• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Building a Aikido preamplifier

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#282
Brit01 said:



:xeye: :xeye: :xeye:

I've set up all the grounds as you described in the pdf.

There is no hum but still getting loads of high frequency static.
😡 😡

Could a leaky tube cause this??

I think I'll try without the pot to see if it is this causing the noise.
Click to expand...


I think I remember reading HF static could be caused by a cold / broken solder joint. Could be a bad tube too, Both L and R speakers or one side only?....just thinking outloud, while typing.
Ron
 
#284
Post #283 brit01, there is a german front panel express.
Click to expand...

Thks for that but unfortunately I'm down in Uruguay, S.America.

I think I remember reading HF static could be caused by a cold / broken solder joint. Could be a bad tube too, Both L and R speakers or one side only?....just thinking outloud, while typing.
Click to expand...

I'll check out all the joints.

I've got 2 spare 6N1P's and 2 5687's. I'll try changing each one.
 
#286
Or this could all be down to the fact that this Aikido has so much gain going through my powerful Carver amp with 380 watts a channel and through Klipsch 93db horns!!

It's pretty quiet through my test speakers. Little active pc speakers.

Anyway I might experiment with DC.
 
#287
Brit01 said:
Or this could all be down to the fact that this Aikido has so much gain going through my powerful Carver amp with 380 watts a channel and through Klipsch 93db horns!!

It's pretty quiet through my test speakers. Little active pc speakers.

Anyway I might experiment with DC.
Click to expand...


I am running AC heaters and, after moving the heater wires far away from the signal wires, running them along the sides of the chassis, tucking them into the corners, and using shielded heater conductors, the hum is barely audible, quieter than the tube rush. My speakers are about 90db.

I did notice a large reduction in noise on the scope when I replaced the heater wires with shielded wires, but I am still quite a newbie at tubes so it could have been from another change. Does your heater transformer have a center tap? If not, have you created a virtual CT?

I was also toying with the idea of a 100 ohm heater balance pot-more for the learning experience than anything, but the last bunch of mods have significantly reduced my hum.

Does your hiss/hum change when you turn the volume knob?
 
#288
Does your heater transformer have a center tap? If not, have you created a virtual CT?
Click to expand...

Yes I created a virtual CT with 2 x 150 ohm resistors. I have this tapped actually where the heater cables enter the main board. Maybe nearer the tranny is better.

Does your hiss/hum change when you turn the volume knob?
Click to expand...

yes as I turn up the volume with music on pause the buzz increases.

And if I take the pot out of the loop and turn up the line level control on my amp the same happens.

Maybe things will change once all is wired up properly inside a chassis.
 
#292
Brit,
Its called a hum-pot, the multi turn unit would be more precise. I can't find where I read it now......I think Sy posted this idea (to use the pot instead of r15)
Parts-------resistors, caps, and especially valves being what they are. Are never what they are calculated to be, the pot lets you adjust the difference. Take a look at JB table of R15 values.


Again, just a thought

Ron

Link to JBs R15 page


edited to correct bad link
 
#293
Hi Ron,

Thks for the link.

In the table he suggests 89,474 for the R15 if I'm using the 6N1P. Does it matter if I'm using the 5687 as output tubes?

I guess as you suggested to use a pot and see if there is any difference as I change the value.
 
#297
Sy has big ears, he hears everything...!
He always shows up when I use his name............. I'm still struggling to comprehend his posts. (personal joke)

Sorry I missed out on a great BA in 2008, 2009 is a new opportunity!

Ron
 
#298
English Sy, English

SY said:
I used a simple trimmer pot, one turn, but put it in the center of the divider, sized to give me about 10-15% adjustment range.
Click to expand...


Sorry SY,
I still got wet solder behind my ears..... could you please explain where exactly? ie: directly after r15 but prior to the bias out;

Thanks
newb,
Ron
 
#300
Brit,
What I think SY is saying: he likes us to figure things out and not just spoon feed us.

Here's what I think....could be wrong of course,

take a 10K pot with 3 legs, one goes to the 84K or whatever the top resistor is.
And the other leg goes to the bottom 100K resistor, the 3rd leg goes to the bias for the heaters.

10K pot because it is 10 - 15% of the value of the larger resistor.
That way it can adjust BOTH with a give and take method.

Pretty sure that's what he meant.

Ron
 
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