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Old 1st November 2004, 11:37 PM   #21
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Location: Denver, CO
Default causes of hum

poor grounding of inputs--very loud!

ground loop--not loud

unfiltered power through filter system--can be loud but usually not, usually 120 cycles

pickup induced current on input lines--usually 120; not loud

high impedance series input -- too much resistance on grid of input/driver tubes; not loud

seems to me you should twist heater wires tightly if not done; position input wires away from heaters or anything with AC including pwer transformer

try moving your amp to another electrical outlet or power strip; keep it away from house currents in walls, on floors

I know all this sounds simple but I always get hum on new amps and manage to find causes and fix--you will too!
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Old 2nd November 2004, 03:36 PM   #22
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Yes the inputs grounding are crossing the filament leads as they going to star earth.
Do you believe my problem is over here? What do you say to place the filament leads L and R of the chassis? So, theyíre away from the center where all the groundings come. Then theyíll be close to output leads but I think itíll be better.

Thanks for this helpful information about causes of hum.I'll check one by one and hope I'll solve the problem.
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Old 3rd November 2004, 05:49 AM   #23
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You might be having what is a confusing but simple problem or effect from ground current that simply escapes the builder.

I built a number of guitar amps. I built the first one and had the "Hummers".

The normal procdure for grounding was to have a short bus-bar from the ground spot on the chassis where the center-taps are grounded, to the filter ca[s on the fiberboard. The tube cathode resistors are grounded to the front copper plate. I ran the bus-bar all the way to the first tube. Hummmmmmm

The spot on any chassis where the PS is is "Hot"

You have to ground the RCa and input gain stage and inverter at the chassis where they are. The chassis is "Cold" there.

You have connected the "Hot" and "Cold" areas of the chassis together.

If you look at the underside of a vintage Heath for example, you will see a bus bar that runs along from the output tube Kathodes to the PS.

The grounds for the input tube stages and the RCA jack are grounded at the chassis away from the PS AC influence.

If the third wire is at the same spot as the center taps, then the third wire will take the AC influence on the chassis away to ground.

Another problem is that the power tranny puts a AC field through the chassis. Where the power tranny is, there will be a AC influence in the chassis.

Does the amp hum by itself?

There could be a AC difference between the chassis line stage and the power amp. Then Ac flows through the ground side of the input cables. See if you can measure a diff between the two with the cables unhooked. If you have the seperate pieces grounded with 3-wire cables, then you only need one side of the RCA cables grounded. Then AC can not flow from chassis to chassis. Or from the power amp to the phono cartridge or CD player.

I have been busy. I am short on time right now. HAng on and I can finger it out...
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Old 3rd November 2004, 12:39 PM   #24
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Hi FranStar (the Tonemaster)

Thank you very much for your help and welcome to this thread.

Iím planning to do a new grounding. So, what do you say for something like this in the attached photo?
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Old 4th November 2004, 03:21 PM   #25
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There was a store in Downey, CA called NET Electronics. It was in a Newberry Drug Store building. It was a old two-story building. The owners bought out all the Plush companys amps when they went under. They were all tube amps like Fender clones. I sat ubstairs and built amps, then put the completed amps out on my display area.

(around 1970)

The biggest amp they had was a six-6550 monster like the big orange and Hi-Watt units. 300 watts RMS.

You learn a lot working on *****y, high-gain Git-Amps.

I looked at the drawing.

Ground the PS stuff over in its area.

Ground the input stages over in their area.


The PS grounding should be by itself. The ground wire from the Ac should go to the PS ground.

Seperate the two top/bottom of the drawing. Ground the PS in it's area, and ground the input stages and the driver over in their area.

Then you won't hook the sensitive input stages to the chassis area where there is AC. The AC on the "Hot" area of the chassis actually raises the Kathode of the input gain stage and influences hum on the tube, the input pot, the RCA jack.

I have had to take the ground wires from the controls on a modified Git-Amp and "wander around" to find a place where I could put it and not get that hum in the background etc...


You can hook a bus bar from the PS ground to the Kathodes of the output tubes but only that.
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Old 4th November 2004, 07:31 PM   #26
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Just wondering - the 220k resistor in the cathode of your first valve seems to be very high in value. Is this a misprint? 220e sounds more like it to me.

Vintage Tube Hobby Group:
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Old 8th November 2004, 01:11 PM   #27
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@Family Dog
Yeap it's 220R. My mistake

I think hummmm is gone!!

I measure a 20mVp-p 120Hz at the output with grounded input. It remains the same with open input but with a little noise on it.
Notice that I havenít balance the outputs of the phase splitter, yet. And also no feedback at the moment.
I donít think it will be audible after these two. With a small amount of FB it goes down to 10mV.

Have you any comments on these? Am I in a good progress?

I think you are the perfect guy to help me build my new guitar amp.
Which tubes do you believe is the best for a guitar amp?
When Iím ready Iíll post a new thread. Hope you will be there!
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Old 8th November 2004, 08:48 PM   #28
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Default Hum gone?

What did it? Please let us learn, too.
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Old 11th November 2004, 07:07 AM   #29
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I follow FranStarís technique with HOT and COLD.
So, I use a multi-star ground technique.
I use one ground point at Left channel, one at Right and one at the PSU.
I grounded these points directly to chassis at each place. I use for the circuit grounds one screw of the phase splitterís socket (for each channel) and another screw (power trannyís) for the PSU. At this screw (PTís) I had connected mains earth, too.
As I said I have only this 20mV 120Hz at the output with grounded inputs. I havenít balance the outputs of the phase splitter, yet. Iím sure that this will reduce this hum dramatically. And with a little FB (about 8dB), hope to have SILENCE!
I will experiment with these today. So, Iíll post the news, soon.
Any further advice is acceptable.
I like very much the sound of this amp and Iíd like to thanks all the guys at this forum that helped me.
Soon, Iíll post a new thread with the schem and voltage measurements on it. Maybe there are some hints and tips that I don't know, yet.
I also don't like very much some measurements on the phase splitter.
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Old 12th November 2004, 02:10 PM   #30
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This little hum that I have is still audible even if I apply a FB and bias the outputs of the phase splitter.


Itís the position of the trannies!

THIS MEANS A NEW ALUMINIUM!Again drilling and placing the components.The base is wooden so, I'll keep it.
Here's the story.
Remember that my PT is vertical and OPTs are horizontal.
I have them like (1)
Then I have done some experiments by listening only at the left channel andÖ
I remove the PT like (2) and, my god SILENCE!! Then I put the speaker at the other channel and hummmmmmm.
The only solution is (3) and (4).
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