Harmony H-400A Project

[Structo]

I tried to draw how it should be wired using my simple windows paint program, not the greatest but it should show the general idea.

[svstee]

Thanks guys, that helps a lot!
So what is the purpose of the isolation transformer on the preamp tube, then?
Also, if I added that transformer, would that also reduce the hum?

[svstee]

Anyone?

[Structo]

I believe that transformer on the heater is a filament transformer to step down the house current to 6.3v.

Since it is a Class A amp, it's mostly likely always going to have some hum.
Kind of the nature of the beast.


[img:319:239]http://i301.photobucket.com/albums/nn71 ... 001096.jpg[/img]

You see the big yellowish brown cap? That is the 10mf/50mf filter cap.

Filter caps only last a certain amount of time before they dry out and quit working efficiently.

You need to replace that and any other electrolytic caps in the amp.

I think the only other one looking at the schematic is the 30mf one on the 50C5 power tube.
That is the smaller brown cap near the tube socket.

Be sure to orient the polarity of the new caps the same as what you took out. In other words the - side goes towards chassis ground.

Also it would be a good idea to measure the carbon comp resistors to compare to there coded value.

Remember you can't measure a resistor in circuit unless at least one end is disconnected. Some of them you can measure with the tubes out since they complete the circuit on the ones connected to the tube pins.

Carbon comp resistors will add noise and drift in value with age.

So if you were to replace those with at least carbon film or better yet metal film caps with the proper wattage rating and the filter cap and bypass cap, you should be able to get rid of quite a bit of the hum and hiss.

[svstee]

Thanks for the transformer info.
I was planning on replacing the filtering cap and the other electro as well, forgot about carbon comps, I should check those... after I get the amp to the point where I'm not risking my life by playing it.

[svstee]

How would I measure the current in this amp in order to figure out what fuse I need? Do I use a multimeter?

[Structo]

You can use a meter or look up the power requirements of the tube filaments and current demands of each tube and add them up.

If you use a meter be very careful.

To measure current you need to have the meter inserted into the circuit.

Set the meter to read at least 10 amps if it isn't auto ranging.

Make sure you have the probes plugged into the right jacks on the meter because most meters require you to have the red probe plugged into a different jack for current measurement.

Then you will have to break the connection of the hot wire from the wall to the amp and have one meter probe connected to the wall voltage and the other one to the amp.

In other words the meter must be in series of one of the wires to measure current.

But, from reading some of the data on those tubes in that amp.
I'm coming up with about .955 ma total current.
I am inclined to say, use a 1.5 amp slo blo fuse.
Try that first then if that blows, go to a 2 amp slo blo fuse.

[svstee]

But doesn't the existing isolation transformer mean I don't need slo blo? It is regulating current, isn't it?

[Structo]

No I don't think so.

Any amp when you first turn it on is going to surge in current until the filter caps charge.
That is why a slo blo is usually used, so it doesn't blow during the quick surge that takes place before it settles down.

[svstee]

OK, thanks, you been a huge help.
I'm buying the stuff now.

[M Fowler]

That 35W4 is a half wave rectifer that supplies 17volts to the panel light. The 12aU6/6au6 has a transformer there suppling the 6.3 volts for the 12au6 and if you use a the 6CA5 pentode. But your schematic states it has a 50C5 this tube of course is 50v at pin 3 & 4.

CaseyJones is correct gut it or toss it. The rectifer supplys the 50V to 50C5 then goes to T2 step down to 6.3V for the filament on 12AU6 why did they do such crazy crap? They weren't building a television here.

Mark