How to reduce hum?

[timp]

My speakers are set pretty good and sound really good. I am so happy. 414s and 806As really rock! Now, the speakers being so efficient I am hearing everything: HUMS. It seems to be around 80hz. I have a home brewed tube preamp, 6dj8 cathode follower and Eico 14s.

I checked all the resistors to see if they are floating. And replaced all the wires and power caps are good. May be this is just natural because all the filaments are AC supplied. Any suggestions would help. Thanks.

Tim

[bfish]

I don't know about your 14s, but most Eicos I've seen had hum pots. Look for a chassis-mt screwdriver pot connected to the filament string.

[timp]

Hi Bfish

Thanks for the reply. Nope. I don't see hum control in my eicos.

Tim

[bfish]

You are aware that HV electrolytics can hold enough charge to knock you silly aren't you?

[timp]

Sure do! I've been shocked just by touching connector that was not shielded. That was painful. So, how do you drain the caps? I just put my voltage meter until it does down to 10v or so. Yeah. I get a cold chill everytime I have to go into TUBE amp. Scawy!

By the way. How do you ground old amps that do not have ground?

Thanks.

Tim

[bfish]

Hummm...Kinda tough over the web. If you had a 'scope...'course then you wouldn't be asking.

If you're sure it's the amp, but can't test to pinpoint the source, the most likely suspects are the main filter caps. They're a common enough source of problems on old gear that I don't even test them anymore, I just replace them. (as well as the rest of the electrolytics).

[timp]

No. I don't have a scope. Me and my ears and ohm meters. Do you think the main filters are leaking? Ok. I am going to start with the main filters. In any case, it will be a major surgery.

Thanks.

Tim

[RonSSS]

I doubt it's 80hz. Ground loops are usually 60hz, power supply caused are 120hz.

AC filaments won't cause hum unless a tube is bad.

First I would disconnect the preamp from the power amp. Short the power amp inputs and see if the hum either goes away or changes level. This will tell you if it's the preamp or not. You did say it was a DIY preamp?

If it's in the preamp, then you will need to make sure the B+ is as well filtered as it can be. A centertap on the filament supply is also required. Either from the transformer, or you can make an artificial one with 2 20ohm resistors.

You really need a scope!

Ground loops can be a pain in the butt. With both the pre amp and amp plugged in and powered on, but not connected together, measure the AC voltage between the 2 chassis. The reverse the power cord on one of the units and measure again. Pick the lowest of the 2.

With horns, you really need the hum level at the output of the amp to be in the less than 5mv range, or you will hear it.

My tube amps are dead silent...but it took some effort to get there.

Ron

[timp]

Ok. I know the hum is coming from the Preamp. When the premap is off the hum is off. Then the right channel hum is louder than the left. It is higher than 60. May be 120H? In fact, I can live with the left channel hum which is really low. I replaced all the tubes. Same result. I replaced all the coupling caps in the preamp. Same result. I replaced all the cables with Mogami shielded lines. Same result.I am definitely losing my mind over here Help. H E L P!

<font color="#FFFFFF" size="1">[ February 01, 2008 04:14 PM: Message edited by: timp ]</font>

[timp]

RonSS

Would you tell me more about the arificial center tap? I don't have it. May be that's what I need for my preamp.

Thnaks!

Tim

<font color="#FFFFFF" size="1">[ February 01, 2008 04:09 PM: Message edited by: timp ]</font>