DC-3 red plating at gig last night

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dodger916

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I noticed the two right hand EL84s (viewed standing behind the amp) were red plating last night. I installed JJs from Eurotubes, and though I requested 20's, they sent me tubes with a "32" rating. I called, and they said those were right for the amp. I find it a bit odd that only the two right-hand tubes were redplating. Is this indicative of something else?

So I want to do the bias mod. I picked up a 100k 1/2 watt resistor, and I've seen pics of which resistor to replace.

Two questions:
1) Is 1/2 watt correct ?
2) Does anyone have a photo or instructions of the adjustable bias mod (76k resistor and 50k bias pot)? I've read the posts, but I'm a "visual" learner and like to see pics.

Thanks again!
 
They sent you WAY to hot of a tube. I cannot run anything higher than a 21 without redplating.

1/2 watt 100K resistor is right, but, I do not think it will resolve your red plating. 32 is too hot for a DC.
 
bendo said:
They sent you WAY to hot of a tube. I cannot run anything higher than a 21 without redplating.

1/2 watt 100K resistor is right, but, I do not think it will resolve your red plating. 32 is too hot for a DC.

Thanks, Bendo. That's what I told the Eurotubes salesguy after I received the tubes. He said they'd be fine in that amp. In my comments on the on-line order, I mentioned I wanted around a 20 rating as it was for a DC-3 with 400 plate volts and -11 bias.

Because only the two right most tubes re-plated, I was thinking maybe the phase inverter is too unbalanced and needs to be replaced, or am I smoking crack here? It's a Mesa Russian-1 12AX7a. For the sake of experimentation, I'm thinking of installing a Mullard 12AT7/CV4024 with balanced triodes. I run this in a couple of amps (including my C+) with good results. It's odd that there was no red plating in the week after installing the tubes.

I have the 100k 1/2 resistor that I will install today, but I'd really like to do the bias pot mod. I wish I could get a photo of the mod.
 
I would try a standard JJ 12AX7 in P/I spot. I have been using those for a few years now with no issues.

In either case, the chances are your tubes are already toast. I have noticed EL84s don't last long after they have been red-plated. Try Doug's Tubes. He will send you a set with the rating you are looking for. I have used him quite a bit in the past.
 
BlackBoxy said:
get the rid of JJ tubes :wink:

There is nothing wrong with JJs. They sound as close to a classic EL84 and with the right rating, last plenty long.

If you are going to boldly bash on something, at least provide a solid explanation.
 
JJ aren't Tesla sweet tone at all, each time you'll compare it to other "reissue" exepted el84, you'll find it dull and dullier ..
thier el 84 and 803s are ok . but there are better el84

the only one i would put as follower would the 803s in the pre ..

only my opinion, and i tested Lots of tubes into different amps.

no bashing at all, get the rid means change the JJ if they are dead , bad transport maybe!
 
BlackBoxy said:
JJ aren't Tesla sweet tone at all, each time you'll compare it to other "reissue" exepted el84, you'll find it dull and dullier ..
thier el 84 and 803s are ok . but there are better el84

the only one i would put as follower would the 803s in the pre ..

only my opinion, and i tested Lots of tubes into different amps.

no bashing at all, get the rid means change the JJ if they are dead , bad transport maybe!

I assume you mean cathode follower. If so, which position is the follower? I was not aware there were any cathode followers positions in the DC-3, and I agree a sturdy tube is needed there.
 
i put a 803s to V3 on dirt channel into my dc5 sometime , i don't know about dc3 V settings!
 
bendo said:
1/2 watt 100K resistor is right, but, I do not think it will resolve your red plating. 32 is too hot for a DC.
You are correct, changing this resistor will not have any effect on the red plating. I tried and all of my brand new Sovtek EL84M tubes still red plated.

The adjustable bias mod is apparently more of a pain in the butt than you might think. To get to the correct component to replace with the pot, it looks like you have to remove the circuit board, and I don't think you can do that without removing all of the jacks and pots, etc. My DC3 has the adjustable bias mod installed, with test points. The test points have a 1 ohm 3watt resistor to ground, and I can measure mV while I turn the pot. Each red test point is for a pair of tubes.

But I'm not sure if the reading I get is a "face value" reading, or if I'm supposed to cut that reading in half to decide what each individual tube is getting. If I read 50mV on a test point, does that mean each tube is getting 25mA? Or is each tube getting 50mA? I emailed the installer, and he said the reading is for "one pair of tubes." Still not crystal clear enough for me. If I set it to 50mv, thinking each tube is getting half that in mA, and I'm wrong, then I just fried a set of tubes for nothing. This is exactly what I did, thinking it meant each tube was getting 25mA, I played for a couple hours this way. Power down, come back the next day, and now no sound and no bias voltage.

I was going to photograph the mod, but the wires go under the board where I cannot see without lifting the board. All I can see is the 1 ohm R's across the test points, next to where the pot is mounted on the rear panel. I can point you to the guy who did the mod on mine if you like.
 
Hi Soundchaser:

1) I'd like the contact info for the bias mod guy;

2) Do you know the rating or idle current draw fo the redplating tubes?

Thanks.


soundchaser59 said:
bendo said:
1/2 watt 100K resistor is right, but, I do not think it will resolve your red plating. 32 is too hot for a DC.
You are correct, changing this resistor will not have any effect on the red plating. I tried and all of my brand new Sovtek EL84M tubes still red plated.

The adjustable bias mod is apparently more of a pain in the butt than you might think. To get to the correct component to replace with the pot, it looks like you have to remove the circuit board, and I don't think you can do that without removing all of the jacks and pots, etc. My DC3 has the adjustable bias mod installed, with test points. The test points have a 1 ohm 3watt resistor to ground, and I can measure mV while I turn the pot. Each red test point is for a pair of tubes.

But I'm not sure if the reading I get is a "face value" reading, or if I'm supposed to cut that reading in half to decide what each individual tube is getting. If I read 50mV on a test point, does that mean each tube is getting 25mA? Or is each tube getting 50mA? I emailed the installer, and he said the reading is for "one pair of tubes." Still not crystal clear enough for me. If I set it to 50mv, thinking each tube is getting half that in mA, and I'm wrong, then I just fried a set of tubes for nothing. This is exactly what I did, thinking it meant each tube was getting 25mA, I played for a couple hours this way. Power down, come back the next day, and now no sound and no bias voltage.

I was going to photograph the mod, but the wires go under the board where I cannot see without lifting the board. All I can see is the 1 ohm R's across the test points, next to where the pot is mounted on the rear panel. I can point you to the guy who did the mod on mine if you like.
 

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