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In Reply to: RE: much better! posted by vinnie2 on February 22, 2019 at 06:21:16
It is usually pretty unambiguous when the clipping starts, in my experience. The scope trace looks like a nice sine wave, and as you crank up the level you reach a stage where the tops (and or bottoms) of the sine wave start to flatten off. The maximum level before clipping is the level just before any sign of flattening of the positive and/or negative peaks begins.
Follow Ups:
This is what 2.4% distortion looks like, you can see one half of the cycle is pretty flat. If you adjust the volume down just enough to make the flat part go away, your at about 1-1.5% distortion which is acceptable for a tube amp. This should get you pretty close without using an analyzer.
I think I have now tweaked the 845 about as much as I can. It sounds pretty good and the op points are close to target values.
Target values
plate volts 430 vdc
plate current 62mA
grid volts -51 vdc
As breadboarded now
plate volts 487 vdc
plate current 62.2 mA
grid volts -52.2 vdc
I like the sound of it in mono, so now it's time to breadboard the other channel and see how it sounds in stereo. After that I will take another shot at the 813. Wish I had some low voltage target op points for it, but I have not found any so far.
It may be my equipment or the operator, but I am having a hard time getting a sine wave like that. Here's what I have been able to get so far. It's kind of weird because it starts to flatten out at about 1/2 volume (2.3 vac) and the flattens more as you increase volume until you get up to almost full volume. That's the top pic (9.8 vac). The interesting part to me is I don't really hear any increasing distortion in the music until almost full volume. So what do you guys think?
Edits: 02/23/19 02/23/19 02/23/19 02/23/19 02/23/19
That top pic is pretty ugly, probably your driver giving up
The sine wave should be symmetrical, whats your driver tube?
1/2 of a 6sn7
Edits: 02/24/19
According to my simulation, your 845 is starting to pull grid current at about 3 volts output (about 1 watt) using your operating points. Your 6SN7 driver is not going to do its job alone. Your sine wave isn't clipping but it sure is producing harmonics (on paper). It would be interesting to see what the real harmonics would be using your OPT. Depending on your music source, your probably not picking up on the harmonics but they are definitely there according to your scope. What is your plate and cathode resistor values.
You should look into a UMC204HD and the free ARTA Software. You get a scope, THD, FFT along with many other features. Its easy to implement and setup, I set it up in an hour. It's not too bad and reads very close to my HP Distortion analyzer.
Edits: 02/24/19
What would happen if I used both halves of the 6sn7 in srpp?
I'm not an expert in SRPP but I don't think so. It may help the output impedance and distortion but it wont provide grid current.
An IT transformer or a mosfet (grid drive) would be better suited for this or run the voltage up a little higher.
With your current driver and voltage, not sure you will get more than a couple watts on a low voltage 845. Those transmitting tubes start pulling grid current early.
You can do better using a 300B and its easier to drive. I would use some DHT's to drive it as well.
Actually, I can live with a couple of good watts with my Altec's. I am playing one channel in mono right now, and I do believe it would play more than loud enough for my tastes with a stereo pair. Sooooooo, before I spend any more effort on this project I am going to breadboard the other channel and see how it compares to my SET 2A3. That is the amp it has to beat to stay in my permanent setup. I already determined a while back that I like the SET 2A3 better than the 300B model 91 clone it replaced.
Edits: 02/25/19
If your are referring to the WE 91 you probably didn't like it because of the pentode drivers.
That sounds like a good candidate for another breadboard project one of these days. Would you happen to have a schematic of a good one you would be willing to share?
You don't really need much of a schematic, simply run any DHT at its op point and couple it with a cap.
Here's one I did back in the day, i got about 6-8 watts depending on transformer
nt
.
Have Fun and Enjoy the Music
"Still Working the Problem"
I got the following values with the amp turned on, no signal.
plate volts pin 2 to pin 3 103.7 vdc
grid volts pin 1 to pin 3 1.31 vdc
bias volts pin 3 to pin 1 minus pin 1 to grnd 1.31 vdc
how much plate current?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It's got a 1.5vdc nmh battery from the cathode to ground. How do you figure the plate current with that arrangement?
Measure the voltage across the plate resistor.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Well there is part of my problem, I thought the flattening would occur on both the positive and negative sections of the sine wave. Thanks, I will try it again.
"I thought the flattening would occur on both the positive and negative sections of the sine wave"
It would if the tube was 1. very linear and 2. biased to idle right at the half way point between cutoff and saturation.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"The maximum level before clipping is the level just before any sign of flattening of the positive and/or negative peaks begins."
I agree. Once clipping is reached, the HD is high and that power is not of much use.
It's a available power BEFORE clipping that vinnie wants to know.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Yeah, that is what def said too, but I am finding very difficult to find a spot where it seems to be clear cut. I mean there is quite a bit difference in voltage between where it is a rounded top and where it is a flattened top. I was hoping to get a better estimate of the power than just 2 to 4 watts.
Edits: 02/22/19
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