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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu Apr 19, 2012 3:06 pm
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Arc-n-spark wrote:
I plan to build a set of guitar hangers if the pole is strong enough to handle the load.


Better to attach some 12-inch strips of commercial-grade slatboard across the rear wall of the wardrobe and use professional hangers as they do in guitar shops.

Trust me, you'll sleep better......

8)

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu Apr 19, 2012 6:48 pm
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I got one of those Tascam DR-5 digital recorders slightly used for around $50
A lot of fun and its makes great sounding recordings.
If I can figure it out anyone can :mrgreen:

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu Apr 19, 2012 9:45 pm
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Uh...Not this caveman! Got my grandson to hit the"record" at church, and it recorded just fine. I can transfer the files to this computer, but can't burn them to a cd. It's the Dr40. This is probably a subject for another forum! My Bad, Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Fri Apr 20, 2012 7:06 am
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Retroverbial wrote:
Arc-n-spark wrote:
I plan to build a set of guitar hangers if the pole is strong enough to handle the load.


Better to attach some 12-inch strips of commercial-grade slatboard across the rear wall of the wardrobe and use professional hangers as they do in guitar shops.

Trust me, you'll sleep better......

8)

Arjay


The more I think about it the more I'm inclined to just put my rack in there, it fits, but I don't like the way it looks. If I feel like I need more guitar space I'll make a nice fuzz lined slotted box stand for 'em.

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat Apr 28, 2012 8:41 am
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Hey fellas,

I gutted an old benchtop power supply and found 2, 1 ohm resistors inside that I want to install for measuring the bias in this amp. I was able to reassemble a terminal strip into a bank of 4 test points and a ground for meter leads. The test strip will find a home on the end of the chassis next to the line in transformer. I plan to use 2 of the test points for checking the bias on two tubes (please suggest which 2) and so I was wondering what to use the other 2 test points for if for anything at all. I have no pictures right now of the test strip but will supply if wanted.

I also have to ask about the bias adjust circuit as it is at the moment. It has been modified to be a 56k resistor from ground to one end of the existing 10k bias adjust pot. The other end of the pot goes to cap on the rectifier circuit area and the wiper goes to the junction of 2 68k resistors. Is this OK? I also recently got a cap kit which contains a replacement 50 50 cap for the bias feed but my schematic has it as a 50 70. There is a 100 100 in there right now. Will the new 50 50 be ok in there? 50 50 is what is on the ab763 which is what this blackface cap kit from MoJo supplies.

Thanks in advance for your advice

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 12:24 am
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Arc, not sure about your set-up. But the output tubes are in two seperate sections: Two on one side(say, left) and the other Two(say, right). So if you're reading each side seperately, the reading would be the two tubes combined. This amp actually has the balance pot, but monitoring the tube current is not a bad idea. As for the bias board cap, I'd try to dig up a 50uf-100vdc. A 100-100, will suffice. But there is the time constant, to consider. The 50uf-100 would be my choice. JMHO, Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 1:03 am
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aclempoppi wrote:
As for the bias board cap, I'd try to dig up a 50uf-100vdc. A 100-100, will suffice. But there is the time constant, to consider. The 50uf-100 would be my choice.


+1

And do not over-spec that resistor on the bias-circuit tag board -- part of its function is to protect the output stage in the event of catastrophic failure.

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 7:06 am
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My questions in regard to the bias circuit is because I thought the resistor value on the pot is too high. Now that I have installed the 1 ohm resistors on each pair of tubes and brought the cathode and plate voltages out to the test points I installed, I took measurements and if I used the weber bias calculator correctly then my amp is only seeing 14ma of plate current when I should see 42ma. Could this explain why I can set my volume to 4 and listen comfortably in my basement? So if you could, please look to my last post about the resistor on the bias pot and tell me what value it should be to get my tubes up to the proper bias current.

Thanks

Arc

this pic is before the 1 ohm resistors and test point wires installed

Image

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 7:38 am
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Arc, To be clear do you have the pot wired as a BALANCE or BIAS pot? Let's say the circuit is a standard (not modified) AA769 circuit. That pot seems to be NOMINALLY 18k-ohm (see schemo):

http://www.webphix.com/schematic%20heav ... _schem.pdf

By the color bands, I'm WAGGING your amp = Blue-Green-Orange-Silver = 65k-ohms, 10% tolerance. If so, your negative voltage reading after the diode in the bias supply is prolly WAY negative of the nominal (-37VDC). Meaning your idle bias is WAY TOO COLD. Like you are finding.

If no other part of that bias supply has been altered, you need to get the reading off the tail of that diode to be around (-37VDC). The diode sits right between the bias filter cap (orange Sprague) and the front of the amp, on the separate breadboard for this circuit.

Good luck!


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 9:14 am
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The bias balance has been changed by a previous person to a bias adjust. The bias adjust pot is a 10k pot which is original. There is a 56k resistor on one end of the pot to ground. There is -54 volts at the diode on the bias supply and i see the schematic calls for -37V. The transformer supply to the bias supply is 52VAC and the schematic says it should be 48VAC. I see the AB763 schematic uses -52V bias. Do you think someone erroneously tried to blackface only this part of the amp? Is the supply diode bad? Do I need to do the entire blackface conversion to fix this?

The schematics also say my plates should be at 405V but mine are at 425V.

As always, Thank you very much.

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 9:30 am
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How does the rest of the bias suplly to the output tube wiring look?

Like this:

http://www.webphix.com/schematic%20heav ... _schem.pdf

or this:

http://www.webphix.com/schematic%20heav ... _schem.pdf

In any event, for whatever reason, you have WAY to much negative voltage being applied to the grid from the bias supply circuit.


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 9:39 am
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BMW2002Ti wrote:
How does the rest of the bias suplly to the output tube wiring look?

Like this:

http://www.webphix.com/schematic%20heav ... _schem.pdf

or this:

http://www.webphix.com/schematic%20heav ... _schem.pdf

In any event, for whatever reason, you have WAY to much negative voltage being applied to the grid from the bias supply circuit.


In the first schematic everything but the bias adjust is the same. If you take the bias adjust from the second schematic and change the resistor to the left of the bias adjust pot to a 56k then you have what's in my amp.

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 10:51 am
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IMHO You might as well go ahead and finish the job, Blackface the whole system. All the way back to the dropping resistors on the power supply. (763) Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 3:14 pm
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aclempoppi wrote:
IMHO You might as well go ahead and finish the job, Blackface the whole system. All the way back to the dropping resistors on the power supply. (763) Art


Dropping resistors?

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 3:16 pm
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And yeah... I think I should do the whole shebang. There isn't much left to change. I hope the tone ends up OK.

Arc

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