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In Reply to: RE: How much current does a -bias voltage need for 6550 tubes posted by DAK on July 18, 2016 at 21:01:41
Dak
It depends what voltage you are running tubes at on the plate but when you are in 440vdc to 470vdc range,I usually bias at 75ma to 80ma.You typically need about minus 47 to minus 57v on the control grid.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Follow Ups:
I don't need the bias current for the power tubes but rather the total current requirement for the bias supply. I need to know this so i can choose a proper choke for the bias power supply. regards, Dak
Keep in mind that G1 draws little to almost no current because if it does,you are in trouble..You could literally get away with less than a 10ma bias supply.It's ok to have a little reserve like 50ma but why not just use the small torroid that Jim has us use on the Deuce? Its a dual 22vac secondary and we connect it to a full wave bridge..You can of course add a choke and filtering and I like the idea of a choke in the bias supply because it's a good way to lessen the chance of any audio that may ride on the C minus rail.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 07/19/16 07/19/16
In fact, I'd keep the supply down to few hundreds of mA, max. So that if the tubes get unstable, for whatever reason --- you won't be sending a lot of current into the grids of the tubes, from this bias PS.
Or way less, the current requirements of a bias supply is basically nothing, it is just a voltage reference. Michael was thinking you were asking about the idle current of the output tubes.
More power to you trying a choke, however using a choke for a bias supply is not really needed. Most amps use a half wave rectification an are dead quiet as there is no current demand. If you want to play with one, you should try a 5 to 10 mA 50 Henry or higher. I would go CRCL with the choke at the end like a grid choke. You are going to want to make part of the R a variable resistor to set the right voltage. Be careful where you put the choke as it can pick up radiated noise. The "best" supply for a bias would be one using a full wave bridge, but that usually requires a separate power transformer for the 50 v secondary.
This made me consider if the bias requirements might be different from a class AB PP amp. The bias supply is a seperate transformer 12.6v filament tranns wired backwards to a Schottky FWB which has a raw output of 70+volts. The bias supply consists of, 47uf x 70v film cap , ?H choke, 47 uf, to 14K , to balance pot, 10uf x 50v to G1 of 6550. The circuit is designed so that if the pot goes "open" there will be full -bias on the outputs instead on no bias. thanks for the suggestions, Dak
How many volts of negative bias are you running onto your grids? Seems like only a few are needed for Class "A" PP conduction.Say, bias voltage needed on grid is 2 VDC. With 1/2 watt:
144mA of current. You are going to need quite a bit heftier bias PS.
You can see, if you increase the watts of dissipation in the bias supply, which may need to be done --- the current flowing increases. Bad for grids.
2 VDC with 1 watt = 288 mA. Plus, you run a big risk of adding more noise to the system.
Is there a reason you are not using cathode biasing?
Thanks!
Edits: 07/19/16
Class A bias voltage is OTO -15V. The current flow is dictated by the resistance of the voltage dividers in the adjustment circuit( pots, etc ), and voltage across them. Plus a little for the grids.
Given it needs so little voltage, one could start with a lower voltage supply for sure. 70V is going to put you near the ends of the pot.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
MY Av is 340 volts. I have the neg bias supply temporarily working with CRC supply of 300uf to 6k 2w to 300uf then to the bias supply divider resistors. It is the first section with the electrolytic caps and resistor that i would like to try the film caps and choke but i am unsure of the current capacity the choke should have. thanks for the help. dak
For -30VDC, 1/2-watt ---> 9.69mA
For -30VDC, 1-watt ---> 19.27mA
50mA capacity should be fine.
I will get the CLC installed and report back any significant results. cheers, Dak
Douglas, a lower voltage supply. But, may draw more current, right? At least compared to a (-40 to -60 VDC) bias supply --- for the same amount of wattage.For -15 VDC, at 1/2-watt = 19.2mA. Build a 50mA-100mA unit?
At 50-100mA, how much does background noise in the bias supply need to be addressed? Versus a 15-20mA bias supply. Prolly need a choke and a full-wave bridge.
Thanks
Edits: 07/20/16
Please describe this boas supply that is consuming this 19.2 mA? That is a little less than 1k at 15V( but that isn't the right voltage, we're dealing with the -30V supply ), that would be the total supply voltage applied to the top of the voltage divider( hopefully including at least one pot ). IOW, with a pair of 10k pots, one for each tube, we have a total bias supply resistance of 5k, or 6 mA flowing through it, plus the grid current( which will be much less than 6 mA ). 1 mA of grid current flowing through a 50k grid resistor gives us 50V...and that for sure isn't happening.Please explain where all this current is going.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Edits: 07/21/16
Yeah, doing the math for 1/2-watt (being generous) at 40VDC...
~ 7.5 mA.
Maybe, with extra robustness... design for 25mA.
That's likely still more than needed. The amount of current through the tube grid is measured in microamps - with the exception of tubes/gear designed to push the tube grids into positive voltages! There is some current through the grid "leak" resistors, and there may be some paths to ground either through bias supply bleeder resistors or the adjustment pots (if present).
For instance a Citation II amp draws just a few ma. of grid current per channel under most circumstances.
It should be noted that some power tubes DO draw a bit more grid current as their grids are driven close to the zero point so some extra current capability is a good idea. But even 25 ma is probably overkill - although harmless...
For xx1 biased tubes anyway. Total every thing between the negative rail and ground, and add some for what the grids will consume. This is the adjustment pot/resistive divider parts. Use Ohm's Law to get current with resistance and supply voltage in hand.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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