50/60Hz Hum - Your Expert Opinion Please

[LinuksGuru]

Hi!

I have continuous problem (or may be not a problem at all) - a very small level of 50/60Hz hum from 7189a push-pull tube amp (28W RMS). It is present even if sound volume pot is at zero point.

On 8-ohm load, its level is 0.003 and 0.009 V, it is audible if I put my ear right on the front of 36sm woofer (with 98 dB/W/m sensitivity).

However, with headphones it is more audible, I can clearly recognize it during pauses. Hum does not change under any circumstances, so it is clearly come from power supply or filament circuit. Phono pre-amp filament is filtered DC (and tubes themselves are covered with grounded metal screens), all other standard, straight from the 6/12V PS transformer. Voltage and idle current of all tubes is in norm.

High-voltage power supply consist of voltage doubler - 2 x fast recovery Vishay BY500 800V/5A diodes, 2x560 ukf Epcos caps connected in series (like in all voltage doublers), + 1 x 100 ukf Panasonic cap right after 450V and ground. There is no room for any filter choke, unfortunately.

Can anyone please measure 50/60Hz hum level in your amp with volume pot at zero? I am just curious to know if I really have a problem, or this is within normal level.

Thanks in advance for any suggestion(s).

[pedroskova]

If it's 50/60Hz, you either have a minor ground loop interaction between source/amp/cable TV, or maybe magnetic coupling between power and output transformers. Still, 3-9 millivolts is pretty low and could be seen as normal.

[richwalters]

Again even though youv'e mentioned it, make sure the output tubes are taking similiar I quiescent currents esp important with fixed bias operation. If not the o/p tranny winding won't cancel the unbalanced 100Hz ripple and it will peter through to the output. This happens in alot of cases. How much 100Hz ripple do you have on the raw B+ ? Be VERY careful how you go about measuring this.

Voltage doubler implies double the current spikes and poor circuit earthing will increase noise and can be hard to eliminate. Make sure the input circuitry grounding is well away from power sections and the power sections is substantially sized.

It sounds like you aren't using a series-shunt attenuator for the headphones. That's very risky for earphones and ears.

Measuring hum is an indication of constructor experience in home made amps and in the case of ready made equipment a reflection of price.
Hum levels also depends on circuit topology, the amount of dB global nfb; wherever an AC source is used for heaters and/or for B+, 2,3,or 4 stage amps all have residual hum levels. It just gets through. A well designed 3,4 stage push pull amp with 20dB global nfb may have hum level just perceptable to the ear (as you mention) i.e around -65 to-70dB down with also electron noise/hiss. However, there are 3 stage amps that can have worse noise figures than a 4 stage amp. Take your pick.
My 1st bit is to check where the earthing of the vol potty is: I've seen it done is places where I wouldn't dare put it.

Catch 2; The decoy: Ye bigger the bass speaker and higher the sensitivity of the drive unit (98dB/w/m to me is exceptionally high ) the lower f response will bring hum out. So a well designed amp with correct leads is imperative for the minimum.
This used to be a big problem in the 1950's where bass reflex enclosures had the lower hump tuned close to mains frequency. However the bass sounded far better than todays units.

richy

[Miles Prower]

Originally posted by LinuksGuru
Hi!

I have continuous problem (or may be not a problem at all) - a very small level of 50/60Hz hum from 7189a push-pull tube amp (28W RMS). It is present even if sound volume pot is at zero point.

On 8-ohm load, its level is 0.003 and 0.009 V

Don't worry about it. I measured a 6.0mVp-p, 60Hz sinusoidal output into 8R with my latest project. It's just barely audible, so that's NBD.

[azazello]

Put choke 10 Hn aprox., instead R83, and cap 1 mkf parallel to choke.

[m6tt]

You could put a low ohm wirewound pot in the cathode of one of the tubes (above the resistor, or between ground if fixed bias) so that you could adjust the balance. Even temporarily, this may rule out tube mismatch. If it's magnetic coupling between the transformers it'll hum with no tubes in it.

[LinuksGuru]

Hi, tube folks !

Thanks a LOT for so many hints. This is not exactly DIY amp, it is Pioneer SM-83 being completely restored by myself.

1) Hum is not caused by output tube mismatch (I have chosen matched pairs by myself measuring idle current). I have removed 6AN8 amp/inverter and hum is gone. Additionally, I took off all 12AX7 (with 6AN8 and output tubes still installed), and hum remained. So culprit is clear - 6AN8 (rolling another NOS 6AN8 have not eliminated hum). I have double checked cold soldered joints - nope, everything is fine.

2) I cannot install 10 Hn choke - there is no physical space for it. I installed small 680 mH toroidal choke - no changes at all in hum level.

3) Does not looks like it is cable grounding issue - moving hand over cables do not changes hum level.

4) Volume pot (NOS vintage Marantz) is screwed into heavy metal chassis.

PS. Unfortunately, I cannot post photos - very small file size limit on this forum.

[Nihilist]

Is the "heavy metal chassis" that the volume pot is mounted to connected to chassis ground of the amp ?

I had a similar problem with my amp. The volume controls and the On/Off sitches are on the same metal faceplate. Had hum at all volumes, but more noticeable when loud. Grounded faceplate to chassis ground, silent as a stone.


Good luck. By the way, I also run 6AN8's.


............................Blake

[azazello]

2) I cannot install 10 Hn choke - there is no physical space for it. I installed small 680 mH toroidal choke - no changes at all in hum level.

It's very, very small. Parts of choke must be Ш & I /like output transf./. There is very big
anode current and the filtering must be very strong!! R 83 doesn't
enough !

[Geek]

Hi,

Since you describe hum and not buzz, there's something you can do that might help - see those grounded centre taps on the 6.3VAC windings? Lift that centre tap and make a voltage divider and tie them at about +35 to 40V above ground. Then bypass that to ground via 2.2uF or larger electrolytic (not critical, just for HF noise bypass).

'Tis a trick I use to knock the hum levels down up to 20dB

Cheers!