Odd imbalance reduces hum

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Odd imbalance reduces hum

Postby ioginy » Sun Jan 27, 2013 4:09 pm

Good day. I have just run into a rather odd scenario that I was hoping you fine folks could help me out with.

I just completed a PPP 6L6 amp with a a split fixed bias adjustment. One bias control for the two tubes on one side of the output transformer and the other bias control for the other side of the output transformer. I am getting proper negative voltage on both sides, however I noticed something strange when I was trying to give it a quick ear balance (yes i know that is not the proper way, I just wanted to see if a little tweak would reduce a little hum that was coming through). Let's call either side of the push pull left and right for the sake of my description. I increased the current to the left side and the hum increased as expected, then reduced the current back to it's original place and increased the current on the right side. Here is where I got a little confused. The hum dropped and continued to get quieter as I increased the current. I tried bringing the left side current up to match the right side and the hum increased with it.
Ideally I would like to have no hum, but having the tubes so imbalanced that one side is glowing blue and the other has almost no blue glow at all causes me a little bit of concern.

Any ideas on this would be greatly appreciated.

Cory.
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Re: Odd imbalance reduces hum

Postby Geek » Sun Jan 27, 2013 9:54 pm

Hi,

The reason for this is DC and AC balance are rarely the same.

Some amps, like old Audio Research ones, had AC and DC balance controls.

By using common-mode AC feedback in the phase inverter, the circuit will auto-balance the AC differential dynamically for you.

Cheers!
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Re: Odd imbalance reduces hum

Postby ioginy » Sun Jan 27, 2013 10:02 pm

Is there an example you could show me of what you mean. The power stage is basically a standard fender style. I'm afraid I don't understand about common mode ac balancing.

Thanks
Cory
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Re: Odd imbalance reduces hum

Postby ioginy » Sun Jan 27, 2013 10:57 pm

I was able to do a little research and I think this is what you mean. I have employed negative feedback with this amp. 820 ohm resistor from the output transformer secondary to the long tail phase inverter. It is exactly like the one that Ted Weber uses on this amp.

https://taweber.powweb.com/store/6a80_schem.jpg

Could this be the cause of the imbalance? I have never run into this issue before so I am utterly lost.

---edit---

I also just found this schematic of an audio research power amp with an AC balance control.

http://www.drtube.com/schematics/ar/d40.gif

If I implement this type of AC balance, would that allow me to level out the output tubes current draw and then adjust out the hum using the AC balance?
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Re: Odd imbalance reduces hum

Postby Geek » Sun Jan 27, 2013 11:03 pm

Ah, guitar amp! that's a bit of a game changer.

Pull V6... does the odd imbalance still occur? If so, replace power tubes with a matched quad.

If not, roll 12AT7's until you find one with sections matched enough the hum goes away. R11 and R14 are mis-matched on purpose to provice AC balance, so if they're OK (and connected to the right anodes and not reversed), problem is in a tube, leaky cap or even wire dressing (layout)

Cheers!
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Re: Odd imbalance reduces hum

Postby ioginy » Sun Jan 27, 2013 11:11 pm

AHA! Ok, i am getting what you're saying now. I will double check and make sure the 82K and 100K AC splitter is wired correctly and then try a run of different tubes. I am using two matched sets of tubes (sadly not a quad), one of each pair on either side which I think is the best way to ensure the closest balance between the two sides. I run them through the tube tester though and see if one is bad.

It seems strange though that such a drastic difference in bias would be needed to quiet them down. I was quite careful with my wire dress but I will give it a look and see if anything is out of place. Is there anything you might suggest to keep an eye out for in this regard?

Thanks a heap!
Cory
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Re: Odd imbalance reduces hum

Postby Geek » Sun Jan 27, 2013 11:23 pm

No problem! :))

ioginy wrote:I am using two matched sets of tubes (sadly not a quad), one of each pair on either side which I think is the best way to ensure the closest balance between the two sides. I run them through the tube tester though and see if one is bad.


That's the way I'd have done it.

It seems strange though that such a drastic difference in bias would be needed to quiet them down. I was quite careful with my wire dress but I will give it a look and see if anything is out of place. Is there anything you might suggest to keep an eye out for in this regard?


It's not the bias so much as you've adjusted the transconductance of the one side so the AC gain matched. Best to keep the output tubes at DC balance.

Let's hope your double-check on the resistors and tube rolling works. Otherwise we'll need pics of the underside of the chassis to see if wiring may be the problem.

Cheers!
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Re: Odd imbalance reduces hum

Postby dhuebert » Mon Jan 28, 2013 8:45 pm

Perfect pair matching from thetubestore.com matches the whole transconductance curve of a set of output tubes. In my BFA I buy matched quads of JJ KT88s and the are within a couple of mA of each other all the way from idle to full power. A good first step is to have a matched quad. Also take a look at the way Shannon did the PI in rev B eiclone with the LM334Z, it makes perfectly balanced PI everytime. You have to scroll down pretty far in the eiclone manual to find it.

Don
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Re: Odd imbalance reduces hum

Postby ioginy » Mon Jan 28, 2013 11:56 pm

It appears that everything is in order. R11 and R14 are placed correctly. I am getting 245 vdc on pin 1 and 233 vdc on pin 6 of V6. I also checked the cathodes and am getting 92 vdc on pin 3 and 8. I rolled a couple tubes and there was absolutely no difference in the hum. Increasing current draw on V7 and V8 reduces the hum while doing the same on V9 and V10 increases hum then turns into squealing.

I took some photos of the layout. It isn't the cleanest amp, but I did my best to make sure everything was placed properly.

Image

Here is a closer photo of the output and PI section.

Image

While I was uploading these pictures I noticed that I had done a little miswiring. I had attached C12 to the intersection of R13, R10, R26 and R12. Oops... Should stop working so late into the night. So, I rectified that problem and that solved the mismatched biasing problem. However, as soon as I powered it up there was a deafening squeal. I wired the output transformer exactly as the color designations are in the schematic, but I have a feeling they are backwards none the less. Not sure if the transformer was manufactured wrong or what, but those colors are right. I disconnected the NFB and the squeal went away but the hum has become radically worse.

For the sake of component reference I am going to put the schematic up here to.

Image

Fixed one problem only to REALLY bring out another one.
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Re: Odd imbalance reduces hum

Postby Geek » Tue Jan 29, 2013 3:41 am

I think I see it ;)

Your OPT wires are **way** too long. You'll need parasitic surpression on them.

Cheers!
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Re: Odd imbalance reduces hum

Postby ioginy » Tue Jan 29, 2013 9:13 am

That seems like a simple fix. I did a little research what a parasitic suppressor might be and near as I can tell, it would be a low rates resistor. 47 ohms or something like that. Could you expand on this a bit for me?

Thanks
Cory
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Re: Odd imbalance reduces hum

Postby Geek » Tue Jan 29, 2013 12:54 pm

You can start with say a 47 or 100 ohm resistor in series with each anode to the wires and try.

If this is still being a pain, a little ferrite toroid scavenged from a cord in a printer on the leads might do wonders.

Cheers!
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Re: Odd imbalance reduces hum

Postby ioginy » Tue Jan 29, 2013 3:18 pm

I will give that a try, thanks!

I was just thinking that if the transformer leads were a problem then it would be apparent with no signal from the PI. When the PI tube is pulled the amp is as quiet as a church mouse. Is that correct or is the signal from the PI magnifying the problem, making it audibly noticable?
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Re: Odd imbalance reduces hum

Postby Geek » Tue Jan 29, 2013 8:17 pm

Possibly.

Move the wires around when it's on, does the hum change?
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Re: Odd imbalance reduces hum

Postby ioginy » Tue Jan 29, 2013 9:04 pm

I switched the transformer primaries and fired it up. The hum was still there to the same extent, but when i tested the NFB the hum dropped slightly. All this tells me is that the wires were wrong, but it isn't the problem. I'll run down to my local electro-shop and pick up some suitable resisters to see if that clears up the problem. There is still absolutely no hum when the phase inverter is pulled though.

When said a ferrite toroid. Would a ferrishield do the same thing? I have enough cables around with those on them to choke a horse. If those would do the trick I am sure I can pry a couple off and give it a go.
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