and how do I fix it?
Trying to determine whether this pre-amp I built is working properly. I cannot seem to get a trace on the O'Scope that seems meaningful. I can get a great trace running the Signal Generator directly into the scope but with the same settings, the pictures show what is displayed. I've used the auto feature a number of times as well as trying manual setting the scope.
A whole bunch of RF noise! You are driving the preamp directly into the input of your scope. The input impedance of the scope is probably on the order of 1M ohm! The output of the pre. is not "loaded" properly.
You need to load the output of the preamp with 20K-100K ohms( if it is a SS pre. can use 10K). Use whatever 1/4w resistors you have laying around. Solder them across an old/used RCA connector and plug those into your preamp outputs. Leave just a bit of the leads exposed so you can hook onto signal hot/ground with your mini-grabbers. Feed that signal into your scope and see what you get.
OK, well that circuit doesn't have much gain and is going into hard clipping. BUT, what resistor value did you choose to load the output with?
Many tube preamps are not too happy driving "low" impedance loads (low in the tube world, that is). My tube pre will drive 10s of volts output into a 10K ohm load all day long, but it uses a cathode follower and insane power supply. Many tube preamps based on medium mu triodes and/or old circuits were meant to drive "tube amplifier" input impedances of 100K and above. Is this your 76/2C22 based preamp ?
So, if you are using load resistors lower than 100K ohms, swap them out. If that does not get more gain before distortion kicks in, then there is a problem.
Was this circuit working good and then all of a sudden you lost your gain and distortion set in? Or, is this a new circuit you are building up?
If all was good and then went downhill, of course the first suspects are going to be the tubes. I kind of doubt it though, since both channels are behaving identically. So, if the circuit is not happy with the higher resistor values (lower load) on the outputs, I am afraid it will be time to post up the schematics and some voltage measurements. The problem would have to be shared between both channels. B+ supply and/or negative bias supply (if so equipped).
So, hang some 100K resistors on there, and let's see what you get!
Thanks. Will relook at it tomorrow. I'll send you the schematic privately as the designer prefers to keep it off public forums. Doesn't care for the criticism
So, I'm trying to determine why my waveform flattens at higher volume. I have the yellow channel on the output and the blue channel on the grid of the second stage tube. Note the difference in shape and PP voltage. There is feedback in the circuit, I wonder if that could be the issue. Both channels behave identically on the scope.
Well, let's see. You are distorting the top of the waveform on the output at about 3V p/p. So, can you get a clean 2V p/p out of it? That would give you 1.4v rms, which should be enough to drive most amplifiers to full output.
Does the output use one tube for both channels? If so, could still be a tube problem. But, the waveform looks like "textbook" too low of an input to the output tube grid. Suppose it could be from feedback, depending on where it is tapped off.
Does the output tube circuit use a grid leak resistor? If so, what is the value? This will affect how much "drive" the output tube grid sees. Now, if so, the designer may have had a very good reason to design it that way. Maybe the circuit doesn't sound as good with the output tube driven harder.
Don't know. I am not a amp. designer. I don't even play one on TV (or the Internet for that matter). I let someone else do the math and mess around with simulation software, I just like to build-up proven designs.
If the circuit uses a grid leak resistor, you can try increasing its value. Less voltage from the preceding stage will be "bleed" off and the grid will be driven a bit harder. But if you want to really start changing the circuit, then you are going to have to "do the math" and consult the tube specifications and characteristic graphs.
