Standby hum/noise?

[Guitarman18]

Hopefully just a quick one for someone out there.....

I've just finished up a 100K Skyline, Vibrolux iron (470v on plates!) build in a hammond 16x8x2 chassis and am experiencing an induced(?) noise in the speakers when the amp is on standby.

I've read other post where people have experienced a similar issue and tried some stuff, such as:

I thought it could be a leaky cap I have over the standby switch, but removing that didn't help.

I loosened one of the bolts holding the OT to see if the orientation would help as per a '67_Plexi suggestion in a previous post, but that didn't help.

I can't see any B+ lines that are running close to AC as experienced by Structo.

Could it be that in this small chassis build the PT is just too close to the OT? In my previous builds with Funk's chassis', they have been un- nervingly quiet in standby and on.

To be honest, when the standby is flipped, the amp is super quiet, the hum seems to dissipate.

Apart from this issue, the build sounds great.

If anyone has any more ideas, they would be greatfully received.

Thanks in advance.

cheers,

Paul.

[talbany]

Hopefully just a quick one for someone out there.....

I've just finished up a 100K Skyline, Vibrolux iron (470v on plates!) build in a hammond 16x8x2 chassis and am experiencing an induced(?) noise in the speakers when the amp is on standby.

I've read other post where people have experienced a similar issue and tried some stuff, such as:

I thought it could be a leaky cap I have over the standby switch, but removing that didn't help.

I loosened one of the bolts holding the OT to see if the orientation would help as per a '67_Plexi suggestion in a previous post, but that didn't help.

I can't see any B+ lines that are running close to AC as experienced by Structo.

Could it be that in this small chassis build the PT is just too close to the OT? In my previous builds with Funk's chassis', they have been un- nervingly quiet in standby and on.

To be honest, when the standby is flipped, the amp is super quiet, the hum seems to dissipate.

Apart from this issue, the build sounds great.

If anyone has any more ideas, they would be greatfully received.

Thanks in advance.

cheers,

Paul.

What kind of noise? a faint hum?

Give this a shot

Pull the power tubes.. If the noise is still there it's most likly
Transformer Prox...Remember flux lines generated very X-former to X-former.. Generally smaller X-formers ( Like you Vibrolux)generate stronger more concentrated fields..Another clue here is since it's not as bad when you power the rest of the amp lowers the voltage on the 2ndary decreases the intensity of the flux lines.. I had one do the same thing happen..
Next would be a bad opt X-former
Next would be Power supply caps or wiring of the caps

If the noise goes away when you pull the tubes you can rule out X-Former Prox...
I would then go to the tubes.or bad output transformer....I assume you have the plates hot all the time and break at the screens.. Good Luck!!!


Tony VVT

[Guitarman18]

Thanks Tony,

I'll give those a try and report back.


Cheers,

Paul.

[Structo]

Not sure what you are referring to in regards to my amp, but I noticed when I had the standby wire from the rectifier board and the wire from the standby switch to the filter board too close to the AC wires of the power transformer, I got a pretty bad level of hum.

So I re-routed the wire, particularly the one from the switch to the filter board and 99% of the hum went away.

I am going to re-wire the standby switch soon and do it as Henry has suggested.
That is, wire it so the first filter cap stage is charged when the power switch is thrown, and then the wire from the cold side of the Stby switch goes to the choke or choke resistor and center tap of the OT.
If I get any popping I will solder a .01uf cap across the standby switch.

[paulster]

As Tony said though, first test is to pull the power tubes.

As soon as they're out there is nothing connected to the OT except the speakers so any hum at that point proves it's magnetic coupling.

Another good reason to use the headphone trick before drilling the chassis!

[Guitarman18]

Thanks for the replies. I've now had a chance to pull the tubes and it still makes the noise, so magnetic coupling it is!

I have my standby setup so that the first stage filters are charged but the plates are after the switch.

From this point, I guess I have a few options;

1, Just leave it as is.

2, Move the OT.

3, Change the standby arrangement, so that plates are hot on power on, which will hopefully reduce the hum due to the increased draw on the PT.

4, ?


If anyone has any more thoughts, they'll be greatly apreciated.

Cheers,

Paul.

4,

[Guitarman18]

paulster wrote;

Another good reason to use the headphone trick before drilling the chassis!

So true!

Cheers,

Paul.

[bluesfendermanblues]

Have a looksee at some of the other builds or at Dumble #124 - the OT is usually in the middle of the chassis.

[heisthl]

Quick and dirty method is to unbolt the OT and with the speaker side disconnected (meaning the secondary leads) rotate 90 degrees clockwise and go as far as you can stretch the primary leads. Mark the new holes and drill for self tapping metal screws. Extend the secondary leads and enjoy.

[talbany]

WOW!! 2' between.... that's some serious flux..Although Vibrolux Power 500v pretty hard workin trans for it's size...2' should be plenty not your fault..Another quick fix would be to mount a piece of Aluminum or even better steel on some angle brackets between the power and opt..Stick a piece between them see if it helps first...Good Luck!!

In my experiances with this rotating the OPT doesn't help much w/open frame...



Tony

[butwhatif]

Heistl is right, extend the leads and move it around 'til u find a good spot/orientation.

[Guitarman18]

Thanks guys,

bluesfendermanblues,
my tranny is pretty central as you can see, I was concerned about lead dress as much as anything. So even though the chassis is smaller, the relative position of the lead dress is pretty much based on all the other Dumbles I've seen gut shots of. You live and learn, I guess! Next time it will be perfect.

heisthl,

I did unbolt the OT completely and tried to see how far I could get it from the PT, but the improvements were negligible. The most improvement was when it was tilted about 30deg off horizontal which just looked too weird! Thanks for the suggestion though.

talbany,

You're right about the serious flux on this one, I think this tranny is a tad over spec! It is an export tranny though, so it weighs some too, for a Vibrolux!
I did try an aluminium plate in between, but it seems to focus the flux like a lens more than anything. I'll try some steel, end bells would have helped, I suspect.


Once again, thanks for your help.

Cheers,

Paul.

[talbany]

Another option would be wire the amp up hot all the time and wire the stby so it lifts the center tap on the power transformer.. Ampeg does this on several amps..so have I works fine and totally quiet and less surge

Tony

[Guitarman18]

talbany wrote;

Another option would be wire the amp up hot all the time and wire the stby so it lifts the center tap on the power transformer.. Ampeg does this on several amps..so have I works fine

That sounds like a great idea! I'll give it a go.

Cheers,

Paul.

[Structo]

On my D'Lite the OT is pretty much centered in the chassis.
No hum unless both volume and master are dimed then there is some.