Need some help in Dallas TX ...scoping and amp (new pics)

[MGW]

I built a JCM800ish head, basically and Egnater seminar head. And I can't get the hum out of it. Bruce has been kind enough to help me with a lot of stuff on the amp, but I have rewired the amp twice, rewired the heaters 3 times, swapped out the tubes more times than I can count.

Is it possible that the PT is introducing noise somewhere? Is it possible that I have a bad PT...if so, how do I tell?

Is there anyone in the Dallas/Fort Worth area with a scope that would be willing to help me track down the hum. I'll bring beer...or other beverage of your choosing

[Colossal]

Have you ruled out the mains as the source of buzz? You could test this by taking the amp to a buddy's house or by using an isolation transformer. Additionally, could there be an appliance or light source such as a compact fluorescent lamp operating on the same circuit? Compact fluorescents typically have high frequency drivers but may be cheaply made and can induce hum nearby. Mark Fowler, a member here, had a similar issue which turned out to be a light source and a suspect appliance (unbeknownst to him) operating on the same circuit as his amp bench.

If you have ruled out the mains or other environmental factors as the possible source of hum, then yes, it's definitely time to look at the transformers and rest of the circuit.

These things can be a bit maddening but stay positive. Approach the problem in a very linear fashion, ruling out one variable at a time.

[LOUDthud]

A few simple questions first.

Does it hum with the Standby off (before the tubes warm up)?

Does it hum in Play mode if the Phase Inverter tube is removed? Any other preamp tubes kill the hum when they are removed?

Listen carefully when you switch the Standby switch to the Off position. Does the hum go away instantly as soon as the switch clicks to the off position or does it fade out over a period of a couple of seconds?

These things will give clues as to where the hum is coming from.

[Phil_S]

Great questions. I have one more. Can you tell if it is 60Hz or 120Hz?

The A or 5th string is 110Hz, so 120Hz is not quite A#.

The E or 6th string, I think that is 83Hz, so you need to tune down to about A# for 60Hz.

You can use your guitar or a keyboard to identify the frequency close enough. You might have both and one may be masking the other.

See if you can offer us something on this. If it is clearly one or the other, it will point us in a particular direction.

[Phil_S]

One more thing, can you post a couple of pictures of the chassis? One from the top so we can see tube and transformer placement, and at least one from the bottom showing us a good overview of the build.

[MGW]

The interesting thing about the mains is that they are at 125 volts. I;ve called the power company and they said this is normal. I put a huge zener on the CT of the B+ to bring the voltage down. It was putting 515VDC on the first cap. Given that it was rated at 500VDC, I figured it would be good to pull them down. Not using the CT on the 6.3 and I put 2 6 amp diodes on one leg to bring down the 6.3 to 6.3 because it was running at about 7 volts. And yes...they are oriented correctly....alternating. Not sure that this had any noise impact in either a positive or negative way. I do have a circuit in the house that I wired and there is nothing on it but the tv and sat receiver. I can unplut everything from that line and check it. But I don't think anything is on the other one....stranger things have happened. Come to think of it, my '66 Bassman hums a little at higher volumes.

Amp is dead quiet in standby...as far as I can tell. The hum goes away instantly when i flip the switch. I may need to stick my ear in the cab and double check. It stops instantly when I flip power only to off...but I would expect that.

I can hear it when I pull the PI. Very faint, but I hear the hum in the speaker. Also , I can turn the gain pot all the way down and hear it with the master turned up. It is quieter when V1 is gone, but I can hear it with all or any combination of pre tubes pulled. I have also tried 3 sets of output tubes. 2 sets of 6L6 and 1 set of EL34. Rebiased when swapping. No noise difference.

pretty sure it's 60HZ. I've tried a new filter cap in the preamp nodes, but the main cap hasn't been swapped. I have another, I may ought to see if that helps.

When I first built the amp i fired it up without the bias supply caps grounded. Fixed that pretty quickly though. Don't know if that goofed anything up though. Bruce didn't seem to think so, but I don't know how well I communicated that to him.

I'll get some pics up when I get home from work.

Thanks guys....for the encouragement and the help.

[Phil_S]

60Hz means its AC not DC. Likely suspects are anything before the rectifier, including the PT, the high voltage secondary, and the filament supply. Also, the outputs from the power tubes to the speaker are AC.

I'm thinking your filament supply is unbalanced. You aren't using the CT. You say you put two diodes on one leg to lower the voltage. It may be this is unbalancing the legs of the filament supply and is a source of hum. If you had 7.0v, lets guess you had 3.5v on each leg. If you took the whole 0.7v reduction on one side, then you've got 2.8v on one leg and 3.5v on the other leg. Or something like that. No amount of rewiring will fix that.

You say the filament supply has its own CT? Try working with that. Take out those diodes and allow for a balanced filament supply. There are a number of ways people approach the voltage reduction problem. What comes to mind is a Zener on the CT. Don't ask me, because I don't know, how to figure exactly what to put there. I've always shied away from this sort of thing, thinking it is better to just get the right tranny. In this case, your problem is the power company.

It won't hurt all that much to remove the diodes and let the filaments operate at 7.0v. I've seen it said that +/- 10% is OK. 6.3*1.1= 6.93V, so that's at the margin. At the very least, put a jumper over the diodes and ground the CT. If the hum goes away, then you can be pretty sure about the problem. Then you can deal with the voltage.

BTW, while 125vac is not unheard of, it is a bit high. I generally see 121 or 122, and I've seen as high as 129v at the wall socket. You can't control this and it can be problematic if the PT is spec'd at 117v.

[tubelectron]

Hi MGW,

LOUDthud asked you the right YES/NO questions :

A few simple questions first.

1 - Does it hum with the Standby off (before the tubes warm up)?

2 - Does it hum in Play mode if the Phase Inverter tube is removed?

3 - Any other preamp tubes kill the hum when they are removed? (with the phase inverter left in place, remove them one by one, starting with the input tube)

Listen carefully when you switch the Standby switch to the Off position :

4 - Does the hum go away instantly as soon as the switch clicks to the off position

5 - or does it fade out over a period of a couple of seconds?

These things will give clues as to where the hum is coming from.

Troubleshooting is a form of art !

A+!

[LOUDthud]

Unfortunately there is a little bit of conflict in the answers. The hum that goes away when the standby is switched off is usually 120Hz rectifier buzz. 60Hz hum is usually from a problem with the heater wiring or possibly the bias supply if it's half wave.

There is almost always some combination of 60Hz and 120Hz hum but I'm guessing the majority is 120Hz. How are the grounds routed?

[MGW]

I have the definitive YES/NO answers below. I'll remove the diodes and use the CT and see how that goes. That being said, I have an artificial CT in there with two 100ohm resistors that are in extremely close tolerance to each other.

Bias supply is half wave. Single 1N4007

Hi MGW,

LOUDthud asked you the right YES/NO questions :

A few simple questions first.

1 - Does it hum with the Standby off (before the tubes warm up)? NO
2 - Does it hum in Play mode if the Phase Inverter tube is removed? YES

3 - Any other preamp tubes kill the hum when they are removed? (with the phase inverter left in place, remove them one by one, starting with the input tube) Nothing kills the hum. I pulled all but the output tubes and still got hum. Pulling V1 lessens it, but doesn't kill it.

Listen carefully when you switch the Standby switch to the Off position :

4 - Does the hum go away instantly as soon as the switch clicks to the off position

5 - or does it fade out over a period of a couple of seconds? It's pretty quick, but I can hear it for about a second after I flip the switch and it is fading. REALLY quickly, but fading nonetheless.

These things will give clues as to where the hum is coming from.

Troubleshooting is a form of art !

A+!

[MGW]

To update...
I swapped out the filter caps, removed the diodes to bring down the 6.3vac, removed the artificial CT & used the actual PT filament CT. Nothing changed. I've attached a clip of the noise. No eq, just iphone. It is with the gain all the way down and the MV about 2/3 up. Bass, Mid are at half, treble is at 3/4, presence is all the way down.

As for grounds.
B+ CT is grounded at the mains ground along with the first filter can. Second can and filament CT are grounded about 2 inches away on the chassis. The bias caps, presence, and bias pot are grounded on a PT lug. Inputs, master, middle, loop circuit, 3 gain and CF are grounded on a lug by the pots. The jacks on the loop are grounded to the output jacks. The output is grounded to a lug on the second PT that also has pins 1 & 8 from both tubes grounded with test points and a 1 ohm resistor between pins and ground. first two stages use shielded cable. MV uses shielded cable. Loop send and return uses shielded cable.

Another note, I do hear the PT humming when powered on.

[rdjones]

To update...
I swapped out the filter caps, removed the diodes to bring down the 6.3vac, removed the artificial CT & used the actual PT filament CT. Nothing changed. I've attached a clip of the noise. No eq, just iphone. It is with the gain all the way down and the MV about 2/3 up. Bass, Mid are at half, treble is at 3/4, presence is all the way down.

As for grounds.
B+ CT is grounded at the mains ground along with the first filter can. Second can and filament CT are grounded about 2 inches away on the chassis. The bias caps, presence, and bias pot are grounded on a PT lug. Inputs, master, middle, loop circuit, 3 gain and CF are grounded on a lug by the pots. The jacks on the loop are grounded to the output jacks. The output is grounded to a lug on the second PT that also has pins 1 & 8 from both tubes grounded with test points and a 1 ohm resistor between pins and ground. first two stages use shielded cable. MV uses shielded cable. Loop send and return uses shielded cable.

Another note, I do hear the PT humming when powered on.

The noise sample sounds to me like "hash", meaning it contains spiky high harmonics of 60/120.

I'd say your ground system needs a bit of cleaning up.
First the AC mains safety ground gets it's own dedicated nut and bolt directly to the chassis, not shared with anything else - no circuit grounds, no supply grounds, not even a transformer bolt. By Itself. This is for safety as much as it is for isolating garbage coming in from the AC.
Next, I'd ground the bias cap and output tube's cathodes to the same place as the main B+ caps and HV CT.
Then the presence, PI tail, speaker jacks to the same place.

Some one correct me if I am picturing this wrong ...

reddog

[Phil_S]

RD: I think I see what you see. If I had known about the ground scheme, I wouldn't have been chasing away the dropping diodes on the filament supply (though I still think that's the wrong strategy). Those grounds are all over the place and need to be solidified into a standardized scheme -- one that's known to work. There are lots of variations on the theme, but what's described isn't one of them.

In addition to what RD says, it's my sense that the use of PT bolts as ground points is a bad practice. I know it's done because it's convenient, but that doesn't make it right.

The mains ground has got to be isolated and near the point where the line cord or IEC inlet penetrates the chassis. Nothing else goes on that, ever. To emphasize the safety aspect, you have at least partially defeated the fuse by tying a circuit ground to the mains ground, not to mention the hash problem it probably creates.

[MGW]

Where would you ground the filament CT...Currently is shares a ground with the preamp cap can. Also, the B+ CT is grounded on a 50v zener bolted to the chassis.

[tubelectron]

MGW,

2 - Does it hum in Play mode if the Phase Inverter tube is removed? YES

Then suspect the power stage and the power supply. The preamp section is not involved - at least not directly.

Nothing kills the hum. I pulled all but the output tubes and still got hum. Pulling V1 lessens it, but doesn't kill it.

Above is the confirmation...

It's pretty quick, but I can hear it for about a second after I flip the switch and it is fading. REALLY quickly, but fading nonetheless.

Then the power stage is involved, indeed, and/or power supply.

Voluntarily, I do not read all the other posts, only relying on your answers on these key questions. Let's of course suppose that all the components are flawless and there is no remaining wiring "joke" mistake. It arise often even to the best skilled of us...

1 - check the heater circuit. It must be made with braided wire, in that order : heater winding NO CT connected > power tubes > phase inverter > preamp stages, going to the input > on the input tube heater (and nowhere else), a 100 ohms wirewound pot with its wiper tied to GND here.

2 - check your GND bus. It must follow that logical order, with no loop, no forward and return : start at the CT of the HV winding > HV PSU and Bias PSU > power stage > phase inverter > preamp stage, going to the input > input jack WHERE the ONLY GND point to the CHASSIS must be, and NOWHERE else. All the GND bus must insulated from chassis elsewhere except at the input jack, the unique GND point.

Following these 2 rules - or trying to get as close as possible - should help you to get rid of the remaining hum.

A+!