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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Oct 18, 2013 5:58 am
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" I also agree that the two prong adapter (lifting the ground) is a big no-no, unless used properly. The way to properly use the two-prong adapter is to connect the little lug sticking out to the center screw in the outlet, which maintains the ground circuit. The only reason to have a ground is for safety. Lift the ground, lift the safety, at your own risk. "

I agree 100%. Do not permanently use a "cheater plug" on any device. Esp one that can draw a lot of current. I use it strictly as a diagnostic tool to rule-out ground loops in house wiring versus ground-loops in amps.

Once you've discovered that the hum or feedback issue is decreased by disconnecting the chassis ground from the outlet's ground, you'll need to investigate both the ground potential in the input section of the amp & the ground potential in your house wiring and outlets.

RFI induced feedback may require a bit more shielding of the pup & control wirings on the guitar and perhaps going to shielded cabling on all input wires of the amp. I've found that most RFI induced issues occur in the very low voltage areas. Like guitars and initial input section of the amp. Areas where mV of noise can be a big percentage of the signal's strength.

Good luck! Keep us informed.


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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Oct 18, 2013 7:34 am
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As I noted earlier, my three high power amps are plugged into a Furman power conditioner. I need to check the specs when I get home tonight but if memory serves, the entire unit is isolated from the house power system and ground. Also, the two banks of outlets are independently filtered. I should think this would address any ground or noise issues between the amp and the house supply.

On the input side, the amp was built using shielded cable between the input jack and the grid of V1 (along with a 68K grid stopper connected directly to V1) as well as a shielded cable between the volume control and the grid of V2 (with a 1K grid stopper connected directly to V2).

It made no difference in feedback whether I plugged the guitar directly into the amp for through the active/isolated/lifted Twin City ABY switch.

The feedback occurred regardless of whether I was using my humbucker equipped guitar (with no shielding of the control cavity, but which I plan to shield very soon) or my SCN equipped Strat (which is very well shielded). Contrary to what some "experts" think, I do know the difference between uncontrolled/undesireable feedback and desired/controllable feedback. The former was the condition I was experiencing last week. The latter is the current operating condition of the amp.

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Oct 18, 2013 2:11 pm
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A common misconception about isolation devices (for three-prong plugs) is that the ground itself is isolated, which is generally not true. The ground is usually maintained the same as if the amp was plugged directly to the outlet. The isolation is usually that the neutral is isolated, meaning that a transformer or other device is used so that the neutral feeding the amp is no longer connected back to ground in the breaker box. A simple isolation transformer is a typical indication of an isolating system, and the ground itself goes straight through to maintain a solid ground connection, while the neutral of the secondary of the transformer is not connected to the primary of the transformer, thus isolating the neutral from any connection to ground, so the secondary no longer has a neutral.

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Oct 18, 2013 2:58 pm
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shimmilou wrote:
A common misconception about isolation devices (for three-prong plugs) is that the ground itself is isolated, which is generally not true. The ground is usually maintained the same as if the amp was plugged directly to the outlet. The isolation is usually that the neutral is isolated, meaning that a transformer or other device is used so that the neutral feeding the amp is no longer connected back to ground in the breaker box. A simple isolation transformer is a typical indication of an isolating system, and the ground itself goes straight through to maintain a solid ground connection, while the neutral of the secondary of the transformer is not connected to the primary of the transformer, thus isolating the neutral from any connection to ground, so the secondary no longer has a neutral.


You're probably right. Here is the Furman conditioner I have:

http://www.furmansound.com/product.php?div=01&id=PL-8C

They talk about "isolated banks" but I can't find anything on their website that defines what that really means.

I checked the house ground as much as I can and everything looks tight. The ground rod wobbles a little at the very top where it pokes out of the ground (edit: I realized after I wrote this that there is about 3" of mulch around the top of the rod which accounts for the wobble.), but I cannot pull it out, nor can I pound it in any deeper (with a 4 lb sledge). I used the little outlet tester to check the outlet that the Furman is plugged into and it checks out OK with all connections correct.

I do think the neck pickup on one of my humbucker equipped guitars has become somewhat microphonic (you can really hear it through the amp compared to the bridge), but that doesn't explain why other guitars fed back so badly also.

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Last edited by bluesky636 on Fri Oct 18, 2013 9:19 pm, edited 1 time in total.

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Oct 18, 2013 3:08 pm
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shimmilou, I sent you a PM.

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Oct 18, 2013 5:20 pm
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Got it. :wink:

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Oct 18, 2013 5:33 pm
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Bluesky, try the amp without the Furman. Conditioners have been noted to have ground loop issues. Rare, but it does happen. Really, anything between the amp and the mains is a possible candidate.

The uncontrolled feedback is definitely pointing to an induced EMF or RFI "pseudo-circuit" within the gain stages of the amp.. Most probably between the guitar/amp and associated ground points. That's my experience. Plus, a very slightly microphonic gain stage tube. Which is induced into sympathetic motion by this loop.

The fact that the strings are vibrating on their own during this process may indicate a feedback loop going back to the guitar from the input stage. Causing resonance in the pups which transmit this resonance to the strings. As you prolly already know, AC signal can ride on top of DC --- backwards in a back flow manner to yield positive feedback.


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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Oct 18, 2013 9:23 pm
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BMW2002Ti wrote:
Bluesky, try the amp without the Furman. Conditioners have been noted to have ground loop issues. Rare, but it does happen. Really, anything between the amp and the mains is a possible candidate.

The uncontrolled feedback is definitely pointing to an induced EMF or RFI "pseudo-circuit" within the gain stages of the amp.. Most probably between the guitar/amp and associated ground points. That's my experience. Plus, a very slightly microphonic gain stage tube. Which is induced into sympathetic motion by this loop.

The fact that the strings are vibrating on their own during this process may indicate a feedback loop going back to the guitar from the input stage. Causing resonance in the pups which transmit this resonance to the strings. As you prolly already know, AC signal can ride on top of DC --- backwards in a back flow manner to yield positive feedback.


BMW,

Thanks for all the suggestions. As of right now, there is really nothing I can do as the amp is back to behaving the way it did before the freak out of last week. I may have to wait for another couple of days of heavy rain to see if the problem reoccurs. If it does, I'll reopen my investigations.

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Thu Dec 19, 2013 10:04 pm
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Was having a problem with popping when switching the bright switch from off to either one of the bright caps (100 pF and 250 pF). Here is a photo of an earlier version of the switch (This is with a Radio Shack center off DPDT. The current switch uses a better quality switch and caps.).

Image

The two center lugs connect across the input and output of the volume control. I thought that maybe tieing the output lugs together as shown in the revised schematic would eliminate the pop. It reduced the pop slightly but not entirely. Oh, well.

Image

Also, here is a photo of the chassis after all the lead dress and parts were finalized. You can also see where I applied (non-acidic) RTV to wires and caps to hold things in place and prevent vibrations.

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Thu Dec 19, 2013 11:39 pm
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Bill, have you tried a cap across the contacts? Like a 0.01mfd/1KV ceramic disc cap?


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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Dec 20, 2013 2:31 am
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BMW2002Ti wrote:
Bill, have you tried a cap across the contacts? Like a 0.01mfd/1KV ceramic disc cap?


Seems to me that would defeat the purpose of the bright cap, wouldn't it?

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Dec 20, 2013 11:43 am
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Bill, I had to go back to your schemo. Since you are switching a 100 pico-farad cap in-&-out of the circuit, you need a much smaller mfd cap, than 0.01mfd. Maybe 1/10th of the 100 pico-farad value? The idea is to have the signal take another path when the switch is near closed --- than arcing between the contacts (which prolly is causing the noise).

Just a thought...


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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Dec 20, 2013 4:35 pm
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BMW2002Ti wrote:
Bill, I had to go back to your schemo. Since you are switching a 100 pico-farad cap in-&-out of the circuit, you need a much smaller mfd cap, than 0.01mfd. Maybe 1/10th of the 100 pico-farad value? The idea is to have the signal take another path when the switch is near closed --- than arcing between the contacts (which prolly is causing the noise).

Just a thought...


I guess I'm just not seeing the value of permanently bypassing the very highest frequencies around the volume control like you suggest. I have never seen another bright switch implemented that way.

The Express first gain stage/tone stack/volume control is essentially the same as a Blackface Fender. Compare my schematic with that of a Blackface Twin Reverb. The only difference between the two (besides component values) is the fact that Fender switches the output of the bright cap while the Express switches the input of the bright cap. Could that make a difference in the noise?

http://support.fender.com/schematics/gu ... ematic.pdf

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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Dec 20, 2013 4:48 pm
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Changing the position of the switch wrt tone shaping cap may make a difference. Guess you need to experiment.

As for the snubber cap, it must be beyond hearing frequencies. You are not going to use it as a filter (part of tone shaping) --- but as a path that the electrons can take to minimize in-rush and possible arcing across the switch's contact.

Similar idea to snubber caps across noisy diodes.

Just a thought, anyhow. Esp if nothing else helps the switching noise.


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Post subject: Re: New Amp Build - Trainwreck Express Clone
Posted: Fri Dec 20, 2013 5:42 pm
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Ok. I think I get it. The pop is essentially an impulse noise which contains an infinite number of frequencies. A very small cap would trim the very highest of those frequencies but still leave the lower ones. So it seems reasonable to expect that the pop would be "softened" somewhat, but still be present to some extent. I'll have to give it some more thought. Thanks.

My theory for having the switch on the input side of the cap is that any charge that accumulates in the cap while it is in circuit would bleed off to the output side of the circuit when it is disconnected. That's my theory anyway. Can't prove it, though. :lol:

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