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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6Π3C Application

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#41
Captn Dave said:
I see your feedback circuit is interesting
Click to expand...
Sorry my drawing is not so clear in this area. As the feedback connections are off the PCB it is poorly captured by my circuit diagram.
I never seem to keep my documents up to date with the actual builds especially over the years when I build a number of variations on a theme.
I will see if I can find time to redraw the feedback connections more clearly and post a update.
This all started out as a rebuild of a heathkit AA-100 amplifier power section. It worked well so I have reused the design over and over in slightly different varations.

For more inspuration have a look at the circuits for the Dyna ST70 and Mark iii amplifer input stages.
They are pretty simple and work well. Only issue is both the 7199 and 6AN8 used in the past are now hard to find at a fair price.
aa-100_PCB.jpg
aa-100_top.jpg
 
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#42
Rechecking what the last version I built and changing the feedback for a common ground here is the circuit with voltages shown.
I produces about 25 watts at about .3% THD and damping is a reasonable 10.
There is a moderate level of feedback of about 20dB.
If the feedback pick up point is moved to the 4 or 8 ohm tap (adjusting the feedback resistor and capacitor at the same time) damping will improve and transient response will degrade.
There is of course plenty of room for adjusting values to suit your specific output stage and transformer.

CIRCUIT-DIA-COMMON GND.jpg
 
#43
Bluesystems said:
The issue I have is with the 6BL8 and all pentode/triode tubes that have the triode plate (pin 1) and the pentode G1 (pin 2) on adjacent pins is this creates a small amount of stray capacitance between the input grid of the high gain pentode and the in phase audio output of the triode's plate.
So at high frequencies this sets up positive feedback from the triode plate output to the pentode G1 grid input. The capacitance is very small but due to the very high gain any stray capacitance is multiplied by the gain.
For 46dB gain this increases the effective stray capacitance by 200 and this positive feedback impairs the stability and transient response of the amplifier.
I discovered this effect years ago working on replacements for the 7199 tube.
Amplifiers that had nice square wave response with the 7199 would have nasty ringing with any tube that featured pin 1 pin 2 pairing.
Click to expand...
That’s interesting. Pitty, as I have some 6U8s at hand. Log ago I rewired a tube socket to accept a 6u8 in lieu of a 7199. I didn’t have much in the way of test equipment back then and wasn’t able to detect any short coming. Ignorance and bliss…
Bluesystems said:
Be warned that a lot of sellers of the 6F12P are selling defective tubes with low emissions and transconductance.
Sadly for me I learned this the hard way. However a good 6F12P is a excellent tube.
Click to expand...
Can you suggest a source for good ones?

How are you making your regulated bias supply? I lack a suitable winding.

And thanks for taking such an interest. Your mastery of the subject is obvious and appreciated.
 
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#46
Captn Dave said:
How are you making your regulated bias supply? I lack a suitable winding.
Click to expand...
There have been posted ANY number of circuits on this forum that can generate the required negative rail from one side of a high voltage secondary.
Sorry never saved any of the links.

Here is one spin I made up based on a shunt regulator for -44 volt BIAS rail.
It has less than 1mV ripple with low output impedance and keeps the bias voltage rock solid.
It works without a dedicated bias winding.
You have to adjust C3 for the correct current in the shunt regulator for the transformer and bias circuits used.
To change the voltage regulated adjust R7.

Yes this is a lot of added complexity compared to self bias resistors.
Many stick to self bias for its simple almost fool proof operation. There are many good arguments in favor of self bias.
Yes self bias costs some power output but you could argue the loss in power is not detectable in listening to music.

Since my design are now only for my personal amusement a bit of complexity is just added fun.

-44V-SHUNT-REG.jpg

Captn Dave said:
Pitty, as I have some 6U8s at hand
Click to expand...
You could try them anyway and always change to a different tube later. How severe the effect is is very dependent on the designs phase margins to start.
 
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#48
I decided to try out a 6U8. Reviewing the ST70 schematic seems appropriate . I notice there is a 330k resistor between B+ and the cathode and I don’t understand it’s purpose, unless it’s a bleed resistor. It’s highlighted in the attached drawing. Can someone please enlighten me?
IMG_1878.jpeg
 
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