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Do I Need To Replace These Tubes?

dstroup

dstroup

Member
image.jpeg A couple months ago I purchased a 1962 Magnavox Danish Classic 1-ST621 with a 9300 series amp (9304). I have been slowly learning about vintage tube audio ever since (fun!). I recently decided to clean the tubes / tube pins / tube sockets on this console and noticed some of the tubes don't look so good. My questions:

1. Do I need to replace these tubes?
2. Is this what "hot plating" looks like?
3. Could this be the (or partially) reason why this console has a "hum" that does not change when the volume is adjusted?

Any advice / information / suggestions would be appreciated. Thanks!

image.jpeg
 
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Those should be fine, despite the stains. Those are "smoky" Mullards and are sought after tubes.
Consider them a bonus.
 
The white haze on the Magnavox rectifier tube indicates a tube that has been run hard and long. I'd think twice about that one. The 'smoky' tube, not so much.
 
I've heard about the 'smoky' Mullard's being perfectly good and I believe it. Anyone know what makes them 'smoky' like that?
 
Oh, noooOOOoos ... only five more years? Imminent doom!! <G>

I'd think about picking up another set anyway. Test your theory on the hum issue, AND have a good set of spares if nothing else. Always nice to have a backup plan.

And ya ... if the amp's not been touched, the clock is ticking on those caps. Had that on my Eico a bit back and traced it back to a weak can cap. Replaced that and quiet as a mouse now.

BTW ... anyone have a good explanation for the smoke rings? I assUme it's something to do with the materials used and how they gas during break in ... maybe even the initial flash?
 
The smoky tubes are from all the cheap British cigarette 2nd hand smoke. :confused: Fields and Meadows was a particularly nasty brand.

The 2nd tube is a Westinghouse tube.
 
Its pretty hard to judge tubes on looks. I've had some that looked roasted but worked fine, and others that looked brand new but were totally useless. I have a couple of Mullards that look like the first pic, they're fine.
 
SMOKING MALLARD

MichaelCullen-Benson--SmokingDuckProductionsPoster-2012-Small.png


SMOKED MALLARD

roast-duck.jpg


Oh! Mullard!
(Never mind) <G>

What's interesting is that a lot of the exotic metals that might have done that sort of thing are either no longer available or have been outlawed for production due to environmental concerns. Also doesn't help that the Chinese cornered the market on said metals some time back and aren't sharing ...
 
I thought getters were just magnesium...do plates or filaments actually use rare earths? I have a couple radioactive Thorium doped tubes but there could be others.

China does have a lot of the world's production of rare earths, but it turns out they are all over the planet, just in tiny amounts. It's the effort and cost of production that makes the expense. China became a leading producer over a decade ago and produces 90% of the world's supplies of the top 5 rare earths. They also consume 72% of the world's production making stuff. Stuff that we buy. Mostly in high tech electronics, and magnets. To say that they're 'not sharing' is not quite accurate. Not that I am unconcerned with their domination in the production of strategic materials, just trying to clarify the situation.

I don't know of any metals whose use has been entirely outlawed, but since we actually have environmental regulations here, and China ignores a lot of that, things can be more expensive to produce. I think that's what is driving our production or lack thereof. Not that we don't have supplies. If it came down to it, we could get it done.

Sorry for the tangent.
 
toxcrusadr said:
I thought getters were just magnesium...do plates or filaments actually use rare earths?
Click to expand...

Getters were often a barium alloy. The cathode coatings are usually where the rarest chemistry is deposited, literally.

I'd test those tubes -- and then plug them in and listen.

Unless the envelope is breached, looks are rarely a reliable guide to condition. Especially old Mullards.

Like someone else said, they could have years left in them.
 
Thanks everyone for your advice and opinions! Very helpful.

I finished cleaning all the contacts / pins / socket holes on the 9304 amp and figured I would try her out before moving on to clean the tuner, multiplexer, and other electrical contact points. I was hoping to notice some improvement in the hum i mentioned in my original post, but didn't expect much.

To my surprise, the cleaning markedly reduced the hum.:)

I'm definetly going to continue with my cleaning plan!
 
Good deal on the hum, but ...

I see no mention of re-tensioning the sockets? Very important as it doesn't take much to spread them lil fingers and lose good contact. A small flat jeweler's screwdriver works well for that - work the blade between the socket body and finger, and give a slight twist. Doesn't take much. I also work the tube straight up and down slightly a few times to polish up the actual contact areas. No rocking allowed!

Standard advice is to "keep a hand in your pocket" while working with tube amps. Always some juice waiting to catch you unawares ... I use a ceramic blade screwdriver myself.

Be interesting to hear if that improves things even more.
 
I am such a newbie at this stuff! So much good information here though! Thanks for your input sKiZo!

Okay, sKiZo: I've never heard of re-tensioning tube sockets, but I can say that a couple of the tubes do seem loose in their sockets to me. I'll try to tighten them up as you suggest. I'v also never heard of ceramic blade screwdrivers, but I'll have to check those out. Finally, I thought it was okay to rock the tubes a little to take them out? Sound like rocking is a bigger risk / no-no than I thought.

I have a couple questions:

1. Why is the standard advice to "keep a hand in your pocket" while working with tube amps? What does that mean and how can it keep you safe / safer from getting zapped with electricity? Is it because if you touch something "hot" with electricity and you don't have your other hand on something metal ('cause it's in your pocket) you don't get zapped with an electrical shock?

2. I've been shutting down my stereo console, unplugging it, and then turning it back on (while unplugged) and letting it sit overnight before I go messing around inside it. I figure any extra electricity would surely have discharged by the time I start poking around the next day. Is my thinking on this correct or am I fooling myself and there's probably electricity still stored in there somewhere and I'm still at risk of getting shocked?
 
The 'hand in your pocket' thing is so that you don't complete a path to ground. You can stick your finger on something that is live, but if you are not contacting the ground with any part of your body it won't really hurt you. Might sting but its not going to fry your heart. On the other hand if your left hand is holding the chassis and the right finger pokes something, the electricity will flow straight across your heart and it might lock it up tight. Doesn't take all that much current to kill you, and its definitely available inside a tube amp. If its unplugged and discharged you're OK, but be extremely careful when messing with any live equipment. I think the pocket idea specifically is so that you know exactly where the other hand is. I've heard it expressed occasionally involving a thumb and a bodily orifice for the same reasons. I'll leave that one to your imagination.


Turning it on while unplugged won't really do anything. Generally tube amps will discharge in 10 minutes or so, but you can always poke the voltmeter in there on the filter caps to confirm. Honestly that isn't a bad idea to do anyway, always be sure of what you're messing with before you put your hands inside.
 
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