can't get rid of hum st 70 + PAS 3x or st70 + PAS-NGT

Hi Guys,
Built a dynakit st70 last year. Bought a PAS 3x (1969) to go with it. Has some hum issues but found that the PAS had about six cold solder joints that I fixed and hum subsided but still there. It's very low but gets louder as the volume is increased. Tried isolation transformer on the mains, put everything on the same outlet. Made no difference. When I connect the PAS to my Harmon Kardin 230 no hum. When I connect the PAS-NGT to the Harmon Kardin no hum. When I short out the inputs to the st70 lno hum. When I connect either PAS to the st70 there's hum. Doesn't matter if I connect an input to the PAS or not it hums the same. I measured a 300micro volt DC difference between the chassis of the st70 and the PAS but when I jumper them together the hum gets much worse but the difference in voltage disappears. I do not understand how the PAS preamps can work fine with the Harmon Kardin but not the st70. And why would the hum increase with I jumper the chassis together? Shouldn't that have eliminated any ground loop? I could not measure any AC difference between the st70 and PAS as the meter I have isn't sensitive enough and I haven't gotten around to putting my scope on it yet. I was hoping I could figure this out but obviously, I can't. So if I do hang my scope on it where should I look? And for what. The hum is obviously 60 cycle. That's for sure. Some times the hum varies with the volume control, other times it doesn't? What's that about? Could this all be a problem with the internal wiring of the st70? The wires from the rca inputs go right to the grids on the 6GH8A's and aren't very far from the power transformer. Maybe a couple of inches (3-4). Would shielded wiring help? This is soo frustrating. Been trying to eliminate the hum for months. Any help would be greatly appreciated.

Best Regards,
Paul

DrBrown wrote:Hi Guys,
Built a dynakit st70 last year. Bought a PAS 3x (1969) to go with it. Has some hum issues but found that the PAS had about six cold solder joints that I fixed and hum subsided but still there. It's very low but gets louder as the volume is increased. Tried isolation transformer on the mains, put everything on the same outlet. Made no difference. When I connect the PAS to my Harmon Kardin 230 no hum. When I connect the PAS-NGT to the Harmon Kardin no hum. When I short out the inputs to the st70 lno hum. When I connect either PAS to the st70 there's hum. Doesn't matter if I connect an input to the PAS or not it hums the same. I measured a 300micro volt DC difference between the chassis of the st70 and the PAS but when I jumper them together the hum gets much worse but the difference in voltage disappears. I do not understand how the PAS preamps can work fine with the Harmon Kardin but not the st70. And why would the hum increase with I jumper the chassis together? Shouldn't that have eliminated any ground loop? I could not measure any AC difference between the st70 and PAS as the meter I have isn't sensitive enough and I haven't gotten around to putting my scope on it yet. I was hoping I could figure this out but obviously, I can't. So if I do hang my scope on it where should I look? And for what. The hum is obviously 60 cycle. That's for sure. Some times the hum varies with the volume control, other times it doesn't? What's that about? Could this all be a problem with the internal wiring of the st70? The wires from the rca inputs go right to the grids on the 6GH8A's and aren't very far from the power transformer. Maybe a couple of inches (3-4). Would shielded wiring help? This is soo frustrating. Been trying to eliminate the hum for months. Any help would be greatly appreciated.

Best Regards,
Paul

Dynaco PAS is sensitive of internal cabling. Do make sure that the internal ground and B+
cables are draws as in the Dynaco manual. Do make sure the groundings on the RCA
is as per the manual including the RCA panel.
Keep the ground connector between the PAS and the ST70, when all sorted out this will
keep the hum away.

If the PAS is changed in any way all bets are off, come back and describe what
different from stock.

Hi Peter,

Thanks for your reply. I don't have a ground connector between the st70 and PAS. Where does it go? What kind of wire/cable/shielded cable should be used? Yesterday, I took the cover off the st70 and pulled the two 6GH8A driver tubes out and turned on both units. The hum was still present. Ugh! Does that mean the PAS is not the problem? I tried to keep the wires inside the PAS as far away from B+ as possible. This is so confusing. Seems like nothing works. What does it mean when you say "make sure the internal ground and B+ cables are draws as in the Dynaco Manual"? I built Kevins Dynakit st70 and followed his instructions exactly. At least I tried. As far as I can tell I have it right. The PAS 3x is stock. The PAS-NGT is a kit I build purchased from ER-HARDT audio just a few months ago. The PAS-NGT is quieter than the PAS 3x but they both hum. How do you suggest I connect ground from the PAS to the ST70? I didn't see that in any of the build manuals.

Thanks again for your help Peter!

Kind Regards,

Paul

DrBrown wrote:Hi Peter,

Thanks for your reply. I don't have a ground connector between the st70 and PAS. Where does it go? What kind of wire/cable/shielded cable should be used? Yesterday, I took the cover off the st70 and pulled the two 6GH8A driver tubes out and turned on both units. The hum was still present. Ugh! Does that mean the PAS is not the problem? I tried to keep the wires inside the PAS as far away from B+ as possible. This is so confusing. Seems like nothing works. What does it mean when you say "make sure the internal ground and B+ cables are draws as in the Dynaco Manual"? I built Kevins Dynakit st70 and followed his instructions exactly. At least I tried. As far as I can tell I have it right. The PAS 3x is stock. The PAS-NGT is a kit I build purchased from ER-HARDT audio just a few months ago. The PAS-NGT is quieter than the PAS 3x but they both hum. How do you suggest I connect ground from the PAS to the ST70? I didn't see that in any of the build manuals.

Thanks again for your help Peter!

Kind Regards,

Paul

Grounding them together is done with a wire connected on a suitabe screw on each chassies.

If the hum is present with the driver tubes removed you have a problem with the "dynaco" st70
What i mean with interlal ground cabling is to check that it is done as in the manual. But
as you seemed to have zoomed in to the st70 i suggest you start there, let the PAS units rest
and concentrato on the ST70.

Thank you both for your reply. Bob, I have tried the tests you suggest and it made zero difference. I shorted out the inputs to the PAS, which made no difference. Shorting out the inputs to the st70 the hum is gone but does that really exonerate the amp? Doesn't shorting the input basically remove the driver board by grounding the inputs? Tell you what's strange, I pulled the 6GH8A tubes, turned on both units, and the hum remained. Short the inputs it's gone. How can that happen? That's why I think the wires going from the rca inputs are picking up signal from the mains. If these wires are grounded out then no signal induced by the mains will get anywhere except ground. But pull the driver tubes the signal still has free run of the board. Might it still get to the EL34's?4 Have you ever seen this before? It's so strange. I put the right speaker on my scope. The hum signal is 100 hertz at 10mv ptp. Not what I expected at all. And the signal is fuzzy...lots of noise on the signal. I was expecting 60 hertz or 120 hertz no 100. Geez this is weird. And why does the PAS work hum free when I run it through my Harmon Kardon 230? No hum at all. I'm grasping at straws here.

Peter, okay I'll give that grounding a try however when I jumper with alligator clips, st70 chassis screw to PAS chassis screw the hum doubles in magnitude. It goes from 10mv ptp to 20mv ptp and it is quite noticeable. Are the dynakit transformers the possible problem here? I had the st70 on the bench last week and the power transformer was buzzing and so were the output transformers. I pressed on them just to see if anything would happen. The power transformer got a bit more quite but it did nothing to the output transformer. After the unit ran for about 10 minutes they stopped making noise. Pressing on the transformers did nothing to stop the hum from coming out of the speakers. I suppose I should attain a different preamp, connect it to the st70 and see what happens. I'm also wondering if I have an impedance match issue. And why do the speaker make a loud POP when I power off the units? So many questions. Sorry guys this is really frustrating and the tests I've run seem to conflict with logic.

I very much appreciate the input both of you have provided.

Kind Regards,
Paul

Have a look at the input circuitry in the st70, especially the grounding of the rca connectors.
If this is a Dynaco st70 ( with a new compatible circuit board) then the pictures in the st70 manual
will show how to and where to ground things.

Will do Peter. I'll check that out today.

Here's another strange occurrence. When I put the st70 on the scope yesterday I found that the volume control on the PAS had no effect on the volume of the hum. It stayed the same no matter what setting the vol control was on. Then I noticed I had disconnected the input to the PAS (an iPod docking station). So I reconnected the input to the PAS only to find the volume control now affected the audio level of the hum. Why would that happen? The input to the PAS was not turned on. Another weird thing I noticed was when I disconnected the scope from the st70 ( I had jumpers with alligator clip on the speaker terminals on the back of the st70) while the jumpers were still connected to the probes of the scope there was still about 5mv of noise showing up on the scope. Once I removed the jumpers this stopped but the jumpers must have been acting like antenna's picking up electrical noise from the wiring in the house. Had no idea there was that much EM noise flying around in the house.

Thanks again for your input. I will check out that wiring and see what I have.

Kind Regards,
Paul

Hello Guys,

Checked all the input circuitry and looks solid. Flipped the st70 over to do some scope work. Hitched up my klipsch speakers and used the PAS as input but no input to the PAS...just had it turned on. With both units on have a hum in the speakers, a 10mv ptp noisy signal on the scope at the speakers, totally flat signal at the rca inputs, flat signal at pins 7 and 17 on driver board and a big noisy signal at pins 1 and 2 going to the EL34's. Where is this signal coming from? There is nothing being input at least not that I could see. The scope showed an almost flat line. Perhaps a few micro volts are there but nothing much. So I pulled out the 6GH8A's and got almost the same thing except this time the signal at pins 1 and 2 was a clean almost sinusoidal wave of about 20mv ptp.
I must be missing something but how can there be a signal there with the 6GH8A's removed? Where's it coming from?

Thanks Guys,

Paul

DrBrown wrote:Hello Guys,

Checked all the input circuitry and looks solid. Flipped the st70 over to do some scope work. Hitched up my klipsch speakers and used the PAS as input but no input to the PAS...just had it turned on. With both units on have a hum in the speakers, a 10mv ptp  noisy signal on the scope at the speakers, totally flat signal at the rca inputs, flat signal at pins 7 and 17 on driver board and a big noisy signal at pins 1 and 2 going to the EL34's. Where is this signal coming from? There is nothing  being input at least not that I could see. The scope showed an almost flat line. Perhaps a few micro volts are there but nothing much. So I pulled out the 6GH8A's and got almost the same thing except this time the signal at pins 1 and 2 was a clean almost sinusoidal wave of about 20mv ptp.
I must be missing something but how can there be a signal there with the 6GH8A's removed? Where's it coming from?

Thanks Guys,

Paul

With the 6gh8 tubes removed the only input is power tube that is not matched i.e. draws
different current . Bad power supply cap could also give hum.

Hi Peter, Thanks for your reply. So the input signal with no input connected to the PAS is 1 millivolt ac. It's a tiny noisy looking signal but it's something. Without the 6GH8A's the only path it through the 470K resistors. Results in roughly 2 nanoamps of current. Input that to the EL34's and what happens? Is that enough to do anything at all?

Thanks,
Paul

DrBrown wrote:Hi Peter, Thanks for your reply. So the input signal with no input connected to the PAS is 1 millivolt ac. It's a tiny noisy looking signal but it's something. Without the 6GH8A's the only path it through the 470K resistors. Results in roughly 2 nanoamps of current.  Input that to the EL34's and what happens? Is that enough to do anything at all?

Thanks,
Paul

If you remove the 6GH8 tubes there is no path to the EL34. If they hum it's due to
B+ hum, and since matched tubes will cancel this you seem to have unmatched tubes.

I assume the board is a new board that is made for 6GH8 tubes ?

Yes. I built the st70 last year from a kit I purchased from Dynakit. It uses 6GH8A's instead of 7199's. I've read bad things about 6GH8A's. The EL34's were supposed to be matched! This is the second batch I've installed. The first ones caused a lot of humming so I bought an entire set of tubes. I don't think any of them are matched despite the sellers claiming they are. How can I measure the tubes to determine if they match? How is that done?

Kind Regards,
Paul

DrBrown wrote:Yes. I built the st70 last year from a kit I purchased from Dynakit. It uses 6GH8A's instead of 7199's. I've read bad things about 6GH8A's. The EL34's were supposed to be matched! This is the second batch I've installed. The first ones caused a lot of humming so I bought an entire set of tubes. I don't think any of them are matched despite the sellers claiming they are. How can I measure the tubes to determine if they match? How is that done?

Kind Regards,
Paul

The easiest way to obtain matched power tubes is to purchase from a serious vendor. Eurotubes.com is one.
Second is to use a dedicated valve testing device ( maximatcher comes to my mind) that
measures tubes at typical working voltages and preset bias and then group similar
tubes.
Or build a device that mimics one of the "typical" working points
( see page D4 in https://frank.pocnet.net/sheets/129/e/EL34.pdf where 400Volt Va and -35.5Vg will result in 30mA Ia and 4.4mA Ig2

There is several variations of "typical working point" what's important is to measure the
tubes the same way and at a voltage close to the voltage in the amp used. ST70 typically
has 430v plate voltage.


If you don't like the 6GH8 try 6U8 instead, they have very similar properties. But start with
examining that you have a working bias system.

Hi Peter,

Interesting. I have two tube testers but that will not work for matching. That sounds like a more sophisticated procedure. Both sets of El34's have the bias set a 1.56 vdc. I think that's what it's supposed to be. Nonetheless that doesn't mean the tubes are behaving. I bought the tubes from antique electronics supply. They were supposed to be matched pairs of J&J. Don't know if those are any good.

Hello Guys,
Well got the old PAS-3 hum issue under control and am able to use it now. But this PAS-NGT is driving me nuts. The hum is bad, increases with volume control and bass controls, isolation transformer did nothing. Anyway, I sent Er-hardt and email but never heard back so I'm not pursuing that. I'm almost positive it's the power supply. The heater power circuit has a half-wave rectifier ripple sitting on top of the it at about 5-10mv. WHen I put the outputs on the scope and crank up the volume and bass there's a half wave rectifier wave on the scope only now it's about 10 times bigger. I did some poking around the power supply board with the scope and what I'm seeing makes no sense at all. In fact in all but one of the diodes I'm not seeing and rectification at all? I'm wondering if this power supply is bad? Here's the schematic:



Sorry, couldn't get the image to post. Please let me know if you can see this schematic. I put the scope on d1, d2 both end. Only one showed any rectification. that was d2 at the positive end. Had a very tiny half wave output. The others all showed full sine waves that looked a bit too straight up and down. The normal sine wave with the slightly curved sides were instead straight lines. I was expecting to see the usual half-wave but it wasn't there. I'm just thinking this supply does not work properly and is making the 12AX7's amplifier that ripple? If you have any ideas what might be going on here please let me know.

Kind Regards,
Paul