What causes 150 Hz feedback ?

[Guus]

Hi Guys,
I have a little problem with a 2 channel tube amp.
I've build:
Pre amp stages:
1e: 1x EF86,
2e: ECC83, ECC 83.
PI: ECC83
End stage:
2x 6V6 fixed biased, no FB loop.

It's an open back combo with 1x 12 inch celestion.
Amp sounds fine/good but:
When a radiotuner signal plugged in and I put the volume of one channel wide open I get a "whoooom" at about 150 Hz.
I thought of the EF86 but it happens to when channel one is turned down and channel 2 is wide open. So I can cancel out the EF86.
When I put the combo near a wall the feedback starts sooner then in the middle of the room.
When nothing is plugged in there is no feedback.

It must be a kind of feedback, but there is only a radio signal pllugged in the input, so no mic!
Anyone knows what can cause the problem?

grtz
Guss

[Stevem]

If you set the volume to a point just below where the feedback starts and tap the tubes with your finger nail will the feedback than start in?

[Guus]

Hi Stevem,

Thanks for reply.
No, tabbing the tubes makes no difference. So I quess no tube failure.

I plugged in a guitar and with everything flat open, the feedback comes on every note frequency you touch or hit. So not only at 150 Hz. So with a guitar the amps works well.

Occurs:
When burning in a new build amp, I mostly put a radiosignal from a tuner in the input, this for many hours at different volumes.
This also to break in the new speaker.

When doing this I find out that a 150Hz feedback comes up at max volume.
Strange thing, I can't explain why.
I thought, maybe something to do with an electrical feedback from the speaker? that gives a resonance in the endstage?

Happely the amp sounds very well with a guitar plugged in and that's where it is build for. So no big problem.

grtz
Guss

[ToneMerc]

Hi Guys,
I have a little problem with a 2 channel tube amp.
I've build:
Pre amp stages:
1e: 1x EF86,
2e: ECC83, ECC 83.
PI: ECC83
End stage:
2x 6V6 fixed biased, no FB loop.

It's an open back combo with 1x 12 inch celestion.
Amp sounds fine/good but:
When a radiotuner signal plugged in and I put the volume of one channel wide open I get a "whoooom" at about 150 Hz.
I thought of the EF86 but it happens to when channel one is turned down and channel 2 is wide open. So I can cancel out the EF86.
When I put the combo near a wall the feedback starts sooner then in the middle of the room.
When nothing is plugged in there is no feedback.

It must be a kind of feedback, but there is only a radio signal pllugged in the input, so no mic!
Anyone knows what can cause the problem?

grtz
Guss

Do you have 50Hz power? Could it be actually a 3rd harmonic of the power line frequency?

TM

[passfan]

Hi Guys,
I have a little problem with a 2 channel tube amp.
I've build:
Pre amp stages:
1e: 1x EF86,
2e: ECC83, ECC 83.
PI: ECC83
End stage:
2x 6V6 fixed biased, no FB loop.

It's an open back combo with 1x 12 inch celestion.
Amp sounds fine/good but:
When a radiotuner signal plugged in and I put the volume of one channel wide open I get a "whoooom" at about 150 Hz.
I thought of the EF86 but it happens to when channel one is turned down and channel 2 is wide open. So I can cancel out the EF86.
When I put the combo near a wall the feedback starts sooner then in the middle of the room.
When nothing is plugged in there is no feedback.

It must be a kind of feedback, but there is only a radio signal pllugged in the input, so no mic!
Anyone knows what can cause the problem?

grtz
Guss

Do you have 50Hz power? Could it be actually a 3rd harmonic of the power line frequency?

TM

Seeing him in the Netherlands , that was my first thought as well. With it not occuring with nothing plugged in at all, check for hum from your source (the radio receiver)

[ampgeek]

Could it be a mechanically coupled feedback from the cabinet?

Maybe try pulling the chassis out of the cabinet to eliminate that possibility?

Just thinking out loud!

Good luck,
Dave O.

[VacuumVoodoo]

Q: is it 150 or 130 or maybe 170Hz?
I'm asking because this could have nothing to do with 50Hz mains. Sounds to me like mechanical coupling between loudspeaker and any or several elements in the amp. This often results in feedback at loudspeaker's resonance frequency or it's 2nd or 3rd harmonic.
I just had a VOX AC30CC do just that and it wasn't a microphonic tube. The culprit was coupling capacitor between 1st & 2nd preamp stage. A lousy 10 cent 470pF ceramic bean gone microphonic. Took a while to identify...

[Guus]

Hi Guy's,

thanks for reply.

Because the problem only comes up with a radiotuner in the input and not with a guitar, the problem must have to do with the radio reciever.

Yep, I live in the Netherlands, so we have 230Volt, 50 Hz.
The feedback frequency is realy 150Hz!
For a very long time I am a FOH engineer, so I've learned to pinpoint a frequency, especially in the lower frequencies.
I dubble checked it this morning with a signal / audio generator and yes it was indeed 150 Hz.

The radiotuner I use has indeed a little 50Hz hum I can hear.
I will follow the advise and check what happens when I put the amp on the workbench and the speaker at distance in the room. When the feedback disappears I can cancel out the 50Hz harmonic and check for a mechnical vibration of a component (capacitor) in the amp.

Thanks again.

grtz
Guss

[Stevem]

Does that radio/tuner have any dc volatge on its output?

[Guus]

Problem solved.
It was the radio/tuner.
Put a CD player in the input and everything sounds well.
Thanks for reply's.
grtz
Guss

[Leo_Gnardo]

Problem solved.
It was the radio/tuner.

Gotta beef up the power supply filter in that radio!