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Old 9th September 2006, 01:41 AM   #1
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Default Marantz 8b operating voltages

I am updating my Marantz 8b amplifier and have put in some new eletrolytics in the power supply and new output tubes (6CA7EH). I have fired up the amp on the bench and recorded various voltages based on the recomended voltages in the scematic and have posted them here
If you prefer PDF format it is this link.

The voltages at the plates of the 6BH6's and the cathodes of the 6CG7'sa seems too be abnormal. (voltage everywhere are slightly high)

Do I have some leaky capcitor's? and if so, which ones should I check first?

Do you see anything else that raises concerns?

It idles fine on the bench (no red tubes, etc) I will have a signal generator next week to do some more tests...

Thanks for your help,
Jeff
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Old 9th September 2006, 02:44 AM   #2
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Quick look: These voltages seem about right to me. Line voltages are higher than what they were back in the day of the 8b. Everything is scaled up by a few percentage (except for the 6CG7 whose plate voltages are a bit lower due to the 6BH6's plate driving their grids a bit high). Actually these voltages are not too far off.
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Old 11th September 2006, 01:12 AM   #3
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Brian,
Thanks for the vote of confidence... Do you think that it effects the sound quality to run at these voltages? I have a variac that I almost never use and could use it to run at the "specified voltages" if that would make any difference.

Thanks,
Jeff
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Old 11th September 2006, 01:23 AM   #4
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Hi Jeff,
I'm going to agree with Brian. You should be fine. Voltages listed in manuals back then were measured with a VTVM at lower line voltages. Do not expect them to be as close as a modern solid state manual might be.

These have a way of balancing themselves, and there is a lot of leeway in circuit operation.

If you were really upset, you could hook up a filiment transformer to "buck out" 6 VAC or so. Make sure it's in a grounded metal box if you do that.

-Chris
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Old 12th September 2006, 01:27 PM   #5
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I have some images of square wave measurements that I took last night. They were taken at 17.25khz across a 16ohm resistor. The scope is set at 2V/div and 10ms I show two measurements for each channel showing the trim capacitor at it two maximum settings...

What frequency (or multiple frequencies) should I be adjusting at?
should I change the fixed value cap parallel to the trim cap to get a wider adjustment range, or should I leave it where it is at? Sorry if these are obvious questions, I am learing as I go...

The left channel is not as flat across the top as the right channel.

Left Channel


Click the image to open in full size.

Click the image to open in full size.

Right Channel


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Click the image to open in full size.
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Old 12th September 2006, 02:47 PM   #6
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Hi Jeff,
Try 400 Hz or 1 KHz. As a rule, you should never run a high level squarewave into an amplifier above about 2 KHz. You can cause damage to some, certainly most solid state amplifiers.

Your LBO 508A is a decent scope. It will serve you well.

-Chris
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Old 12th September 2006, 03:22 PM   #7
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Ooops...

Thanks anatech, that is what I needed to know. I could hear a very low level of the sound through the tubes. Hopefully I didn't stress it too badly.

I will post the the new measurements when I take them. From what I recall, the trimmer caps did not seem to make much difference in the 400hz to 1khz range while I was playing around with it (didn't take pictues in that frequency range)

Thanks,
Jeff
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Old 12th September 2006, 05:20 PM   #8
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Howdy,

As Brian pointed out, the line voltages at the time of the 8B were in the 105-107 range.

I would be more concerned with the filament voltages...a few percentage points of voltage rise shouldn't effect the operating points much, but it CAN shorten your tube life if your tube filaments are ran hot.

I have a Fisher 800C, and took a cheapo variac and soldered the wiper lead onto the windings at the 105v point...works like a charm. I didn't notice any sonic difference, but it does give me a little piece of mind

-Casey
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Old 12th September 2006, 06:59 PM   #9
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Casey,
I measured a steady 6.41VAC (with fluke DMM). How close to 6.3volts does the heater element like to be?

Thanks,
Jeff
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Old 12th September 2006, 07:22 PM   #10
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Hi Jeff,
You are fine at 6.41 Vac for your heaters. You are 1.7 % over and the tolerance is much wider. 5% at least. Don't lose any sleep over this.

When adjusting those caps, look carefully at the leading edge when adjusting. If your output is not terminated into a resistive dummy load, you will not get the results you want. They specify a 16 ohm resistive load on the 16 ohm tap and a 10 V output (rms) at 1 KHz. That's probably for a sine wave for your voltage measurements. The resistors marked with an asterix need to be in tolerance. They will vary with production run mostly.

Do you have the adjustment procedure? I am missing the one for the trimmer caps (factory adjustment).

-Chris
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