What do these tube readings mean on Hiwatt?

[frank57]

Okay, I've learned a lot and decide to take some readings on that pesky Korean Hiwatt custom 20 tube monstrosity.
It's an amp with big hum problems, maybe this is why? Aside from bad layout and everything else.
I have the bias at about 60%.
When I measured stuff on the 12ax7 tubes I was surprised:
With my auto ranging meter I got:

Pin 9 to Ground- 3.875vac
Pin 4 0r 5 to ground-3.85vac
pin 9 to 4 or 5 - 7.73 vac.
That's not right at all is it?
Pins 4 and 5 are not connected on this amp either.
With my other meter, not as accurate,
I got
Pin 9 to Ground- 3.4vac
Pin 4 0r 5 to ground-3.4vac
pin 9 to 4 or 5 - 7.2 vac.

Did I do it right? I took the readings fairly quick after turning it on.
Does the amp have to warm up for awhile?

Can the voltages vary or should they be right on 6.3 and 3.15?

[Dave]

Those readings sound a little high to me. Some might argue that they are not  that bad. I think it should work ok like that but it might make your tubes have a shorter life.

Now, if there was a "Virtual CT" on the filaments that had burnt,.........
It might address both your problems.
It would definately cause hum problems and certainly might account for the high readings on the heater pins.

Check along the path of the 6.3 volt leads and at some point there should be (50 50) a pair of resistors (one from each leg) to ground. Probably 100r resistors, but not necessarily. If they are there and burnt, that's probably your problem.

Dave

[eleventeen]

Excellent thoughts, Dave. Could be an easy fix!

[frank57]

Nope, no resistors there at all.
The transformer does have a center tap going to the pcb from the standby switch.
Wonder why they didn't connect pins 4 and 5?
Wouldn't that help an ac powered amp reduce hum?
I imagine reducing the voltages would help with the hum.
Yes this amp eats preamp tubes that's for sure because of the voltage.

[Dave]

Does it have a hum balance pot? Hum balance pots are typically wired in exactly the same way the resistors would be.
If it does have a hum balance pot and it were burnt, it would present the same symptoms.

Look at it really closely, because filament windings would generally have their very own CT or, at least,  some sort of virtual CT.

Now that you have referenced 1 CT for the HT winding, I am even more suspicious that an artificial CT is the problem.

Dave

[frank57]

No, none of that stuff is there.
You can see pics and schematic here:
http://www.el34world.com/Forum/index.php?topic=8727.msg78728#msg78728

[Dave]

That schematic does not address the filaments in any way. However, it does show that the high voltage winding does have a CT. So...... If you can only find one CT from the PT, then there definately should be some sort of artificial CT for the filament winding somewhere. You need to keep searching until you find it. It is there.

Dave

[frank57]

You might be onto something here.
Could it be on the board itself?
If it's not there at all then what..
Can you explain it better?
You're talking about the yellow heater wires?
Maybe this drawing will help.
From the power transformer itself there are 2 red hv wires, the two yellow heaters and
what I was told is the center tap going to the board.

[VMS]

Do you have a picture of bottom side of that TUBE PCB? And where does those yellow heater wires go from the PT?



ps. I think that biggest cause for the hum and buzz is that the tubes are too close together, but then again I could be wrong.

[frank57]

Here's the pcb bottom of the tubes.
As you can see nothing there.
4 and 5 are not connected either.

[frank57]

Heater stuff that phil drew out awhile back.
 

[frank57]

Yellow wires and power transformer

[VMS]

4 and 5 are not connected either.

Yes, those aren't even supposed to be connected on EL84's, but they are connected on 12ax7's, as you can see on the PCB. Can you show me the other end of the yellow heater wires?

[tommytornado]

Huh?  Looking at the pic posted, pins 4 & 5 on thare tied together by the trace on the 4 tubes on the right.. and the 2 tubes on the left are the EL84's. 

[VMS]

Also, I think that the bias pot should be located inside the chassis.

[sluckey]

The fact that you measure filament voltage from pins 4/5 or 9 to chassis means that there is a ground reference on the filament string. It may be a hard wire CT or artificial CT, but it's there.

I don't usually think of filaments as a source of 'big' hum. I'd look to the power supply and filter caps.

[frank57]

Anything I should measure?
You can see the inside of the amp by clicking on the link I posted before.
It's I believe 120hz hum.

Where would this ct be exactly?

The filter caps are all new.
Diodes too.
I have a sound clip but don't know how I can post it.
It's a big file.
The problem some people have seen is the crap coming around on the earth track, hence the suggested fix by Phil Abbot.
Don't know if it's the exact problem though.
The rectifier buzz is coming around along with the ripple.
But I can't confirm that.
As for the bias pot it is where it is for now, worry about it later.
Hum goes away with v2 out.
I think the power section is ok.
If I disconnect efx loop there is little hum and buzz.

[Dave]

I don't usually think of filaments as a source of 'big' hum.

I do. Well, in the context of a missing or busted filament CT, I do.

Where would this ct be exactly?

Who knows, but its in their somewhere. If there is not a real CT on the transformer to ground, then there is an artificial CT in there somewhere. It will present itself as either a resistor from each leg of the filament supply to ground, or a pot with the outer legs connected to each leg of the filament supply and the middle (wiper) connected to ground.

Either way, its the same thing.

Dave

[Dave]

If it's not there at all then what..
Can you explain it better?
You're talking about the yellow heater wires?

If its not there, then it will hum loudly and the load that would have been presented to the filament winding by the resistors to ground would likely cause the filament voltage to increase slightly.

Can I explain it better?
No.

Yellow heater wires? Yes, heater wires.

I am not saying that a busted (or missing) artificial CT is your problem, but it sure fits the symptoms.

Dave

[frank57]

yes the symptoms do fit, because it sounds like something isn't grounded.
The buzz goes up with the gain pot.
But there is no pot that i can see anywhere connected to the filaments.

The only center tap is the wire going from the standby switch to the pcb.
It's a wire coming from the transformer. Black on the outside, blue on the inside.
Should there be another center tap for the heaters then?

I see on the bottom pcb the 4 and 5 trace going to the power tubes which are connected to the pcb ground?
So we're looking for something connecting the filaments to the ground somewhere?
If it's there maybe the implementation is the problem.

[Dave]

1. Should there be another center tap for the heaters then?
2. So we're looking for something connecting the filaments to the ground somewhere?
3. I see on the bottom pcb the 4 and 5 trace going to the power tubes which are connected to the pcb ground?

1. YES!!!!!!!!!!!!!!!
2. YES!!!!!!!!!!!!!!!
3. 4 and 5 on the power tubes are seperate filament pins. They will not work if they are connected to ground or to each other and for that matter it would probably blow up the PT in short order. Look again.

Dave

[frank57]

Could they be hooked up to pin 3 on the power tubes?
It looks like the trace is going there.
I'll try to check on the amp.
Would it help any to get all the filtering off the main board onto a terminal strip maybe?
Off to the right with all the hv etc?

[frank57]

Could it simply be the green wire in the transformer pic?
From the transformer itself?
It goes to the bolt on the inside of the amp.
Black wire goes to standby switch.