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
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
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.
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
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
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
Right Channel
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
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Right Channel
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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
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
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
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
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
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
Hi Casey,
Tube heaters were designed for a much wider range. Higher or lower than that range doesn't do a tube any good and will shorten it's life. Within 5% you will be just fine either way.
I would not touch a thing there. Older line specs were closer to 115 Vac than 107 Vac. Besides, the operating voltages should have been measured while the amplifier was under load. Your voltages will be a little higher idleing.
Don't worry, be happy ......
-Chris
Tube heaters were designed for a much wider range. Higher or lower than that range doesn't do a tube any good and will shorten it's life. Within 5% you will be just fine either way.
I would not touch a thing there. Older line specs were closer to 115 Vac than 107 Vac. Besides, the operating voltages should have been measured while the amplifier was under load. Your voltages will be a little higher idleing.
Don't worry, be happy ......
-Chris
Chris,
I do not have a factory adjustment proceedure with respect to the trimmer caps (and the resistors marked with a *). I will double check the resistor values just to be sure they are in spec.
I had made the assumption that they were used to fine tune the square wave response and not some other stability issue. I have not seen any stability issues on the bench.
I measured the trimmer caps out of circuit (with LCR meter) before I adjusted them in case I needed to set them back. I believe one measured 340pf and the other 440pf, I would need to check my notes.
I do not have a factory adjustment proceedure with respect to the trimmer caps (and the resistors marked with a *). I will double check the resistor values just to be sure they are in spec.
I had made the assumption that they were used to fine tune the square wave response and not some other stability issue. I have not seen any stability issues on the bench.
I measured the trimmer caps out of circuit (with LCR meter) before I adjusted them in case I needed to set them back. I believe one measured 340pf and the other 440pf, I would need to check my notes.
Yup, Those square waves at 17KHz look pretty good to me (many tube amps and some solid state ones look much worse). Sure they could be tweaked to perfection, but you're close. Your circuit voltages are in range. With all due respect, may I suggest that you quit worrying and connect a pair of speakers, pour some wine, and play your favorite discs?
Hey Brian,
Too bad we aren't neighbours. BBQ and wine (or beer) listening to some good tunes.
Jeff,
In the 60's, the test gear wasn't near as good as it is today. They still managed to build and maintain some great amps without worrying too much about a few percent in voltages. As Brian said - you're close enough and many amps don't do as well.
Now tell us what it sounds like.
-Chris
Too bad we aren't neighbours. BBQ and wine (or beer) listening to some good tunes.
Jeff,
In the 60's, the test gear wasn't near as good as it is today. They still managed to build and maintain some great amps without worrying too much about a few percent in voltages. As Brian said - you're close enough and many amps don't do as well.
Now tell us what it sounds like.
-Chris
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