diytube.com

Thanks for that tip - tried my iPod and the hum was all but gone. Just a tiny amount which can probably be eradicated by moving some wires around inside the ST70.

However ANYTHING which is mains connected seems to make this thing hum. Even using my laptop (which isn't earthed) through a USB to SPDIF converter, then onto a (again, not earthed) DAC I get awful noise. Strange. It seems worse if I un-earth the ST70 as well.

Perhaps my KingRex pre-amp (being solid-state) meant for Class D amps is not a good match for the ST70 - impedence mis-match perhaps? I will try my re-built Quad 34 on it later today and see if that fares better.

Cheers,

- John

Well this just gets weirder and weirder Yellow_Light_Colorz_PDT_21

Used my Quad 34 (with output levels changed to 1.6V) which is earthed, and using my Shangling CD-T80 CD player (also earthed) through this, and into my ST70, and the hum levels were barely audible! This was with the ST70 earthed as well - so three items earthed and barely any hum! What's going on there?!?

Interestingly when I removed the earth from the ST70 the hum got alot worse...

VERY confusing......

What have I done wrong here?

EDIT: My mistake - the Quad 34 mains lead isn't earthed. I'll try earthing the Quad 34 and taking off the earth from the ST70...

Your hum issue vaguely reminds me of this post:

http://www.diytube.com/phpBB2/viewtopic.php?t=1775

The original 50s - 60s vintage Dynaco gear had two line power cords, no third wire safety earth / ground wire. Two wire line cords were pretty much standard consumer equipment practive back then...

The more modern 3 wire power cords can introduce strange ground loop situations...

/ed B

The weirdest thing is if I plug my ST70 in, with nothing else, and I unearth it like the stock unit, I get hum (even with RCA cheater plugs).

If I earth it the hum lessons considerably.

I'm sure I must have made a mistake somewhere, either that or the 240V transformer has something to do with it (i.e. not having a shielded base plate).

My ST 70 rebuild hums also. I have exactly the same problem with the same board, quad cap and Triode Ele. transformer(120V). After hours of trouble shooting and frustration I found it to be very sensitive to 7199 tubes, I bought a NOS pair of 7199's and they hum like crazy, I installed an old set and they are much quieter, yet one is still noisier than the other. Bias is solid and it sounds much better than I expected. I am going to try another NOS set, go figure. Please post your changes and results, I am very interested. I still think it is ultimately a grounding problem?????

Good luck.
I am new to DIY tube, this is my first post.

cheers,
David

Hi David.

Sorry to hear you've got a hum problem as well.

I'm also using a pair of NOS RCA 7199's - later ones if the more modern RCA logo on the tubes is anything to go by. I am going to try rubber washers under the transformer.

However, I have a feeling the problem could be down to excess DC on the 240V AC mains. Noticed anything with a torroidal transformer ALWAYS hums in this house, which tends to point a finger towards DC. This would explain why the transformer hums, and also why it gets hotter than usual. I am going to build a 'DC trap' (basically a 35A 1000V bridge rectifier and two 4700uF 10V caps) to try and solve this.

From what you said the 7199s seem very susceptable to microphonics, so perhaps if I can lesson the transformer hum I can lesson the hum from the speakers. And if that doesn't work it has to be a ground problem somewheres.

Cheers,

- John

Just a quick thought on hum... Would placing the 15.6 ohm resistors diagnally across the valves bases cause any hum? I notice in the original Dynaco diagram the resistors go around the octal valve sockets. I wanted to keep things as tidy as possible so wired mine across the center of the octal base...

I see the safety ground connected. Unbalanced gear with safety grounds will have more noise/hum. See if disconnecting it solves your problem and work from there.

Hi Mugsy,

The 15.6 ohm resistors on my St70 are mouted across the the valve, good observation on your part, it my be worth trying Dyna's layout. I have rubber isolation washers from Dynakit parts under the output transformers. I will try damping rings on the 7199's also. I do think the wiring in my house may add to the hum. My St70 is still on the bench, I will also try with a Richard Gray 400 pro. to see if that helps reduce the hum. I was looking through my tube inventory last night and found 2 pairs of Mullard xf2 dual getter EL34 valves. Yellow_Light_Colorz_PDT_12 One pair tested good the other tested as NOS. I put them in and the hum went down a notch. I have never had an ST 70 that need this much time of tweek and burn in. I am ready to move it into a system and see what happens.

Hi.

Separks - have tried with earth and without, and - bizarrely - it was worse without!

Boycephoto - Something I noticed which may help, when I touch either of the 7199 tubes, the hum goes up in volume. Take finger away and it's just a low 'purr' again. Both are NOS and it seems unliley they could both be bad?

Something else, the valve bases I'm using have bits of metal in the center. On the underside there is no earthing spigot though. I know with some B9A bases they advice to earth the center point. Wondering if the metal in the center of the ceramic base is somehow causing the hum in the sensitive 7199s?

- John

Something I noticed which may help, when I touch either of the 7199 tubes, the hum goes up in volume. Take finger away and it's just a low 'purr' again.

You mentioned that the power transformer hums. Can you feel a vibration in the chassis from it? Tubes are all microphonic to some degree and the pentode section of a 7199 has significant gain. Maybe soft rubber washers under the PT or between the driver board mounting screws could help.
Do this! Loosen or remove all four driver board screws so there is as little mechanical coupling as possible and see if the hum decreases.

Wondering if the metal in the center of the ceramic base is somehow causing the hum in the sensitive 7199s?

Did you try grounding the metal center of the base

I will try using grommets and see if that solves anything.

However discovered a REALLY weird phenomina... if I have the shorting plugs in, and i place my hand NEAR to either 7199 - NOT touching - the hum level increases. What th eheck is going on there?!? Is my ST70 turning into a theramin?!? Haha! Truly bizarre.

Burned fingers: nope the sockets cannot be grounded - they are of the type shown here:

http://www.dynakitparts.com/store/product.aspx?id=227

There is no ground tab - the underneath is the same as the top - just that bit of metal in the center...

Getting real tired of this bastard hum - taking all the enjoyment away from owning a ST70. Going back to basics again. All other posts I have made should be forgotten. Starting again here:

1) Dynaco ST70 unearthed, with RCA cheater plugs - low level hum. Not audible from normal listening distance.

2) Plug in my Quad 34 (or KingRex) pre-amp - hummmm! Unplug all inputs to the Quad 34. Nothing else plugged into the mains block. STILL hums. Disconnect Quad 34 earth - hums, but not QUITE as badly.

3) Plug in iPod directly to ST70 - as no.1, hardly any hum. Sounds fine.

4) Disconnect ground to Quad 34, PUT a ground on the St70 - hardly any hum.......

I have a migraine! Yellow_Light_Colorz_PDT_20

- John

P.S. I have the ST70 earth (when connected) attached to the mounting bolt of one of the mains trannies mounting bolts. Would it be better to try it at the star ground point near the quad cap?

Secondly, if I use rubber grommets under the transformer, I presume this now means it not as safe as it's isolated from chassis ground?...

Secondly, if I use rubber grommets under the transformer, I presume this now means it not as safe as it's isolated from chassis ground?...

The screws holding the tranny to the chassis should ground the tranny case just fine. Make sure that any paint under the screw head is sanded to bare metal and then do a ohmmeter check to make sure the tranny case and chassis are connected.

If you have ever had an RCA plug inserted into the amp but open on the other end and then touched the center prong you have noticed that the human body is a giant antenna for all kinds of stray EMF so I'm not surprised that bringing your finger near a high gain tube like a 7199 will cause some hum. You could make your own temporaray jerry-rigged tube shield with some aluminium foil and a bare wire touching it going to ground.
Or you could watch on ebay for a Dynaco ST70 driver board metal shield/cover . They show up from time to time and also add to the looks.