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ST70 hybrid finally up and running !!
ST70 hybrid finally up and running !!
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Old 19th December 2006, 03:19 AM   #1
zigzagflux is offline zigzagflux  United States
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Default ST70 hybrid finally up and running !!

Finally got my ST70 (or what's left of it) running. Basically, I took the xfmrs out of a ST70, and built everything else from the ground up, chassis, power supplies, you name it. Had a ST70 many years ago, so I wanted to relive the glory days, but kind of take it to the extreme.

Haven't had too much auditioning time, but I have some 60Hz noise going to the speakers. It's not excessive, and doesn't bother me at all, but I am a little miffed as to its source.

It's not 120Hz, checked with a scope. Level is about 50mV peak-to-peak with a 4 ohm speaker connected to the 8 ohm tap (long story-don't ask). Output B+ has 80 mV ripple, driver stages less than 5 mV ripple.

All heaters are DC, regulated with LM317 as current source. Star grounding throughout, and only one connection to chassis (IEC ground) through a 100 ohm resistor. If that resistor is lifted, I get open circuit with Fluke 87.

Ideas on where this hum is coming from? I can submit schematics, but will need a little time to post. Basically 6922 cascode input followed by 6922 concertina, driving EL34's.
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Old 19th December 2006, 03:35 AM   #2
zigzagflux is offline zigzagflux  United States
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Here's what were dealing with.....
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File Type: jpg st70.jpg (70.1 KB, 605 views)
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Old 19th December 2006, 05:17 AM   #3
Audio_idiot is offline Audio_idiot  Malaysia
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Default Hum?

Hi,
Great work!!! I never have the patient to build such handsome looking chassis.

2+1 places I normally look into when hum come out from fresh amp,
1) Layout of the wires, make sure all wire crossing are perpendicular to each other, avoid wires running in parrallel, and twist all AC wires.
2) check if any small signal wires are close to PSU or other RFI sources in the chassis...
+1) if 1 & 2 fail, increase thepower supply's first capacitor size....

As for the grounding scheme, my personal preference is to separate the ground of the power supply from the signal circuit... at least the 1st cap's ground is connected directly to the ractifier's/diode's/bridge's ground pin. and I like to separate the ground for small signals from bigger signals...
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Old 19th December 2006, 01:03 PM   #4
zigzagflux is offline zigzagflux  United States
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Paid very close attention to wire layout. There is a star point for input stage, then a star point for output stage, and a star point for power supply. These get tied together in one place.

What is most confusing is the relative absence of 60Hz carrying wires inside this amp. They are confined to two areas, and are run with at least 3 inches clearance from all significant circuit paths.

I used a distributed PCB layout of power supplies, followed by point to point wiring of the tube sockets. All heaters and power supplies, even though DC, are tightly twisted pairs.

I will try lifting vs shorting my 100 ohm resistor to chassis, see if that makes any difference.

I've got quite a bit of capacitance already, and like I said, my noise is not 120Hz. It's either got to be coming in through the ground point, or possibly coupled from the power xfmr.

Source for testing is an iPod, so no loops there.
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Old 19th December 2006, 03:33 PM   #5
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by zigzagflux
possibly coupled from the power xfmr.
If it's the power xfmr influencing the OPT's, then you will hear the hum loudest right at turn on (current draw, hence flux, is greatest then) - even before the tube filaments are warm and the amp is amping. In fact, you can try it with no output tubes.

Another easy check is to take a plate of metal (steel is good, but thick aluminum is also fine) and slide it between the power xfmr and OPT, then listen for a change in hum.

Also, if you have a steel chassis, try isolating the transformer from the chassis by a mm or so, with non-conductive spacers.

Sheldon
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Old 19th December 2006, 03:54 PM   #6
zigzagflux is offline zigzagflux  United States
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Good suggestions, but already done, my friend.

Power transformer is electrically and mechanically isolated from the top plate through rubber mounts and teflon shoulder washers. Single point ground to chassis from there. Didn't want mechanical vibrations to be an issue.

I use a delay circuit, such that tubes are heated before B+ is applied. Hum only starts after B+ is up, so I guess the chances of xfmr coupling are reduced? I do know that the physical spacing on my top plate is greater than that found in the original ST70 chassis.

I will pull the tubes tonite and see what happens. Will also poke around with the o-scope to try and find the leakage source.

Thx
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Old 19th December 2006, 04:31 PM   #7
pedroskova is offline pedroskova
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Use shorting plugs on your inputs and see what happens. 60Hz could signal a ground loop between various components.
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Old 19th December 2006, 06:44 PM   #8
stokessd is offline stokessd  United States
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Given that your transformers are mounted in the same orientation and even farther apart than the original ST-70, I would be really surprised if the mounting is an issue.

Another Sheldon
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Old 20th December 2006, 05:20 PM   #9
zigzagflux is offline zigzagflux  United States
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Well at least I've had some progress.

As far as I can tell, the 60Hz is entering at the passive input stage. I will be using a powered subwoofer, so have a first order BP filter at 90Hz and 50 kHz prior to the first amplification stage. The LP at 50 kHz was based on Van Alstine recommendations.

When the RCA input is shorted, there was essentially very little reduction in output hum. However, when the input grid was shorted (post-filter), All is Quiet on the Western Front. So the filter is picking up 60Hz EMI.

There was a 120V twisted pair run within say 2 inches of this passive filter, which surprisingly was a partial culprit. I was surprised because this pair feeds only my bias circuit, which draws essentially no current. Current carrying conductors produce stronger fields than only voltage carrying, but it is what it is. Moved it to the other side of the case, and cut my hum by about 50%.

The rest of the hum can still be seen on the scope, but is very difficult to remove. There is literally nothing nearby. The closest transformer is maybe 4-5 inches (sorry, 10-13cm) away, and is small signal.

My next choice is to remove the LP filter entirely, and decrease the resistance of the HP filter (and increase cap accordingly). This will hopefully reduce hum even further.

Ultimately, the level is very low, about 3mV rms at the speaker terminals, but it bothers me that its there, and it can be heard with the ear up to the speaker.

Anyone have good results with the Van Alstine recommendations?

http://www.avahifi.com/root/equipmen...uild_plans.htm
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Old 20th December 2006, 06:20 PM   #10
EC8010 is offline EC8010  United Kingdom
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60Hz hum is almost always electromagnetic. I note that the hum field from your mains transformer points directly at your input valve. This may be the one instance where a (steel) screening can on the valve is useful.
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