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Discussion Starter #1
Just finished a 6G3 build (brownface deluxe), did my tubeless checks and then powered it up with tubes in it for the first time. When the tubes came live, I was greeted by a horrible, loud, high-pitched, feed-back like squeal. Double checked everything again, all looks good.

I remember reading something here about swapping the leads from the power transformer to the power tubes (switch the blue and brown). Was that a fix for this same problem I'm having, or for some other god-awful noise?
 

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That should be all you need to do. By swapping the plate wiring to the output tubes you reverse the phase of the signal. The feedback loop is now in phase and causes the amp to oscillate. Reversing the leads will make the feedback out of phase (as it is intended) and the amp should work normally.
 

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Discussion Starter #3
That should be all you need to do. By swapping the plate wiring to the output tubes you reverse the phase of the signal. The feedback loop is now in phase and causes the amp to oscillate. Reversing the leads will make the feedback out of phase (as it is intended) and the amp should work normally.
110% success sir! Amp is nice & quiet now and the oscillation is gone. I had a sound in the tremolo that would vary with the speed and intensity controls, but as soon as I grounded out the RCA jack for the trem foot switch, it went away.
Thank you for your support.
 

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Discussion Starter #5
6G3Barker3.jpg
Full shot of my new friend

6G3Barker4.jpg
The money shot. I departed from norm on this build, ran the heater wires along the chassis and didn't wind them around each other. Much easier, and in this case at least quieter than running twisted wires overhead.
 

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Much easier, and in this case at least quieter than running twisted wires overhead.
This is very interesting!
Is this something new/recently advised?
Why would it not be done regularly instead of the twisted wires approach?...especially when it is quieter.

Thanks for the pics...Beautiful work!
 

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Discussion Starter #7
This is very interesting!
Is this something new/recently advised?
Why would it not be done regularly instead of the twisted wires approach?...especially when it is quieter.

Thanks for the pics...Beautiful work!
Thanks Dave.
It's nothing new, just a different approach. Fender for sure didn't do it like that, but I've seen a lot of Marshall stuff built this way. For me, the heater wiring is the worst/hardest part of a build. (A skinny Tweed chassis makes it even worse) I've had a few disasters along the way in my quest to build the perfect amp, so I backed it up to doing brownface stuff. Keeping it simple till I get act together and my confidence back up. This time I put the power supply and tube sockets into the chassis first, and ran the heater wires. I built the rest around those things. It worked for me, so my next one will be made the same way.
 

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Makes sense to me, paths to tubes are physically reversing over each other, causing field cancellation. Plus you maybe save a significant percent of your heater wire. Bonus !
 

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Discussion Starter #10
This 6G3 build works right, sounds great, but I was startled to find it was running at a way higher voltage then what the schematic quotes. It's sitting at 465 volts on the B+ when it should be about 375. I'm running a GZ34, I haven't tried a 5U4 or 5Y3.....yet. I used a Deluxe/Deluxe Reverb PT in this build. Should I have used a 5E3 Deluxe transformer instead? Can I put a large capacity resistor inline before the filter caps to knock her down a few notches? Bias voltage is -31.5 and the tubes are seeing 65 mA of current. Nothing is going to live very long unless I cut that voltage back somehow.
Thoughts?
 

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What is the model # of the power transformer you used?
If you mean 65mA per tube, I would not run it like that. At 465V that is about 30W idle dissipation per tube.
 

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Discussion Starter #12
What is the model # of the power transformer you used?
If you mean 65mA per tube, I would not run it like that. At 465V that is about 30W idle dissipation per tube.
It's a Hammond 290BX. Tubes are Electro-Hamonix 6V6EH, I haven't noticed them red-plating but they do make some serious heat.

a 5U4 should help things, right?

I shut it down as soon as I smelled the tubes, and I haven't left it run for any length of time since.
Yes, 65mA per tube is what I measure. Is that even possible?
 

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I don't know, I'm hoping you have a meter problem. :) I've had some that read high with weak battery.
30W per tube for tubes rated at max 14W is a lot. As you lower the idle dissipation, your B+ at the plates is going to rise, so that will just make things worse.
So I suspect your readings are high somehow. Is this one of the newer Hammonds where you have a choice of 120 or 125V on the primary?
How about measuring the heater winding coming out of the PT. That will give an idea of relative voltages.
Unfortunately, you need to be taking these readings out of standby.
Yes, 5U4 will bring down the B+ a bit.
 

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a 5u4 should drop the b+ in the ballpark of 30-40 volts and the 290bx has 3 amps of heater current on the rectifier winding so you don't have to worry about overheating. Jb welders advice is spot on. the 290bx doesn't appear to have a universal primary so we can rule that out. I am starting to wonder if its a meter issue. another thing to think about might be using a using a 5y3 rectifier to drop another 10 volts down from a 5u4 value.
 

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Discussion Starter #15
I don't know, I'm hoping you have a meter problem. :) I've had some that read high with weak battery.
30W per tube for tubes rated at max 14W is a lot. As you lower the idle dissipation, your B+ at the plates is going to rise, so that will just make things worse.
So I suspect your readings are high somehow. Is this one of the newer Hammonds where you have a choice of 120 or 125V on the primary?
How about measuring the heater winding coming out of the PT. That will give an idea of relative voltages.
Unfortunately, you need to be taking these readings out of standby.
Yes, 5U4 will bring down the B+ a bit.
no, this is a normal version of the 290BX. Primary is simply black and white. There is a blue secondary tap that I didn't use and I left open.

I'm hoping I have a meter problem too. Gotta be.
 

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That current reading is still crazy, how are you measuring it?
Can you see what your meter says for the wall AC, and the heater voltage that should be 6.3VAC at the pilot light?
 

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Discussion Starter #18 (Edited)
That current reading is still crazy, how are you measuring it?
Can you see what your meter says for the wall AC, and the heater voltage that should be 6.3VAC at the pilot light?
heater voltage is showing 6.9V, wall is 123.5V

I'm using a tube socket base thru type adapter that plugs into a multimeter on mA scale. It's a new, cheapie Greenlee meter I don't really trust. I need to bring my good Fluke home from work.

Edit: I played it long enough last night that the tubes would have gone into melt-down mode if they were seeing 60 or 65 mA current. I've got a package of 1%, 1 ohm resistors on order. I'll get to the bottom of this when they come in.
 

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Discussion Starter #19 (Edited)
Well, I couldn't be waiting around for those 1% 1 ohm resistor to arrive, so I used what I had. 5% 1 ohm resistors. And I brought home my good Fluke meter.

At an idle,
with the 5U4, I'm getting a voltage drop of .044 volts across the 1 ohm resistor. Plate voltage is a calm 393. I think that's still on the upper range of what a 6V6 can handle.

with the GZ34, I'm getting a voltage drop of .054 and plate voltage is 420. a little over the top?


And that green meter reads all over the dam place. Even the ohms are wacky. The 100K ohm bias supply resistor measures 82K ohm with the Greenlee, 100.2K ohm with the Fluke.
 
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