Marshall 6100 simple repair and How to Bias
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- Опубліковано 4 вер 2021
- This one was a quick job I should have uploaded months ago but better late than never.
The work on this one was quick and easy. Client wanted new tubes and apparently had some trouble with trying to replace them at home. Replaced a blown fuse and bypassed the two 5.6K grid stoppers on the PCB for two of the output tubes and placed them directly on the tube sockets rather than lifting the circuit board.
For those that came here for learning how to bias their 6100 I will include some extra information here.
I used a bias probe available from Eurotubes.
www.eurotubes.com/store/pc/bi...
There are many different methods to determining the proper bias for your tube amp and if you're a beginner I would highly suggest starting with a probe. I still use them frequently as it's a simple, non-invasive, way to measure bias current and requires no modification of the amp circuitry to use unlike methods such as the "1ohm resistor" method. Bob the owner has always been extremely helpful and he's passionate about tubes! Give their site a visit.
I like to use two meters when using a probe so one can be dedicated to the probe itself and the other I can use to measure the DC plate voltage on the amp. It is helpful to have the specification sheet for the particular tube you are biasing as the max plate dissipation can differ between types and manufacturers. Generally, you will see EL34's at 25W and current production 6L6 tubes at 30W. This amp uses 6L6 tubes so we will use the JJ spec for 30W in these equations.
You will measure the plate current from the probe with you meter set to the lowest milliamp setting, usually around 20mA or 200mA. You measure the plate current with a meter set to the highest DCV setting, usually at 600VDC or so.
When measuring plate voltage keep one hand in your pocket and carefully touch your probe to pin #3 of any output tube socket. Your negative lead must be grounded to the amp's chassis.
For this amp, we started with a plate voltage of 485 so let's plug this into our formula.
(plate dissipation / plate voltage) = maximum idle current
Your max idle current is pretty much your "do not exceed" number but you would never really want to bias that high for any reason. A typical value to shoot for is 70% to 60% of your max idle current. This is where crossover distortion is minimized and tonally the amp performs the best here. 60% will lead to longer tube life and in some cases slightly more clean headroom and punchier chords and notes. Your mileage may vary but in between this range is a safe place to be.
Back to our formula...
(30 / 485) x 70% = 0.043 which is 43mA
(30 / 485) x 60% = 0.037 which is 37mA
The bias can be adjusted via a trim pot on the circuit board as shown in the video. The process is the same for many other amplifiers. Some manufactures such as Peavey and Mesa Boogie do not give the user an option to bias their tubes without a modification to the amp's circuitry.
2 videos in 1 night. This is awesome!!
Derek finally got his amp to you. Those 6100's are so damn cool. Really a lot more under the hood than I imagined though.
This was actually way back in March. Lotta ribbon cables in there. Does not look fun to work on!
@@purgasound Yeah, quite a bit more daunting than an 800. I'll get you the Hammer (5150) soon to check out that preamp socket.
Short and sweet. Purge baby!
PURGASOUND, general question its common for biasing cathode biasing or grid biasing amplifiers that when adjust the biasing voltage for NO crossover distortion while the biasing current is in the range of 30mA to 45mA idle current but the problem is either the power tube dissipation is way below 70% or way above 70%, what can you do to fix this problem? because the plate voltage is already max out and you don't really want to decrease or increase the plate voltage because its set to the specs of the datasheets of the power tubes. The correct way of biasing amplifiers either cathode bias or grid bias is that the rule of thumb is NO crossover distortion, idle current is 30mA to 45mA, plate voltage is spec to data sheets, Power tube dissipation is 70%. Now the common problem with amplifiers 2000's is that the tube dissipation is way below or way above 70%, so what can you do to fix this problem?
I bought one of those Bias Probe tools a few years ago..it’s got two cable probes that plug into a switcher box so you can select tube A,B,then that plugs into a multimeter…each of the probe cables have a 1ohm,1%2 watt shunt resistor built into the cable…I need 2 more cables and was just going to make them myself,but was wondering the purpose of the resistor?I have seen some probes like yours that do not have a resistor in the cable….
Can you tell me please how to connect the probe test in your amp. Where the connection. I really need to know where you put the two lids of probe test in your amp, thanks a lot
Do you need to discharge those caps manually before starting?
I did that for old crt televisions when I worked on them.
A lot of modern amplifiers have a bleeder resistor somewhere in the circuit but it is a good rule of thumb to discharge them manually before poking around in there. I've had my fair share of zaps from being impatient.
Hello! I can't bias my 6100 hot enough, I'm planning to add a resistor in parallel to the r212 resistor to achieve it. Is it safe to do it with the amp off without draining the caps? It looks really difficult to turn the board around! Thanks for your time and your videos man!
You can easily drain the caps with two leads with a resistor between them. Most Marshall's should bleed the capacitor's charge when turned off but it's a good rule to discharge them manually in every case. You can also verify the charge with a multimeter. You can piggyback a resistor across the bias range resistor to get within the proper range. Having a colder bias on these amps isn't terrible since all of the distortion comes from the preamp stage. As long as the amp is no lower than 55% or 50% max plate dissipation there shouldn't be too much crossover distortion.
@@purgasound Thanks a lot for your answer! I piggybacked another resistor and im now able to properly bias it. It was 11ma instead of 37ma... needless to say that the amps now sings better than ever!
Hey you should add some like really quiet guitar playing along with the video like not loud enough so you can still hear wat your saying but i just think itll really fill in the empty space
That's a good idea. I'll give it a shot
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Creepazoids!!!!