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In Reply to: RE: Example: Don't trust schematics on the internet posted by deafbykhorns on August 08, 2015 at 17:39:35
If I read the sim correctly (I might not!) you have positive feedback, not negative feedback. That would exacerbate distortion rather than reducing it ... :^)
Follow Ups:
I don't see the mistake in my schematic against the published.
The notes below are correct. But to be honest, the high distortion is what made me look for positive instead of negative feedback.
Yeah, swap the output terminals and see if the gain goes down along with the distortion.
There are three stages (well, three that invert), so the output is out of phase with the input but it (the feedback line) is connected to the cathode of the input stage.Since the cathode follows the grid the feedback needs to be in phase with the grid for it to be negative feedback.
In this circuit that can be accomplished by wiring the output transformer so that it reverses phase.
I hope I said that right.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/09/15
Per the discussion below, the OPT in the published schematic must invert phase, otherwise FB will be positive.
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Buy Chinese. Bury freedom.
Reversing fixed the pos feedback problem, amp still distorts bad.
Last stage seems like its biased wrong. Doesn't look good on 6sn7 tube curves (if I'm looking at it correctly)
Output is the same, -130v is about 45ma at 845
LT Spice? Cant believe schematic could be that far off.
Ah, I understand now. Not experienced on FB until now.
What would be a good way to setup a simulation of the transformer inverting phase? Would these be as easy as moving the connections around?
Yes, just swap the two output connections at the secondary.
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Buy Chinese. Bury freedom.
I don't see the coupling directive in the original sim but it can be adjusted to determine phase. Typically it reads something like:
K1 L1 L2 1
this tells spice that inductors L1 and L2 are ideally coupled together. Taking the coupling factor to a value lower than 1 introduces leakage and iirc a factor of .999 represents 1mhy of leakage. Another nice point of the K command is if you change the 1 to a -1 you reverse the phase of one of the coils.
Since I do not see the K command in the original I suspect some for of complete transformer model is being used at which point the value for K could be made into a {parameter} and varied with a directive.
dave
I know how to couple but have not learned how to input/calculate an output trannys inductance, series resistance, etc...Wish I knew how to do this for power supply transformers as well.
Edits: 08/10/15
It is really simple. The inductance is simply the square of the voltage ratio so a 120:240+240 transformer would be a 1:2+2 voltage ratio or a 1:4+4 inductance ratio. It is also important to remember that spice treats all AC voltages as peak so for 120Vrms you need to put in around 170V.
dave
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It's not that complicated. If FB is +, the circuit will oscillate. You will have a 22W tone generator. Reverse FB polarity, the circuit will work.This is DIY, right?
Edits: 08/09/15 08/09/15
CIrcuit still distorts bad after first stage.
Bias has to be way off by the looks of tube curves on 6SN7
The schematic is using the OPT to reverse phase, although the OPT itself isn't marked. The OPT in the SIM is in phase.While we're at it, what's the purpose of VB2? Why didn't the designer simply connect the cathode of VB1b to coupling cap C4?
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Buy Chinese. Bury freedom.
Edits: 08/08/15
Second generation of the amp gets rid of the V2 cathode follower.
Dan Santoni
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