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Post subject: 84' Concert Amp
Posted: Wed May 03, 2017 10:10 am
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Roadie
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Just got this in with a "hum" complaint. But, the reason for this posting is what mysteriously happened with the last tech who had it a couple of years ago. The owner of this Concert, honest and all around good guy, brought it to a local tech in his area to be serviced. He had recently put in a new matched set of 6L6's in the amp. When the amp was finished and he went to pick it up the tech told him the 6L6's were bad so he replaced them. When asked for the "bad" tubes the tech told him, "I don't keep bad tubes around, I tossed them". The first thing I noticed when I pulled the chassis is in the 2nd and 3rd pictures. Even I can put 2 and 2 together on this one.

Don't like the thought of pulling the board to straighten this out but I don't see any other option. I'll try the vice in the garage with a couple of wood "levers" first.

UN

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Post subject: Re: 84' Concert Amp
Posted: Wed May 03, 2017 1:15 pm
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Roadie
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The vice trick with the help of a 2x4 worked.

UN

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Post subject: Re: 84' Concert Amp
Posted: Wed May 03, 2017 1:44 pm
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That's what happens when a chassis is *DROPPED*. That the impact also destroyed the customer's previous tubes is not surprising.

While you've got the vice handy, stick that other tech's head in it then crank the jaws closed until it pops like a big pus-filled zit.

:evil:

Arjay

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Post subject: Re: 84' Concert Amp
Posted: Wed May 03, 2017 3:08 pm
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Roadie
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Good one Arjay. But I may have to clean this up upon completion.

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Post subject: Re: 84' Concert Amp
Posted: Wed May 03, 2017 3:43 pm
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Hobbyist
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Back in the 80's we had a band together with my wife's 2 sisters, and would get a lot of help on load in/out!
Transformers crooked, reverb pans with broken wires, even lost a speaker to coil rub once!


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Post subject: Re: 84' Concert Amp
Posted: Wed May 03, 2017 3:59 pm
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I never let anyone touch my gear.

NEVER!

Haven't had a piece damaged or lost yet.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 84' Concert Amp
Posted: Wed May 03, 2017 4:54 pm
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Retroverbial wrote:
I never let anyone touch my gear.

NEVER!

Haven't had a piece damaged or lost yet.

Arjay


+1K! I own an '84 Concert 2x10 that has done 150+ gigs, still looks practically new. Be your own roadie and only trust your gear to tech you've checked out!

Seeing those Illinois caps on the board, tells me everything I need to know about that tech!

T2

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Post subject: Re: 84' Concert Amp
Posted: Wed May 03, 2017 6:48 pm
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Roadie
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He also did this wiring change to the tube matching pot. I'm still trying to figure out why, feeble attempt at adjustable fixed or the only way to get his "unmatched" replacement tubes close? Who knows. Any thoughts?

UN

Update - I typed the wrong resistor value, it's a 51k not a 15k.

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Post subject: Re: 84' Concert Amp
Posted: Wed May 03, 2017 10:32 pm
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That mod appears to be an attempt to alter the circuit to a traditional bias adjustment pot, a la AB763. The original configuration is a balance/matching design.

Arjay

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Post subject: Re: 84' Concert Amp
Posted: Sat May 06, 2017 2:43 am
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Exactly. How centered is the pot, with that dropping resistor?

Thanks!


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Post subject: Re: 84' Concert Amp
Posted: Sat May 06, 2017 2:58 am
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BMW2002Ti wrote:
How centered is the pot, with that dropping resistor?


+1

That would determine how closely matched the output bottles should be.

Arjay

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Post subject: Re: 84' Concert Amp
Posted: Sat May 06, 2017 12:48 pm
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Roadie
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Here's what I tried. I put everything back to the stock schematic which seems cold to me.

Plate Voltage 475v
Plate Current 23mA
Grid Voltage - 51v

Then I tried the following change and bumped up the pc.

Plate Voltage 475v (Can't remember exactly but it was close to the same, give or take a couple of volts)
Plate current 36mA
Grid Voltage - 52.5

I know I need to watch the grid voltage and this will only work with a set that's matched closely.
UN

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Post subject: Re: 84' Concert Amp
Posted: Sat May 06, 2017 4:26 pm
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How does it sound now?

Arjay

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Post subject: Re: 84' Concert Amp
Posted: Sat May 06, 2017 5:02 pm
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Roadie
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Sounds better than it did already. I was just a little concerned about the grid voltage. I want to replace the electro's with some better ones, check some resistors and clean up the pots next.

UN


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Post subject: Re: 84' Concert Amp
Posted: Sat May 06, 2017 5:45 pm
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-52.5 VDC on the signal grids seems fine. Measure the VDC just past the second 70mfd/100VDC cap in the bias circuit (from the bias circuit diode). It should be around -50 VDC (+/- 20%), according to schematic.


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