Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on February 09, 2013, 03:24:21 pm
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18 watt, cathode biased amp with one el84 slightly red-plating. The tubes are old NOS Russian round plates and sound great. I'm assuming the tubes aren't matched well. The bias R is 150 ohms. I guess I could go up to a 180 ohm but any other fix?
Could I bias one tube with a 300 ohm and the red plating one with 340 ohm? Use two cathode caps, one for each tube?
al
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If there aren't other problems, you can use two separated resistors and cap
What happen if you swap the tubes ? the tube that previously redplated redplate also now ?
K
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No other problems, other el84's play well no red plating. These round plates sound the best, just one runs a little hot.
I did switched them and the red plating carries with the tube, a little better but still too hot.
Since I usually use 140/150 bias R in my 18 watt (cathode biased, PP amps) shared, if I separate them and without getting into setting up separate bias pots, etc what single bias R would work for each tube? Would I be good to go with say a 280 ohm on the non-red plating tube and maybe 300 ohm on the hot one?
I can always do the math and change the R's, but a starting point.
al
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I think you can start with those values
you can also try to double a circuit like this
(1 pot + 2 resistor + 1 cap for each tube = R20 - R21 - R22 - C8)
as to trim the bias at the level you prefer
K
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I think a new set of tubes may be what you need. I have seen yugoslavian EL84 tubes redplate right out of the box and also not last very long. I swapped them with a new pair of sovteks and no red plating. I dont think you want to use different cathode resistors values like that to cure a replating tube. If you ever sell the amp it may come back on you later. As long as the cat r are good then you shouldnt have any redplating going on anyway unless grid coupling cap or other bias parts are bad
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All other power tubes play good why change an amp for one set of tubes...?
Even thought they're sweet, not worth the consequences. They're burnout eventually and I'll be back to square one.
BTW, I have played mix matched el84s that sounded incredible. Two old stock, good tubes of different makers. Mix and match forget the mods.
al
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I think a new set of tubes may be what you need. I have seen yugoslavian EL84 tubes redplate right out of the box and also not last very long. I swapped them with a new pair of sovteks and no red plating.
That is why you need to check bias each time you replace Output tubes . Those Yougoslavian tube you try need another Cathode resistor for proper Bias .
Russian are not red plating , this doesn't mean bias is OK.
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How long have you been using the redplating tube?
I'm wondering if an issue of wear is leading to it redplating and not the other.
Easiest path is to slightly increase the cathode resistor. While you're at it, add 1Ω resistors between the tube socket cathode pin and the top of the cathode resistor. Then you can know exactly how much current each tube is drawing.
The tube may operate just fine in a fixed-bias circuit, depending on the nature of the cause of redplating.
Cathode bias should be self-limiting, so I wonder if the tube were put on a transconductance tube tester that it might show less Gm than the tube not redplating. If Gm were much lower, it would require more negative bias to control the tube current (more so than a tube with higher Gm), and I wonder if that might yield redplating. If this whole theory were true, it might indicate the tube is worn out.
But there could be other reasons for redplating.
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These Russian, Yugoslavian tubes red plate in all my cathode biased el84 amps not just the one I'd like to use them. All my R are 150. I have three of tubes, bought them new from Gary Gilmore, I'm sure you guys heard of him. He was big on selling tubes once over at 18 watt.com They're nice and full sounding with a tight bass and not too bright. I have three of them sold as NOS, they looked new.
No matter which ones of the three I use, same two always slightly red plate. I guess I could take the two red plating tubes and match them with a 180 R ? Other el84s at 150 play well no problem, it's the tubes.
I haven't played them longer than to start see the glow. My stash is getting low on old el84's and lately the prices on flea-bay are way high. I like the Holland Philups.
I guess I could use the 1 ohm, and check the current but than to change I'd have to change the bias R, I like to see about 11v on el84s.
al
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Dude ,
if tube are redplating
1- Tubes are "short " / defective
or
2- Bias resistor are wrong
This is not a rocket science. Read bias ( cathode current ) with a 1 ohms resitor. You'll see.
All amp need a bais check , ALL include Cathode bias amp or self biasing .
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These Russian, Yugoslavian tubes red plate in all my cathode biased el84 amps not just the one I'd like to use them. All my R are 150.
Then if you'd like to use Russian or Yugo tubes, increase the cathode resistor a bit. There is nothing sacred about a cathode resistor value, especially if the stock value yields redplating.
I guess I could use the 1 ohm, and check the current but than to change I'd have to change the bias R, I like to see about 11v on el84s.
No, you wouldn't need to change the cathode resistor, unless you chose to do it because the tubes were redplating, as above.
You mentioned your present R is 150Ω. 1Ω/150Ω = 0.67% The resistor you're using isn't that tight a tolerance.
If you mean you'd have to change the cathode resistor because now you'd know the individual tube current, that's not exactly correct either. I only recommended adding individual 1Ω resistors so you know what each tube is really doing. Right now, you're assuming each tube is splitting the current through the cathode resistor equally, yet one redplates so you know that can't be true. So the 1Ω resistors only add a way for you to diagnose what's happening rather than guess.
There are other ways to go about this, but those ways are either less convenient (unsolder OT lead from plate and insert d.c. milliammeter) or require specialized test equipment that is not common (clip-on d.c. milliammeter; a.c. version is common).