Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bruce on October 30, 2016, 01:11:08 pm
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Trying to find bias resistor on ds50 danelectro...it has a split amp set up pre on top power on bottom. Totally separate chassis that plug together with 8 pin socket.now the question there is a 27k resistor in tremolo circuit in the preamp chassis on the strength pot on schematic I listed a week ago goes to 8 pin plug to the power chassis i to resistor 100k and 2.2k which goes to the 2 330 k resistors to grid.....is the 27k on preamp chassis the bias resistor.or the2.2k or 100k on the power amp chassis .....schematic is already posted..None of the amps I have made have tremolo circuit or two chassis please help..I think it is the 27k but can't understand why it's not on the power amp chassis next to the 100k and 2.2k ????
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The only resistors in the bias supply circuit are the 100K and 2.2K that are on the power chassis. A very small negative voltage is sent from the bias circuit to the tremolo circuit on the preamp chassis, but that has nothing to do with the operation of the bias circuit.
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Bias is not adjustable.
Does it really need adjustment? It is clearly intended to idle "cool", like many large 6L6 amps, and should be OK with any healthy pair of tubes.
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Thanks guys
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You can install a bias pot but your amp is already running with -37v to the output tube grids and that is the most that bias supply puts out. You can lower that -37v to make the output run hotter though.
To do that... replace the 100K with a 50K pot and a 47K in series. Top of the pot connects to the diode, bottom of the pot to one end of the 47K, the other end of the 47K connects to the 2.2K. The wire that goes to the 330K grid resistors will connect to the pot wiper.
You wont be able to run the tubes any cooler than they are now, but you can make them run hotter, even redplate, so be careful if you do this. I don't think it's really worthwhile unless you just want to play with bias. It will probably warm up the sound though.
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Just to know if it is feasible.
And add a doubler ? Will this allow to run the tube hotter but also cooler ?
(http://i.imgur.com/CjuaQrG.jpg)
(http://i.imgur.com/1hxZZ3x.jpg)
Thanks
Franco
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And add a doubler ? Will this allow to run the tube hotter but also cooler ?
yes.
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Is the polarity on the 22uf cap, correct?
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Is the polarity on the 22uf cap, correct?
NO, it was wrong, now it is correct (I modified the previous post)
+ must be connected to ground (excuse me)
Franco
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Sorry for not responding but it was my 61 Birthday... thanks for all the input.. last thing I have to know and the more I read the more I get screwed up,the most bias is -37 volts is more bias -38 or - 36 volts,half of what I read is cooler is more current less negative say -30 so -38 would be hotter more - volts but less current.or am I getting it wrong because of the one ohm resistor at cathode to ground that I put it in with test points....To me less negative say -30 or even -20 means hotter because it raises the current.Or is more negative say -38 and up -40 more current it can't go both ways ..I really can't get this figured out because people write conflicting articles on biased and they all sound for sure .. most of what I learn is trial and error cause I have lot of gear to mess with but I have 4 good books and they say the reverse of the other guys wiew on bias.. so when I bias a amp at - 40 when math says -35 am I hotter or cooler and which is for more current And is up and hot the same ...make it simple is something I have been stuck on for a long time please help
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Bias voltage is the negative voltage that is applied to the control grid of a tube. This negative voltage controls the current flow through the tube. A higher bias voltage (-40v is higher than -30v) reduces the current flow through the tube and the tube runs cooler. A lower bias voltage increases the current flow through the tube and the tube runs hotter.
If you try to read anything else into that statement, you will likely get confused.
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.. last thing I have to know and the more I read the more I get screwed up,the most bias is -37 volts is more bias -38 or - 36 volts,half of what I read is cooler is more current less negative say -30 so -38 would be hotter more - volts but less current.or am I getting it wrong ...
Bias voltage is negative w.r.t. ground potential. So increasing the amount of negative voltage (i.e. going from -37 to -47V) would make the output tube(s) run colder.
Edit: Oops - Steve replied while I was happily typing away.
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Thanks I really do appreciate it ...this should be a basic fact on google but it only screws me up too many people with a opinion and not fact . So this is fact ?? God I hope it is
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> To me less negative say -30 or even -20 means hotter because it raises the current.
Yes.
If you apply "zero" volts to grid 1, the tube tries to conduct "infinite" current.
For most of our tubes, negative 100V on G1 will turn the current "off", or much too small to be any use to us.
This is, what? A 400V 6L6 job? Figure a target of 1/10th of the Screen grid voltage. This will be a nominal "slightly on" point. 400V/10= 40V. G1 (control, bias) grid voltage is the negative of that. So -40V as a very rough initial guess.
We would expect -50V to be "very cold", -30V to be "pretty hot", -20V to be "burning up".
Why do we care? High current with this high plate voltage is high heat and also shorter tube life. Low current is "nearly OFF" so small faint sounds are "rough".
Hi-Fi fanatics use lower plate voltage and higher current so their small soft passages are as sweet as possible. Stage amps have other goals. We didn't drag a 40-Watt across town to play 0.5 Watts. If we did, we would not be heard above a drunk crowd. We like over-volted (plate volts) amps for Power, cooler bias for decent tube life, and let the real soft sounds be imperfect.
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Thanks I really do appreciate it ...this should be a basic fact on google but it only screws me up too many people with a opinion and not fact . So this is fact ?? God I hope it is
Look at some schematics of Fender 6L6 amps (see the schematics section here) for examples. E.g the brown vibrolux or tremolux, which runs the 6L6s at ~365V on the plates (on the low side for a 6L6GC) and uses a bias voltage of around -36V, whereas a twin reverb runs the 6L6GC at 460V on the plates and runs the bias voltage at -52V.