hum, no way to get rid of it

[franckM]

Hi,

I have built a SE amplifier, witch was dead quit when using DC heater on the tubes. (CLC filter)

I have changed to AC on the heaters, in order to hear if there is any sound difference as many people believe and I get it..., not really that is a bad joke, I get about 20 mv hum peak to peak on the speakers terminals .

I use à hum pot balance on the DHT tubes and 2 x100 ohm resistors with a centre tap to ground, on the IDHT tubes.

The circuit is as following.
6SL7-->ECC99/LL1660-->300B/LL1623, with negative bias supply.
The 6sl7 is loaded by a CCS and have DC connection to the gird of the ECC99.

Any suggestions on how to reduce the hum, please ?

Best regards,
Franck

[jarthel]

can you tell me what choke you use for the DC heater? a link to a manufacturer's product page would be very much appreciated.

thank you.

[kevinkr]

My experimentation with ac heating on dht output tubes has indicated that hum is a serious problem with heater voltages above 2.5V.

There is an article on the Steve Bench website regarding hum canceling techniques that might help.

I use dc constant current heating in my own designs at 5V and above, it's simple and less complicated than any of the hum canceling circuits I have encountered.

[zarniwoop]

This is a big topic for me lately... there is another thread here about this and also on AA.

This really should be more widely known. If you AC heat a SE DHT amp that uses filament voltage of more than 2-3 volts you will probably notice hum on many efficient speakers, even using a hum pot. Its the harmonics of the heating frequency that are not removed using any normal balancing circuit.

20mV p-p hum? Wow, thats about exactly what I get too. It does vary depending on the brand, but this is almost all heating induced hum (120hz and up) that gets on the output. And that is not too cool with 100dB speakers.

I'm going to try the steve bench circuit again and see if I can get the supposed 20dB reduction here. I'm sick of the hum and I'm not going to buy more filament transformers and make more DC supplies. Will report back on the results.

steve bench hum cancel

[franckM]

Hi,

"can you tell me what choke you use for the DC heater? a link to a manufacturer's product page would be very much appreciated."

I am really sorry, I have it since many years and nothing can be read on it. I know they are rated @0.47mH 5 amps, small size and heavy.

"There is an article on the Steve Bench website ", yes I know this article, If I remember, the circuit inject in the system a portion of hum out of phase in order to cancel the hum...

I read some times on the net that some people are able to use AC and without hum. I wonder how ?

A+,
Franck

[kevinkr]

With very well matched dht you can run pushpull with 5V tubes and not have a big problem which is what I did in my first pp 300B design, however I have not encountered any amplifier using ac heated 300B or 50 that had acceptable hum except in the case where the amplifier was used with low efficiency speakers or ones with high a high bass roll off.

My observation in the pp amplifier was that most of the filamentary hum was 120Hz and cancelled quite effectively in the output transformer.

In my system anything over 1mVrms is quite audible..

[zarniwoop]

Yeah, PP can work fine with AC. I'm using 6B4Gs in PP and can get the hum acceptably low.

I'm just trying this cancellation circuit again. I'm using a 15turn 10k pot as the adjustment pot. Definitely not a straightforward process. There is not a easy to find null point. I'm just using a multimeter on the output so far. The best I can do is about 4.5mV rms at the output. Still sounds like primarily 120hz. Apparently there is some phaseshift to deal with or something. I don't know. Where is steve bench?

[SHiFTY]

For 6B4g I just used a small toroid with 2x 9VAC windings, rectified with Schottky diodes to 2x12VDC, into a 5000uF cap and then a 4-5 ohm dropper resistor into another 5000uF cap for 6.2VDC. Result: silence.

[Gerrit Boers]

I would let go of the idea of using AC for DHTs, AC isn't what it used to be:

AC supply noise

In my opinion the (supposed?) superiority of AC heating is the result of the absence of diode switching noise.

John Atwood did some research on this topic:

DC filament supply test

In my amp I use a CRC filter (no room for an extra choke) followed by a voltage controlled current source:

DC heater supply part 4

Total hum & noise on the output is <70µV, the peaks in the spectrum for 50Hz, 100Hz and 150Hz are below 10µV.

[zobsky]

Originally posted by jarthel
can you tell me what choke you use for the DC heater? a link to a manufacturer's product page would be very much appreciated.

thank you.

FWIW, allied electronics (alliedelec.com, I think) had some suitable chokes.