ST70 tone playback warble

Playing a test tone back through my ST70 ( A recently acquired stock unmodified factory build) instead of a solid tone it’s almost warbling. I get no audible hum or noise from the speakers. Tried several different test sources and same result.

I got the amp with original Mullard el34 an rectifier tube ad rca 7199 in it. I replaced the el34 with Mullard reissue. Same result. If it was ripple would I hear hum through the speakers?

Any ideas what this might be?

kiwidad wrote:Playing a test tone back through my ST70 ( A recently acquired stock unmodified factory build) instead of a solid tone it’s  almost warbling. I get no audible hum or noise from the speakers. Tried several different test sources and same result.

I got the amp with original Mullard el34 an rectifier tube ad rca 7199 in it. I replaced the el34 with Mullard reissue. Same result. If it was ripple would I hear hum through the speakers?

Any ideas what this might be?

Hard to say without more information.
What is the frequency spectra of the source and after the amp ?
What is the source ?
What do you mean with "warble" ?

peterh wrote:

kiwidad wrote:Playing a test tone back through my ST70 ( A recently acquired stock unmodified factory build) instead of a solid tone it’s  almost warbling. I get no audible hum or noise from the speakers. Tried several different test sources and same result.

I got the amp with original Mullard el34 an rectifier tube ad rca 7199 in it. I replaced the el34 with Mullard reissue. Same result. If it was ripple would I hear hum through the speakers?

Any ideas what this might be?

Hard to say without more information.
What is the frequency spectra of the source and after the amp ?
What is the source ?
What do you mean with "warble" ?

The sound reminds me of the intermodulation tests on the old ortofon test record. that was a 60hz/4k test. I hooked up my old oscilloscope and that seems to introduce a ground loop and I get a faint but audible buzz from the speakers. but I can see a pattern moving through the signwave output. I hear this at everything from 400hz up.
Source is a streamed test record.
Sound is clean from headphone output on preamp.
Warble sound is like two frequencies mixed together or a lower frequency modulating the higher frequency to breaking it up which made me think of the old tracking test sound again from ortofon on the test record.
Could ripple not audible at no signal cause this?

kiwidad wrote:Playing a test tone back through my ST70 ( A recently acquired stock unmodified factory build) instead of a solid tone it’s  almost warbling. I get no audible hum or noise from the speakers. Tried several different test sources and same result.

I got the amp with original Mullard el34 an rectifier tube ad rca 7199 in it. I replaced the el34 with Mullard reissue. Same result. If it was ripple would I hear hum through the speakers?

Any ideas what this might be?

Ok time for the laugh of the day!

Ceiling fan is the culprit. It’s essentially modulating the air and thus the sound. Fan off and nice clean tones...

Oops

kiwidad wrote:

peterh wrote:

kiwidad wrote:Playing a test tone back through my ST70 ( A recently acquired stock unmodified factory build) instead of a solid tone it’s  almost warbling. I get no audible hum or noise from the speakers. Tried several different test sources and same result.

I got the amp with original Mullard el34 an rectifier tube ad rca 7199 in it. I replaced the el34 with Mullard reissue. Same result. If it was ripple would I hear hum through the speakers?

Any ideas what this might be?

Hard to say without more information.
What is the frequency spectra of the source and after the amp ?
What is the source ?
What do you mean with "warble" ?

The sound reminds me of the intermodulation tests on the old ortofon test record. that was a 60hz/4k test. I hooked up my old oscilloscope and that seems to introduce a ground loop and I get a faint but audible buzz from the speakers. but I can see a pattern moving through the signwave output. I hear this at everything from 400hz up.
Source is a streamed test record.
Sound is clean from headphone output on preamp.
Warble sound is like two frequencies mixed together or a lower frequency modulating the higher frequency to breaking it up which made me think of the old tracking test sound again from ortofon on the test record.
Could ripple not audible at no signal cause this?

It _could_ be intermodulation with hum, possibly a grounding issue. Hard
to tell. Any connection to cable-TV is a possible source of hum via the ground.

I would repeat this with a test tone from a local generator. Or listen
on "slow piano music", it's very revealing.

kiwidad wrote:


Ok time for the laugh of the day!

Ceiling fan is the culprit. It’s essentially modulating the air and thus the sound. Fan off and nice clean tones...

Oops

:D

Always nice to fix a problem.