distard
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Post by distard on May 6, 2015 2:10:57 GMT -5
I would like to learn how to BIAS my amp if I can. Its a Randall RG50TC, A Mike Fortin 50watt EL34 beasty, or so I have been led to believe. Mike Fortin works for Randall all the really savage high gain stuff is mostly Mike Fortin designed. Right now it is equipped with a set of YellowJackets(still sorting out exact model as the stickers have been removed or possibly fell off), the Yellowjackets allow it to run with EL84's with a slight reduction in power. I would like to try a set of EL34s in it, and want to set the BIAS correctly, these amps are known for getting very muddy if the BIAS is not high enough. Apparently the YellowJackets 'self-BIAS' ? There is a jack on the back of the amp which I assume if for the multimeter, it is a small three pronged plug(male) with two small trim-pots one on each side of the plug, I assume these are the pots to adjust the bias. Im well aware of the hazards of working with this kind of equipment, Im not planning on modding this particualr amp, but I though I might be able to learn how to BIAS, Im fairly adept electronically. So is AMP BIAS something I might be able to accomplish, please do not worry, there's not need for safety disclaimers, I wont be doing anything until I am 110% sure of what I am trying to accomplish, and confident in my knowledge of how to achieve desired outcome. Basically I want to build an amp one day, and this is sort of the first step Ive taken towards that ultimate goal. Appreciate anyone's comments, or criticisms, and thank you for your consideration. Have a great one all.
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Post by haydukej on May 6, 2015 13:21:30 GMT -5
Distard, Disclaimer: I am not an amp technician and far from the least electronics-savy nut here, so please let someone else chime in before you actually do anything. My goal here is to just hopefully add some useful information to get things rolling. From what I've seen available by a quick google, yes, your amp does have a convenient access point(s) to adjust the bias. You have the three prongs along with 2 trim pots on each side as shown below. My assumption, would be that of the three prongs, the middle is the common/ground and the two outside prongs are for each respective tube. Since you're wanting to try out the stock tube configuration (back to the EL 34's) instead of the EL84s, you would remove the yellow jackets and plug in your replacement EL34s directly to the stock tube sockets. If I had to guess, I would imagine you have the Yellowjackets YJS which is pretty common for converting an 8-pin EL34 configuration to utilize a 9-pin EL84. If you have a little black wire coming off of the yellowjacket connected to the chassis (or not), then it's likely a YJC. For biasing, I've seen the numbers between 30 - 40 millivolts (mV) thrown out there for this specific amp. And that would be measured for each side. I'm thinking alligator clips are going to be essential here as you don't want to try to be holding the probes on the prongs while adjusting the trim pots. As first stated, please wait until someone else confirms (or flat out refutes) my information/thoughts. Stay safe!
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fluro
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Post by fluro on Jul 24, 2020 4:08:58 GMT -5
Hey guys!
I was going to bias my amp as well but I was destructed by the prong thing. With my amp off I just checked cn20 and found out that two prongs out of three are in short circuit. I didn't expected them this way since the middle one is common gnd. How is that possible? And are we really sure how to connect the prongs? Can anyone describe/illustrate the whole procedure on biasing?
Thanks guys!
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Post by thetragichero on Jul 24, 2020 9:31:30 GMT -5
hello fluro and welcome to the forum! here's the power section of the amp so that we can see what we're talking about if you'll notice right there in the middle, r107 and r108 are 2 watt 1 ohm resistors from the pin 8 cathode (and technically also the pin 1 suppressor grid, but we can leave that out if this discussion) the ground. these will measure as a short on most continuity meters (i forget what the threshold is on mine but it's on the order of several ohms) but should measure out right around 1 ohm (depending on how good the meter is, it might be slightly lower or higher. that's okay) what these allow us to do is to measure the voltage drop (mV scale if your meter is not auto ranging) across the resistor and give us a good approximation of the bias current (Ohm's law V = I * R). if it reads 40mV that's ~40mA of bias current (technically it also include cathode current, so it'll read slightly higher than the actual bias current... setting slightly cooler bias is better for tube life anyway) to get the most accurate bias information in YOUR mains voltage, you'd want to measure the plate to cathode voltage (pin 3 to pin 8). NOW i am assuming you understand you'll be measuring 400v+ so keeping fingers/paperclips/etc out of the sockets is the bare minimum for safety. read up on tube amp safety. don't die. if you're not comfortable, find somebody who is so with the plate to cathode voltage, I'll plug it and the other relative information into this handy calculator: robrobinette.com/Tube_Bias_Calculator.htmand get my target figures. this is of course class ab fixed bias
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fluro
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Post by fluro on Jul 27, 2020 6:14:53 GMT -5
thatnks thetragichero for this! i'm still kind of confused about which prong goes where but what i found out is some weird stuff on v5. why this happened? here's the picture.
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Post by newey on Jul 27, 2020 6:18:28 GMT -5
fluro-
Your image doesn't show up because the forum software does not support direct linking of an image, it must be hosted elsewhere first. There is a tutorial to do so in the reference section, there are a couple of options you can use.
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fluro
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Post by fluro on Jul 28, 2020 5:15:55 GMT -5
I hope this works now...
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fluro
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Post by fluro on Jul 28, 2020 5:17:14 GMT -5
apparently it works.
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Post by thetragichero on Jul 28, 2020 11:15:30 GMT -5
if you're on a pc imgur can work for hosting
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fluro
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Post by fluro on Jul 28, 2020 14:23:59 GMT -5
thanks!
so, any ideas how this can happen?
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Post by thetragichero on Jul 28, 2020 14:47:47 GMT -5
a lifted trace? often caused by poor desoldering. potentially too much heat if the tubes are hung upside down like fender often does. can be repaired with a wire connecting the two solder joints together
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