dave
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Posts: 7
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Post by dave on Nov 29, 2007 4:21:07 GMT -7
I am thinking of checking the bias in my Z-28 - I am pretty handy with a soldering iron and multimeter and happily poke around stuff at mains voltage, but the higher voltages in amps make me a little more nervous! I recently changed the mains tranny in my Z-28, an operation that went pretty well (ie it still works!), and after a recent tube change its time to think about the bias.
There are dummy sockets with flying least that enable the grid voltage to be measured, or the method the Doc posts that involves measuring the plate voltage from the output transformer centre tap. The latter seems straightforward and saves spending money on a probe - but a question:
The optimum voltage is given as 375v for GT 6V6's, but I am running Brimars - so will setting the plate voltage to 375v necessarily bias these tubes properly? Am I better off using a probe and setting the grid voltage instead?
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Post by dixiechicken on Nov 29, 2007 6:28:44 GMT -7
DC here!
I'd say follow Doc:s instructions. If the Briamars follow specs that should work.
Ask Doc and Myles what they think and follow their advice.
Cheers: Dixiechicken!
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Post by myles on Nov 29, 2007 9:46:23 GMT -7
You are much better off with a bias tool or bias probe as output tubes vary widely in their current requirements off a given power supply.
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Post by Telemanic on Dec 1, 2007 14:01:52 GMT -7
Absolutely, if you want to really know where your set at. Doc recommends the 375V based on using Groove Tubes in i think the 4 - 6 range. He knows then that in THIS amp circuit, using THESE tubes, setting 375V will yield a predictable plate current. It does because of the consistent measuring and grading done by GT. The Brimars could be all over the map, and while chances are you'd probably get lucky and be within max plate dissipation, you really wont know where your set at. The probes work great and are much safer than the other methods like current shunting, etc. Remember that if you use the probes to bias in the Brimar's, the voltage may go up or down from the 375 depending on the plate ma you set via the bias pot. I believe that is called IR drop.
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Post by hdahs143 on Dec 5, 2007 14:41:06 GMT -7
Just to clear things up, you dont adjust the plate voltage when biasing the amp. The plate voltage is supplied by the power transformer, regulated by the amp circuitry, and isnt adjustable. What you are adjusting is the negative voltage applied to the control grid of the tube. This in turn controls the amount of current flowing from the cathode to the plate of the tube, which is measured in milliamps. This is the measurement you are setting when you set the bias. The plate voltage measurement ( which you need to know to set the bias for optimum plate dissipation wattage) will fluctuate up/down as the current being drawn by the tube rises/falls. Usually this is set to 70-75% of max dissipation. You should use a bias probe and set the current to the measurement Doc suggests for his amps. Or in the case of a Non Z amp, refer to this page from Weber's site. Hope this helps! www.webervst.com/tubes/calcbias.htm
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Post by dixiechicken on Dec 5, 2007 16:27:57 GMT -7
Yes that's perfectly true - BUT - if there is a resistor in series witht the anod plate - changing the idle current by changing the bias voltage - will also affect the the plate voltage to some degree.
Cheers: Dixiechicken!
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Post by hdahs143 on Dec 6, 2007 10:05:09 GMT -7
Actually I did mention that at the end of the first paragraph, but it doesnt hurt to point it out again. Thanks DC. +1 Harold
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dave
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Post by dave on Dec 8, 2007 10:44:22 GMT -7
Well I got a bias probe from Torres Amps, fitted it , turned the amp on and got violent humming and the tube plates started glowing!!!! Aaaaargh! Something wong, turn off amp PDQ. The probe was faulty and had a short inside it. Pretty poor really. Got a replacement and insisted it was tested hot before it was sent out. Arrived today and job done. For the record the Brimars were drawing 32ma at the old setting, back off to 26 ma and the Z-28 sounds really sweet, in fact the best I have heard it.
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Post by hdahs143 on Dec 8, 2007 15:27:44 GMT -7
Glad to hear everything is working out. It's cool to be able to fine tune the bias to your tubes and your ears. Harold
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Post by Telemanic on Dec 11, 2007 11:00:45 GMT -7
Good job! nice to have a precisely tuned amp! Just to be clear, .. this discussion came about in regard to Doc's instructions to set the bias in the Z28, by VOLTAGE only ( read off of either the center tap or the PLATE ) ... via the bias pot. I think this is where the reference to "adjusting the voltage" came about, because if you are only looking at the voltage when adjusting the bias pot, then in a sense you ARE adjusting the voltage, Just that it is via the inverse relationship between voltage and current, ... and yes, current at the plate is what you are actually setting with the pot. It is NOT how you would go about biasing any old amp with any old tubes! This thread was a discussion of Doc's recomended way of setting the bias on the Z28 with GT's and would it work with Brimars.
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Deleted
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Post by Deleted on Dec 11, 2007 15:20:35 GMT -7
DRZ had mentioned somewhere not to use those voltages on his web sight as the voltage in his shop is higher than normal. That's why Myles recommended to use a bias probe.
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dave
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Posts: 7
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Post by dave on Dec 15, 2007 8:35:29 GMT -7
Thanks for the comments. I am still trying to get my head round the merits of all these different methods. Have I got this right? The Doc's voltage method works only for a specific voltage-tube-rectifer set up ( 375v-Groove Tube 6V6-R and ZX-34), so this is applicable to checking out stock Z-28's only? The bias current method involves setting the current to a target range (24-28 mA for a Z-28) and tweaking the final setting by ear? But here I get confused again. I have seen tables relating anode voltage to target bias current for class AB operation for different tubes. These indicate that for 6V6's operating at 375v the bias current should be much lower, around 20mA. Why this difference? The point of all this is to eliminate crossover distortion, right? So then, is the only surefire way to tune the bias correctly is to use a signal generator and an oscilloscope? Or once grid bias is in the right range to give good tube life, measuring and setting bias to milliamp precision is pointless? ie just let your ear's tell you when you hit the sweet spot?
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Post by dixiechicken on Dec 16, 2007 6:16:33 GMT -7
Yes to a certain degree that's correct. But crossover distortion is an an integral part of a class AB/B amplifier. You cannot get rid of it - you can diminish it to the point of acceptance. (i.e. where your ears stop complaining about it ) Myles mentioned in thread here that some old Marshall models or if it was HiWatt - had a fairly large amount of cross-over distortion and this contributed to their "classic" agressive in your face sound. Thus players liking that vibe - will want to have their amps ever so slightly "miss-biased" if you will. Cheers: Dixiechicken!
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Post by hdahs143 on Dec 16, 2007 6:58:40 GMT -7
It boils down to getting the amp to sound the way you like, while keeping the tubes from exceeding their designed operation parameters.
All of the above methods are kind of different ways of arriving at the same point, with the exception of the oscilloscope method. The oscope method does not take any measurements into account, and directs you to adjust the bias until the crossover notch disappears. As stated above, this notch is always present on some amps. Even on the amps you can adjust it out, you still have no idea how much current the tube is drawing without measuring.
Using the bias probe, measuring the plate voltage, and using a chart like the one I posted earlier on this thread(or the mathematical formula if you prefer)to determine the proper current draw for safe and toneful operation of the amp, is the simplest, safest way to go.
And it works on all Class AB amps.
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Post by dixiechicken on Dec 16, 2007 8:58:37 GMT -7
You pretty much nailed it.
Some Music Man models the HD-130 Watt models have very high plate voltages around 700 volts - as has my RD112-50 - 620volts.
This means you can get 130 watts out of four EL34 tubes - but it also means a shorter tubelife. Some touring pro musicians had to do a tube swap every 6 months or so.
Cheers: Dixiechicken!
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