So testing my new tube amp. I'll post schematic if this goes further. Was chasing down some hum and ended up with twisted pair going to input jacks on back panel. With a shorting jack loaded it is dead quiet. Same condition with Mogami coaxial with shield and would hum to much. Anyway everything good but then I pulled the shorting jack and went to squealing terribly. So all the guitar amps I work with use a 68k loading resistor soldered right to the input jack. Do I need to add something like that?
Squealing is usually too much gain or incorrect input wiring. Your schematic would help.
If you turn the volume down, does it then behave I wonder.
If you turn the volume down, does it then behave I wonder.
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That's strange, they usually have an input impedance of about 1 Mohm.
Could there be any capacitive coupling from output to input? For example, does the twisted pair get close to the wiring of the output transformer?
Parasitic capacitance coupling can occur if the wire from the power supply to the OPT is too close to signal wires. If you have a squeal above a certain volume point, you need to fix your wiring.
So here is the latest. I have stopped the squealing by replacing the mogami with tightly twisted pair used in industrial controls wiring. But when I pull the shorting plugs and just leave the inputs open circuit, I get humming. I do not know if that is unexpected or not, or a problem. When I connect an input device CD, MP3 Player any high level source it is dead quiet. When the input device is off or on or paused there is dead quiet until I play music. So is that a problem?
I don't see why it would be a problem, unless you like to listen to amplifiers with open input. You might get hum if you ever connect a source with a series muting switch, but then there is probably a resistor to ground in the source, after the muting switch, that makes the hum much softer than it is now.
Since this seems to be a HI-FI amp, and you are asking if there should be some sort of resistor on the input... like a guitar amp, is it safe to assume that there is no resistor from line in to ground there now? Is it a tube driver there with no grid resistor? Your amp seems to be only marginally stable if it ''squeeled'' when you triggered some noise at the input by pulling the shorting plug. That would be oscillation. Is there a feedback line in the amp?
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So there is a resistor to ground I believe it is like 1M. So maybe just like an open circuit anyway. It has a low pass circuit at input. I'll post schematic.
R14 and R1 are effectively in parallel, so it's about 340 kohm. That is already a bit low for a guitar amplifier, so I wouldn't reduce it if the amplifier is to be used with electric guitars.
Assuming it's a soft hum and not a hum so loud that it scares you, I'd say there is nothing wrong with the amplifier anymore.
Assuming it's a soft hum and not a hum so loud that it scares you, I'd say there is nothing wrong with the amplifier anymore.
So next question. As far as biasing for class AB I'm running 518 on the plates @ 40mA. It is a matched Quad so things are closely matched. This is at about 45-46% bias based on a plate max power of 46W. I want it to be on the cool side. I know guitar amps will shoot higher like 70%. So what do people think about the bias level I'm using??
I think your problem with the oscillation is the experiment with the OPT's and tubes. Made from 2 torroid PT's ? I don't see how any accurate bias point can be normally plotted with this config. Out of my wheelhouse.
Oh well this is the original amp made by another DIYer. I'm only using the circuit layout. My outputs are KT90s with fixed bias and OPT on outputs are Edcors. hammond power XFMR. So not exactly like the diagram. It is connected as UL in mine. I'm using 100W output OPT at 4200 Ohm primary.