful of vintage Ampegs I've owned or worked (about 5 1959-1964 amps) on the filter caps have never been an issue.

[guildguy]

Reverb bleed is vibration on pan and reverb pot letting some signal through.

The switch for reverb is on the send side, I usually move it to the return side so footswitch kills reverb completely.

Do you have a picture or hand drawn schematic to show exactly how you do this mod? Thanks!

[drewl]

Do you have a picture or hand drawn schematic to show exactly how you do this mod? Thanks!

No, yours is different as the footswitch is connected to the cathode of the reverb return amp stage.

[Chuck H]

For the reverb bleed issue, you might try replacing C24. If that cap is leaking DC it could allow the reverb driver tube to bias up even when the cathode resistor is lifted. No promises.

[guildguy]

Is this the correct schematic?
https://tubeamplifierparts.com/schem..._schematic.gif

I was wrong, this is not the correct schematic. Mine is the 7591 version with solid state rectifier. At first glance I thought it was because the schematic inside the amp is exact, I took some more time and started looking at it and realized it was different. Sorry!

[guildguy]

Is this the correct schematic?
https://tubeamplifierparts.com/schem..._schematic.gif

This is the right schematic.

[guildguy]

Here is a picture of the tremolo circuit as it came.

After going through the Tiny Chief caps, they had all drifted so I replaced them and a resistor that was out of value.

The tremolo still did not work, even after confirming all other components were in value, switcing tubes and checking the footswtich. This weekend I went through and replaced all components in the tremolo circuit value for value - it still does not work.

After comparing these side by side I realized there is a discrepancy between R28. I did not replace the original 150k for it's value - probably watching one too many Simpsons episodes while soldering. Will replace that one tonight.

[Randall]

Do you have any modulation at the plate of V5A?

Have you verified the speed and intensity pots are working?