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Finished My ST-70

PostPosted: Tue Nov 25, 2008 12:48 am
by kt88pppamp
I finally finished my stereo 70 kit. It was hard work but it works. The bad news there is a slight hum, inaudible outside of three feet. The hum is heard via upper harmonics from the tweeter (I use two way test speakers) and the woofer. Is this from the AC filament most likely?

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PostPosted: Tue Nov 25, 2008 1:49 am
by mesherm
Its a long shot but try this. I believe the power supply board has two small value film caps, one running from each 6 volt filament line to power supply ground. I dont know if your power supply ground is also connected to the safety ground but if it isnt then try wiring a 0.02 uf cap (a cheap ceramic will do fine) from the power supply ground where the junction of those two filament caps are connected to the safety ground. If only one set of filament lines has those two caps to ground then install a pair of caps on the one without them. I usually install them at the furthest point in the filament train from the tranny.

PostPosted: Tue Nov 25, 2008 3:37 am
by kt88pppamp
Just to clarify, it is wired accordingly (see picture).

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PostPosted: Tue Nov 25, 2008 11:17 am
by mesherm
OK, if the third wire safety ground is isolated from the power supply/circuit ground try connecting the two grounds with a 0.02uf cap.
If no hlep then temporarily lift the safety ground and see how that affects the hum.

PostPosted: Tue Nov 25, 2008 11:24 am
by snitch56
Let us know how that Blackburn driver sounds. The 6SN7’s look great on the board. Unfortunately, I think Matt stopped making those boards this month.

PostPosted: Wed Nov 26, 2008 5:05 am
by TerrySmith
Very nice!

You might try re-routing the 6.3v filament wiring away from the speaker jacks, could have some inductive coupling there.

PostPosted: Thu Nov 27, 2008 8:01 am
by burnedfingers
Quote:

The bad news there is a slight hum.

I built the same board earlier this year and had a slight hum that went away when I used nylon fasteners in place of standard steel screws to fasten the driver board to the chassis.

Not a bad board as the 6SN7's look impressive but doesn't come close the the modified Mapletree design I am using now in my opinion.

My board is sitting on a shelf if anyone is interested in getting a ready made one.

PostPosted: Thu Nov 27, 2008 10:36 am
by EWBrown
Isolating the driver board's "ground" from the chassis would eliminate a ground loop, and that, in turn, reduce or eliminate any residual hum problems.

Best to "star ground" from the negative side of the PSU cap(s) and, if using the old style can cap with the twist lugs, use a short jumper wire and ring lug to make a "hard" electrical gound connection to the chassis.

/ed B

PostPosted: Thu Nov 27, 2008 11:44 pm
by kt88pppamp
If you looked closely at the pictures in my first post, I did use nylon fasteners. I think the hum is coming from the filaments, since they are AC. I do not know this, but some of you claim that zero hum is achievable with AC filaments. Is that true or do you all not have sensitive ears, do not care about faint hum, or not try placing their ears next to or into the speakers to try and hear hum? I have a circuit I am going to build that rectifies the 6.3 volts AC into DC. That will hopefully kill away that hum.

PostPosted: Fri Nov 28, 2008 12:43 am
by mugsy
It appears from your first picture that your mains transformer is mounted directly to the chassis. The transformer hum (the physical vibration) is transmitted from the chassis to the sensitive valves which MAY be contributing to your problem.

If you haven't done so already mount the transformer on rubber washers and then to the chassis and see if this helps any.

- John

PostPosted: Fri Nov 28, 2008 8:57 am
by TerrySmith
I actually built this circuit on an aluminum plate I cut to fit the ST70 chassis, wired it point to point. The plate was recycled from a previous project years ago. Anyways my amp is dead silent, plus I have the on-off switch next to the input jacks.

This circuit sounds really good, I played this amp all day yesterday before replying to this post. The coupling caps are Russian K75-10's, and the rest of the circuit was built around them because of their size.

Your amp will be hum free, just have to find it.

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PostPosted: Fri Nov 28, 2008 5:33 pm
by Sal Brisindi
This is a long shot but I noticed your power transformer does not have an end bell on the bottom, is it a 60 hz hum or a 120 hz hum? Also, possibly a heater/cathode leakage in one of your driver tubes.

Sal

PostPosted: Fri Nov 28, 2008 10:20 pm
by TerrySmith
Kingneb's tranny is a Triode replacement, they don't have bottom covers. My tranny is an older Triode unit with PVC leads instead of teflon.

PostPosted: Sat Nov 29, 2008 3:59 pm
by kt88pppamp
I tried some tube swapping with the 6SN7's. The hum gets louder and softer with different ones. I have Electro Harmonix ones i tried and they made the hum so incredibly loud and horrible. I have a feeling Sal is right, filament to cathode leakage. How do I find 6SN7s that do not have this problem? The ones in the picture are NOS GE tubes. The one in the center (i think thats the one) hums more than the others. The others still have an audable hum.

PostPosted: Sat Nov 29, 2008 6:02 pm
by EWBrown
I've had some less than satisfactory results with the Electroharmonix 6SN7s, and I found that the cheap "generic" Russian 6H8Cs (6SN7 equivalent) worked and sounded far better.

My application was in a modified Bottlehead "Foreplay 2" linestage, and the EHs sounded rough, harsh, and grainy. The cheapo "Russkis" sounded very nice, about the same as some vintage RCA 6SN7GTAs. Maybe the EH's just needed some burn-in time, but I haven't bothered to try them again.

/ed B