B+ Plummets When Power Tubes Warm Up

[The New Steve H]

I didn't have 16uF caps. I stuck 20s all the way across.

I don't suppose you'd care to give me a clue what "break point response" is.

[The New Steve H]

Now I'm thinking the cathode follower can't be the problem, because it has no gain. It can't cause breakup if there is no gain, right?

[The New Steve H]

I have been studying up on this, and somewhere I saw someone claiming the grid resistors on the output tubes can affect headroom. I have been playing this thing with the grid resistors rigged up so four tubes are using two resistors. That was just to get it running. I decided to stick the PPIMV back in and take the resistors out.

Now the amp sounds much better. Still breaks up somewhat earlier than I would like, but it sounds like an amp now. The PPIMV is crazy. You can get insane distortion at very low volume.

[The New Steve H]

Can't resist posting a baby picture.

[John_P_WI]

Steve,

The break point that I spoke of is the 3db down point on the "RC time constant cutoff frequency" and is found with the equation fc = 1 /( 2 pi R C ). This is the point that the knee "breaks" on the curve and falls at a slope of -6db per octive. I usually try to have the break point at least 10 times lower than the lowest fundamental frequency to be amplified, in other words for a guitar we would look for approximately 7 hz.

[The New Steve H]

I am now starting to be convinced that what I thought was bass distortion is actually compression. I turned the preamp up and turned the PPIMV , and I noticed that the sustain was practically endless. Also, the PT gets pretty warm. I think I'll get a PT with at least twice the current and see what happens. It sounds great, but I just can't believe it should break up this early.

[The New Steve H]

I was going to get a Trainwreck-style PT, but I decided to go with an Edcor from RJ Guitars. It's rated at 250 mA with a fairly small footprint. We'll see if it changes anything. Meanwhile, I still have the old PT in case I want to do a 2-tube amp.