Unidentifiable hum with Kenwood tube receiver kw 33l

[Benji01]

hello forum,

I'm currently working on a tube receiver (kw 33 l) made by Trio or now known as Kenwood.
When I got the receiver I preventively changed all electrolytic capacitors and all paper/wax caps because the old
one was leaking and the caps behind the rectifier already had bulges.
But now I'm facing a problem that I don't have any solutions for.
The receiver has quite a loud hum in it's output and the hum is not affected by the volume control, the hum or rather the peaks are when measured on the speaker outputs around 30mv.
I am quite unsure where the hum is coming from because the negative half wave that I would expect to see when the hum is coming from the power supply or the mains in general
is missing.
The picture shows the signal measured on the speaker terminals when no input is connected.
Could this hum be caused by the heater supply leaking into the signal or a bad tube ?
Why is the hum missing the negative half wave ?
So any ideas or information is greatly appreciated

best regards Benji

[xwarp]

Rotate the plug and see what the trace looks like. Also, if that is main's frequency, you still have filtering issues.

[Audiorepair]

This amp has an unusual thing where the cathode circuit of the output valves drives the heaters of V9 and V10.

This has come up on here before, and I can't remember now why anyone ever thought this was a good idea.



https://www.hifi-archiv.info/Kenwood/Trio%20KW-33L%20Manual/10.jpg

[Benji01]

Thank you all for your replies.
I tried rotating the plug and it does not change much only the amplitute of the 2nd spike slightly increases or deceased
 depending on the plug.
@ Audiorepair: so do you mean that the hum is induced by the design circuit?
You don't happen to have the link to previous post or know how to get the hum reduced or completely out of the signal ?

kind regards Benji

[Audiorepair]

I just posted this as a heads up.

I have seen this, or a similar circuit discussed on here before, though I have no link for that particular discussion, nor whether it was about hum or not.
I don't repair hi-fi.

Someone else on here should have had a few bells rung by now and could probably help you more than I can.

[TimFox]

This amp has an unusual thing where the cathode circuit of the output valves drives the heaters of V9 and V10.

This has come up on here before, and I can't remember now why anyone ever thought this was a good idea.



https://www.hifi-archiv.info/Kenwood/Trio%20KW-33L%20Manual/10.jpg

Driving the heaters of the preamplifier section from the cathode of the output tubes is a time-honored tradition, also used in Fisher amplifiers.
Besides driving the heaters with DC, this has the advantage of a soft start for the heater current as the output tubes warm up.
A little tricky to set up, but it was used by reputable manufacturers.

[Circlotron]

Is that hum signal on the scope screen 50Hz or 100Hz? Knowing that will help us track down the cause.

[wasedadoc]

That is a stereo amp.  Is the hum on both left and right outputs or only one?

[Benji01]

The hum is a 50 Hz hum and it is present on both channels with the same intensity.

[wasedadoc]

The prime suspects are the two diodes (especially if they are the old metal oxide type) and the two electrolytics on the high voltage secondary of the mains transformer.  The output of those has no more filtering before going to the centre taps of the audio output transformers.  (The schematic erroneously does not have that line to the OPT2 transformer.)

When that hum is sorted, VR7 is adjusted for minimum hum from the input valve/tube.  You do need the volume up for that one.

[Roehrenonkel]

Hi Benji,
 
have you tried switching the output-tubes?
Have you checked the tubes for leakage (Filament to Kathode)?
Is there maybe a ground-loop (ger.: Masseschleife)?
Try disconnecting all inputs and see if there is a change.
Is there a big transformer adjacent?
 
Pleas post a picture of this nice little amp (inside and outside). :-)

Good luck, best regards.

PS.: Have a look at "GFGF.ORG" and maybe consider becomeing a member.

[Benji01]

Thanks for your reply, I swapped the power tubes from left to right and the signal changed slightly.
The amplitude got smaller and the waveform changed slightly but I noticed something else.
When I start knocking one some of the power tubes the signal goes crazy so does that mean that the power tubes pick up the hum ?
I sadly don't have a tube tester at hand so I can't check the tubes.
What I forgot to mention, the rectifier diodes in the power supply I already changed for  new replacements.

[Benji01]

I found the source of at least 90 % of the hum.
I shielded the power tubes in aluminum foil and grounded the foli on the chassis and the hum nearly vanished.
Now my last question is how are the metal pipes called that are on some tubes to shield them and can I use them to shield the power tubes or do I need to get new tubes?

kind regard Benji