Seduction motorboating

[nerdorama]

A friend owns a Seduction with C4S upgrade which he got from another friend who got it from who knows where. It seems to be built well and matches the pictures I can find on the web. BUT, is has a very low level putt-putt-putt sound in both channels which I assume is motorboating. Has anyone had this before with a Seduction. I don't want to start tearing into it if this is a know issue from the past. It sounds quite good. You have to be within a couple of feet from the speaker to hear it.
Thank you for any help.

 

[Paul Birkeland]

Is it both channels? Have you tried an alternate set of tubes?

This could end up just being a simple soldering issue.

 

[nerdorama]

Yes both channels. I have gone in and touched a few solder joints but haven't gone through the entire circuit. Don't have another set of tubes unfortunately. The tubes in it are unmatched yet they both produce the putt-putt so I'm guessing not the tubes.

 

[Paul Birkeland]

I'd poke around with a chopstick to see if you can change the noise at all by poking around in there.

 

[nerdorama]

A bit more poking around, I think it is actually in only one channel. I swear I heard it in both channels yesterday. Today I can only get it in the channel farthest from the power transformer. I've tried a couple of different tubes in that channel but the motorboating is always there. I'll go through and touch up the solder connections in that channel, those I can reach with the iron. And poke around with a chop stick.

 

[Paul Joppa]

Are you sure it's not a radio signal, from a nearby cell phone/router/computer/etc. ?

 

[nerdorama]

Not absolutely. I've heard that sort of noise before, but it doesn't seem to change from one place to another in my room. And I did have the idea of holding my cell phone near to it but that didn't change it. I'll have more time to work on this tomorrow. Thanks for the suggestions.

 

[Doc B]

Has a film capacitor been nicked with a hot soldering iron? That can cause a periodic short in the cap.

 

[nerdorama]

Hi Doc,
Not that I've noticed. They all seem to be ok externally.

I've reworked most of the solder connections, especially those on the ground bus around the input jacks. At first I thought this had made a difference as only one channel had audible putting. But after more fiddling around and swapping tubes the putting is still there. The tube shields make a large difference but don't completely kill the noise. There were no shields on it when I received it so I found a couple in my box of parts. I've also swapped tubes with the few that I have in my collection but no luck. I tried setting the entire preamp inside the perforated top of a Dynaco ST-70 and grounded it thinking possible an overall shield would help but can't tell the difference. A couple of times one channel seemed to be quiet but the next power up it came back. Setting it down for now as I have some errands to run.

John

 

[Doc B]

OK, if the shields make a big difference then it is almost certainly from an external source. Usual suspects are routers, phones, etc., but it could also be some other appliance, lights, electric heaters, anything electrical that might generate some periodic interference. Unplugging electrical items that are near the Seduction and listening for a change might be the place to start.

 

[nerdorama]

10-4. It was also happening at the owner's house so he probably has something going on there as well. I'll move it around and see what I can find.

 

[Thermioniclife]

A poorly or not soldered grid load resistor can give you that. Don't ask me how I know.

 

[nerdorama]

That appears to be the problem/solution. Moved it farther from my wifi router and music streamer and only getting tube rush now. Thanks again for the help. The owner says it sits pretty close to his router so I believe we have a solution.