ST70 humming with inputs shorted

Hey Guys,

Well, still have a hum problem. Rewired all the grounds to one point. Made no difference. Checked all the tubes. All good. Don't know how well matched the EL's are. This is my second set of tubes as I replaced all my tube shortly after building this st70 because of hum problems. At that time it helped lower the level of the hum but it's still there. So using Jensens method of tracking down the source of the hum I determined it was the st70. Okay so with the st70 attached to my polk towers and shorting out the input I could hear nothing. But when I put the st70 on the bench and connected my bookcase klipsch there it was. The EL's I'm using are J&J. Are these any good? Could this be the source of my hum? Not balanced. The bias is set exactly to 1.57vdc on both sides. But if the tubes aren't matched how can you tell? Scope? Or could there be a dc offset sneaking in there? There has to be an explanation for this hum. It's been there to varying degrees since I built this unit. Any thoughts would be welcomed.

Kind Regards,
Paul

DrBrown wrote:Hey Guys,

Well, still have a hum problem. Rewired all the grounds to one point. Made no difference. Checked all the tubes. All good. Don't know how well matched the EL's are. This is my second set of tubes as I replaced all my tube shortly after building this st70 because of hum problems. At that time it helped lower the level of the hum but it's still there. So using Jensens method of tracking down the source of the hum I determined it was the st70. Okay so with the st70 attached to my polk towers and shorting out the input I could hear nothing. But when I put the st70 on the bench and connected my bookcase klipsch there it was. The EL's I'm using are J&J. Are these any good? Could this be the source of my hum? Not balanced. The bias is set exactly to 1.57vdc on both sides. But if the tubes aren't matched how can you tell? Scope? Or could there be a dc offset sneaking in there? There has to be an explanation for this  hum. It's been there to varying degrees since I built this unit. Any thoughts would be welcomed.

Kind Regards,
Paul

measuring cathode current with individual cathode resistors is a good way to see if tubes are matched. But it is not a good way to match tubes as you need > 25 tubes to get a reasonable
matched pair. Thus this is best done at the vendor/importer.


Unmatched tubes in a p-p amp will increase hum as B+ ripple is no longer balanced out.

How do you get matched tubes ? Easy, ask your (seroius) vedor for matched pairs or
even better a matched quad. If the tube vendor cannot do this, swicth vendor.

You can also experiment with lifting the amp ground with a cheater plug and then lifting ALL grounds with cheater plug at duplex outlet. If quiet, then you might have a ground loop somewhere. Ground loops are pesky critters to run down...

Is this a VTA version?  If so, the center tube on the driver board could be the source of your hum.  It got me on my ST-120 years ago.  Ended up with a tested quiet Mullard 12AT7 for that spot and the amp has been quiet ever since. (I have the older high gain driver board. The newer ones use (I think) 12AU7s for the driver tubes)

Hi Guys,

Thanks for your replies! Peter, can't I just read the voltage across the 15.6 ohm resistor and calculate the current? Hate to unsolder these connections to measure the current. Are you thinking that even though I have the bias set to 1.57 on each pair of cathodes the current draw might be different? How would you go about making this measurement? There's a company here in the US named Upscale Audio that says the cryo the tubes, burn in the tube, and match the tubes. Sounds impressive do you think this is the way to go? And which EL34's are most likely to work well? Upscale Audio has Mullard, Gold Lion, Prima Luna, and Electro Harmonix. Thanks again for all you help Peter.

nmchiefsfan, Thanks for your reply. No it's not the VTA but I have the kit. Haven't put it together yet. This is the board that comes with the Dynakit ST70 kit. What's strange is when I pull the 6GH8A tubes the hum is still there.

j.click, thanks for getting back to me. Yes I've done the cheater plugs and it did help. The hum I'm chasing is very low level. You have to put your ear very close to the speaker to hear it. When everything is connected, that is the iPod docking station connected to the PAS-NGT connected to the ST70 connected to the Speakers, and is all turned on the humming increases with the volume control and can be made very loud. If just the PAS is turned on then the hum is low level. Seems like a ground loop doesn't it? I tried Isolation transformers. Did nothing. I used both the Jensen inline isolation transformers for the connecting between components and power Isolation transformers for the various mains connections. None of it helped at all? Isn't that strange? I keep going back the those EL34's. Just seems like I'm missing something obvious but what? Thanks for your help.

Any ideas come to mind please share.

Best Regards,
Paul

Hi Paul, I rebuilt a Dyna ST35. When it was done it hummed.In the rebuild I installed brand new gold input jacks. In the old Dyna's the jacks were grounded to the chassis the way they were installed. I forgot to ground the new gold jacks I installed.I had installed them like the old jacks which had no grounding tabs. The new gold jacks are isolated from the chassis with plastic washers so they must be grounded. The old Dyna jacks were designed grounded to the chassis without the use of those tabs. It took a trip in the mail to Van AlStine's to figure that one out.It may be a shot in the dark but I had not heard that mentioned.The PAS also uses the old style 2 prong outlet plug. There is a reason for that explained on a You Tube video by D Lab Electronics. Take care Paul....sincerely Mark Korda.

DrBrown wrote:Hi Guys,

Thanks for your replies! Peter, can't I just read the voltage across the 15.6 ohm resistor and calculate the current? Hate to unsolder these connections to measure the current. Are you thinking that even though I have the bias set to 1.57 on each pair of cathodes the current draw might be different? How would you go about making this measurement? There's a company here in the US named Upscale Audio that says the cryo the tubes, burn in the tube, and match the tubes. Sounds impressive do you think this is the way to go? And which EL34's are most likely to work well? Upscale Audio has Mullard, Gold Lion, Prima Luna, and  Electro Harmonix. Thanks again for all you help Peter.

nmchiefsfan, Thanks for your reply. No it's not the VTA but I have the kit. Haven't put it together yet. This is the board that comes with the Dynakit ST70 kit. What's strange is when I pull the 6GH8A tubes the hum is still there.

j.click, thanks for getting back to me. Yes I've done the cheater plugs and it did help. The hum I'm chasing is very low level. You have to put your ear very close to the speaker to hear it. When everything is connected, that is the iPod docking station connected to the PAS-NGT connected to the ST70 connected to the Speakers, and is all turned on the humming increases with the volume control and can be made very loud. If just the PAS is turned on then the hum is low level. Seems like a ground loop doesn't it? I tried Isolation transformers. Did nothing. I used both the Jensen inline isolation transformers for the connecting between components and power Isolation transformers for the various mains connections. None of it helped at all? Isn't that strange? I keep going back the those EL34's. Just seems like I'm missing something obvious but what? Thanks for your help.

Any ideas come to mind please share.

Best Regards,
Paul

Obviously a common cathode resistor won't allow you to measure individual current.
The easiest way is to disconnect the 15.6 ohm resistor and connect 2 10ohm resistors
one freacj cathode to ground. This as a temporary work to figure out if tubes are matched.
10 ohms will correspons 50mA to 0.5V across the resistor. The amp will be unaffected as
the difference in resistance is small.
When you know that you have two matched tubes you can reconnect the 15.6 ohm resistor again.

If the amp hums when the 6GH8 tube is removed all that can create hum is the power tubes.
Stay away from hypes ( cryo etc) and buy from reutable dealers. Eurotubes is one, this will
save money and in return give warranty if anything is wrong with the tubes.

Hi Mark, thanks for your reply. The ST70 I built has the rca's isolated both input and output. Too bad that would have been an easy fix.

Peter, Thanks for the information. Would also have to remove the jumper wires from v2 pin1 to v3 pin8. Same with the other channel. No big deal but if the only thing left to hum is the tubes it seems I need different tubes....again. A third set seems like I'm not having much luck at matching these tubes. What about the transformers? With everything interconnected, that is the PAS to the ST70 to the speakers, the hum is fairly loud and I can hear the output transformer on the right channel vibrate. When I put my hand on the transformer the hum decreases quite a bit. But, the hum is still there. Seems the problem reduces to the EL34's and output transformers. Looked at Eurotubes website. Their prices are low. They have 4 matching J&J EL34's for $100. That much less than I was paying. They didn't show any other brands though. Are these what you use? All of the EL's I have are J&J. Okay so I have some work to do. Thanks again for your help.

Best Regards,
Paul

Paul, my Dyna Stereo 70 (Purist Mod) was making some low level humming. It was not noticeable except when I got closer to the amp. I went as far as removing a dimmer switch in the room with my amp and also buying this AC filter from Emotiva for 95 bucks. It has 2 outlets in it. I noticed 1 thing with Dynaco transformers. They scrimped on the bell housing mounting screws. Look at your transformers, they will have empty holes where screws,nuts,and bolts should be. On my 70 as an experiment I took a carpenters wooden door shiv and shoved it under the center transformer. The noise was reduced. I think oxidation can happen if the transformer iron plates are not compressed tightly together. I saw a video on McIntosh transformers and they poor some tar material in the transformer so this does not happen, why else would they, vibration dampining? I will try sending a picture of what I did...take care Paul.....Mark Korda

DrBrown wrote:Hi Mark, thanks for your reply. The ST70 I built has the rca's isolated both input and output. Too bad that would have been an easy fix.

Peter, Thanks for the information. Would also have to remove the jumper wires from v2 pin1 to v3 pin8. Same with the other channel. No big deal but if the only thing left to hum is the tubes it seems I need different tubes....again. A third set seems like I'm not having much luck at matching these tubes. What about the transformers? With everything interconnected, that is the PAS to the ST70 to the speakers, the hum is fairly loud and I can hear the output transformer on the right channel vibrate. When I put my hand on the transformer the hum decreases quite a bit. But, the hum is still there. Seems the problem reduces to the EL34's and output transformers.  Looked at Eurotubes website. Their prices are low. They have 4 matching J&J EL34's for $100. That much less than I was paying. They didn't show any other brands though. Are these what you use? All of the EL's I have are J&J. Okay so I have some work to do. Thanks again for your help.

Best Regards,
Paul

eurotubes is the JJ representative. I know that make good matching, i also know they warrants their
tubes just keep the reciept.
And yes, buying direct from the importer saves money as noone else is involved.

Hi Guys, Just did some poking around with my scope after hearing a faint hum with the inputs shorted out. At input there was no signal just a horizontal line on the scope. Scope voltage set to 5mv. Then I checked pins 1 and 2 (left channel) which is the output right after the 6GH8A phase splitter. Look at this:


Then I checked the right channel pins 20 and 21:



Images are about 15mv to 20mv ptp. Then I checked at the speaker outputs. I put both channels on the scope.



Smaller signal? Is the 15mv to 20mv to low for the EL34's to amplify? IS this just noise? I am hearing a hum. It's there. How can there be any signal at all after the phase splitter?
These are definitely 60 hertz signals. Have the horizontal set to 5ms/div. Comes out to 60 hertz. Are these 6GH8A's no good?

Any comments would be welcomed.

Kind Regards,
Paul

Oops. Forgot to say for the speakers the top trace is the right speaker the bottom left.

As it still hums with 6GH8 tubes removed you are down to 2 possible sources :
- unbalanced EL34
- bad can cap

If the can cap is the original it might be dried out and lost much of it's capacity. A simple
test is to (temporarily) solder a new electrolytic cap in parallel with the "C" cap ( after the
choke). 20 - 40 uF 450Volt will do.

Hi Guys,

So spent a lot of time in the lab running tests on my entire setup. Put the St70, PAS-NGT, and PAS-3X plus docking station input on the bench. I connected the oscilloscope to see what I could see. Turned out the majority of the hum ( about 90%) was coming from a bad rca to rca cable from the PAS output to the ST70 input. I accidentally touched it while reaching from a probe and the scope went nuts. This was supposed to be a high-quality shielded cable. HAHA. Not so much. I replaced it with a cheap amazon purchased connector and the humming immediately dropped way down. Isn't that nuts? What I discovered during this time was that my old house is extremely EMI-prone. The old wiring and cable tv connection make a ton of noise and the equipment picks it up so there's always going to be a bit of noise (hum) in there. On the scope, the hum was always 60 hertz (where ever it was measured) and varied in amplitude from 5mv to 20mv.
What really surprised me is that my old PAS-3X is far quieter than my newly built PAS-NGT attained from ER-HARDT. Last night I re-wired the PAS-NGT to replace the 3-prong power cord with a 2-prong. Also, re-wired a few AC connections inside the unit and disconnected the chassis ground wire. Made absolutely no difference. The ST70 is fine. Has a tiny hum that's so low as to be negligible. But that PAS-NGT is awful. Hums like crazy and hums even louder if I put my hand on the box. Guess I'm acting as an antenna. So the setup now is dynakit ST70+PAS-3X+iPod docking station with extremely low hum only noticeable if you put your ear right on the speaker. That's not bad! I have a brand new Yamaha receiver that also has a faint hum. It's the wiring in the old house. Does anyone have any experience with the ER-HARDT PAS-NGT? I really would like to use it but the humming is terrible.

Thanks again for all the input. Much appreciated.
Kind Regards,
Paul

Hi Guys,

Well I just discovered something strange. It seems I have a PAS-3 that's been wired the same as a PAS-3X. Spent some time tracking down a problem I was having (other than a very low level hum) with balance. The left channel was way louder than the right channel. Put it on the scope and put a 1k sine wave signal into both left and right inputs. Sure enough, The left was a much higher amplitude than the right. Then I switched off the filter switch and presto the signals were exactly the same. Hmm. Then I noticed the treble controls greatly increase/decrease each signal's amplitude. As did the volume control. The bass controls did little to nothing. So then I turned off the signal generator and after a while the 60 hrz noise signal appears. The magnitude of the noise signal increased proportionately with the bass controls. Turn the signal gen back on the noise disappears as the signal takes over right. Works well at all frequencies. Nice clean signals same amplitude. But turn off the signal gen and there's this noise. So there's a twisted pair (AC) right next to the right bass control running the "ON" light. Oh, also the right channel has a much bigger noise signal than the left. Anyway, started checking around and some of the wiring just didn't look to be in the right place. For instance the wiper wire for the bass control is supposed to go to eyelet 6 on PC-5. Instead, it went to the lower end of the bass control. Strange. Anway, I did a little looking on the web and I'm pretty sure I have a PAS-3 not a PAS-3X and I cannot find a schematic for the PAS-3. I found PAS-3 series II but that's a different preamp from this one. And I saw that the PAS-3 bass and treble pots are different on the PAS-3 from the PAS-3X. This is all very confusing. I'd like to be able to get the wiring correct but I have no schematic for this unit. Is the PAS-3 wired the same as the PAS-3X? Any advise would be greatly appreciated.l

Kind Regards,
Paul

P.S. I forgot to mention the majority of the hum problem turned out to be a bad RCA cable from the output of the preamp to the st70. Replaced it and almost all the hum disappeared. Crazy?

PAS3 and PAS3X are very similar ecept for the actual pots and a capacitor

Visit http://www.audioregenesis.com/ for all
facts regarding PAS3/PAS3X

Thanks, Peter! What a great website. The article about PAS line stage, PAS compatibility, and PAS Tone control explains exactly what I am seeing with my bass controls. The behavior I'm seeing is how the PAS 2/3 are designed. The output impedance changes with frequency and position of the bass control pot. The author does a great job explaining how it works and now i understand why so many owners opt to remove the tone control circuits' influence on the output impedance. It flattens the response and keeps the impedance fairly steady. What I do not understand yet is where this 60 Hz noise is coming from. It can be quite large if I turn the bass controls all the way clockwise and the volume control all the up it's a 1 volt signal at the speaker output. I'd like to get rid of it. Can't help but think the proximity of the AC wires to the bass controls might be the problem. Thanks again Peter.


Best Regards,
Paul

DrBrown wrote:Thanks, Peter! What a great website. The article about PAS line stage, PAS compatibility, and PAS Tone control explains exactly what I am seeing with my bass controls. The behavior I'm seeing is how the PAS 2/3 are designed. The output impedance changes with frequency and position of the bass control pot. The author does a great job explaining how it works and now i understand why so many owners opt to remove the tone control circuits' influence on the output impedance. It flattens the response and keeps the impedance fairly steady. What I do not understand yet is where this 60 Hz noise is coming from. It can be quite large if I turn the bass controls all the way clockwise and the volume control all the up it's a 1 volt signal at the speaker output. I'd like to get rid of it. Can't help but think the proximity of the AC wires to the bass controls might be the problem. Thanks again Peter.


Best Regards,
Paul

Hum is dependent on routing of wires. Try to follow the manual exactly !

As for output impedans, if that is a problem ( trying to drive less then 250k ) you need an external
buffer. ( there was a gentlemen that sold a FET board to be mounted inside the PAS powered by the
filament voltages) This worked fine and was less intrusive that amputating the controls.

Thanks for the reply Peter. The routing is exactly as it is in the manual. I wish it were not that would make fixing this noise issue a lot easier. Nonetheless, there exists 60hz signal at the outputs with no signal input. It varies with the bass controls and volume controls. With both sets of controls turned completely clockwise (full volume full base) the output signal is over 1 volt peak to peak. Looking at the schematic this is no surprise dues to the way the bass controls are connected to the circuit. I'm going to remove the power switch and indicator light from the front panel and put them to the side of the unit, turn it on and see it that has any effect on the noise level. I can't help but think that with these wires being so close to the bass controls they are inducing a voltage into the bass pots. But that's a stretch since I haven't heard anyone else complain about this arrangement. When I scoped out the voltages coming off the bias capacitors (which also goes to the on/off lamp) the signal is very choppy. That's supposed to be DC but it's not. There's a 20mv 60hz wave sitting on top of the DC. Maybe what I need is a new power supply. These are all original parts in this PAS. The ECC83's are so old the printing is gone. Furthermore, the tubes never get hot, only warm. The tube diode is the only tube that gets noticeably hot. I checked all the voltages and they seem fine however the bias voltages are 1/2 the value that dynaco put on the schematic. I do not see how they could have missed this but it's a DC voltage and it's being split between two tubes so naturally your only going to have 5.5 volts on each end.

The articles at AudioRegenesis are priceless. Lots of excellent information as to the circuit behavior of all the PAS preamps. Thanks again for that link!

Paul

DrBrown wrote:Thanks for the reply Peter. The routing is exactly as it is in the manual. I wish it were not that would make fixing this noise issue a lot easier. Nonetheless, there exists 60hz signal at the outputs with no signal input. It varies with the bass controls and volume controls. With both sets of controls turned completely clockwise (full volume full base) the output signal is over 1 volt peak to peak. Looking at the schematic this is no surprise dues to the way the bass controls are connected to the circuit. I'm going to remove the power switch and indicator light from the front panel and put them to the side of the unit, turn it on and see it that has any effect on the noise level. I can't help but think that with these wires being so close to the bass controls they are inducing a voltage into the bass pots. But that's a stretch since I haven't heard anyone else complain about this arrangement. When I scoped out the voltages coming off the bias capacitors (which also goes to the on/off lamp) the signal is very choppy. That's supposed to be DC but it's not. There's a 20mv 60hz wave sitting on top of the DC. Maybe what I need is a new power supply. These are all original parts in this PAS. The ECC83's are so old the printing is gone. Furthermore, the tubes never get hot, only warm. The tube diode is the only tube that gets noticeably hot. I checked all the voltages and they seem fine however the bias voltages are 1/2 the value that dynaco put on the schematic. I do not see how they could have missed this but it's a DC voltage and it's being split between two tubes so naturally your only going to have 5.5 volts on each end.

The articles at AudioRegenesis are priceless. Lots of excellent information as to the circuit behavior of all the PAS preamps. Thanks again for that link!

Paul

What do you mean with "bias voltage" ? IS it the cathode(s) to ground that is half the value ? If so
you might need new tubes.