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distorted hum

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disorted hum

What could be the cause of distorted hum ? Everything measures out ok in my new amp although the B++ is a bit low (460 instead of 494 as per schematic) After switching on in stand-by mode I have 4 mV hum on the speaker terminals even though the end tube is DC fed. After flipping the stand-by switch I get about 13 mV very ugly distorted hum from my speakers.
I would very much appreciate some feedback.
Thanks in advance

Martin
 

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I'm not sure if I understand the term "distorted hum".
If it's just hum:
How much hum?
Please measure the AC voltage across the speaker terminals with the amp input shorted.
A value of 5mV or less would be expected on this design.
Try removing the 0.33uF coupling cap. Does the hum disappear?

Alternately, do you mean that the signal is modulated by hum?


Have you asked Mr Van de Merwe;)
 
John,

By distorted I mean that there is an alarming, scratchy sort of modulation on the hum when listening to it on the speakers. This is in stand by mode. No signal whatsoever. It measures 3 mV at the speaker terminals. Also the 300b heaters are fed DC, so wouldn't you suspect the hum to be much less ? And in stand by mode, totally absent ? I also noticed that in stand by mode I still had 34 V DC on the B+ and 126 V on the B++.
Does this mean anything to you ? After flipping the stand by switch, the hum level rises to 12-13 mV.

thanks very much for your input.

Martin
 
diyAudio Senior Member
Joined 2002
Hi,

The cathode bypass cap in the schematic is underrated. It is shown as a 100V cap with 92V DC accross it. Remember that the AC signal is added to the DC, so the cap should be a 200V type minimum...

I respectfully agree.:angel:

This the same cap that tended to blow up and was replaced with a same value but 160VDC BG.

Cheers,;)
 
mcs said:
The cathode bypass cap in the schematic is underrated. It is shown as a 100V cap with 92V DC accross it. Remember that the AC signal is added to the DC, so the cap should be a 200V type minimum...


And I respectfully agree and disagree at the same time :angel:

The capacitor is shorting the AC component to ground which it is supposed to do.

But 92V on a 100V capacitor is way too close to my liking. I want 20-25% headroom. A 160V should do.
 
Martin,

By removing the cap, we proved that most of the hum was caused by the output stage. There are a number of places it can enter:

On the HT (+B) line as ripple.
On the heater supply.
Static coupling by proximity.
Magnetically coupled to the output transformer from the mains transformer or chokes.

Interestingly, some SET designs have more hum with DC heaters than with AC.
The reason is that residual ripple on the HT line is cancelled by the heater hum.
However, Dick has tried this and it should work.
If you can't turn the transformer, then the next thing is to measure the HT ripple. This can be done with a meter on the AC range, if you have no scope. A 0.1uF in series with the meter lead will protect the meter against the DC.

Drop me an email if I don't respond. I was waiting or you ;)
 
John,

I measured B+ between last filter cap and earth with the meter on AC and a 0,1 uF cap giving a reading of about 5 mV. It's a bit erratic, swinging between 4 and 6 mV.
How do I measure the AC on the heaters ? I tried just measuring between - and + but this gave about 170 mV. That can't be right, can it ? Then I tried measuring AC between the heater plus and chassis earth, and this gave about half, 85 mV.

Martin
 
Well Tomatito,

Since you said "highly distorted hum" I would guess it's the ripple on the heater supply you hear. You could try adding another 10,000 uF to the heater supply just to see if the hum changes.

Diodes can be rather dirty in their job.

Have you tried AC heater supply? While it may still hum some, pure AC hum is more pleasant than the highly distorted hum from the rectifier.
 
Martin,

The HT rail looks OK. Remember that the 5mV will be divided by 2K4/8 before it gets to the speaker: very low.

Jax may be right about diode noise, but If it is, I don't think that adding uF's will help. In fact be very cautious as adding a big cap will raise the heater voltage, and will necessitate an increase in the 0.27R resistor, or adding another.

The hum - or buzz, couls still be getting in at the grid. Try grounding the grid with the removed 0.33uF cap to prove this. Ground it to the common earth point* or to the negative terminal of the cathode cap.

*I'm presuming that you have a proper earthing (grounding) scheme, such as a central earth point or an earhing bus.
 
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