Power tube hum

Still having issues with this little GA-5 amp, for a bedroom amp it hums away a bit too loud. It is linked to the power tube..pull it out hum goes away...pull the preamp tube hum still there. I've read up on the heater wiring as a cause and I could apply a 100ohm trim pot. I've tried the heater CT on the power tube cathode and makes no difference, also poss beef up the first filter cap to 22uf or 30uf from the 16uf at the mo. I've done the chopstick poke around and tried grounding varies parts of the amp and the only thing that changes is touching the heater wiring...the hum increase. Is there any other possible cause to check while I'm waiting for bits in the post.

that much heater hum from only the power tube would be unusual, but then again it is a EL84, so it actually has some voltage gain. Make sure it is 50/60hz hum (I forget which you have in Australia) and not 100/120. If it is not the lower frequency, then it is not the heaters. If it is the the lower frequency, the likely it is the heaters, so how is the heater winding referenced to ground now?

Pulling the power (output) tube kills the hum because pulling the power tube kills all of the output.

That the noise does not go away when you pull the pre-amp tube thankfully tells you the problem is not in that section.

Single ended output sections tend to be naturally prone to noise and so are very sensitive to less than ideal layout.

Sight unseen my guess is that the heater wires are probably not nice, neat twisted pairs. If that is the case then this is where you want to start. Then try adding the trim pot as a humdinger.

It's a 6V6 power tube, the heater wires are twisted and raised above the sockets with the center tap on the 6V6 pin 8 at the moment, it was on the nut of the PT but like I said made no difference. I'm pretty certain it's low frequency hum, I'm taking it in to show an amp buddy he may be able to confirm it.

Pulling the output tube from my amp and then turning it on I get a faint hum from the speaker. This is due to the output transformer picking up noise from the power transformer. Putting the tube in and then turning on the amp I get the minimal hum as without the tube and it increases until the tube is warm. I am getting about 0.62 Vrms on the first cap and about 0.53 on the screen and 0.1 on the 12AX7. My voltage is about 167 Vdc so you could scale it up for yours. (I am using a 15CW5 tube which is a low voltage EL84.)

I clipped on 270 mF on the first cap (which is somewhere in the 200 mF range, can't tell now the way it is built) and the hum went down quite a bit. I could tell what is more left over now is the power line frequency rather than it doubled from the rectifiers.

Rather than leave the the 270 mF cap across the first cap and say all is good I clipped it across the screen cap which is 22 mF. The noise dropped down to the level of the amp without the power tube in. To make the comparison.

As original Vac ,
0.62, 0.53

With added 270 mF cap on first stage
0.23, 0.20

With added 270 mF cap on the screen
0.61, 0.11

So in my case I have a good chunk of capacitance on the output and increasing it did drop the noise level but after adding the capacitor to the screen I see that the added capacitance on the first stage was causing less ripple on the screen. So in my case adding capacitance to the screen is the way to go.


Thanks for having a problem. It got me to re-look at my amp and reduce the hum to a minor level.

Thanks for the input printer2, glad to be of help..lol. However my poor brain can't comprehend what your saying here...I am getting about 0.62 Vrms on the first cap and about 0.53 on the screen and 0.1 on the 12AX7.

I clipped on 270 mF on the first cap (which is somewhere in the 200 mF range

Did you measure Vrma on a scope or calculated it?
First cap as in the first filter cap?
A 200mF is 200uf...that is huge....a 20uf filter cap is more to spec?

Anyway, this is the wiring as it was...

http://i757.photobucket.com/albums/xx217/telenut62/GA-5%20Champ/GA-5hEATERWIRING001.jpg

http://i757.photobucket.com/albums/xx217/telenut62/GA-5%20Champ/GA-5hEATERWIRING002.jpg

I took the amp for a second opinion and I proved to my mate that it wasn't PT hum. All I've done today is rewired it like I did with a 5watt Marshall type amp....with the heater wires going down along the chassis. It seems to have made a wee bit of improvement, I'll check it all tomorrow with a fresh set of ears.

Thanks for the replies I'm soaking this all up :wink:

Output tube cap had 0.62v, screen cap 0.53v, preamp cap, 0.1v at the start. Ignored the ipreamp stage after that as I have a master volume control and I can turn that stage's contribution out.

Increased the filtering on the output cap and hum went down but it was almost eliminated by adding filtering to the screen. The voltage was measured with a Fluke RMS meter, I was too lazy to get the scope going. With the increased filtering on the screen the power supply ripple was about the same on the output stage capacitor as when I started so it had less effect than the screen ripple (which makes sense). Just saying you could give increasing the screen capacitor value also and see what happens.

Yup was slowly getting the drift of what you were doing but the highest cap I had was 45uf, I didn't think that would make much diff

I'm guessing those are 'before' photos. I'd have thought re-doing those heater wires would have done the trick.

Call me retentive if you wish but what is in those photos is not what I'd call twisted pair. That's what I'd consider pseudo twisted pair (which can actually be worse than using plain parallel.) IMO about three turns per inch is twisted pair.

The heater wiring also looks to be sitting directly on (and parallel to) that 22k power resistor.

Nope not near the 22k resistor, the depth of field doesn't show up on the pic. I will experiment more today, certainly with higher value caps in the filter stage. It's def getting better but still room for improvement.

Yup was slowly getting the drift of what you were doing but the highest cap I had was 45uf, I didn't think that would make much diff

Try it on the screen. You have to remember I am running at a lower voltage with more current which means a lower resistance. Because of that I need a lot more capacitance to do the same filtering.

Ok success right away, the filter caps will be upgraded

Ok success right away, the filter caps will be upgraded

Not to be a spoilsport, but I think from your pictures that you are running a tube rectifier. Some have a maximum capacitance value they want to see. I would not go over it for the first cap but the second has a resistor to go through so a large cap here will not bother the rectifier and will give the screen a nice quiet supply. The one thing about having a large capacitor here, I think it might make the amp a little less dynamic as the screen voltage would not fluctuate as much. I am still new at this tube thing so take that with a grain of salt.

From the couple of minutes that I have played through mine after the addition that is what I think I hear. I would guess if you want the same feel from the amp you could put a resistor between the second cap and have a small value cap on the screen. That way you feed it with a quieter dc and yet the cap supplying the screen could sag but without the hum. I will have to check that out some day. For now I am pleased as heck on the low hum level I am not changing a thing. The amp was built to test some ideas also to find out if the transformer and tube would work together. Now that I know I like it I might build a higher value amp with the other transformer which I got from an old tube stereo.

Noted thanks :wink:

New caps came today....three 22uf F&T's. Much better result and has helped stiffen up the tone when the amp is pushed i'm sure.

I see 3 wires (blue, red, brown) on your output transformer.
Isn't that a Push Pull OT?
Looks like you are using 1/2 (blue to red) with the brown unused.
Probably about 4k to 3.3k ohms for the tube.
Can you use a PP OT for a single end Champ ?
I have read that SE OTs are larger per watt than PP OTs because of saturation.
Having that other half of the input windings floating might be weird also.
Someone else here will know more than I do about using PP OT as a sub for SE.
It was the taped off brown wire that got my attention. It's not grounding is it ?
Hope this helps,
Good Luck !

Not a PP OT, it's a ClassicTone 40-18031. The brown wire is 5Kohms and is capped. The red wires are for a NFB mod, I had an on/off/on switch lying around so I wired it up for three diff NFB levels. Another TDPRI'er highlighted that Weber seems to have a PP OT in their 5F2A kit and muchxs reckons it's doable, the thread is here...

http://www.tdpri.com/forum/shock-brothers-diy-amps/325538-tweed-champ-princeton-ot-questions.html

I honestly would not have expected an increase from 16 uf to 22 uf to make that much of a noise difference, but certainly cannot argue with success. Glad it worked out for you.

I was surprised to