This is my 6J6-based LTP. For the time being, try to overlook the tail, cathode, and plate resistors - I'm still working out bias and balance after scrapping the CCS.
My question is about NFB, which is coming in via the 8k2 and getting divided by the 100R. That's only pushing 1.2% of the OT signal into the lower grid. For comparison, Fender AB763 uses 820R and 47R for about 5.5%.
The question is, if I want to increase the NFB, should I reduce the 8k2, or increase the 100R? The former would put more NFB onto the tail resistor, while the latter would push up grid and cathode voltages (albeit only slightly). Or should I drop them both down ala Fender? Is there a best practice here?
My question is about NFB, which is coming in via the 8k2 and getting divided by the 100R. That's only pushing 1.2% of the OT signal into the lower grid. For comparison, Fender AB763 uses 820R and 47R for about 5.5%.
The question is, if I want to increase the NFB, should I reduce the 8k2, or increase the 100R? The former would put more NFB onto the tail resistor, while the latter would push up grid and cathode voltages (albeit only slightly). Or should I drop them both down ala Fender? Is there a best practice here?
Dropping both by the same ratio would have no affect. Reducing the 8.2K resistor will increase NFB.
Reducing the 100 ohm resistor will decrease NFB.
Also the schematic appears to be drawn wrong.
The bottom of the 5,1K goes to ground and the 8.2K resistor goes to the 100 ohm and .1uf point.
Reducing the 100 ohm resistor will decrease NFB.
Also the schematic appears to be drawn wrong.
The bottom of the 5,1K goes to ground and the 8.2K resistor goes to the 100 ohm and .1uf point.
Gotcha.
Are you sure? Unless I'm missing something, the AB763 has the tail going to the grounding resistor, not directly to ground:
Aiken's LTP example looks like it does the same. Or am I totally missing something?
I mean, I don't see a reason why it couldn't go directly to ground - it's not like an extra 100R makes a difference. Unless there's some reason that we want the NFB signal to be present on the tail resistor, I guess?
The linked Aikens LTP seems to also add a presence control into the loop.
If you were designing an audio amplifier with a LTP, feedback would really be going to the opposite grid through a coupling capacitor after being divided by the 8.2K and 100 ohm resistor.
Looks just like a typical input LTP on a solid state amplifier.
What are you building? Usually NFB is injected onto the cathode side of a single tube gain stage and then a LTP phase splitter comes next.
If you were designing an audio amplifier with a LTP, feedback would really be going to the opposite grid through a coupling capacitor after being divided by the 8.2K and 100 ohm resistor.
Looks just like a typical input LTP on a solid state amplifier.
What are you building? Usually NFB is injected onto the cathode side of a single tube gain stage and then a LTP phase splitter comes next.
The el-cheapo amplifier is a good example of LTP and NFB on the front end.
El Cheapo, builders thread
El Cheapo, builders thread
If you were building a typical HiFi amp, then yes, this drawing is wrong.
This "error" can be seen in some of Leo Fenders earliest amps. Rumor has it that wiring it this way saved him a terminal or two on the fiber board. It worked, but since some of the feedback is common mode, he simply turned it up.
This version has been seen in countless guitar amps ever since.
I honestly don't know. I put that in over a month ago and haven't the foggiest idea where it came from.
I put in a pot and experimented to find something that made sense, but it mostly made it more prone to oscillation unless I sent it in full blast. So I'm going to leave NFB out completely for the time being while I work on finishing up the rest. Hell, it's a guitar amp, right?
There is a schematic of an Orange Dark Terror in post #13 of this thread. Notice the similarity to your PI. Either way, I always leave any GNFB disconnected until the amp works and is stable without it.
VOX style EL84 PP amp questions
VOX style EL84 PP amp questions