Ampeg V4 Hum Challenge

[Raw Amateur]

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 should 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.

[beans-on-toast]

There are several things that your tech can check out.

Changing the tone controls is not increasing the hum. This is just revealing what is already there. Not using a variac to power up a new capacitor should not be an issue. They never did this when they were manufacturing the amps. It is a good idea, but not necessary.

Look at the recap that the first tech performed.

1) An insulator must be installed between the chassis and the can capacitor (C17) or there will be hum. Make sure that they didn't ground the can to the chassis with a wire. The cap can ground return should be only be connected to the ground bus.

2) Check all the solder joints in the power supply and ground bus. Reflow them. A bad solder joint can cause noise in the power supply.

3) Check the main ground point for the amp at the end of the ground buss at the input jack. Make sure there is a clean contact to the chassis and that the connection is tight. Clean the jack contacts, including the ground shunt, with deoxit.

4) Hum pots can be a problem. It is hard to tell from the video but when they are blown, the hum is usually louder. You can disconnect the pot and test it with an ohm meter. An alternate solution would be to disconnect the pot and replace it in the heater circuit with two 100 ohm 1/2 watt resistors.

5) Plug an instrument cable into the ext amp jack and plug into the input of another amp. If it is hum free, this tells you that the problem is not in the preamp.

6) Remove the pre-amp tubes from one channel at a time and check the amp for hum. This can help identify a problem in specific locations. In general, a bad tube can have different symptoms. Swap out all the tubes.

7) Twisting the heater wires whenever possible helps reduce hum.

8) A scope or a signal tracer can help identify the source of the noise. Use these after the more obvious possibilities have been eliminated.

9) In some cases, the design of the amp is the cause of hum. The V4 hums. The power supply can be made quieter by increasing the value of the capacitors. For example, C17 can be increased from 40 to 100uF. The main capacitor, parallel 100uF and 40uF in series with to 100uF (=58uF) can be increased with two series 220uF caps (=110uF). Depending on the amp, you have to make sure that the board will fit. I use SDS Labs Dynaco MK3 upgrade capacitor boards, available from Triode Electronics in amps for this. I use high current capacity Panasonic snap-in EETEE2G221KJ (220uF 400V, EE series) that are available from Newark. See post 87 SVT recap for an example. The SVT and the V4 use the same first cap configuration. This is a major overhaul but, in my opinion, makes quite a difference. The cap values can be boosted to improve the amps performance without going to the extent of using a cap board. That's just my approach.

[KramerBassFan]

How is it grounded? Two prong or three?

[Raw Amateur]

B on T,
Wow, that's great. Thanks for those ideas.

A question about the external amp hook up.

On the back of the V4 there is a "Preamp Out" jack. Is this what you are talking about?

[Raw Amateur]

@Kramer,
It is a three prong plug. I did troubleshoot that a month ago, it appears to be properly grounded, but it was definitely add on three prong, so I'll have the tech double check that too.

[MAMMOTHvolume]

That hum isn't really all that bad, you're not gonna hear it when your playing. I'd recomend putting bigger caps in the PS and see if that helps. It'll change the distortion characteristics though.

Another solution is a lower gain valve in the mid range circuit.

[beans-on-toast]

Originally Posted by Raw Amateur

On the back of the V4 there is a "Preamp Out" jack. Is this what you are talking about?

Yes.

Feeding the pre-amp out into another amp allows you to isolate that part of the amp to some extent and test it separately. The one common part is the power supply so you can't isolate things completely. If you test the pre-amp out and it is hum free, then the problem is not in that stage. If you test the pre-amp and there is hum, that doesn't mean it is in the pre. It just means that it is there as well. More than likely, the hum originates in the power supply.

You have to be methodical. The idea is break the stages of the amp down into subsections as much as possible to simplify what you are looking at. You can then look at each subsection in-turn and isolate the problem. Experience can tell you what you should look at first. Generally, look at the easy stuff that doesn't take much time before getting into the more complex areas.

Something that I should have mentioned earlier. Try the amp in another room on a different power circuit (breaker). Or point the amp in a different direction. It could be noise from an electrical wiring problem in your house or a fluorescent lamp or dimmer. I assumed that these sort of things were tried and that the noise is indeed from within the amp.

[solodb]

I have a V4B and one thing that made it a lot quieter was to have a tech check all the lead dress as this model is known to have parassitic oscillation. Don't do this yourself as it involves moving live wiring around and risk of lethal voltage.Enjoy your V4- they are special amps!

[Raw Amateur]

Originally Posted by MAMMOTHvolume

That hum isn't really all that bad, you're not gonna hear it when your playing. I'd recomend putting bigger caps in the PS and see if that helps. It'll change the distortion characteristics though.

Another solution is a lower gain valve in the mid range circuit.

My tech in talking with him suggested lowering the gain on the amp, but I don't think he was talking about just the mid range circuit, he was talking about the whole amp.

Originally Posted by beans-on-toast

Feeding the pre-amp out into another amp allows you to isolate that part of the amp to some extent and test it separately. The one common part is the power supply so you can't isolate things completely. If you test the pre-amp out and it is hum free, then the problem is not in that stage. If you test the pre-amp and there is hum, that doesn't mean it is in the pre. It just means that it is there as well. More than likely, the hum originates in the power supply.

I hooked my guitar cable to the preamp out and into the passive input of my GK MB112 combo, I would say it's silent, yes when you crank the gain all the way up there is something there, but at normal gain I don't hear any hum. I also tested the active input, and tried it on my uber crappy Ibanez IBZ1B amp and heard absolutely no buzz/hum.

If the hum originates in the power supply would MAMMOTH's suggestion of bumping up the PS caps be a solution? How could you figure out if the the transformer was involved?

I guess one thing would be to redress the leads out of the power supply?

Originally Posted by beans-on-toast

Something that I should have mentioned earlier. Try the amp in another room on a different power circuit (breaker). Or point the amp in a different direction. It could be noise from an electrical wiring problem in your house or a fluorescent lamp or dimmer. I assumed that these sort of things were tried and that the noise is indeed from within the amp.

In my basement where I play there are two dimmer switches. This got my tech's attention talking to him. I shut the breakers off in that part of the basement and took it to another part of the basement and it still had a hum. Moving it around in place does seem to make it vary a bit. I took it out to our garage and it had a hum there too that didn't seem much different.

Again, I am really appreciative to you guys helping me on this.

[johnk_10]

i wouldn't recommend changing/raising the uf value main power supply filter caps, unless you want to change the voicing of the amp. IMO, the type of hum that you're getting doesn't really have much, if anything to do with that anyway. IMO, it's not the hum balance pot either.

as i stated in another forum yesterday, it's more likely the lead dress to the input jack, a bad ground or a ground loop on it's shielding to it, or an open switch contact on it's shorting switch on one or both of the jacks.

[WingKL]

Sounds like your filament supply is being polluted by the 120 hz ripple from the high voltage plate supply if they both share the same transformer. This noise is affecting your preamp tubes. You can test this by pulling a preamp tube or temporarily interrupting the filament supply to your preamp tubes with a switch. Another way to see this is to look at the filament supply with a scope. It won't be a nice sine wave. Depending on if you have a standby switch and where it interrupts your high voltage supply, you will see a difference in the shape of the filament supply when you put the amp on standby.

There's a series of youtube videos of a guy experimenting with this very same issue. If you have a separate transformer for your filament or go DC, I'm pretty sure the hum goes away.

[Raw Amateur]

For some reason, my earlier post didn't come up.

I ran a cable from my preamp jack to my GK 112MB combo amp and there was no noise unless you truly cranked the gain to full and it sounded more like snow not a hum. I also tried it on my uber crappy Ibanez IBZ1B amp. There was no noise on that at all.


@Mammoth,
The tech I'm working with mentioned lowering the gain on the amp but he wasn't talking about just the midrange section. He was thinking turning down the whole thing. I'm not so sure about that, I feel like you shouldn't have to completely rework the gain to get it sounding right.

@Johnk
I am definitely going to have him look at the grounding first. This seems like an easy fix. I'm going to start a list of things from easiest to hardest in terms of fixes.

WingKL,
I understand about half of that , I'm sure the tech will know what you are talking about, and I'll pass it along. That seems like a more in depth troubleshoot, hopefully we don't have to go that far.