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

Those are beautifully built!
As for the scope, should you need it again, we can help with your setup.
Good catch on the 6BH6. Might find yourself a spare pair.

I printed the service manual. Unique circuit for sure. I noted that the EL34 current is measured in the plate circuit. Was thinking it would be good to clean those switch contacts with DeOxit for good measure.

Enjoy that classic setup. Most of us will never hear one.
 
RonSSNova said:
I noted that the EL34 current is measured in the plate circuit. Was thinking it would be good to clean those switch contacts with DeOxit for good measure.
Click to expand...

I was planning to do that, but I was so pleased to have solved the noise problem that I got excited and skipped doing it.
 
yeah, the heater runs on 60hz AC, the cathode is where the electrons that make the tube do it's thing come from. In this one its also a second input for the feedback. If there is leakage, basically you get 60hz AC connected to an input, and it just amplifies it. In some circuits it doesn't matter, but it does in this one.
 
Pete1729: I had the exact thing happen to one of the triodes in a 6SL7. Hum started pretty suddenly, and it took me a moment to think of swapping it in the other amplifier. Hum moved to that mono block. Replacement cured the hum for me, too, but I was a bit heartbroken that this was a way that a valve could fail, at least in this circuit.

My experience with 6BH6s that I had gotten from eBay and various established vendors was a pretty mixed bag. No H-K leakage, but far too many of them were microphonic, which was a problem when it came to using them in the power supply and vertical amplifier of a Tek 310A. You might want to get a decent number of them so that you will have some quiet ones to use.
 
I had light hum on my AES Superamp, I rotated it out of my system. Since I also have other nice amps to use. . . . two years later I was thinking, it only hums after a couple minute of warmup, light click sound then hum in one channel. I changed the output tubes, no more hum.
 
Change those electrolytics and bumble bees! Even if this makes the amplifier no longer original, you will still have a functioning amplifier that will sound better, last a long time, and will be much easier on the valves. I know that some say that re-forming the electrolytics will work, but it's not worth the gamble when you have expensive to replace transformers (and valves!) on the line. The electrolytics did look like the originals, too.

If you do sell it, you can furnish the original parts to the next owner, who will hopefully put them where they belong - in the rubbish.

(I am writing this with some experience from rebuilding several amplifiers from this era. Any time I left original resistors in place, for example, they would end up becoming noisy. The capacitors had always been the first thing to replace because of their age, many hours of use, and to remedy the residue of probable abuse over the years.)

I'll conclude this long-winded missive by noting that these components are over fifty years old. Even if they test as good, do you want to depend on capacitors and resistors that have been in service this long? Also, if you do rebuild the amplifier correctly, there's a very good chance that you won't have to take it out of service to repair it next week or month.
 
This~~ ^--^

I've replaced many many hundreds of the "Bumble Bee" caps and measured tons of leaky ones.

And it's Far worse for Electrolytics. A temperature probe can be useful but if it shows a Hot cap it should have shorted already.

New capacitor builds have Superior specs in every possible category and amplifiers sound improves across the board.

It takes a very well educated tech to be able to tell if a capacitor is bad without removing it and subjecting it to full rated voltage.

And that's still just a guess.

If you have used the amp for a long time recapping it would be a jaw dropper.

:)
 
The saving grace of those bumblebees in this amp specifically is that if they are leaky, it really doesn't matter all that much. The two big ones by the octal sockets go across a 6.8 ohm cathode resistor. Leakage in parallel with that low resistance isn't going to do much

The smaller ones towards the input jacks are part of the high pass network between the input jack and the test jack. Value shifts would skew that response, but simply using the test jacks will bypass them and make it not really matter. So long as the preamp doesn't leak DC leakage isn't a problem there either.

sonically I don't exactly know what all that would do, nothing helpful I'm sure, but at least they aren't tube destroyers like they are in most locations.

the one I rebuilt got new ones as a matter of principal. One of the output tube coupling caps was leaky, one was just barely leaky and a sloppy replacement so all 4 of those got changed. The ones in the middle were OK so I left it. They are either mylar or a di-film mylar/paper type that doesn't go leaky at least, though they may degrade in other ways. There were a few resistors off value too, I think the cathode resistors on the output tube and the plate resistors on the 6CG7, maybe something else? Forget. The electrolytics had been done by someone before me and weren't old so I let that be too.

All in all it was in pretty darn good shape for its age, a testament to the original parts being very good quality. Also the neat layout in these makes them not difficult to work on. Flip over any German tabletop radio from the 1950s and its way more crowded.
 
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.View attachment 3823656View attachment 3823657
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From what your saying about hum, I can believe its not in the pre amp, is either piece two pronged plugged for power if so shut it down pull plug reverse plug turn on any less hum ? Ive even found on one amp years ago if I spin the RCA plugs without removing them from the input they connected to, I had hum after days of trying everything I found by accident a RCA input to plug if I had spun it that the connection would come clean no more hum..I try the easiest thing first. just my 2.5 % cents increase due to the market fluctuations. LOL
 
pete1729 said:
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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As long as you keep an ear open for noise and your eyes open for tubes glowing too bright, chances are good you'll get a few years of enjoyment out of the amp before you have issues. I'd guess power supply will need to be restored first.
 
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