Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Draxsonimy on July 30, 2018, 02:16:02 pm
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Just finished doing a 70's plexi and with the bias pot turned all the way i'm getting 30ma at my bias test port and -38v on pin 5
Anything that I should look at first?
I've added a drawing I made of the bias section of the circuit.
The transformer has a dedicated bias supply which according to the specs is 98vac but i'm reading 105vac.
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Where's the other power tube? How are you measuring 30mA? Would you like to set the bias pot for more tube current but the pot is already maxed out? Is that the issue?
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not sure what you mean about where the other power tube is. If you mean reading wise it's basically the same if you mean in the schematic I just drew it as simple as possible.
measuring mV from pin 1,8 through a 1ohm resistor
yes I would like more range on the bias pot. My plate voltage is 470v. So i'm just a little cold on the bias but would prefer to have much more of a sweep.
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Maybe the range is large enough, but it isn't in the right place.
To shift the range for hotter bias and increase its size a little, leave the 27K and the 15K resistors and change the 47K resistor to 27K.
To shift the range for hotter bias and decease its size a little, leave the 15K and 47K resistors and change the 27K resistor to 47K --or-- leave the 27K and 47K resistors and change the 15K to 68K.
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I reduced the 47k to a 38k and was able to get a sweep up to 60ma which is more than enough sweep.
I posted that in a Facebook group and had multiple people agree that I shouldn't be looking at how to increase the current draw but instead look at why my negative voltage was so high and that I should be more focused on determining how to decrease the negative voltage to then get to the correct current draw, but to be honest I was lost in that concept and was unable to get any better of an explanation.
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The 27K resistor was calculated to give you the desired bias voltage at somewhere around the middle of the rotation on the 50K pot. The 38K may be a better choice if you aren't confident about what you are doing because it doesn't allow you to fry the tubes.
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38k was just what came from running a pair of resistors in parallel.
I added designators to the schematic to reduce confusion.
I apologize what is the difference in increasing R1 or decreasing R3 if they both achieve the same thing?
I increased R1 to a 37k and got a good sweep that was hot enough.
I replaced R1 to the original value
I decreased R3 to 38k and got a good sweep that was hot enough.
Would one option be preferred over another?
I feel like increasing R1 is a better option since the original transformer was supposed to supply 98v and mine is supplying 105v.
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I apologize what is the difference in increasing R1 or decreasing R3 if they both achieve the same thing?
Increasing R1 decreases the current through the circuit because there is more total resistance. The range available decreases because the 50K pot becomes a smaller proportion of the total resistance.
Decreasing R3 increases the current through the circuit because there is less total resistance. The range available increases because the 50K pot becomes a larger proportion of the total resistance.
Increasing R2 will have the same effects as increasing R1, but it will also reduce ripple.
Would one option be preferred over another?
Sometimes, but a lot of the time its what resistors you have on hand that decides the circuit.