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RM100 biasing tubes question

 
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supercop9907
RM20


Joined: 12 Jul 2006
Posts: 6
Location: central,ca

PostPosted: Wed Jul 12, 2006 1:14 pm    Post subject: RM100 biasing tubes question Reply with quote

Hey everyone I'm new here and the RM100m is my first tube amp. Checking the bias on the tubes seemed pretty simple and straight forward, but when I plugged my voltmeter in it read that each tube was biased at around -15mA. The manual for my voltmeter said that if there was a hyphen (-) in front of the number it meant that the polarity was reversed. Anybody know what that means? The EL34's should be around 30-35 mA correct? Any reason why they would come biased at 15 mA?

Any help would be appreciated. Thanks
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bduersch
RM100


Joined: 20 Apr 2006
Posts: 454
Location: Cincinnati, OH

PostPosted: Wed Jul 12, 2006 2:15 pm    Post subject: Reply with quote

That just means you've got the red & black probes on the wrong test points... if you reverse the probes, it should read positive.

No harm done, tho, by mixing 'em up.

--B
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supercop9907
RM20


Joined: 12 Jul 2006
Posts: 6
Location: central,ca

PostPosted: Thu Jul 13, 2006 9:13 am    Post subject: Reply with quote

Thanks...my voltmeter has a couple different places to plug the wires into it, so that's where I must have went wrong. I'll try switching em around and see what happens. Thanks again.
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Drew
Donating Member


Joined: 26 Mar 2006
Posts: 135
Location: New Haven,CT

PostPosted: Thu Jul 13, 2006 10:39 am    Post subject: Reply with quote

Some of those places are probably for measuring current (amps and milliamps).

Be careful.

*Updated*
Heres a link that may help

http://mechatronics.mech.northwestern.edu/design_ref/tools/multimeter.html
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supercop9907
RM20


Joined: 12 Jul 2006
Posts: 6
Location: central,ca

PostPosted: Thu Jul 13, 2006 12:23 pm    Post subject: Reply with quote

Thanks Drew...yep, I had the red lead in the "com" hole and the black lead in the "mA" hole which is backwards. Once I get home I'll give it another try.

Any idea what it's a good setting for playing heavy metal would be? I think the EL34's can be biased between 25mA and 35mA.
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Tom
RM20


Joined: 10 Jun 2006
Posts: 18
Location: Wales England

PostPosted: Fri Jul 14, 2006 11:21 am    Post subject: Reply with quote

Just to throw another point in. Anyone know what the manual is on about when it says measure between tube one and two, and it should be between 5ma, higher than 8ma the tubes aren't matched. Im ok with this - but what about tube three and four. Sent Randall an E mail but alas no reply. Whats the best bias figure for 6l6's. Also check out the video clips at the address below.

http://www.randallamplifiers.com/products/amplifiers/mts/index.asp

Cheers
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