Packard Bell Hum Problem

[JacobsNova]

I pulled this out of a console(Model DPA-30-3 amp) and pictures are attached. Everything is new or NOS except the transformers and tubes which are original. I also did some modifications. I put in a fuse an on off switch, input switch for 2 sets of stereo inputs, indicator light which is wired to the filament circuit, new speaker posts and a 100 ohm 4 watt wire wound pot for hum adjustment.

On my first start up using my variac half-way up the dial my main voltages were getting too high, the output tubes would start glowing too much and I would get a hum out of both speakers. Being a novice and not knowing what I am doing I figured out that the power supply was over driving the amp. So through trial and error I got my two mail rail voltages within specifications(see attached wiring diagram). So I got my voltages right and the tubes have stopped glowing too much but I still get a hum out of both speakers past the half way dial mark on my variac. Plus when I hook it up to an input source it sounds terrible. I don't think it is any Rf or NF noise coming in through the input jacks or ac induced noise being picked up by the input wiring because I have shorted the jacks out. The hum is pretty profuse.
I was planning on using this amp for my tv or home stereo, hoping it would make a good home stereo for listening to records. I also have a pas 3 that I can use with it too.
Any help in figuring out my problem is much appreciated...
Packard Bell Schematic
[IMG:640:480]http://i407.photobucket.com/albums/pp15 ... CN3327.jpg[/img]

Part 2 of Schematic
[IMG:640:480]http://i407.photobucket.com/albums/pp15 ... CN3328.jpg[/img]

Adjusted Power Supply Schematic
[IMG:640:480]http://i407.photobucket.com/albums/pp15 ... 3322-1.jpg[/img]

Chassis Before Pic
[IMG:640:482]http://i407.photobucket.com/albums/pp15 ... 0002-1.jpg[/img]


Wiring
[IMG:640:480]http://i407.photobucket.com/albums/pp15 ... CN3231.jpg[/img]


Wiring
[IMG:640:480]http://i407.photobucket.com/albums/pp15 ... CN3236.jpg[/img]


Back of Chassis
[IMG:640:482]http://i407.photobucket.com/albums/pp15 ... G_0109.jpg[/img]


After Picture of Chassis
[IMG:640:480]http://i407.photobucket.com/albums/pp15 ... CN3285.jpg[/img]

After Picture of Wiring
[IMG:640:480]http://i407.photobucket.com/albums/pp15 ... CN3232.jpg[/img]

[Structo]

When you are using the variac are you scaling the whole amp?
In other words, you are supplying the amps primary voltage from the variac?

If that is the case, then you are also scaling the heater supply.
If you have that too low the tubes will not heat sufficiently to work properly.

If your power tubes are redplating it is a bias problem.

I can't read the schematic very well, is it cathode bypass on the power tubes?

[Structo]

Also on the schematic what does it list the supply voltage to the PT as?

110v? 117?

[xtian]

Wow! I salute your decision to rehabilitate that amp! You can see what I did to mine: http://ampgarage.com/forum/viewtopic.ph ... =30#121881

[Alexo]

Those freewheelin' filament wires sand giant hum pot right next to the preamp might be worth checking out, as regards the hum. ..

[JacobsNova]

Yes, it is cathode bypass on the power tubes and the supply is 117.

When you are using the variac are you scaling the whole amp?
In other words, you are supplying the amps primary voltage from the variac?

If that is the case, then you are also scaling the heater supply.
If you have that too low the tubes will not heat sufficiently to work properly.

If your power tubes are redplating it is a bias problem.

I can't read the schematic very well, is it cathode bypass on the power tubes?

[JacobsNova]

Yeah, I moved around all my filament wiring and it didn't make any difference..

Those freewheelin' filament wires sand giant hum pot right next to the preamp might be worth checking out, as regards the hum. ..

[20to20]

100 ohm 4 watt wire wound pot for hum adjustment

Why did you change the original heater circuit from a 100R/100R balanced centertap to ground to a 100R pot? Don't modify anything until you have a working original circuit. The original unit didn't hum when it came from the factory.

[Cliff Schecht]

I never go below 200ish unless I'm using one or two tubes total.

[Structo]

So what I was saying is if you are only feeding the amp with roughly 50vac, then you are only giving the heaters roughly 3 vac when they want 6.3 vac.

And, I don't understand why you think you have to drop the B+ when you are only going to see roughly 3 more volts on the wall voltage.
That won't increase the B+ a whole lot.
So I'm thinking you have other problems going on there.

Also if your power tubes are way out of balance, that is not matched close enough, that can cause bad hum as well.

[JacobsNova]

I am running my variac up to 100% since I modified the power supply to get my main voltages right. As far as the heater voltages are at 100% measuring ac across the two legs of the hum control pot I get 7.8 volts, if I measure from each leg to ground I get 5.6 volts.

Secondly, I measured the resistance to the primary of the power transformer and I get 2.6 ohms. The wiring diagram shows it should be 17.5 ohms. So I'm wondering if I have a bad power transformer.

As well as matching the output tubes I have a tube tester and I can check them. I think I checked them a long time ago but I don't remember what the readings were.

So what I was saying is if you are only feeding the amp with roughly 50vac, then you are only giving the heaters roughly 3 vac when they want 6.3 vac.

And, I don't understand why you think you have to drop the B+ when you are only going to see roughly 3 more volts on the wall voltage.
That won't increase the B+ a whole lot.
So I'm thinking you have other problems going on there.

Also if your power tubes are way out of balance, that is not matched close enough, that can cause bad hum as well.

[20to20]

Secondly, I measured the resistance to the primary of the power transformer and I get 2.6 ohms. The wiring diagram shows it should be 17.5 ohms. So I'm wondering if I have a bad power transformer.

Look closely at diagram for the primary resistance. Could it be 1.75 ohms? That is more likely for a primary. You should set your variac at @120v and then troubleshoot. Your heater voltage is too high even for 125v. line. Should not be much over 7.0-7.2v.

Rebuild your circuit according to the diagram. Don't add any extra capacitance no matter what anyone tells you.