Ampeg V4 Hum Challenge

[Cochise Rembrandt]

So, I bought a mid-late seventies Ampeg V4 amp this summer. I am trying to get some ideas to take back to the tech in terms of troubleshooting the hum issue.

The basic issue is that there is a hum at start up that is minimally improved with the hum balance. Moving the controls on the tone stack shows the midrange affects the hum the most. With careful adjustment of the hum balance I can get the thing completely quiet if I turn the midrange all the way down. Turning it up makes it worse.

Some history is necessary here too. After buying the amp I took it to a tech #1 and had the preamp and power tubes replaced and I also had the electrolytic caps replaced as well. I was there when he powered it up, and I'm pretty sure he didn't use a variac to do it. Only after a lot of research afterwards did I discover that you should probably use a variac for this. Anyway there was no improvement in the problem.

I took it to tech #2 who found several burnt out resistors on the preamp board and they were replaced. He also found that the preamp tube closest to the transformer was microphonic. After a lot of switching of tubes he got some reduction in the hum. Not much.

Tech #2 is a good guy and wants to get this thing right. I am going to take it back to him but I wanted to get some ideas as to where he might be looking. He thinks their might be noise from the transformer on that preamp tube.

At the minimum, I think the hum pot should be replaced and I have one for him to install I got from fliptops.net. Whether the lack of variac use means the cap job is screwed, I'm not sure. Ideas about the transformer too would help. My gut says that since things are noticeably worse and better with the midrange means that it lies in the preamp circuit dealing with this control.

Here is a video of my amp doing its hum thing and a quick walk through of what I'm experiencing.

Ampeg V4 Hum Challenge - YouTube



Any help would be greatly appreciated.

[johnk_10]

IMO, not using a variac to form the new caps has nothing to do with it and shouldn't have hurt the new ones at all. i do use one when i replace mine, but that's just an extra measure that really isn't necessary unless you're using older NOS caps.

judging from your video, i would check to make sure that the input jack's shorting switches are clean and making contact, shorting the input to ground when nothing is plugged into them since from the video it sounds like they're open. does it hum the same way when a guitar or bass is plugged into it when the instrument's volume is turned all the way off? if it doesn't hum, then it's a dirty/faulty shorting switch in the input jack (pretty common).
lead dress or a ground loop from the jack to the preamp pcb can also contribute to a V4 humming. IMO, your hum balance pot is probably working fine and they rarely go bad. it would be much simpler to test it first with a volt ohm meter than to just replace it. all it does is balance the 12.6V filament voltage.

2 cents..............

[Cochise Rembrandt]

Johnk,
Thanks for the input. It does hum the same way with the bass plugged in. I will still have the tech double check the grounding on the input jacks.

[Prairie Dawg]

All the hum balance does is diddle the filament circuit a little. It doesn't need replacement. Powering the amp up w/o variac doesn't make a difference either. And there's probably nothing wrong with the transformers that causes this-when they go bad there is usually an evil smelling cloud of smoke or a pile of melted gunk underneath them.

On the other hand not spotting a couple of burned resistors is reason for concern. What I would do if you brought it to me would be to start with a known good, known very low noise preamp tube like a Phillips 6681, which is a mobile radio equivalent to a 12AX7. If that didn't get the job done it'd be time to start looking inside. What you're hearing sounds a lot like an unshielded input as JohnK has indicated. Also, validate your residential wiring with a socket tester to make sure it's set up right. You do have a 3 prong cord on that amp, right?

In my experience a high powered amp like a V4 demands a set of carefully matched power tubes as well. I assume that you have this matter well in hand.

[Cochise Rembrandt]

Prairie Dawg,
I will keep the Phillips 6681 in mind. I've checked the socket's in my basement which was renovated a few years ago. The socket is grounded thank goodness. The cord is three prong, but looks like it was an add on to the original amp. The tubes are JJ/Tesla's and were matched according to fliptops.net. Thanks for your help. I really appreciate it.

[Cochise Rembrandt]

On the other hand not spotting a couple of burned resistors is reason for concern. What I would do if you brought it to me would be to start with a known good, known very low noise preamp tube like a Phillips 6681, which is a mobile radio equivalent to a 12AX7. If that didn't get the job done it'd be time to start looking inside.

Anyone have recommendations on where to locate a Philips 6681? I've done some googles and found some leads but was wondering if there was anywhere people preferred.