Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Auke Jolman on December 13, 2021, 01:33:53 pm

Title: COS tubes flickering
Post by: Auke Jolman on December 13, 2021, 01:33:53 pm

Hi All,

Sometime ago I've build the COS (schematic is attached). Yesterday I was playing thru it and suddenly there was a thumping sound coming out of the speaker. The outputtubes lit up blue in rhythm with it. You can look and listen to it on ytube:

.

At first I could get rid of it by lowering the volume. Bet now as I turn the amp on it is quiet until I play something. Lowering the volume does not make it go away anymore.

I've never came across this and would appreciate some insights as to what it is and how I can make it go away.  :help:

Title: Re: COS tubes flickering
Post by: brewdude on December 13, 2021, 01:40:07 pm

Have you checked the bias?

Title: Re: COS tubes flickering
Post by: Auke Jolman on December 13, 2021, 02:25:03 pm

Thanks for the reply.

The amp is cathode biased. I don't have 1 ohm resistors in there to measure the bias. I checked de value of the cathode resistor and it is as per schematic.

Title: Re: COS tubes flickering
Post by: tubenit on December 13, 2021, 05:22:58 pm

I am wondering if it's parasitic oscillation?  Perhaps an output tube has gone bad? 


I'd measure voltages and make sure they are OK?    And I'd try other tubes that are known to be good.


with respect, Tubenit

Title: Re: COS tubes flickering
Post by: brewdude on December 13, 2021, 09:11:53 pm

I would check cathode voltage and use the resistor value to calculate dissipation.

Title: Re: COS tubes flickering
Post by: PRR on December 14, 2021, 12:04:48 am

Thumping is motorboating which is very often B+ filter caps.

Title: Re: COS tubes flickering
Post by: Auke Jolman on December 14, 2021, 01:47:26 pm

Had some time to do some measurements (see the attached voltage chart).

With a shared cathode resistor @ 300 oms =bias=> 57 mA. I changed the cathode resistor to 450 ohms =bias=> 37 mA. Changing the cathode resistor is likely a good thing, but did not change the motorboarding at this point.

I swapped in known good powertubes, but the motorboarding persisted. Als swapped the PI-tube. That did't help either.

The measured voltages are taken without the motorboarding. Once it starts, the voltage on the PI-node changes with the thumping (approximate 5 V). When it is motorboarding and I pull the PI-tube, the motorboarding stops. Don't know if that's a clue?

Any pointers as to what to do next?

Title: Re: COS tubes flickering
Post by: PRR on December 14, 2021, 05:22:34 pm

https://el34world.com/Forum/index.php?topic=28284.msg311609#msg311609

Title: Re: COS tubes flickering
Post by: Auke Jolman on December 16, 2021, 12:33:45 am

I searched the internet for known causes of motor boating. Came across all kinds of causes. It's a rather new build, so I wasn't suspecting the filtercaps. Still I tagged in a new one whilst disconnecting the original at one side. But that didn't solve it.

In the B+ line the first resistor after the choke is not that big. I read somewhere that sometimes, if the resistor value is to low, it can cause motorboating. So I upped that resistor to 10K. No change.

Since I had voltages on both plates of the output tubes, I did not suspect the OT.

After setting it aside for a day, I went over it again yesterday evening. Suddenly I noticed that the resistor coming off of the preseance pot had become loose. So I guess the problem was caused by  parasitic oscillation?

Thanks again for the help.

Title: Re: COS tubes flickering
Post by: tubenit on December 16, 2021, 05:40:04 am

I am understanding your last post to mean that the issue is resolved.  Is that correct? 


With respect, Tubenit

Title: Re: COS tubes flickering
Post by: Auke Jolman on December 16, 2021, 07:54:54 am

Hi Jeff,

You're correct. The issue is resolved I'm happy to say.