DC lowering Resistor Issue

Thread Starter

Iceberg321

Joined Dec 1, 2023
10
I’m making a tube amp circuit based on a known schematic, and I’m having trouble with higher DC voltage than expected after the rectifier and voltage doubler network. I’m supposed to be getting around 210 V DC, and have about 260 V DC present. I changed both of the 10 microfarad voltage doubling network caps for 2.2 microfarad and that reduced the output voltage slightly. Schematic says to reduce value of a 470K ohm 1/2 watt resistor to ground after this section if the voltage is too high, but I’m not getting any significant volt drop unless I reduce the value of that resistor to under 10K…and then it’s getting way too hot. This resistor is before a 2.2K ohm 2 watt voltage resistor that is followed by a 33 microfarad 350 V smoothing cap to ground. Which is where the 210 VDC is supposed to be. Ive added a second 2.2K ohm resistor there in series, and it didn’t effect the voltage. I’ve measured and it’s been as high as 305 VDC. I’m using a 30 microfarad 500 volt smoothing cap, because that’s what I had available, could this difference be causing this issue? The AC voltage feeding the rectifier and voltage doubling network is 100 V. Schematic is attached. Any and all suggestions or check points would be greatly appreciated, thanks!!
 

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dl324

Joined Mar 30, 2015
16,679
Welcome to AAC!

There is no need for duplicate posts. I flagged the other one for removal.

Your post would be easier to read if you used paragraphs to organize your thoughts.

Schematic says to reduce value of a 470K ohm 1/2 watt resistor to ground after this section if the voltage is too high, but I’m not getting any significant volt drop unless I reduce the value of that resistor to under 10K…and then it’s getting way too hot.
I've not worked on many tube circuits, but that seems like a dumb way to lower the voltage. All that does is load the transformer secondary.

How much current is involved? I'm inclined to suggest a voltage divider in place of the 470k resistor...

I’m using a 30 microfarad 500 volt smoothing cap, because that’s what I had available, could this difference be causing this issue?
Not likely.
 

LowQCab

Joined Nov 6, 2012
3,937
Are You doing your Voltage testing with no Tube in place ?
The Power-Supply design expects a Load from Tube-Bias.

If You are testing with the Tube in place,
and the Voltage is still to high,
eliminate the Voltage-Doubler arrangement.

Tubes will quite often operate just fine with the Plate-Voltage "too-low".

In fact, there are many "Effects" Circuits that operate on ridiculously low Voltages,
( low for Tubes that is ),
to create Dynamic-Range-"Compression" or Distortion-Effects.

"Compression", in this particular case,
refers to "sagging" Power-Supply-Voltages when excessive Input-Gain is applied.
.
.
.
 

Thread Starter

Iceberg321

Joined Dec 1, 2023
10
You are reading the no-load voltage. At least I think that is the case.
But if the amplifier is complete and working , that difference will not matter
The circuit has already been completed and functions fairly normally aside from too much DC voltage powering tube
 

Thread Starter

Iceberg321

Joined Dec 1, 2023
10
Are You doing your Voltage testing with no Tube in place ?
The Power-Supply design expects a Load from Tube-Bias.

If You are testing with the Tube in place,
and the Voltage is still to high,
eliminate the Voltage-Doubler arrangement.

Tubes will quite often operate just fine with the Plate-Voltage "too-low".

In fact, there are many "Effects" Circuits that operate on ridiculously low Voltages,
( low for Tubes that is ),
to create Dynamic-Range-"Compression" or Distortion-Effects.

"Compression", in this particular case,
refers to "sagging" Power-Supply-Voltages when excessive Input-Gain is applied.
.
.
.
I am testing with the tube in place and the circuit complete. I’ve tried a couple different “known good” 12AX7’s.
 

MrChips

Joined Oct 2, 2009
30,476
The circuit has already been completed and functions fairly normally aside from too much DC voltage powering tube
So what are the symptoms?
Is the circuit functioning well?

Measure the voltage across the 2.2kΩ 2W resistor. This will tell you how much current is being drawn and what effect a second 2.2kΩ would have.
 

MisterBill2

Joined Jan 23, 2018
17,800
One very simple option is based on the fact that a 12AX7 amplifier will function quite well on a lower voltage, is to change the power supply from a voltage doubler to just a half wave rectifier. The tube will last longer and the power dissipated will be reduced. So unless you need a very large output voltage swing, using about 130 plate volts will work very well.
 

Thread Starter

Iceberg321

Joined Dec 1, 2023
10
Welcome to AAC!

There is no need for duplicate posts. I flagged the other one for removal.

Your post would be easier to read if you used paragraphs to organize your thoughts.

I've not worked on many tube circuits, but that seems like a dumb way to lower the voltage. All that does is load the transformer secondary.

How much current is involved? I'm inclined to suggest a voltage divider in place of the 470k resistor...

Not likely.
I’m
So what are the symptoms?
Is the circuit functioning well?

Measure the voltage across the 2.2kΩ 2W resistor. This will tell you how much current is being drawn and what effect a second 2.2kΩ would have.
The tremolo section of the circuit seems to be functioning quite nicely. I’m concerned that I’ll be burning through tubes running them at 230-250 volts, or that I’ll burn up the 470K resistor (it’s 470K on schematic, but I’m using a 10K 1 W in its place).

I’d say the symptom is the heat on that resistor, and the seeming lack of ability to get the voltage a bit lower as designed.
Maybe I will calculate and insert a voltage divided before the 2.2 K resistor and see where that gets me. I measured a 1.7mV drop across it, so that puts the current at 7.5mA which seems pretty low to me.

I calculated the plate current to be 231mA.
 

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Thread Starter

Iceberg321

Joined Dec 1, 2023
10
One very simple option is based on the fact that a 12AX7 amplifier will function quite well on a lower voltage, is to change the power supply from a voltage doubler to just a half wave rectifier. The tube will last longer and the power dissipated will be reduced. So unless you need a very large output voltage swing, using about 130 plate volts will work very well.
It honestly seems like my best bet, and an easy alteration. For the sake of understanding what went wrong here as a learning experience I’m still trying to figure out why the DC volts are so high in that section of the circuit. Pin one is receiving 160-170 volts, which would be fine except for the super hot resistor putting lots of voltage to ground before that.
 

MisterBill2

Joined Jan 23, 2018
17,800
It honestly seems like my best bet, and an easy alteration. For the sake of understanding what went wrong here as a learning experience I’m still trying to figure out why the DC volts are so high in that section of the circuit. Pin one is receiving 160-170 volts, which would be fine except for the super hot resistor putting lots of voltage to ground before that.
It may be that you havd some voltage step up with that two-transformer isolation scheme. If you have a means of measuring AC voltage, check the secondary of the transformer feeding the rectifiers, It might be more than 120 volts. The last mains-fed voltage doubler I built delivered about 270 volts. It was a lot more than twice the 120 volt mains source. OR, maybe the original version of the circuit used selenium rectifiers. Much lower efficiency.
 

Thread Starter

Iceberg321

Joined Dec 1, 2023
10
It may be that you havd some voltage step up with that two-transformer isolation scheme. If you have a means of measuring AC voltage, check the secondary of the transformer feeding the rectifiers, It might be more than 120 volts. The last mains-fed voltage doubler I built delivered about 270 volts. It was a lot more than twice the 120 volt mains source. OR, maybe the original version of the circuit used selenium rectifiers. Much lower efficiency.
I have checked the AC voltage at the rectifiers, and it measures only 100 volts. I will check again though and make sure.

it does now occur to me that the indicator light I’m using is coming off the 12.6 secondary side of the first transformer, which is directly linked to the second transformer. It’s a 12V 5W bulb. I decided to use this option instead of a 120V neon indicator wired to the primary mains input of the power section as in the schematic.

Thanks for your input
 

MrChips

Joined Oct 2, 2009
30,476
Here are my thoughts.
High voltage on the tubes will not burn them out. High current will.
Your numbers are way out in left field.
I = V / R = 1.7mV / 2.2kΩ = 0.8μA which does not sound right.
Your calculated plate current of 231mA is crazy.

Here are the 12AX7 specs.
Plate Voltage max = 330V
Plate Current max = 3mA
Typical operating plate voltage = 250V
Typical operating plate current = 1mA

I would get rid of the 470kΩ resistor. All it does is generate heat.
If you are uncomfortable with 300V plate voltage then get rid of the voltage doubler.
 

MisterBill2

Joined Jan 23, 2018
17,800
I am not sure what tube manual post #13 used. My RCA manual gives "typical conditions for a class "A" amplifier as 100 volts on the plate (Vp), which is what I used, or close to it, back when the 12AX7 was what we used for a first stage audio amplifier, where a large voltage output is not required. An application driving an output stage would be different.

As for usingthe back-to-back transformers, that is a scheme for areas where the only transformers available are doorbell transformers. In some areas electronic parts are not available locally. The scheme will work and it is a bit more tolerant of wiring errors.
 

Thread Starter

Iceberg321

Joined Dec 1, 2023
10
I am not sure what tube manual post #13 used. My RCA manual gives "typical conditions for a class "A" amplifier as 100 volts on the plate (Vp), which is what I used, or close to it, back when the 12AX7 was what we used for a first stage audio amplifier, where a large voltage output is not required. An application driving an output stage would be different.

As for usingthe back-to-back transformers, that is a scheme for areas where the only transformers available are doorbell transformers. In some areas electronic parts are not available locally. The scheme will work and it is a bit more tolerant of wiring errors.
That makes more sense to me as well, and because this isn’t driving an output tube section it seems more reasonable. This essentially is a glorified guitar effects pedal using a 12AX7.

As far as the transformer scheme goes: I specifically chose this circuit for the two LOW COST ($13 each) transformers. I actually got them from that website mentioned in post #14. I’m transitioning from building and repairing guitar effects pedals that typically run on 9V batteries, or 9V wall wort adapters. That means I’m still learning about using higher voltages and dealing with transformers or AC mains powered circuits at all. This circuit allows me to learn and operate these high voltages and filter caps safely so I can more confidently move on to building actually tube amps with power tubes and output transformers. All that being said using two low cost transformers like this circuit is designed for gives me a foot in the door without having to fork out 80-90 bucks just to run one 12AX7.
 

MisterBill2

Joined Jan 23, 2018
17,800
Fir effects pedals, probably a single IC regulator will be the way to go. With an adequate input voltage they should provide totally hum-free DC.
And my more current RCA tube manual described the 12AX7A amplifier as having a gain of 100 with either 100 volts or 250 volts,
with either 0.5 mA or 1.2mA plate current., depending on the voltage.
 

Thread Starter

Iceberg321

Joined Dec 1, 2023
10
Welcome to AAC!

There is no need for duplicate posts. I flagged the other one for removal.

Your post would be easier to read if you used paragraphs to organize your thoughts.

I'm inclined to suggest a voltage divider in place of the 470k resistor...
I’m happy to report that this solution has worked. After debating making a voltage divider, or scraping the voltage doubler and putting in a half wave of full wave bridge rectifier I decided to give the voltage divider a try before dismantling the voltage doubler.

This worked perfectly, and it sounds amazing! I’ve added a second smoothing cap for better audio performance. The attached image is the change to the schematic that I’ve used. (Edit: upon reviewing my sketch of the circuit change I’ve realized I forgot to tie the lower 2.2 uF cap and diode to ground in the voltage doubler network) Voltage and current are stable after tube warms up, and the whole secondary power section is now operating at relatively low temperature. Once the circuit has discharged I’m able to touch every resistor without feeling any significant increased heat above room temp.

Thanks everyone!
 

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Thread Starter

Iceberg321

Joined Dec 1, 2023
10
Fir effects pedals, probably a single IC regulator will be the way to go. With an adequate input voltage they should provide totally hum-free DC.
And my more current RCA tube manual described the 12AX7A amplifier as having a gain of 100 with either 100 volts or 250 volts,
with either 0.5 mA or 1.2mA plate current., depending on the voltage.
Thanks for your encouragement and input! It’s all sorted and working now. See post #18 for schematic revision and the solution I’ve opted for.
 

MisterBill2

Joined Jan 23, 2018
17,800
One caution here, which is that while your original drawing shows isolation transformers to avoid a serious shock hazard, the circuit shown does not provide isolation from the mains. ( I am obligated to provide this warning about the hazard.) At the very least, direct mains connection will cause hum and noise problems in the audio system.

With the resistor values shown the amplifier circuit will work very well and all of the components will be working within their ratings.
 
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