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Old 11-14-2012, 04:24 PM   #1
loki_lulamen
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Calling amp techs! Bias test point problem.

So after my thread last week on power tubes. I settled on the E34Ls.

However a problem occurred. One side of my bias test prongs are ****ed.

Inside my Marshall JVM410h there are 3 bias test prongs. One for each pair of tubes and a middle common. One side will read DCA and DCV fine, but on the other when I read DCV my multimeter has a spaz out.

Any ideas on the problem or fixes?

I'm pretty good with a soldering iron and not afraid to tinker. So I can fix it myself if a solution is available.

Thanks
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Old 11-14-2012, 04:57 PM   #2
Dick Foster
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Looks like all the schematics are here. Lucky you.
http://forum.metroamp.com/viewtopic.php?f=25&t=21932

You're not over-volting your meter are ya? Just a question because I don't know what meter you're using, which test point you're testing or what the TP is meant to test. Could it be DC decoupled with caps for an AC scope type test or a signal injection point?

It's been awhile since I've messed with tubes. And I didn't bother to download the schematics since I don't own that amp. They would not open up from the link in Acrobat without downloading them first.
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Old 11-14-2012, 08:32 PM   #3
R45VT
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Free bump. I will post back later with some info if no one else does. On my iPhone.
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See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
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Old 11-14-2012, 10:24 PM   #4
loki_lulamen
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I dont think im over volting my meter as it works on one side fine and im using the same setting. As for the AC scope test.... no idea. I was just following what i learned from vids on youtube and info on the web over the years.

(btw this is a pic i quickly found online, ill get a gut shot of mine in the morning)


the three prongs sticking out, just above the marshall logo and trim pots. the center is a common point. the left works fine.

when i touch the right one. my meter dosent like it and i get a "scratchy popping" sound through the speakers.

ps thanks for the bump.
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My Rig:
Maverick F-1, Ibanez RG1527, Schecter Omen 8
Marshall JVM 410H,
Hand built 4x12 w/ V30s
Current board:
PXL LIVE
TU2
WHAMMY IV
MXR M132
MXR M101
TIMMY
NS2
MXR M108
XOTIC EP
TC DREAMSCAPE
DL8
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Old 11-14-2012, 10:27 PM   #5
R45VT
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What settings do you have your meter?

Where do you have your probes hooked up? Please try to go into detail.

Thanks.
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Originally Posted by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
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Old 11-14-2012, 10:46 PM   #6
loki_lulamen
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well ive tried reading in both 200mV DC and 200mA DC settings. Amps works both sides and Volts only on the left.

i dont have bias probes just a standard multimeter with crocodile clips. the black one goes on the center prong and the red on either side. The prongs are located in between the two big blue resistors near the top of the pic. where it says "1 CON1"
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My Rig:
Maverick F-1, Ibanez RG1527, Schecter Omen 8
Marshall JVM 410H,
Hand built 4x12 w/ V30s
Current board:
PXL LIVE
TU2
WHAMMY IV
MXR M132
MXR M101
TIMMY
NS2
MXR M108
XOTIC EP
TC DREAMSCAPE
DL8
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Old 11-14-2012, 11:02 PM   #7
311ZOSOVHJH
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Quote:
Originally Posted by loki_lulamen
the three prongs sticking out, just above the marshall logo and trim pots. the center is a common point.

I don't think those are trim pots. Those are just test points. Where in the hell is the bias trim pot?


Don't poke around too much or at least stick one hand in your pocket.
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Old 11-14-2012, 11:23 PM   #8
loki_lulamen
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the trim pots are the two black things just below the prongs. theres two, one for each pair of valves. genius design if it worked
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My Rig:
Maverick F-1, Ibanez RG1527, Schecter Omen 8
Marshall JVM 410H,
Hand built 4x12 w/ V30s
Current board:
PXL LIVE
TU2
WHAMMY IV
MXR M132
MXR M101
TIMMY
NS2
MXR M108
XOTIC EP
TC DREAMSCAPE
DL8
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Old 11-14-2012, 11:26 PM   #9
311ZOSOVHJH
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sorry, I see the VR2 now
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Old 11-14-2012, 11:55 PM   #10
R45VT
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Quote:
Originally Posted by loki_lulamen
well ive tried reading in both 200mV DC and 200mA DC settings. Amps works both sides and Volts only on the left.

i dont have bias probes just a standard multimeter with crocodile clips. the black one goes on the center prong and the red on either side. The prongs are located in between the two big blue resistors near the top of the pic. where it says "1 CON1"


Why are you on 200mV DC? What other settings do you have?? It should be higher if I remember correctly. I am going to look it up.

Yeah it's lower than a DSL which is what I was thinking. 65-70mV


Should be amp on, master at zero(no guitar plugged in, speaker hooked up), let it warm up and should set around 65-70mV.


Turn up the scale on your meter and see if you get a Voltage reading. It sounds like you have you're meter hooked up properly as long as its onDC.

Are your tubes glowing?

Why are you biasing? New tubes?
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Quote:
Originally Posted by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.

Last edited by R45VT : 11-15-2012 at 12:03 AM.
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Old 11-15-2012, 12:12 AM   #11
loki_lulamen
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65-70mV was what i was aiming for, but with one of the prongs not working i can only bias one pair of tubes.

I bought a set of JJ E34Ls last week and discovered this whilst installing them. I put the old EL34s back in and still the same thing.

i have tried reading on 200mV, 2000mV and 200v settings on my multi meter but still nothing.
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My Rig:
Maverick F-1, Ibanez RG1527, Schecter Omen 8
Marshall JVM 410H,
Hand built 4x12 w/ V30s
Current board:
PXL LIVE
TU2
WHAMMY IV
MXR M132
MXR M101
TIMMY
NS2
MXR M108
XOTIC EP
TC DREAMSCAPE
DL8
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Old 11-15-2012, 01:05 AM   #12
R45VT
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Easy. Buy a current style bias probe. Tube depot has them for sale, 15 bucks.

Your meter would need to be set and hooked up for an in-line current A reading. Are you firmiliar with that??
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Originally Posted by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
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Old 11-15-2012, 02:11 AM   #13
nickdohle
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Man after looking at that picture, my brain hurts. I wonder if my DSL is that crazy inside.
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Old 11-15-2012, 02:25 AM   #14
R45VT
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Quote:
Originally Posted by nickdohle
Man after looking at that picture, my brain hurts. I wonder if my DSL is that crazy inside.


Yes. Find the gut shot threads.
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Originally Posted by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
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Old 11-15-2012, 06:46 AM   #15
Blktiger0
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The DSL and TSL are MUCH worse than that.

Also, I'm not the most knowledgable about this, but I know with my amp, you have to use a Multimeter, most likely because those are more common than bias probes.

Honestly, though, I don't really know the difference

I just figured I would offer another bump.

Really, though, the way my amp works, there is a 1 ohm resistor in the cathodes of the output tube pairs that make it so you can read the current in mV instead of mA. Could it be that there is an issue with this resistor, which is why he can read it in mA, but not mV? Also, due to the location, wouldn't it be those two big blue resistors that are the resistors in question? TS, does the resostor beside the problematic test point look like it's messed up?
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Old 11-15-2012, 10:04 AM   #16
311ZOSOVHJH
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^ just to help clear that up (I'm not a tech)

The probes and meters are usually separate but sometimes they are combined.

The JVM looks to have its own test points so all you need is a meter. Some amps, like mine, do not have test points. You need some kind of meter either way. Some probes have the meters built in and a switch so you can check plate voltage and bias current at the same time.

A separate tube probe is still good to have in situations like this when you are not sure of the readings you are getting.

TS - I'd get a probe like the one R45VT said so you can be sure. Otherwise, I suppose you could swap the tube pairs around to the side you feel comfortable with and get a general average. Should be close enough
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Old 11-15-2012, 04:50 PM   #17
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Quote:
Originally Posted by 311ZOSOVHJH
^ just to help clear that up (I'm not a tech)

The probes and meters are usually separate but sometimes they are combined.

The JVM looks to have its own test points so all you need is a meter. Some amps, like mine, do not have test points. You need some kind of meter either way. Some probes have the meters built in and a switch so you can check plate voltage and bias current at the same time.

A separate tube probe is still good to have in situations like this when you are not sure of the readings you are getting.

TS - I'd get a probe like the one R45VT said so you can be sure. Otherwise, I suppose you could swap the tube pairs around to the side you feel comfortable with and get a general average. Should be close enough


Ah, okay. That makes sense.

I still think it might be that resistor, just because of the circumstances.
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Old 11-15-2012, 08:55 PM   #18
loki_lulamen
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Thanks for the help so far guys.

Ill check out that resistor and see if its busted. I'll let you know what I find.

Also I should definitely get a bias probe as it will save some cash in the long run.
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My Rig:
Maverick F-1, Ibanez RG1527, Schecter Omen 8
Marshall JVM 410H,
Hand built 4x12 w/ V30s
Current board:
PXL LIVE
TU2
WHAMMY IV
MXR M132
MXR M101
TIMMY
NS2
MXR M108
XOTIC EP
TC DREAMSCAPE
DL8
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