Vibrato Tabs have a loud thump when switched on or off

[Tony1949]

I have a Hammond A-100 manufactured late 1964/early 1965. After starting the organ and sound is produced, I can switch on and off both vibrato tabs with out a thump. Any additional use of the tabs produce a loud thump either switching on or off. The following maintenance has been completed:

- All pre-amp tubes were replaced less than a year ago.
- Vibrato scanner cleaned.
- Replaced C2, C5, C7, C9, and C11.
- Cleaned both Vibrato tabs.

I'm not sure where to go from here. In a previous thread, someone had mentioned a 6J7 can that has a cap in it. I don't think the A-100 has such an item or I can't seem to find it on a schematic. Additionally, I think the vibrato doesn't sound right. It appears to sound choppy. The organ plays fine without the use of the vibrato.
Any help/ideas will be appreciated. Thanks. Tony1949.

[muckelroy]

You've replaced every capacitor that might cause this to occur, if it is caused by a leaky capacitor, the exception being C3 and C8, which are doubtful to cause this problem.

You may have one or more bad 6AU6 tubes. They can develop grid emission, which is a small DC voltage buildup on the grids that goes away when it gets a path to ground, in the form of a pop sound. And yes, even tubes 1 - 2 years old can do this. 6AU6 tubes haven't been manufactured in quite a while, so either the tube is very tired, or it was simply defective from the start.

Good luck!

[Tony1949]

Thanks for taking the time to reply. I'll try replacing them.

[Tony1949]

Muckelroy: I managed to find the old tubes that I took out of the pre-amp and installed the 6AU6 tubes and the thump went away. It was the case of the newer tubes checking out OK on the tube tester but not in the circuit. Thanks again for your input.

[muckelroy]

Good to hear!

My tube tester is a Hickok 532. It does not have a grid emission test. Later tube testers do have one, though, and I have heard that many of these testers are not sensitive enough to detect grid emissions for all tubes, the 6AU6 being one of them.

The couple of times I've come across this, the grids have such a tiny amount of DC voltage - in the tens of millivolts at most. Also it may be that until you put the tube in circuit where the plate / screen are energized in the way they are in the preamp, it won't develop the same problem.

[geoffbrown]

This situation happened on my C3, the 12bh7 developed grid emission so every time the soft volume switch was used. a pop occurred due to a buildup of about 20v DC which increased with time resulting in a much louder pop,all resolved by changing out the 12BHY7 tube.

[David Anderson]

As the old saying goes, "The best tube tester is the circuit in which the tube operates."

This thread also shows why I don't generally shotgun all the tubes in a preamp, assuming that new = better. You may very well retire a tube that has plenty of life left and replace it with one that has problems.

[geoffbrown]

Quite correct changing out components without first proving them bad, breaks all the rules of trouble shooting

[David Anderson]

Quite correct changing out components without first proving them bad, breaks all the rules of trouble shooting

Trouble-shooting, yes, but there are cases where, for the sake of reliability, you go ahead and replace a component (other than a tube) because you know from experience that it's likely to fail in the near future. Or, especially in SS amps where it's harder to track a noise source, it can be more efficient, time-wise, to shotgun several resistors rather than to replace them one at a time, hoping you've got the right one.

I've made the mistake of being too conservative in repairs a few times where it came back to bite me.