Help with El84 SE design

[then1xon5]

How does this look? Based on your previous post, this shouldn't sound like a paper sac full of bees. HAHA! That was awesome. I have the gain before the first gain stage, is that correct?

Thanks!

Scott

[then1xon5]

I didn't have the power supply labeled correctly. here is the new drawing. What I meant to say in my last post was that I have the gain control before the first gain stage. and the volume control before the second gain stage. Is that correct?

[mooreamps]

I would not put a volume control in front of the first gain stage. That would put it in parallel with the volume control on the instrument.....

-g

[then1xon5]

So, would it go after the first gain stage and before the tone control?

[Bruce / Mission Amps]

So, would it go after the first gain stage and before the tone control?

Just remove it completely and sub a 250Ka to 500Ka pot (wired as a variable resistor) in place of the 220K grid load resistor on the EL84.
Now you have a master volume

[then1xon5]

I understand the master volume what I was trying to do was ad a gain contol. Would this do the trick?

[mooreamps]

Second question, I still need a 900k resistor to ground before the fouth stage correct?

Scott

Well, I guess that just all depends.......
Do you want to learn how to do a really good design, like a seasoned professional ; or not ?

-g

[Bruce / Mission Amps]

I understand the master volume what I was trying to do was ad a gain control. Would this do the trick?

Yes the volume control is in the right place now... the other pot could be used as a variable grid load on the EL84 and, the first preamp stage has enough gain, that would sound like a master volume...that allows you to turn the volume control up further can possibly creating some preamp tube distortion in the second triode.
The values you have chosen for your coupling caps, cathode bypass and the tone control are such that the amp is going to have too much gain the bass frequencies and might sound muddy.
I didn't change that part but I did change the grid load on the EL84.
See my altered drawing attached here.

[Bruce / Mission Amps]

I understand the master volume what I was trying to do was ad a gain control. Would this do the trick?

Yes the volume control is in the right place now... the other pot could be used as a variable grid load on the EL84 and that could sound like a master volume...that might allow you to turn the volume control up further (all the way) and possibly create some preamp tube distortion in the second triode.
The way you have the cathode bypass caps, tone control and coupling caps now seems like it will be too high of gain in the bass frequencies and might make for a muddy sound...


See my altered drawing attached here.

[then1xon5]

Fantastic. I didn't realize that there is that much difference between the .022u cap and the .01u cap in the tone circuit. I think I have what i need to get started. I plan on throwing this all on a temporary board and experimenting a bit with the tone and gain. Hopefully I will have it completed by Christmas time. I will post the final drawing and my findings when complete.

I would like to thank all of you for your help again. I know everyone is busy but still you took time from your days (and nights) to teach me. I have learned a great deal and know I have a long way to go. Maybe someday I might be the one helping.

My Kindest Regards,

Scott (Grasshopper)

[then1xon5]

concusion, well almost

I finished putting together this amp today according to my last drawing. It sounds great except.... there is always an except. There is a high pitch ringing when it's on. The ring gets louder with the amount of volume I give it until it's all the way up then it goes away. Also, the tone control dosent seem to be working that hot. I'm guessing that's either the caps that I used or I did something wrong wiring it. I will have to take a closer look at that one.

Any ideas about the high pitch squeel though?

Thanks!

Scott

[tubeswell]

This is very common with SE builds. You could have +ve feedback from your NFB loop going on if the primaries are wired back-to-front (which is not always easy to spot at a glance ;-0). Try swapping the primaries around by moving the plate-wire to the ground, and the ground-wire at the plate.

[then1xon5]

Thanks for the tip on where to look. You were correct and it solved part of my problem. The Microphonic squeal reduced significantly however it's still there and while I was checking the signal with the scope I turned up the volume a bit more than I had previouslybecause the squeal happens a little further up the volume pot. This was a big mistake. It started squealing and the plate terminal on the EL84 socket decided it wanted to arc weld which produced a awesome looking blue glow but I'm sure it can't be good. I will replace the socket now.

I tested the voltage off the power supply just before the welding incident. They looked a little high for the EL84 and the output primary but nothing outragous and the 12ax7 plates seemed low. The output primary was 360v, the EL84 G2 voltage was 342v and the 12ax7 plate was 220v I believe.

Ideas?

I know it will be worth it when I'm done but I'm feeling in over my head tonight.

Thanks!

Scott

[tubeswell]

Re-check all your wiring. Sounds like your re-soldering may have something shorted which shouldn't be

[Jag]

Also, there's no NFB shown on that schematic so that wouldn't be the problem. I'd be checking wire routing for an input wire being too close to an output.