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 Post subject: Re: Homebrew mono audio amp project
PostPosted: Feb Thu 18, 2021 2:44 am 
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black85vette wrote:
I don't have all the parts I need for the 6L6 amp so I moved ahead with a 6V6 version because I had all the parts on hand. I got it going and it sounds pretty good without the first stage. I don't have enough heater current to add the other tube but tomorrow I will hook up a second transformer and put in the other tube. I wanted to go ahead and at least see what this configuration is able to do. Sound is clean and as loud as I need it to be. It is running on my bench test supply with variable B+ so I can confirm the range of voltage it will work with and not have any issues. Right now it is running at 350 vdc and that will be my target as I source a power transformer. I swapped a 12AT7 for the 12AU7. It worked without any noticeable difference. Here is my starting point:


Referring to the schematic on this post:

One thing you could do which is an improvement is to put a resistor in the transformer secondary winding's center tap, and bypass that with an electrolytic, and use that for fixed bias, for the output tubes, as many circuits do.

In the case of cathode bias, if there is just the one cathode resistor and one bypass capacitor to save on two parts, if one of the tubes has a higher standing current than the other, it will be responsible for more voltage drop across the single cathode resistor than the other tube. The other tube will also then have a decreased cathode current with a higher bias level, increase the imbalance in the cathode currents and in extreme cases one tube hogs the current. So Ideally each output valve would have its own cathode resistor and bypass capacitor, and also this way its easy to check the cathode current of each tube separately by measuring the voltage across each resistor with a meter.


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 Post subject: Re: Homebrew mono audio amp project
PostPosted: Feb Thu 18, 2021 4:37 am 
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ACORNVALVE wrote:
One thing you could do which is an improvement is to put a resistor in the transformer secondary winding's center tap, and bypass that with an electrolytic, and use that for fixed bias, for the output tubes, as many circuits do.

In the case of cathode bias, ....... So Ideally each output valve would have its own cathode resistor and bypass capacitor,


OK. Got the cathodes separated with their own resistors and bypass capacitors. I had some 200 ohm 10w so I used those.

Not sure about the center tap part. Are you talking about the taps for the grids? If so, my transformer has no taps and I am using the B+ that feeds the 12AU7 which is about 250 volts and biasing the grids with that.

Thanks for your pointers and improvements!

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Thu 18, 2021 4:46 am 
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If you converted one 330 ohm resistor for both cathodes to separate resistors, those should be around 680 ohms each. If you used 200 ohm ones, those tubes are going to be cooking, possibly to the point of damage. I'd recommend changing those out ASAP.

IlikeTech


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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Thu 18, 2021 4:50 am 
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IlikeTech wrote:
If you converted one 330 ohm resistor for both cathodes to separate resistors, those should be around 680 ohms each. If you used 200 ohm ones, those tubes are going to be cooking, possibly to the point of damage. I'd recommend changing those out ASAP.

IlikeTech


I will get right on it. I think I have some 750 ohm. Fortunately I did not run the amp for very long. Thanks for catching that.

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Thu 18, 2021 4:51 am 
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750 should be fine!


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 Post subject: Re: Homebrew mono audio amp project
PostPosted: Feb Thu 18, 2021 4:53 am 
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black85vette wrote:
ACORNVALVE wrote:
One thing you could do which is an improvement is to put a resistor in the transformer secondary winding's center tap, and bypass that with an electrolytic, and use that for fixed bias, for the output tubes, as many circuits do.

In the case of cathode bias, ....... So Ideally each output valve would have its own cathode resistor and bypass capacitor,


OK. Got the cathodes separated with their own resistors and bypass capacitors. I had some 200 ohm 10w so I used those.

Not sure about the center tap part. Are you talking about the taps for the grids? If so, my transformer has no taps and I am using the B+ that feeds the 12AU7 which is about 250 volts and biasing the grids with that.

Thanks for your pointers and improvements!


Have a look at the circuit posted by Mr. Highlander from the "RC20 handbook"

The transformer's high voltage winding center tap, is not connected to ground, instead via a wire wound resistor call it R to ground, which is bypassed by an electrolytic capacitor. A negative voltage is developed across this resistor, with respect to ground. The output tube g1 grid resistors are then returned to this negative voltage, rather than ground, and the output tubes are biased this way and their cathodes can be grounded.

One trick is to still use some low value cathode resistors for the output tubes, like 10 Ohms to act as current sense resistors. Then, you can measure the voltage drop across these with the meter to determine the tube cathode currents and adjust the value of R until it is correct.


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 Post subject: Re: Homebrew mono audio amp project
PostPosted: Feb Thu 18, 2021 5:26 am 
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ACORNVALVE wrote:
Have a look at the circuit posted by Mr. Highlander from the "RC20 handbook"

The transformer's high voltage winding center tap, is not connected to ground, instead via a wire wound resistor call it R to ground, which is bypassed by an electrolytic capacitor. A negative voltage is developed across this resistor, with respect to ground. The output tube g1 grid resistors are then returned to this negative voltage, rather than ground, and the output tubes are biased this way and their cathodes can be grounded.


OK. Thanks for the clarification. I got it now. The power supply for B+ I am using is my bench test rig with a variable output and it has no center tap. However, I am looking for a 300-0-300 transformer for this amp and will implement that change when I get it.

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Sat 20, 2021 4:48 am 
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Since I can't start the actual build until some parts arrive I have been just working to improve the performance. The gain is just a little low to use with a phone or tablet. I noticed at the grid of the 12AU7 driver that putting a resistor to ground got the instability to settle down at some settings. I don't think that is the solution but it did help localize part of the issue. I tested with an LM386 chip hooked to pin 7 (grid) of the 12AU7 and was able to fix all the issues. There was plenty of gain, very low distortion at higher volume and no instability or motor boating. The 6V6s seem to be working fine in their current configuration and the driver portion of the 12AU7 seems OK. I don't want to leave it like this since this was to be a tube project and not another 386 project. So I am assuming the rest of my issues are from the two stages prior to the driver. I am not stuck on this design and just picked a starting point using a schematic using mostly what was on hand. I would like to rework the 12AU7 (or use something else) to get the gain up, distortion down and remain stable. I need to confess that this is in a breadboard prototype with long leads, no ground planes and sort of spread out on the bench. Any suggestions or thoughts?

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Sat 20, 2021 5:14 am 
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You could swap the 12AU7 for a 12AT7. A bit more gain, and the exact same pinout. You might want to rework the stage loading resistors though, as a load for a 12AU7 isn't going to be optimal.

It should also improve distortion, though I would actually add a negative feedback loop, as that'll likely reduce the distortion, and you'll have more gain so it won't be such an issue.

IlikeTech


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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Sat 20, 2021 2:51 pm 
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IlikeTech wrote:
You could swap the 12AU7 for a 12AT7. A bit more gain, and the exact same pinout.

You might want to rework the stage loading resistors though, as a load for a 12AU7 isn't going to be optimal.

It should also improve distortion, though I would actually add a negative feedback loop....
IlikeTech


At first the 12AT7 swap produced runaway feedback. I took out the resistor feeding the second stage and replaced it with a pot. That looks like most of the problem. I can turn that down until the feedback stops and then the distortion cleans up. I ended up at about 16K ohms and the volume at that point is adequate. Certainly more than with the 12AU7s.

The feedback loop is still a puzzle. I can hook it up but it sounds like it is affecting the higher frequencies much more than the lower ones. However the sound is clean without the feedback loop connected. So that is how it is running right now. I tried it at different taps on the output transformer without much difference.

Making progress now. Thanks for all the good input.

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Sat 20, 2021 3:30 pm 
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No problem! Do you have an oscilloscope? If you do, it's much easier to adjust the NFB loop. You said you were getting oscillations with the 12AT7 with the loop disconnected?

IlikeTech

EDIT: Which 12AU7 did you replace with an AT7? If it was the second one, It probably will require some tweaked resistor values. If it was the first one, you can probably get away with it. What's the voltage like after the 10K resistor in the power supply?


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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Sat 20, 2021 4:40 pm 
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IlikeTech wrote:
Do you have an oscilloscope?

You said you were getting oscillations with the 12AT7 with the loop disconnected?

Which 12AU7 did you replace with an AT7?

What's the voltage like after the 10K resistor in the power supply?


I have a good dual trace scope

I had oscillation initially. It cleared up when I adjusted the resistor on the grid of the second stage (was 470k)

Both tubes were replaced with 12AT7

Did you mean the 15k resistor feeding the 12AT7s? 265 volts there. Plate on first stage is 209 v. Plate on middle stage is 128 v. Plate on 3rd stage driver is 151. Supply output is 350 vdc.

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 Post subject: Re: Homebrew mono audio amp project
PostPosted: Feb Sat 20, 2021 5:18 pm 
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black85vette wrote:
<edit> I just thought of a question. In the dual 12AT7 the plate on pin 1 is directly to the grid on pin 7 without a capacitor. It seems to work OK but is that common?

Since the 2nd triode is operating as a phase splitter with equal cathode and plate resistors, it will be floating in the V/2 region. The DC value of the first triode plate sets the 2nd triode grid voltage which sets the 2nd triode cathode voltage (cathode follower), and the current through the cathode resistor sets the 2nd triode plate voltage. With equal plate and cathode resistors, and the triode floating in the V/2 region, the stage will give equal and opposite signals to the two output tube grids.


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 Post subject: Re: Homebrew mono audio amp project
PostPosted: Feb Sat 20, 2021 7:15 pm 
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black85vette wrote:
two input taps going to the grids of the tubes.
Ultralinear...

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 Post subject: Re: Homebrew mono audio amp project
PostPosted: Feb Sat 20, 2021 8:20 pm 
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KK6GM wrote:
Since the 2nd triode is operating as a phase splitter with equal cathode and plate resistors, it will be floating in the V/2 region. The DC value of the first triode plate sets the 2nd triode grid voltage which sets the 2nd triode cathode voltage (cathode follower), and the current through the cathode resistor sets the 2nd triode plate voltage. With equal plate and cathode resistors, and the triode floating in the V/2 region, the stage will give equal and opposite signals to the two output tube grids.


Good explanation. I understand it now.

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Sat 20, 2021 9:07 pm 
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Is it just me, or do the choices of resistor value in the schematic posted seem really bad? Those tubes are not running in a particularly linear area if B+ is really just 265 volts. Also, 220k of loading is a huge stretch for even a 12AT7 and seems totally out to lunch for a 12AU7. Please someone correct me if I'm being an idiot, but I don't think I am.

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Sun 21, 2021 12:18 am 
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IlikeTech wrote:
Is it just me, or do the choices of resistor value in the schematic posted seem really bad?


Let me know what they should be and I will swap them out. Any change to the B+?

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Tue 23, 2021 3:28 am 
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I moved the amp to a fixed power supply which lowered the B+ slightly. It is now 335 vdc. I took some measurements on the 6V6 tubes and this is the current readings:

1. Voltage to pin 3 / plate = +329
2. Voltage drop across the 750 ohm resistors on each cathode = +18 vdc which puts tube current at 24 ma.
3. Voltage from pin 5 G2 to the cathode is -18 vdc
4. Voltage on pin 4 G1 is +257 vdc

How does that look? Any suggestions?

<edit - error above; Pin 4 is G2 and Pin 5 is G1>

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Last edited by black85vette on Feb Tue 23, 2021 3:09 pm, edited 1 time in total.

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Tue 23, 2021 4:34 am 
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I may have been wrong on my last post, so I'll hold off on that.

Pin 4 is G2, not Pin 5. Pin 5 is G1. Those voltages seem decent to me, what's the actual g2 voltage like?

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 Post subject: Re: Homebrew mono audio amp project - Push Pull 6V6 - 12AU7
PostPosted: Feb Tue 23, 2021 7:44 am 
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black85vette wrote:
IlikeTech wrote:
Is it just me, or do the choices of resistor value in the schematic posted seem really bad?


Let me know what they should be and I will swap them out. Any change to the B+?


If you want a better phase splitter I can post a schematic from work tomorrow.

It only uses a single 12AU7.


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