PI problem

[Matty_V]

Having an issue with my 18 watter I can't figure out. I prodded it with my listening amp and discovered the signal going bad at the PI. I tested around there with the multimeter and found v3b plate to be ~100v less than v3a plate. I tried swapping tubes, checking resistor values, checking coupling caps for dc leakage, and all sorts of combinations. The only thing that makes the v3b plate load not drop so much voltage is to disconnect it from the socket. Does this sound like a bad socket? Some other issue?

[sluckey]

You ommitted a very important part to that LTP PI... You must use a coupling cap between the wiper of the treble pot and the grid of V3B. That 470K must remain connected to the grid.

[DummyLoad]

You must use a coupling cap between the wiper of the treble pot and the grid of V3B

V3A

[sluckey]

Oops!

[Matty_V]

Doh! I see. Thanks. I'll try that out. I knew something was wrong. 

[stingray_65]

Hey Matty,

Am I missing something, or are you using only half of V1?

Looks like you have a whole triode waiting to be utilized.

Ray

[HotBluePlates]

For others not following why you need that coupling cap:

You might look at the tone stack and think, "the caps in the tone circuit perform the function of blocking the voltage at V2B's plate. Why do I need another coupling cap?"

The tonestack pots provide a path to ground for V3A's grid. Normally, that's a good thing. But the LTP grids need to be able to sit at an elevated voltage defined by the inverter's 47k tail resistor, the 820 ohm bias resistor and the 470k grid reference resistor. Without an additional coupling cap between the tonestack and V3A's grid, the grid moves to 0v and passes big current. That current is pulled through V3A's plate resistor, thus the big voltage drop.

You add a coupling cap, and the grid voltage rises back up to where it should based on the cathode bias resistor and the tail resistor, and the current becomes similar to V3B.

[Matty_V]

Thanks for that explanation HBP! I was going to ask what was going on without the additional coupling cap but you beat me to it. I get it now.

Stingray, v1 is paralleled. Maybe my representation of that could be better

[stingray_65]

Stingray, v1 is paralleled. Maybe my representation of that could be better

I had assumed so, but had to ask! wasting a whole triode in a hi gain amp like yours would have been a sin.

HBP!

Thanks from me too for your explanation about the coupling cap. When looking at schematics I've often thought it was redundant and was vestigial from past engineering practices.

It had been in the back of my mind for a long time to ask, but it never came up.

Ray

[Matty_V]

Stuck in a .022 there before leaving for work and we're rockin' and rollin' now! Can't wait to crank it later. You guys rule

[John]

I'll chime in with my thanks for HPB's explanation as well.

[jojokeo]

Stuck in a "_____" there before leaving for work and we're rockin' and rollin' now! Can't wait to crank it later.

  That's got to be one of the great experiences by most all of us. The anticipation of playing & testing something just makes it that much better when you finally get to "flip the switch" and get rockin'.

[Matty_V]

Its those "flipping of the switch followed by smoke, fireworks, and burning components smell" moments I try to avoid as much as possible. Sometimes they happen