Are you missing any tube shields? Photos of the interior might be helpful.
Harman Kardon A-30K (A-300) power supply
- Thread starter depaj
- Start date
I think all the 9-pin tubes in my A500 are shielded. Images on Google indicate the same is true of yours. This could be all that's wrong.
Jack
Jack
buickguy62
Active Member
It's worth a try, my experience is that they seem to make little difference, however your situation may be different, with some AC wiring close to the amp.
I have found that having the chassis bottom installed does make a difference however.
Primotubes
Active Member
The Harman Kardon tube amps were somewhat unusual having a ground bus wire running the length of the chassis. This bus should only be grounded at one end or if you want to be really precise in the center of the bus wire.
The noise in V3 or V4 could be a noisy resistor or capacitor or a cold solder joint. Have you replaced all the old carbon comp or original resistors and caps?
FlaCharlie
Super Member
Yeah, the A-300 definitely had shields on all the input tubes. I've also had noise issues that disappeared after the bottom cover was attached and the amp was turned right side up. Sometimes something like a fluorescent light can also be picked up by the amp circuitry.
I replaced only the power supply caps so far, there are a few carbon comp resistors (they measure fine resistance wise) but I might try replacing them if putting the shields back on doesn't help (I contacted the previous owner and he forgot to put them back but offered to send them.
The way the bus wire is connected is definitely original but as this is the 30k version it was sold as a kit and thus could have been wired wrong. I did try removing both sides (on at a time of course) and that didn't make a difference. The previous owner didn't notice the noise though and the amp came with an acrylic transparent bottom cover so that probably won't help much.
I will start by putting the shields back on and see where that gets us.
The way the bus wire is connected is definitely original but as this is the 30k version it was sold as a kit and thus could have been wired wrong. I did try removing both sides (on at a time of course) and that didn't make a difference. The previous owner didn't notice the noise though and the amp came with an acrylic transparent bottom cover so that probably won't help much.
I will start by putting the shields back on and see where that gets us.
FlaCharlie
Super Member
You might want to try covering the acrylic cover with something like aluminum foil. Lots of console stereo amps used a piece of foil covered cardboard between the amp chassis and the wood on the bottom of the cabinet to accomplish the same thing. Guitar amps often shielded their chassis like this too.
Obviously, you need to make sure the foil stays flat and doesn't contact any of the electrical connections.
On one or two A300 or A500 I worked had a ground loop. I was surprise bc I think they are factory wired units.
I’m not sure but I think that where the voltage doubler was grounded to chassis (the diode and second filter cap) was ALSO tied to the buss, which was then grounded at the phono inputs.
Thats right out of the Dept of Redundancy Department.
Take a look and see if yours is wired like this. If so, try it with those grounds separated.
I’m not sure but I think that where the voltage doubler was grounded to chassis (the diode and second filter cap) was ALSO tied to the buss, which was then grounded at the phono inputs.
Thats right out of the Dept of Redundancy Department.
Take a look and see if yours is wired like this. If so, try it with those grounds separated.
As it wasn't original when I got it I don't really know how it was but now nothing is tied to the chassis except the phono inputs. All the other grounds join at the first filter cap.
I tried putting a different bottom cover on it but that didn't change anything.
Ok, I see that you don’t have the original PT and aren’t using a voltage doubler.
Something I recommend is to make sure your PT center tap and the first filter cap negative terminal are attached to the chassis together. And separate from the ground buss. Everything else can ‘get on the bus’.
Search Valve Wizard’s site for his article on grounding. Great stuff.
Something I recommend is to make sure your PT center tap and the first filter cap negative terminal are attached to the chassis together. And separate from the ground buss. Everything else can ‘get on the bus’.
Search Valve Wizard’s site for his article on grounding. Great stuff.
Welllll, I put the tube shields back on and, no difference 
.
How do you mean separate from the ground bus ? as it is now the negative terminal of the first filter cap is attached directly to the center tap of the power transformer, from there a single wire is connected to the ground bus as well as to the rest of the filter caps for the different voltages. The ground bus is also connected to chassis at the rca inputs through the shielding of the phono input cable.
.How do you mean separate from the ground bus ? as it is now the negative terminal of the first filter cap is attached directly to the center tap of the power transformer, from there a single wire is connected to the ground bus as well as to the rest of the filter caps for the different voltages. The ground bus is also connected to chassis at the rca inputs through the shielding of the phono input cable.
What you describe above works fine as long as there not a connection to the chassis there at the first filter cap AND a connection to the bus bar wire which is also grounded.
I connect the HT centertap and the negative of the first filter cap together at the chassis, with no other components connected there. Then everything else gets connected to the bus which is grounded at the input.
I don’t know if this will solve your issue but it is a common building practice.
http://www.valvewizard.co.uk/Grounding.pdf
I connect the HT centertap and the negative of the first filter cap together at the chassis, with no other components connected there. Then everything else gets connected to the bus which is grounded at the input.
I don’t know if this will solve your issue but it is a common building practice.
http://www.valvewizard.co.uk/Grounding.pdf
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Have you confirmed whether the hum is 60Hz or 120Hz? Maybe I missed that in the thread.
Jack
Jack
Nothing is connected to chassis at the first filter cap. I tried just to be sure the way you recommend, the noise stays the same on one channel and gets somewhat worse on the other but just barely.
It's really more a buzzing sound than hum. There is some 50Hz hum mixed up but I can only hear it pretty close to the speaker.
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I'm still nowhere on this amp. I tried removing the whole tone control section but that just got me more hum (probably due to different impedance). the hum itself is dependent on the orientation of the power cord and also slightly better on one channel and worse on the other when I turn the amp upside down 
. Also how normal is it for the amp to transmit mechanical noises through the speaker ? What I mean by that is that when I activate the switches it makes thumping/crackling through the speakers. First I thought it were dirty switches but than I tried just gently tapping the chassis or faceplate or transformer, anywhere really, the same noise is heard through the speakers. Is there any chance those ECC 803 tubes that are there instead of the 12AX7 are more sensible to that and are just picking up the vibration coming from the transformer ?
. Also how normal is it for the amp to transmit mechanical noises through the speaker ? What I mean by that is that when I activate the switches it makes thumping/crackling through the speakers. First I thought it were dirty switches but than I tried just gently tapping the chassis or faceplate or transformer, anywhere really, the same noise is heard through the speakers. Is there any chance those ECC 803 tubes that are there instead of the 12AX7 are more sensible to that and are just picking up the vibration coming from the transformer ?I guess these modern transformers are just emitting more EMI that is being picked up by tubes or other circuitry (I had to change the transformer in a quad 33 preamp and even though its characteristics were the same and all voltages are correct in the preamp, it now hums more than it used to and when I use the high filter, the coil in the circuit picks up the emi and buzzes like cray). I added a copper foil around the transformer (perpendicular to the lamination) which helped a little but far from what I needed. I'm going to try also adding some foil in the same direction as the lamination to see if that helps some more.
Primotubes
Active Member
I have not gone through all the comments yet, but, i just wanted to mention "mu metal" as a way to block EMI, and help to control stray magnetic fields. I have seen people construct boxes that use that material which fit over the power transformer. I can't say that it will cure your problem but it may help if the hum is not the entire transformer humming.