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I posted this on 18watt as well, but thought maybe there would be some takers here as well. BTW, Stephen and all who helped indirectly, you made it easy to troubleshoot my hum problem. I LOVE THIS SITE!!!

Ok. I finished my 1st build, got the dreaded hum, troubleshot and resolved almost all of my hum. The one thing I did that helped the most was to create a center tap for my 6.3v heater supply by using 100 ohm resistors tied to the cathodes of my EL84s. I built a lightning btw. I also ran these resistors straight to ground, but there was no noticeable effect. I still have the slightest of hums but I want all of it gone. I have separated my grounds and done all of the recommended things. I guess this leads me to my question. I know this doesn't make sense electrically, but thought I'd ask it anyway. The PT I'm using is a Hammond 270DX and doesn't come with center taps for my 5v and 6.3v. Explain to me why my mind knows it's not a good idea to create a center tap for my 5v like I did for my 6.3v. I imagine I would be putting my B+ straight to ground. Anyone out there care to comment? Also, I'm open for suggestions on the hum. It is most pronounced when gain and master volume are turned all the way up. In other words, turn it down and the hum goes away.....Thanks.

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~~~~~always grateful for the help....

Adding a derived ground to the heaters for the signal tubes is a must if you dont have one on the transformer. For the rectifier, it is not carrying signal so not to worry. But if you did that with an octal rectifier like the 5Y3 or 5AR4 etc, you would be grounding your B+ through a 100 ohm resistor, which is a No-No.

Did you use a choke across the 33uf caps? The Lightning has a hi gain front end. No choke & it could hum.

Any hum is going to be most pronounced when gain and master volume are turned all the way up. What if you dont turn the master all the way up & just the volume?

A picture would really help if you can post a link to it. Then we could look it over.

You can look over the TC-15 post and see how to wire a 15. In that build, I didn't have a choke to use! and it's pretty quiet.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Here are some pix.....

http://www.18watt.com/modules.php?name=Forums&file=download&id=6532
http://www.18watt.com/modules.php?name=Forums&file=download&id=6533
http://www.18watt.com/modules.php?name=Forums&file=download&id=6534

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~~~~~always grateful for the help....

Very clean build. Nice job. Here's some comments:

The output transformer lines go right over top the hi gain input stage;
The wire to the OT have to go across the chassis under the board;
Cant see where the Power supply caps are grounded (separate from pre-amp ground);
The OT & PT are not at 90 degrees opposed, even though they far apart;
The OT is Right above the input jacks.
These could be contribute to some hum.

Not sure you want to spoil a nice build like that to get rid on the last bit of hum.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks for the compliment as far as cleanliness. I am working on orienting my transformers 90 degrees from each other right now, taking a break to check the forums. I don't know what else to do about the OT over my inputs. The power supply ground is there and is grounded at the same place my PT center tap is grounded. The OTwires to pins 7 and 9 go well there, I think.....Moving them around either gives me more hiss or even oscillation. The OT leads going to my outputs have been moved around all over the place and have no noticeable effect no matter where they end up. Maybe I sould run them along the edge of my chassis as opposed to under the board and over my preamp? I don't know....I really appreciate all of the help everyone is giving me here. Will continue to keep you up to date on this.....

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~~~~~always grateful for the help....

Transformers are now 90 degrees to each other. Still have the hum. Preamp tubes and PI tube is pulled, stull hums. When I switch it to standby, the hum goes away. I guess I will tackle the OT location next. Maybe move some of the wires for the OT around. I have such a strong desire to get this thing right because it's my first build. I want this dude to be as perfect as possible.

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~~~~~always grateful for the help....

That being the case, it's probably wiring/layout in the output section. I dont usually connect my output jacks to ground. I connect to the wires from the OT direct. I noticed yours is connected at both. Try disconnecting the lead that goes to the chassis.

You're not going to get hum with the standby on.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

To determine where the best place to mount two trasnformers is, here is the Headphone technique.

This is the best way to find the quietest place to put a reverb, output, driver, or any other signal transformer to get the least hum. CAUTION: THIS INVOLVES WORKING ON THE AMP WITH WALL VOLTAGE PRESENT. WALL VOLTAGE CAN BE FATAL.

First set up the power transformer. If it's not installed yet, all the better, but if it is, this will still help. If the PT is not installed, wire up the primaries to a power cord, but do not wire up any secondaries-- tape the ends of the secondaries. If the PT is installed, fine, just make sure there is no circuit on any of the secondary leads (pull all tubes, and disconnect leads and tape them as necessary).

Now set up the signal transformer (output, reverb, whatever). Hook one set of OT secondaries (your choice, but I usually go from ground to the highest impedance tap, don't worry about impedance matching here!) to a set of headphones. Tape the primary leads.

Put the headphones on, plug in the power cord, and start moving the signal transformer around. Try it all over the chassis, and also rotate it. While you might expect the least hum with the transformers as far from each other as possible and at right angles (and that may be the case for you), it might come at an odd angle and/or position. Sometimes the least hum occurs when the transformers are side by side. When you find the quietest spot, use a permanent marker to mark where the mounting holes should be.

If you have more than one signal transformer, repeat this for each one.

source: AX84 site

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Last post on the above subject, I think....Thanks to all who helped out. I hooked up my o-scope looking for ripple on my power tubes. Hooked up to pin 9 (screen grid) and noticed about 1.2vac ripple on both power tubes. Also noticed that I forgot to put in my 100 ohm screen resistors. OOOPS....Anyway, I have a hammond 1608 OT with the linear taps. I had those linear taps hooked up to pin 9 instead of coming straight off my power supply. I looked at my power supply and it was smooth as ever on my scope. I put my power supply to my screen resistors to pin 9 and bam! no more hum. I guess the OT ultra linear taps were bringing in just a hint of AC, enough to cause me frustration and some sleepless nights!!! Moral of the story....for my next build, I will look at getting my iron from some other manufacturer so I don't have to deal with the ultralinear taps and also finding a PT with center taps for my heater supply. Man does this thing sound sooo good now. Now all I hear is that sweet small hiss that comes from cc resistors. Thanks for hanging in with me everyone!

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~~~~~always grateful for the help....

Yes, you should use iron designed for the amp. I know a lot of people use Hammonds etc., but it's a compromise and I've heard these amps both ways.

Congrats on your success!!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps