DC-3 red plating at gig last night

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dodger916
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DC-3 red plating at gig last night

Post by dodger916 » Sun Mar 30, 2014 10:45 am

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!

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Re: DC-3 red plating at gig last night

Post by bendo » Sun Mar 30, 2014 11:17 pm

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.
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Re: DC-3 red plating at gig last night

Post by dodger916 » Mon Mar 31, 2014 8:41 am

bendo wrote: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.

Mk IIA->C+ DRGX head "Cold Sweat" in GTS shell
1966 Fender Pro Reverb
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Re: DC-3 red plating at gig last night

Post by bendo » Mon Mar 31, 2014 10:45 am

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.
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Re: DC-3 red plating at gig last night

Post by dodger916 » Mon Mar 31, 2014 1:32 pm

Will do. Thx again!

Mk IIA->C+ DRGX head "Cold Sweat" in GTS shell
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Re: DC-3 red plating at gig last night

Post by BlackBoxy » Thu Apr 03, 2014 7:12 am

get the rid of JJ tubes :wink:

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Re: DC-3 red plating at gig last night

Post by dodger916 » Thu Apr 03, 2014 10:47 pm

BlackBoxy wrote:get the rid of JJ tubes :wink:
and replace with....?

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Re: DC-3 red plating at gig last night

Post by bendo » Fri Apr 04, 2014 12:13 am

BlackBoxy wrote: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.
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Re: DC-3 red plating at gig last night

Post by BlackBoxy » Fri Apr 04, 2014 4:38 am

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!

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Re: DC-3 red plating at gig last night

Post by dodger916 » Fri Apr 04, 2014 7:45 am

BlackBoxy wrote: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.

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Re: DC-3 red plating at gig last night

Post by BlackBoxy » Sun Apr 06, 2014 5:30 am

i put a 803s to V3 on dirt channel into my dc5 sometime , i don't know about dc3 V settings!

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Re: DC-3 red plating at gig last night

Post by soundchaser59 » Sun Apr 06, 2014 10:56 am

bendo wrote: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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dodger916
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Re: DC-3 red plating at gig last night

Post by dodger916 » Mon Apr 07, 2014 1:35 pm

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 wrote:
bendo wrote: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.

Mk IIA->C+ DRGX head "Cold Sweat" in GTS shell
1966 Fender Pro Reverb
DC-3 head
Orange Dual Terror
EVM loaded Thiele; MS-12 1/2 back
71 SG; 72 Tele; 76 White Strat; 77 LP Custom; 81 Goldtop; 05 Am Dlx Strat; 11 G0 plaintop

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soundchaser59
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Re: DC-3 red plating at gig last night

Post by soundchaser59 » Wed Apr 09, 2014 1:25 am

The Audio Monk did the bias adjust mod install on mine.

http://www.audiomonknyc.com/

Anthony is the one answering my emails.
Rivera R55 and K55, Frenzel 5E3-40, MIA Strat, Reverend 6 Gun, Weber, Reaper.

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