Please Help me learn to Tweak

[bcmatt]

So on an 18Watt T-Rex build, the T-rex Channel is sort of fizzy sounding at any higher gain, and when turned up with the VVR not holding it back, it will squeal like crazy.
The normal channel seems to be fine so we are thinking there is too much gain too fast somewhere in the t-rex channel.
I measured the plate voltages and this leads me to believe it could be at tube stage V2b:

[IMG791]http://i27.tinypic.com/3492gd1.jpg[/img]

What do I do in this scenario? Should I lower the plate resistor value to something, or perhaps raise the cathode to ground resistor to something on that stage?
What about a snubbing capacitor on that plate resistor?
What do you guys recommend?
What is the way to go about this?

[dartanion]

When you use the Wreck channel, make sure you have the NFB engaged as it really needs NFB to be stable.

[bcmatt]

When you use the Wreck channel, make sure you have the NFB engaged as it really needs NFB to be stable.

That is very true, but it's not quite enough. it just allows a little more volume before the squealing.

[Firestorm]

Fizziness and squeal can both be oscillation issues, so you need to look at layout and lead dress especially. It's also useful to measure both the supply voltage and the plate voltage for any given stage. The difference between them is the maximum swing voltage that can come out of that stage before clipping. BUT, depending how the stage is biased, you could still clip only the positive-going portion of the signal if the signal tries to drive the tube above the source voltage (saturation), or clip only the negative-going portion of the signal if the signal drives the tube into cutoff. We think of triode preamp stages as "Class A" 'cause we design them that way, but they can be driven into Class B with a big signal or with the wrong bias, or both.

[bcmatt]

well, part of my problem here is that I have no idea what sorts of voltages to expect.

So, on this gain stage that I am suspecting I am reading 0 voltage on the grid. (this is good, right?). Across the cathode resistor, I am reading 2.88 Volts. (i have no idea what I should be expecting there. And, of course, I have 256 Volts on the plate.

Are things all wacked out, or is this something I should be able to fix with the altering of a resistor value?

[bcmatt]

Fizziness and squeal can both be oscillation issues, so you need to look at layout and lead dress especially.

By the way, it could be a lead dress issue, but I'm hoping not, otherwise I need to rip out all the components and start from scratch with the eyelet board material that arrived after I finished the amp.
[IMG1200]http://i29.tinypic.com/2znr1ur.jpg[/img]
I'm embarrassed of how I built this sort of, but I was stuck trying to do it with just terminal strips, because it had to be done before Christmas and the turret-board was MIA.

I will redo all the wiring if necessary, bu I'm hoping that it won't come to that...[/i]

[dartanion]

By the looks of the pic, I would guess it is a layout issue.

Nothing to be embarrassed about with your build. Even with "proper layout" you can still have oscillations. However, following the original layout makes sense to help ensure a stable build.

Try chopsticking and see if you can isolate the source. Also post some larger chassis/gut shots as this helps us help you.

[Structo]

I don't know that amp but if it is a sort of wreck preamp then point to point probably isn't your best solution.

You say the gain stage you are getting 2.88v on the cathode.
To my small knowledge isn't that a pretty high bias for a 12ax7?
Is that on V2b?
That voltage seems pretty high then to hit the PI with that sort of gain might be the culprit.
Go smaller on the 10k cathode on V2b.

If those are your plate voltages written on the schematic, the preamp plates look OK but I think I would try to raise the PI plates nearer to 300v.

You also may want to change the PI input coupler to a lower value than 100nf.

[jjman]

2.88 volts thru a 10k cathode resistor is only 0.288ma. That's "cool" and close to the expected idle for the 3rd stage in an Express. In the Express that stage will clip from reaching cutoff (since it idles cold) but only after the output and the PI are already clipping. I'm not familiar with your amp nor if the PI is supposed to be clipping before the stage you refer to. Looks like there is also a voltage divider reducing the input to that stage.

What is the voltage on the PS side of the plate resistors in the preamp?

[bcmatt]

What is the voltage on the PS side of the plate resistors in the preamp?

Here are a few more voltages taken:
[IMG1224]http://i30.tinypic.com/2ebe4cz.jpg[/img]