DC supply on driver tube hums..

[Ciscokid]

Hi,

I've asked this over on AA but thought I would ask here as well..

I have been experimenting with one of my jelabs300b mono block amps today and thought I would try DC on the 6sn7 driver to see if it would remove the last bit of hum. Keep in mind that I am rather the novice when it comes to electronics. Anyhow, I created the supply using a SS full wave rectifier, .2 ohm resistor and a 10,000uf cap. I attached the positive and negative leads to the filament of the 6sn7. Voltage is about right at 6.23 volts.

However, when I fire up the amp, I get a really bad hum. Not sure I did this correctly? What am I missing? Do I need to ground one leg of the supply?

Any help with this would be most appreciated.


Regards,
Danny

 

[Richard Ellis]

Hum...

10K mikes of capacitance isn't enough to hold the ripple down enough to keep some forms of AC off the line. Schematics please??
Let me do the numbers....MJ..I'll get my book, OK?
________________________________________Rick........

 

[Sheldon]

Do I need to ground one leg of the supply?y

The supply must have a ground reference. This can be done by grounding the center tap of the supply winding. If it's not center tapped, you can ground one leg, or better, create a virtual center tap with two 100R in series across the output, and the center to ground.

Before you try any of that, I would suggest that you try AC again, but lifting the heater reference about 20-30V above ground. This is done with a voltage divider from B+ to ground. Let's say your B+ is 300V. Assuming a bleeder resistor of 200k, you'd use something between 20-30k between the 200k and ground. Connect a 10uf cap from that point to ground, and from that point to one heater leg. You won't need DC for a driver, unless you have a poor tube.

Sheldon

 

[koolatron]

What's this DC supply referenced to? If it's floating with respect to everything else, that could be your culprit.

If it's not, make sure that you've used sane grounding practices.

Also, I agree that 10,000uF / 0.2 ohms is probably an insufficient filter to smooth 60Hz AC. Increase your capacitance. Also, an RF choke may remove any high-frequency switching artifacts introduced by your rectifier.

-k

 

[kmaier]

Well, a couple things:

1- If you built the JE Labs 300B amp in stock trim (at least from the schematic I've seen) it uses AC for the 300B filament. So if this is the case, you'll never get the hum balanced out with these. The 300B really needs a DC filament to be quiet.

2- A good 6SN7 should be quiet with AC filaments, but as pointed out already, you should reference it to ground either via a center-tap on the winding or a pair of resistors (I prefer 30ohm 1%). Alternately, if you are having some filament/cathode voltage issues (or want a safety margin) you can make the divider network and "lift" the filaments off of ground.

Regards, KM

Note: full-wave rectified DC will have 120Hz ripple, not 60Hz.

 

[koolatron]

My mistake. 10000uF / 0.2 ohm yields a corner frequency around 79 Hz. It'll kill 120Hz hum pretty significantly.

 

[Ciscokid]

Hi Kmaier, Sheldon,

You're correct about the humming 300B Anyway, I went to DC on my 300B months ago. Actually used the Tent DC Current regulated supply and most of my hum is gone. However, I am using 97.5 db efficient speakers and during quiet passages, am hearing noticeable hum. Thinking it is the driver tube wanted to try DC.

Anyhow, Sheldon makes a good case for elevating the filament on the driver. I think I will give that a try before doing anything else...

Thanks to all for your input and I will let you know how it turns out..

Regards,
Danny

 

[kmaier]

I always put a mute switch on my DHT SE designs... simply grounds the grid of the output tube, be it a 45, 2A3 or 300B. This easily allows you to fine tune the hum balance pot and also determine what hum level is coming in from the input/driver stages, along with a shorting input plug.

Have you measured the total output noise of the amplifier?

Regards, KM

 

[Ciscokid]

Hi Sheldon,


I built the voltage divider circuit and have about 35 volts. If I connect it to one of the heater legs it doesn't seem to make much difference.

One thing I am confused about, my AC winding has a center tap to ground, instead of connecting to a heater leg, should I connect the CT to the voltage divider instead?

Regards,
Danny

 

[Sheldon]

Yes, in that case, use the center tap to your divider. If that doesn't reduce your hum, it's not the driver filament. I have an 801 amp with 6SN7 driver. AC on the driver, Tentlabs for the DHT. It has no hum on 106dB speakers - none. Schematic, pictures? Let's have a look at your power supply and grounding scheme.

sheldon

 

[Ciscokid]

Hi Sheldon,

Call me dead quiet Dan

I now have dead quiet amps, at least while on the bench using lifted AC on the driver and shorting the input (ask me how I figured that one out ) jack.

I now measure less than a milli-volt of noise on the outputs. However, when connected into my system, am picking up about 10-12 mili-volt (did I mention how bad the noise had gotten ) of noise. Anyhow, I'm thinking ground loop or patch cable problems or something. So I am going to tear everything out today and build it back as simple as possible.

Everyone,

Thanks for all of your help

Regards,
Danny

 

[kmaier]

Yes, I would suspect a ground loop... try hooking up one amp (or channel) only and see if you still have the large increase... could be grounding between your gear, could also be your source that's creating the noise.

Also, define "less than 1 mv of noise" on the output (with the input shorted). If you have 1mv of output noise into an 8 ohm load then your signal/noise ratio is only 69dB below 1-watt. With speakers that have 97dB sensitivity that would be fairly audible in a quiet environment.

My 45 SE amps (all AC filaments) are typically less than 150 micro-volts of output noise, which is better than 85dB below 1-watt.

Regards, KM

 

[Ciscokid]

YES!!! Sound of silence !!

Found the remaining noise and it was a ground loop. Both the mono blocks have three prong plugs and I used one each of the 3to2 plugs and the hum is gone!!

I'm so happy.. Going to leave these alone and simply enjoy the music!!

Thanks to all for the suggestions in resolving my issues..

Best regards,

Danny

 

[Renron]

YES!!! Sound of silence !!

I used one each of the 3to2 plugs and the hum is gone!!


Danny

Does this mean there is NO ground on your equipment???
Hmmmmm could be just masking the problem (ground loop)

Ron

 

[Ciscokid]

Yes,

Exactly the problem! Am about to order a pair of input transformers in order to nip the ground loop problem!

Regards,
Danny

 

[audiowize]

If that 2 to 3 prong plug is really the issue, consider going into your amps and separating the safety ground from the signal ground. The safety ground will connect to the chassis, output transformers, power transformer(S), choke(s), etc. The signal/electrical ground will connect to the ground tab on your insulated RCA jacks, the - side of your power supply, the bottom of your cathode resistors, etc. The other option is to go into your preamp and do the same procedure, although I find having them tied there works best.