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Marantz Model 8b, slight hum in one channel. Where to start looking.

pete1729

pete1729

Super Member
I have just finished building a set of speakers with Altec 604e’s. I figured out what was wrong with my Model 7 preamp. I have the thing all wired up. It sounds glorious.

I have a little hum in one channel.

It is not volume dependent, and does not move when I select 'stereo reverse' on the preamp. What did affect it at one point was turning the 'bias' knob. When I turned it to position A2 there would be a tiny pop from the speaker, and then the hum disappeared. Now when I do it there's a pop, but the hum doesn't go away.

This amp is entirely unmolested. Note the Delco branded output tubes, they carry markings that suggest Mullard Blackburn manufacture.20260822_201544.jpg20260822_201550.jpg
 
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Delco re-branded all sorts of stuff, no idea why they would do this tube specifically but they did sell tubes for the radios they produced.

Check the bias on it, if the two tubes within each channel aren't balanced it will tend to hum. Could be filter cap problems or a lot of other stuff given it's age but thats about the only thing you can do without opening it.

The one I worked on had slightly leaky coupling caps on 2 tubes, off-value resistors on the cathodes of the output tubes, a couple off-value resistors at the phase inverter, and someone prior had replaced the power supply caps so they were still OK. It didn't hum when I got it but I suspect that had more to do with the reasonably fresh power supply caps than anything I did.
 
I had a quad of Delco 6L6GC's and they were fine, Don't remember who made them.

First action is to swap Right and Left tubes to eliminate a vacuum tube.

Next you can pull the preamp tube and see if it goes away, Then pull the 6CG7 driver to isolate location.

Of course there are Lots and Lots of filter capacitors...

[This amp is entirely unmolested] Shivers...

I think the term is Unmaintained.
 
pete1729 said:
I have just finished building a set of speakers with Altec 604e’s. I figured out what was wrong with my Model 7 preamp. I have the thing all wired up. It sounds glorious.

I have a little hum in one channel.

It is not volume dependent, and does not move when I select 'stereo reverse' on the preamp. What did affect it at one point was turning the 'bias' knob. When I turned it to position A2 there would be a tiny pop from the speaker, and then the hum disappeared. Now when I do it there's a pop, but the hum doesn't go away.

This amp is entirely unmolested. Note the Delco branded output tubes, they carry markings that suggest Mullard Blackburn manufacture.
Click to expand...
Remove the bottom plate and show us what 60yrs of not being touched looks like.
 
Buck Rogers said:
I think the term is Unmaintained
Click to expand...

I understand.

I have rebuilt the power supply and replaced the coupling capacitors in the aforementioned Model 7. It had been mildly butchered inside.

The 8b was just so super sanitary inside and out that I didn't want to fix something that wasn't clearly broken.
 
I'm pretty sure we can guess where the hum is coming from...

Since your using high efficiency speakers it only takes a little ripple.

Had to replace a power transformer in an 8b once, I Never want to do that again!
 
may want to try to determine if the hum is 120 hz or 60 hz. If 120 one option would be to measure the AC mv on the power supply caps in the preamp and power amp.
Do you know if these caps are OE or have they been replaced?

May also want to make sure you have a good ground connection on all your RCA jacks.
 
Hum in one channel is not power supply cap related.

OP said turning the bias knob affected it, my question would be, did they set the bias in the first place and/or how is it set now? Also assuming the hum is the same with the Model 7 disconnected?
 
pete1729 said:
I understand.

I have rebuilt the power supply and replaced the coupling capacitors in the aforementioned Model 7. It had been mildly butchered inside.

The 8b was just so super sanitary inside and out that I didn't want to fix something that wasn't clearly broken.
Click to expand...
A dirty BIAS switch is a common issue in the Model 8 and 8b. Very infrequent use and the age of the amplifiers makes a dirty condition inevitable. That said, 60+ year old electrolytics are past their prime, IMHO........
 
I would clean all the pots, switches, connectors, and connections. Deoxit D100L for the switches and connections. Fader Lube for the pots.

Work them in for a good 30 seconds each. Remove the excess.

See if that helps. Good luck! :thumbsup:
 
I mentioned earlier that my amp had some noise in one channel. Well, I have an oscilloscope and have decided to try to use it.

I have dummy loads on the speaker taps. I have the positive and ground leads hooked to the speaker taps. One channel shows me a pretty flat line, the noisy channel shows me a wavy line.

The idea, as far as I understand it, is to check the signal at various points upstream until I find the quiet spot. Any problems, e.g. bad components, will be in the segment just immediately down stream of that test point. Is that basically it?

20260827_115309.jpg20260827_113634.jpg
 
generally yes, but handily with tube amps the "sections" easily unplug for isolation.

If swapping tubes channel to channel did nothing, then its not going to be a tube, but if pulling one shuts up the noise you have an idea what point the noise is upstream of.

may want to put some shorting plugs on the input to eliminate any possible noise caused by open inputs.
 
We can’t tell anything from your scope trace. Does that scope not have readouts for volts/division?
Doesn’t seem set up properly.
 
Well alright then. I started with tube swapping.

First the power tubes. No change.
Next the 6cg7's. No change.
Last the the 6bh6. OK now we're getting somewhere the noise followed the tube.

Oddly enough I had another 6bh6. I dropped it in and the noise is gone off the screen.

I got the amp back in the system, dialed in the bias, and now it sounds just fine. Really great in fact, airy and spacious but authoritative.

I know it's risky, but I'm still leaving it entirely unrestored at this point.
 
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