50W Plexi, Dual Rectifier, PPIMV Budget Build

[martin manning]

The drawing looks correct, but I'd bet your jumper goes to to the wrong end of the resistor connecting the two caps. The voltage matches what you measure on the first filter right?

Thinking about it a bit more, a better way to bring the voltage up would be to put about 1k in front of the diode, and 33k between the two filters. That will limit the current surge through the diode on start-up.

[Guitarnut]

The drawing looks correct, but I'd bet your jumper goes to to the wrong end of the resistor connecting the two caps. The voltage matches what you measure on the first filter right?

Thinking about it a bit more, a better way to bring the voltage up would be to put about 1k in front of the diode, and 33k between the two filters. That will limit the current surge through the diode on start-up.

The jumper is in the right place. The voltage that was showing at the other end of the jumper was in between readings at either end of the 15K. That was the stumper and likely had something to do with the MV.

I replaced the 15K before seeing your post. I used a 43K and it brought the voltage up to -53V. The bias pot now gives me a range of about -53V to -42V. I might still add the 1K to bring it up a bit more.

I also replaced the errant 470R mix resistors with 470K and removed the MV and soldered in 220K resistors.

Peace,
Mark

[Guitarnut]

I powered up last night. The amp was quiet while the voltage came up but then made a horrible sound like an eagle screeching at about 120dB. It was a loud, half second burst, then pulsed getting faster every time while I scrambled for the plug. I powered down and realized I had left a ground loose when replacing the 470K mix resistors. It was the ground for the 4 first components on the board...2 caps and 2 Rs at the input stage end. I soldered it in place and powered up again. Now it's just a constant 120dB assault.

I'm unsure it's related to the ground. I think with the bias in the proper range, the power tubes are now conducting and revealing a problem. Is this is the parasitic oscillation I've been expecting?

When I get the dogs out this morning, I'll record a sound clip.

Peace,
Mark

[Guitarnut]

Here's a clip. It made the sound for just a second and then blew the fuse.


http://www.crenshawweb.com/plexi50/sound.mp3

[martin manning]

The jumper is in the right place. The voltage that was showing at the other end of the jumper was in between readings at either end of the 15K. That was the stumper and likely had something to do with the MV.

The only way the bias voltage can get more negative is if the jumper or some later wire or component gets connected to some earlier part of the bias circuit. Anyway, the bias range looks good now.

Try it with just the power tubes in, and check the bias.
If it's quiet that way, then add the PI tube and see if it makes the noise.
If it does, try disconnecting the negative feedback.
If it stops, then try reversing the OT primaries.

[Guitarnut]

Try it with just the power tubes in, and check the bias.
If it's quiet that way, then add the PI tube and see if it makes the noise.
If it does, try disconnecting the negative feedback.
If it stops, then try reversing the OT primaries.

Will do this evening.

Thanks!

[Guitarnut]

Curiosity got the better of me and I gave this a try before going to work.

It was quiet with just power tubes, Bias is -45v. Added the PI and the original pulsed screeching came back. Removed the NFB and it's quiet again. Should I add the remaining tubes and try again?

[martin manning]

I think it's safe to say the problem is you have positive feedback, so you need to reverse the OT primary leads (blue and brown? ...leave the CT alone) or the reverse the leads to the PI coupling caps.

[Guitarnut]

Yep, that would make sense. I removed them yesterday when I took out the MV. Must have reversed them going back in. I'll swap them this evening.

I added the other tubes and it's whisper quiet. I got brave and plugged in and it's ALIVE! It sounds pretty decent considering the speaker is open air right now. Gotta tie up a few details and get the cab finished this weekend.

Thanks again Martin!

[Guitarnut]

Here are a couple of down and dirty sound clips. Recorded on iPhone with open air speaker.

httP://www.crenshawweb.com/plexi50/plexi01.mp3

httP://www.crenshawweb.com/plexi50/plexi02.mp3

[martin manning]

Sounds good! Kicks ass, even, and not too shabby for a "junk-box" amp!

This is with no negative feedback? Which rectifier?

For the benefit of all here, can you document the voltages, and how they change when you switch from vacuum to SS rectification?

[Guitarnut]

Sounds good! Kicks ass, even, and not too shabby for a "junk-box" amp!

This is with no negative feedback? Which rectifier?

For the benefit of all here, can you document the voltages, and how they change when you switch from vacuum to SS rectification?

Thanks Martin!

This is without NFB and with a GZ34 tube.

Sure, I'll do a full wrap up when I get the SS recto back in and have the feedback restored...if I restore it. And better sound clips when I get it buttoned up in the cab.

Peace,
Mark

[Guitarnut]

...or the reverse the leads to the PI coupling caps.

To make sure I understand this, you're referring to the leads at the caps with the 47pf cap between them?

[Guitarnut]

I just added the SS recto back in and it definitely has a different personality than the tube but it's hard to say where they differ without the speaker in the cab. I'll know more this weekend.

Voltage comparison and sound clips to come.

Peace,
Mark

[martin manning]

...or the reverse the leads to the PI coupling caps.

To make sure I understand this, you're referring to the leads at the caps with the 47pf cap between them?

Yes, the ones that go to the power tube grid stopper resistors (used to go to your MV).