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Amp bias 900

Hi This amp had 3 of the output valves damaged. A friend of mine brought 4 of these new valves and asked me to replace them. I have downloaded the schematic, but unsure how to set the bias for this amp. On the schematic, it is noted to set the bias between V to V between R27 and R26, but how do I set the bias without any trimmer pot to adjust bias. Please assist. Thanks Carlos.


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WATCH RELATED VIDEO: Marshall JCM 2000 - TSL 100 Thermal Bias Problem

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JCM Bias Problem. Thread starter Strongrad Start date Jul 19, Strongrad Member. Messages 8. Hello everyone, first post so let me know if I'm doing anything wrong here. I wanted to replace the power tubes since I have no clue when they were replaced last.

I have no way of testing these but the ebayer said he only used them 1 time. I'm using eurotubes bias-probe with my Fluke and got some odd voltages and mA.

Started at mA so I immediately cranked it down to bottom level, being 54mA. From what I've read, these normally set in the area so I'm a little confused! I didn't check wall voltage but let's just consider that ruled out for now and I'll check when I get home. Also worth mentioning, I made an oopsy and connected the bias probe while in vDC mode and heard a pop. No voltage changes after this though. Keep in mind this is also my first time opening an amp, and Biasing but I am an electrician with plenty of experience in high voltage.

Sorry for the long post, any help would be appreciated! Thanks, Nick. Messages 8, You are keeping the amp properly loaded with a cab whilst doing this? You may have noticed some weird audio effects. Last edited: Jul 19, Steppin' Wolfe Silver Supporting Member. Messages 5, That aside That initial current draw was high in both tubes The tubes may not be 'matched'. High current draw will pull the voltage down AS current draw increases, plate voltage goes down.

As current draw decreases, plate voltage goes up. Way hot for fixed bias So, you need the current draw and plate voltage for each of those power tubes. IF you adjust the bias, you have to measure each of those things in each tube again to know where things are. I don't understand. If you have the probe why are you pinning it. I have the same amp and probe. Never had and issues Are your tubes a matched set. I'm around 38ma off the probe reading A little cold but that's how it sounds best to me.

Btw great head and too bad it does not get good reviews around here. Steppin' Wolfe said:. UPDATE: Last night I threw in the old tubes, and then a different set of tubes to see what voltages and current I'm getting, roughly the same on both sets even though 1 set is a GT6L6b wxt and I'm getting even voltage on both sets around v.

Bias still only goes down to 50mA which is pretty high for the 6L6b's. I set them to 55mA just to keep off the bottom limit and they seem to be holding up well. I cranked it yesterday for an hour or so, and ran an overdrive pedal for about 30 minutes, only sounds like it's clipping once in a while, tubes aren't glowing like the possibly shorted ones did.

I noticed that there's a larger resistor piggybacked on R30 next to the bias pot, also pins 1 and 8 are jumped together on both bases. Starting to sound like my amp may be set up for EL34, being a '92 made in '90 that's pretty probable. This would explain the bias pot having a higher low limit and range right? Stop me if I'm making myself sound stupid here, trying to take what I've read on the forums and use my electrical knowledge to understand tube amps and describe this issue better.

Messages 12, Strongrad said:. I would think that the amp is set up to run either 6L6's or EL's. IN a 6L6, the suppressor grid is tied to pin 8 internally. Pin 1 is 'open' and some amp builders Fender for instance In the El, the suppressor grid is NOT tied internally; so that connection is made externally by connecting pins 1 suppressor grid and 8.

ONE can run either a 6L6 or an EL in the amp you have there, Strongrad, with concessions made for the biasing differences. I agree, that any newly acquired amp should be evaluated in hand by a tech so that one knows what is there.

AR Member. Messages The bias voltage could only run EL34 20ma. This schematic show your using EL I was also told of the european tube shortage and many early JCM were modded and shipped with Setup for EL34s by who? You might want to take it to a tech just to figure out what's going on in there. AR said:. Thank you all for replying! I appreciate the help. Last edited: Jul 21, You must log in or register to reply here. Trending Topics.

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Fender Stratocaster Guitar Forum. Messages: 1, I thought I read some place that the JCM and have a solid state diode based preamp section. The 12ax7's run the loop and reverb.

On the JCM series amplifiers there is the fail safe fuse which is built in tone until new tubes were put in and biased by an authorized technician.

Tubeset for Marshall JCM900 100 Watt (5881)


If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. Use just one hand when working on a powered amp. See more tube amplifier safety info here. The following mods are not specific to any particular amp model and apply to many tube guitar amplifiers. See these links for modifications specific to the 5E3 Deluxe , 5F6A Bassman , blackface and silverface amps. Adjustable Bias Adjust your power tube bias for max output and sweetest tone.

Brit 900 SLX2100 Bias Amp Matching

amp bias 900

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Warning: The adjustments and processes described in this article require interacting with dangerous levels of voltage. Even unplugged, an amplifier can retain dangerous levels of charge. We encourage you to take extreme precautions when performing these adjustments or interacting with your amplifier. When in doubt, seek the help of a trained professional. You can follow this process with our companion video on how to bias your amplifier here:.

Generic Tube Amplifier Modifications

Moderators: pompeiisneaks , Colossal. Post by madmartigan » Thu Jun 24, pm. Post by dukeamps » Thu Jun 24, pm. Post by billyz » Thu Jun 24, pm. Post by es » Thu Jun 24, pm.

Features · Power Switch · IEC AC Input Socket · Line Fuse · Tube Fuse · Recessed Tube Bias Control · 5-way binding post outputs · Common, 2, 4, and 8-ohm speaker load.

Marshall JCM900 Tune Up

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Also used in many others brands of amps. With nut and washer. Marshall part number J3BA or S Used in many brands of amps and other equipment. Same footprint as J jack - may be used in place of J to add a touch of "chrome" class to your amp!

This wonderful old Marshall JCM lives in a recording studio. It was due for a set of tubes and a million-mile checkup.

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  1. Tygora

    In my opinion, you are on the wrong track.

  2. Donnally

    Spring!!!

  3. Tony

    It not meant it