buickguy62
Active Member
All of the capacitors for Capacitor 3 (A,B,C,D) have a common ground lead. Capacitors C1 and C2 are in the voltage doubler circuit and are wired differently.
Help needed with Harman Kardon a220 “the lute”
- Thread starter Odizzieus
- Start date
Odizzieus
Member
Just a question, which input do you use as the aux on this?
buickguy62
Active Member
The A220 has an interesting source input philosophy. Really there are only two sets of stereo inputs, one lo level set for magnetic pickups (phono cartridges) and the other hi level set for everything else. Ok to complicate it,on the hi level side, there are selector switch positions for AM, FM, Tuner Stereo and Tuner Reverse. If you switch to AM, pretty sure it will play the left hand stereo input only and play it through both Left and Right Speakers. In the FM position, it likely will play the right hand stereo input through both Left and Right speakers. Tuner Stereo plays the LH input to the LH speaker and RH input to RH speaker. Tuner Reverse just swaps the signals. So, a short answer to a short question; I suggest for regular stereo operation select Tuner Stereo for your high level input.
Odizzieus
Member
So can the mag phono input work for Moving magnet cartridges? That’s what I intend to use it with, but I assumed the preamp wouldn’t work with MM. do you have any idea what the S/N ratio would be?
buickguy62
Active Member
Odizzieus
Member
Fantastic, thank you so much. I was having such a hard time finding info on this thing. I only have one other question, and it’s that the unit doesn’t appear to have a grounding post. How would you go about grounding your turntable to the unit?
buickguy62
Active Member
A couple of places for grounding comes to mind. Could loosen one of the output transformers mounting screws and sandwich it there or else sandwich it between one of the bottom cover mounting screws. That works better if you have rubber feet on the amp, otherwise it's a tight fit. The G speaker terminal also is at ground potential, but I avoid using that terminal, I like to use those just for the speaker connections to keep it neater. In another post you asked about the tube shield for the 12AX7. My experience is that likely it won't be needed, if you can find one, fine, if not run without it, also if looking for one it needs to be a full ring snap on type, see pic. Sorry about the first two pics. they get rotated in the upload.
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Odizzieus
Member
Alright I guess I’ll try the transformer screw. Thanks for your help once again!
Odizzieus
Member
Alright, the capacitors have been replaced, but the unit still has a hum. It is not the same kind of hum as before, it sounds more like a ground loop than anything. When I tried connecting a ground wire from the chassis to the earth of an outlet, the hum persisted, maybe getting slightly quiteter. According to a signal analyzer, it’s 250hz, which seems strange. I’m waiting for the 6v6 tube this unit is missing to come in the mail. Could the missing tube be the cause of this hum?
century tek
Addicted Member
Is the hum coming out of both speakers? Is it quiet with the volume all the way down? Make sure your new capacitors are solidly grounded and oriented properly (- to - / + to +).
And yes, hum in one channel would be contributed to not having the other tube as it needs both tubes in each channel. This configuration of tubes is called "Push Pull". Read up on it.
One missing power output tube in a push pull configuration will cause hum in that channel as it is only amplifying 1/2 of the cycle. Two fairly matched tubes in push pull will cancel hum.
For testing purposes and since you don't have the missing tube at the moment, just remove the other tube. It won't hurt it to run one channel working, and voltages won't rise that much to be of any concern for short period testing.
Mind you I said "SHORT PERIOD" testing!
I ran a Bogen amp for years on two output tubes compared to the four tubes it what made for.
And yes, hum in one channel would be contributed to not having the other tube as it needs both tubes in each channel. This configuration of tubes is called "Push Pull". Read up on it.
One missing power output tube in a push pull configuration will cause hum in that channel as it is only amplifying 1/2 of the cycle. Two fairly matched tubes in push pull will cancel hum.
For testing purposes and since you don't have the missing tube at the moment, just remove the other tube. It won't hurt it to run one channel working, and voltages won't rise that much to be of any concern for short period testing.
Mind you I said "SHORT PERIOD" testing!
I ran a Bogen amp for years on two output tubes compared to the four tubes it what made for.
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Odizzieus
Member
The hum is quiet with volume all the way down, the tube should hopefully arrive tomorrow. I’ll report back after I install it and see if the issue persists
century tek
Addicted Member
There ya go!
Put the two good 6V6/7408 tubes in the other channel and have a listen. If its quiet with the volume all the way down, then it is the missing tube or simple ground loop. You're getting there!
PS, you're going to want to have a pair of matched tubes and not one 7408 and one 6V6 otherwise you are going to have hum or other weird things that happen with mis matched tubes.
Both should be 6V6GT's or 7408's but not mixed.
EDIT: You should probably use a pair of 7408's as they have a little higher voltage specs than the 6V6. An amp designed for the 7408 tube will probably have more voltage on it than one using 6V6's so if you were to use 6V6's in place of the 7408, you might get red plating of the 6V6 tubes.
Put the two good 6V6/7408 tubes in the other channel and have a listen. If its quiet with the volume all the way down, then it is the missing tube or simple ground loop. You're getting there!
PS, you're going to want to have a pair of matched tubes and not one 7408 and one 6V6 otherwise you are going to have hum or other weird things that happen with mis matched tubes.
Both should be 6V6GT's or 7408's but not mixed.
EDIT: You should probably use a pair of 7408's as they have a little higher voltage specs than the 6V6. An amp designed for the 7408 tube will probably have more voltage on it than one using 6V6's so if you were to use 6V6's in place of the 7408, you might get red plating of the 6V6 tubes.
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Odizzieus
Member
Yeah, I ordered 7408s so all the tubes will be the same, that won’t be an issueThere ya go!
Put the two good 6V6/7408 tubes in the other channel and have a listen. If its quiet with the volume all the way down, then it is the missing tube or simple ground loop. You're getting there!
PS, you're going to want to have a pair of matched tubes and not one 7408 and one 6V6 otherwise you are going to have hum or other weird things that happen with mis matched tubes.
Both should be 6V6GT's or 7408's but not mixed.
century tek
Addicted Member
That's great!
Odizzieus
Member
Alright new tubes installed and chassis attached to ground, and the hum is inaudible except at highest volume. I noticed that when I started the unit up, one of the 12ax7s glowed very brightly for a second. Is this normal?That's great!
century tek
Addicted Member
YUP! Classic Mullard flash. It's the tube itself and not your amplifier. That tube will do that in any location or amplifier or receiver.
Odizzieus
Member
Alright, the unit is hooked up, playing music, and it sounds awful. Sounds extremely flat and muffled. Probably means the 12ax7s need to be replaced right?
Also, the ground loop is still at a high enough volume to be annoying even with a ground wire connected to chassis and then to outlet earth. There is also other kinda background static other than the ground loop
Odizzieus
Member
also, in a YouTube video about this unit, it was mentioned that the person installed a capacitor across the main in to increase s/n. Should I try doing that? And what value caps should I use?
Here is the video I’m referencing:
Edit: I’ve also noticed the 12ax7s to the left of the unit don’t glow nearly as bright as the 12ax7 between the 3 7408s
Here is the video I’m referencing:
Edit: I’ve also noticed the 12ax7s to the left of the unit don’t glow nearly as bright as the 12ax7 between the 3 7408s
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century tek
Addicted Member
Boy, you really are a novice.
The heaters in those two tubes (can't make out V1 or what) are tied together in a series connection and are part of the phono preamp circuit. It is normal.It sounds like you just need to pay attention to your hum issue. Tubes are fine!
I would disconnect that ground wire. It isn't doing any good.
If it is quiet with the volume all the way down, a capacitor on the mains side isn't going to help as it sounds to me like you have a microphonic tube. Pull the 12AX7 between the 7408's and have a listen. Turn the volume up and down and check it out. You can also swap those 12AX7's around.
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