In an effort to reduce B+ (chifi 110v ps in a 120v country) i made some changes most notable was the cathode resistor of 680 ohms to 1k ohms. The cathode current dropped from 90ma to 70ma.
Was working on whats required to get it to the 80ma range and thought now might be a good time to see if i could add in a pot on both cathodes making the bias somewhat adjustable.
Knowing I know only enough to make many mistakes figured i should see how its been done before.
My search terms are giving me nothing too usefull..
My thinking was to add a 20k pot in parallel with the cathode to ground, figuring I probably forgot something...
Anyone have any scematic examples of how this is done?
Was working on whats required to get it to the 80ma range and thought now might be a good time to see if i could add in a pot on both cathodes making the bias somewhat adjustable.
Knowing I know only enough to make many mistakes figured i should see how its been done before.
My search terms are giving me nothing too usefull..
My thinking was to add a 20k pot in parallel with the cathode to ground, figuring I probably forgot something...
Anyone have any scematic examples of how this is done?
Not ideal as with variable fixed bias , you can get that cathode cap out of the signal path . You could always put a variable (well-heatsinked) CCS in the cathode in place of the resistor , such as a depletion MOSFET or LM317 wired as CCS with resistor in series to take into account Vmax with this part . High impedence CCS not so important is it will be bypassed by the cathode cap
OK, I'm gonna throw another suggestion, using a rough SWAG that your amp pulls ~1.5A on the line side and you want to drop 10v, think about the most basic and reliable, cost effective, one shot solution for your B+ and heater voltages, you should just add @7-8R 25W resistor on the neutral/common side of your PT primary, drop the line voltage 10V and call it done. If there's not a good space in the chassis for it, then a small external box or terminal strips would be the mounting method. Placed on the neutral side the resistor would only have the 10v on its input side exposed.
Sounds like this is still cathode bias with a bypass cap. If that's the case I'd be more concerned with the quality of the parts - a really good wirewound resistor, vitreous enamel rather than a white box one. And a DC Link cap like a Kemet 40uF. Once you choose the bias you want, size the resistor appropriately. I know CCS have their fans, but personally I'm not one of them. I like good quality resistors.
1. The best answer so far has been the series resistor in the power transformer primary.
Just my opinion.
But do not expose the resistor leads. I have seen neutral be as much as 8V above the ground return.
If you short neutral to ground at the amplifier, you may get sparks, and hot wires too. Ouch!
Neutral and Ground are only the same voltage at the power entry of your power breaker panel.
Neutral passes current, there is a voltage drop on the Neutral wire to your wall outlet.
Ground does not pass current, so there is no voltage drop on the ground wire to your wall outlet.
Safety First!
If your ground wire is passing current, then you have a major problem; find the cause and fix it!
House wiring? Amplifier, Toaster, Vacuum Cleaner, other appliance, etc.?
You could also build a Buck Transformer. An 8V or 10V transformer will get you in the ball park (depends on whether you use a 115V or 120V "buck transformer). Get the phase right, or you will increase the voltage to the amplifier primary, instead of decrease the voltage to the amplifier.
2. A hint to many who open threads (not this thread):
In general, for many of the common questions of brand new threads:
Please post a complete and accurate schematic of your amplifier. Please include:
Power Supply
Amplifier Circuit
A good schematic is worth 500 posts of questions, and 500 posts of answers to those questions.
I am not a good Archer, but when blindfolded, I am an even worse shot, I can't even hit the target, and I certainly can not hit the bulls eye.
You will get many answers on most threads, but many of them will be based on assumptions that are incorrect.
Just my opinion.
But do not expose the resistor leads. I have seen neutral be as much as 8V above the ground return.
If you short neutral to ground at the amplifier, you may get sparks, and hot wires too. Ouch!
Neutral and Ground are only the same voltage at the power entry of your power breaker panel.
Neutral passes current, there is a voltage drop on the Neutral wire to your wall outlet.
Ground does not pass current, so there is no voltage drop on the ground wire to your wall outlet.
Safety First!
If your ground wire is passing current, then you have a major problem; find the cause and fix it!
House wiring? Amplifier, Toaster, Vacuum Cleaner, other appliance, etc.?
You could also build a Buck Transformer. An 8V or 10V transformer will get you in the ball park (depends on whether you use a 115V or 120V "buck transformer). Get the phase right, or you will increase the voltage to the amplifier primary, instead of decrease the voltage to the amplifier.
2. A hint to many who open threads (not this thread):
In general, for many of the common questions of brand new threads:
Please post a complete and accurate schematic of your amplifier. Please include:
Power Supply
Amplifier Circuit
A good schematic is worth 500 posts of questions, and 500 posts of answers to those questions.
I am not a good Archer, but when blindfolded, I am an even worse shot, I can't even hit the target, and I certainly can not hit the bulls eye.
You will get many answers on most threads, but many of them will be based on assumptions that are incorrect.
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In this case all voltages -including heaters- 9% bigger!
This is more over the conventional 5% filament voltage tolerance.
If the 300B doesn't have stabilized 5V DC filament PSU, will break sooner!
Instead of any modification try to use 120V->110V power transformer.
A Buck Transformer is far less expensive than a new power transformer.
Get a metal chassis box, and build a buck transformer circuit.
A 2 Amp 10V transformer is lighter, smaller, and less expensive, than a new amplifier power transformer.
Your B+ and filament voltages will be correct.
Get a metal chassis box, and build a buck transformer circuit.
A 2 Amp 10V transformer is lighter, smaller, and less expensive, than a new amplifier power transformer.
Your B+ and filament voltages will be correct.
I have the B+ voltages under control, and the heaters solved.. no need for modifications to my modifications yet lol.
That said, i simply want the convenience of adjusting the bias if even a little. Sure i could try reducing the value of the cathode resistor until i hit my mark but that again gives me no leeway in the future.
Forgive my lack of knowledge, but i dont understand how a cathode resistor of 1k with say another cathode resistor (or pot) in parallel both combined worth more than the originally installed value could lift the ac signal off the bypass capacitor. I do understand how choosing a 2nd parallel bypass resistor of lower resistance could be a problem. Basic parallel resistance says 2 at 1k ohms act like 500 ohms.. my plan is to not drop below 700 ohms.. From my very elementary understanding, the capacitor is more or less like a dead short to the ac.
With (1) 1k and say a 3k in parallel the total is 750 ohms... going to a 9k gets me to a 900 ohm equivalent. i can understand that a budget pot might wreak havok on the sound, but one designed for audio use of proper wattage shouldn't be too horrible (i think)
Im just struggling to understand how adding overall resistance could affect this.. but is why im here talking to you folks!
That said, i simply want the convenience of adjusting the bias if even a little. Sure i could try reducing the value of the cathode resistor until i hit my mark but that again gives me no leeway in the future.
Forgive my lack of knowledge, but i dont understand how a cathode resistor of 1k with say another cathode resistor (or pot) in parallel both combined worth more than the originally installed value could lift the ac signal off the bypass capacitor. I do understand how choosing a 2nd parallel bypass resistor of lower resistance could be a problem. Basic parallel resistance says 2 at 1k ohms act like 500 ohms.. my plan is to not drop below 700 ohms.. From my very elementary understanding, the capacitor is more or less like a dead short to the ac.
With (1) 1k and say a 3k in parallel the total is 750 ohms... going to a 9k gets me to a 900 ohm equivalent. i can understand that a budget pot might wreak havok on the sound, but one designed for audio use of proper wattage shouldn't be too horrible (i think)
Im just struggling to understand how adding overall resistance could affect this.. but is why im here talking to you folks!
To get all the resistor values figured, you need to decide on how much voltage or current adjustability you want. 10mA ...? 10v...? Set a maximum and minimum allowable for the pot to control. Then the calculations can be made for the tail resistors and the pot. The current drain through the pot needs to be kept to a minimum and designed so that if the pot wiper fails then the cathode current cannot rise to dangerous levels.
20to20,
A properly wired Rheostat . . . If the wiper opens, the resistance will be the maximum end to end resistance.
It will not be a short.
But I never use an adjustable resistor / rheostat if I do not have to.
AudioFanMan,
The beauty of Self Bias is that when you plug in a variety of 300B tubes, the plate current tends to be nearly the same.
My experience with self biased JJ, Art Audio, Electro Harmonix, and Svetlana 300B tubes is they all draw about the same plate current in a self biased circuit using a resistor (And I used a bypass capacitor across the self bias resistor, to retain the low plate impedance, rp, and the gain of the tube).
My experience with all those tubes in self biased mode was "Plug and Play".
Unfortunately, some of the early Svetlana tubes after some hours of play time, would develop a Grid to Filament short (they were really good before they failed).
I am not talking about the other various brands of "Super 300B" tube designs, they often will conduct more plate current, and usually they will have more filament current too.
A properly wired Rheostat . . . If the wiper opens, the resistance will be the maximum end to end resistance.
It will not be a short.
But I never use an adjustable resistor / rheostat if I do not have to.
AudioFanMan,
The beauty of Self Bias is that when you plug in a variety of 300B tubes, the plate current tends to be nearly the same.
My experience with self biased JJ, Art Audio, Electro Harmonix, and Svetlana 300B tubes is they all draw about the same plate current in a self biased circuit using a resistor (And I used a bypass capacitor across the self bias resistor, to retain the low plate impedance, rp, and the gain of the tube).
My experience with all those tubes in self biased mode was "Plug and Play".
Unfortunately, some of the early Svetlana tubes after some hours of play time, would develop a Grid to Filament short (they were really good before they failed).
I am not talking about the other various brands of "Super 300B" tube designs, they often will conduct more plate current, and usually they will have more filament current too.
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6A3sUMMER,
Thank you... I do have the cathode bypass cap as well. I was hoping to be able to try various bias points to see what I liked..
I hear many like the sound of the hotter side at the sacrafice of tube life..
I also assumed (probably incorrectly) that if one of my 300b's wasn't performing as well I could somewhat adjust the under performing tube.
I figured with the help of Lt spice that at 2k ohm the rheostat would dissipate about 1.3 watts and at 10k about half a watt.
Again, this is in parrallel with the existing 1k 50w cathode resistor...
Im gonna search to see if there is a 2k-10k (or more) 4 watt pot that might fit the bill...
Thank you... I do have the cathode bypass cap as well. I was hoping to be able to try various bias points to see what I liked..
I hear many like the sound of the hotter side at the sacrafice of tube life..
I also assumed (probably incorrectly) that if one of my 300b's wasn't performing as well I could somewhat adjust the under performing tube.
I figured with the help of Lt spice that at 2k ohm the rheostat would dissipate about 1.3 watts and at 10k about half a watt.
Again, this is in parrallel with the existing 1k 50w cathode resistor...
Im gonna search to see if there is a 2k-10k (or more) 4 watt pot that might fit the bill...
But not stated here? I had to make it up.
DigiKey:
91R1A-R16-A15L $6.36
91A1D-C24-A15L $7.45
91A1A-B28-A15L $8.19
91U1A-T22-A15L $8.19
Wow, a lot of work! Thank you do very much.
I was hoping to add at most a 2-3k ohm safety resistor before the pot so if I do me, the way I typically do me, I cant cause too much trouble ...
That said, I see now that I may have to swap out the cathode resistor after all. Using the installed 1k cathode resistor
and burning some brain cells, my figuring wont get the the parameters right for a 300B (B+,Grid bias, plate current)
One step forward, 15 steps back... This EE stuff is not as simple as it appears!!!
That said, I want to thank everyone for getting me this far! Once I figure out the issue I have now, I will circle back to this for certain!!!
I was hoping to add at most a 2-3k ohm safety resistor before the pot so if I do me, the way I typically do me, I cant cause too much trouble ...
That said, I see now that I may have to swap out the cathode resistor after all. Using the installed 1k cathode resistor
and burning some brain cells, my figuring wont get the the parameters right for a 300B (B+,Grid bias, plate current)
One step forward, 15 steps back... This EE stuff is not as simple as it appears!!!
That said, I want to thank everyone for getting me this far! Once I figure out the issue I have now, I will circle back to this for certain!!!
AudioFanMan,
I am guessing you have only one type of single ended output transformer to try.
Different 300B current levels often work best with a limited range of output transformer primary impedance.
And SE transformers have ratings for maximum quiescent DC current.
The tradeoff of an air gapped transformer is the quiescent DC current, versus the power level at low frequencies that will not result in saturation of the laminations. The more the quiescent current, and the lower the frequency, the earlier that saturation will occur.
Knowing the output transformer specifications is helpful in determining a realistic range of 300B current, versus the performance of the amplifier.
Global Negative Feedback can not fix lamination saturation, the error correcting signal just adds more current, making the saturation worse.
But many 300B SE amplifiers tend to work better without global negative feedback, so that is usually not a concern anyway.
I am guessing you have only one type of single ended output transformer to try.
Different 300B current levels often work best with a limited range of output transformer primary impedance.
And SE transformers have ratings for maximum quiescent DC current.
The tradeoff of an air gapped transformer is the quiescent DC current, versus the power level at low frequencies that will not result in saturation of the laminations. The more the quiescent current, and the lower the frequency, the earlier that saturation will occur.
Knowing the output transformer specifications is helpful in determining a realistic range of 300B current, versus the performance of the amplifier.
Global Negative Feedback can not fix lamination saturation, the error correcting signal just adds more current, making the saturation worse.
But many 300B SE amplifiers tend to work better without global negative feedback, so that is usually not a concern anyway.
One of the first mods i made was switchable gnf and local at well. While local feedback disconnected sounds pretty much the same, shutting off global made a huge difference. At the listening levels i mostly use, i much prefer gnf off. When the house is filled with people gnf must go on as someone invariably in a spirited mood turns the volume past clipping ...
One of the things i was planning on was adding a variable resistance gnf, maybe 4 positions from where it came stock to off.. the EE on that is still a bit above my head..
Having a difficult to service pcb is somewhat limiting as the only way to try different things easily would require 12 wires a foot or two long linking the chassis to the displaced board.. quite a lot to go wrong with these voltages!
Yet, to accomplish in any reasonable time frame may be something i have to work through...
The dcr of the opt is about 260 ohms and it measured about 5.5k at the 8 ohm tap.. i guess the huge irons of yesterday presented much less dcr, allowing more at the plate?
Garage heater on, wifey away time to get back to the work at hand lol...
One of the things i was planning on was adding a variable resistance gnf, maybe 4 positions from where it came stock to off.. the EE on that is still a bit above my head..
Having a difficult to service pcb is somewhat limiting as the only way to try different things easily would require 12 wires a foot or two long linking the chassis to the displaced board.. quite a lot to go wrong with these voltages!
Yet, to accomplish in any reasonable time frame may be something i have to work through...
The dcr of the opt is about 260 ohms and it measured about 5.5k at the 8 ohm tap.. i guess the huge irons of yesterday presented much less dcr, allowing more at the plate?
Garage heater on, wifey away time to get back to the work at hand lol...
AudioFanMan,
I still have not seen a schematic of your amplifier and power supply. It would be very helpful.
A schematic can help us from making assumptions about your amplifier topology, preventing us from making wrong answers to your questions.
Instead of you trying so many variations, we might find one or two changes to make, that will meet your top 3 amplifier performance desires.
There are design tradeoffs, such as good damping factor, low distortion, maximum power out, musical presentation, etc.
Normally, you can not fit all 4 of those in one box.
With a schematic and the ranked order of the 3 or 4 of your top criteria, we might find you are already at optimum, or that a small change or two would get you there.
The 260 Ohms DCR on a 5.5k primary is not really bad; it causes about a 0.5dB insertion loss in the transformer at midrange frequencies.
If the 8 Ohm tap DCR is 0.4 Ohms, it will give an additional 0.5dB insertion loss at midrange frequencies, for a total loss of 1dB.
A 1 dB insertion loss with a 300B tube that is putting out 5 Watts of clean power, there will be 4 Watts at the 8 Ohm tap.
8 clean Watts from the 300B will give about 6.4 Watts at the 8 Ohm tap.
Etc.
Many builders wonder why they do not get the predicted power out; they forget about the insertion loss of the output transformer.
The Western Electric 300B data sheet lists output power, generally at about 5% Total Harmonic Distortion, and the Watts are listed Before the insertion loss of the output transformer.
What is the approximate weight of your output transformer?
______ kG
Or
______ Lb.
What is the rated maximum DC current of your output transformer?
300B tubes do have some variations from tube to tube. But if you use equal self bias resistors, and one tube has more than about 20% to 25% current variation versus the other channel's tube, I would be looking for another 300B.
In that case, changing the one self bias resistor is probably will not make the less performing tube come up to the same performance as the other tube.
Have fun building, modifying, and listening!
I still have not seen a schematic of your amplifier and power supply. It would be very helpful.
A schematic can help us from making assumptions about your amplifier topology, preventing us from making wrong answers to your questions.
Instead of you trying so many variations, we might find one or two changes to make, that will meet your top 3 amplifier performance desires.
There are design tradeoffs, such as good damping factor, low distortion, maximum power out, musical presentation, etc.
Normally, you can not fit all 4 of those in one box.
With a schematic and the ranked order of the 3 or 4 of your top criteria, we might find you are already at optimum, or that a small change or two would get you there.
The 260 Ohms DCR on a 5.5k primary is not really bad; it causes about a 0.5dB insertion loss in the transformer at midrange frequencies.
If the 8 Ohm tap DCR is 0.4 Ohms, it will give an additional 0.5dB insertion loss at midrange frequencies, for a total loss of 1dB.
A 1 dB insertion loss with a 300B tube that is putting out 5 Watts of clean power, there will be 4 Watts at the 8 Ohm tap.
8 clean Watts from the 300B will give about 6.4 Watts at the 8 Ohm tap.
Etc.
Many builders wonder why they do not get the predicted power out; they forget about the insertion loss of the output transformer.
The Western Electric 300B data sheet lists output power, generally at about 5% Total Harmonic Distortion, and the Watts are listed Before the insertion loss of the output transformer.
What is the approximate weight of your output transformer?
______ kG
Or
______ Lb.
What is the rated maximum DC current of your output transformer?
300B tubes do have some variations from tube to tube. But if you use equal self bias resistors, and one tube has more than about 20% to 25% current variation versus the other channel's tube, I would be looking for another 300B.
In that case, changing the one self bias resistor is probably will not make the less performing tube come up to the same performance as the other tube.
Have fun building, modifying, and listening!
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Ha ha,,,,,, you can either CCS or Power Zener circuit for use as cathode bias of the 300B. Then, you can adjust the amount of current of the CCS to change the bias point. With use of the Power Zener circuit, the bypass capacitor no longer require.
Johnny
Johnny
Update,
Amp back up! The pair of 300B tubes I thought were toast are performing as a pair again!
Took a quick run through on the voltages and the 300B heaters are at 5, the cathode resistors both have exactly a 72v drop (1kohm), both 300Bs anode to cathode are both 365v, B+ 455V B+ after the opt 434 to both tubes, B++, B+++ all really close or dead nuts.
Absolutely zero hum..
Been so long since I listened to SE, have to re-calibrate my ears! Anyway needed to gloat a little....
EDIT
A big TY to all those who helped!!
Amp back up! The pair of 300B tubes I thought were toast are performing as a pair again!
Took a quick run through on the voltages and the 300B heaters are at 5, the cathode resistors both have exactly a 72v drop (1kohm), both 300Bs anode to cathode are both 365v, B+ 455V B+ after the opt 434 to both tubes, B++, B+++ all really close or dead nuts.
Absolutely zero hum..
Been so long since I listened to SE, have to re-calibrate my ears! Anyway needed to gloat a little....
EDIT
A big TY to all those who helped!!
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