Rackmount Poweramp Build, A few Questions....

[nandrewjackson]

One channel, 4-6L6 or EL34.

Volume, Presence controls.

I'm not sure if I should use Audio or Liner taper pots for the Volume and Presence controls.
They are both being used as Variable Resistor setup, not voltage divider.

The Value for the Cathode resistor on the phase splitter is bugging me.  I've seen schematics that use 470 OHM and some that use 475 K Ohm.  That seems like quite a big difference to me. Any thoughts?

Also, The NegBias circuit resistor values may be wrong, but I figure they will need to be tweaked anyway to get the right value of Bias.

I have the Boards 90% done. One board is the pre section and Caps and bias circuit, the other board is the 4 powertubes and their resistors.
Chassis has not even been started yet.

[nandrewjackson]

BTW, my references for this circuit are Fender Twin, Mesa Recto (and Mesa Rack Poweramps), EVH 5150, and Hoffman Plexi Circuits.

Any other Comments, Concerns are welcomed.
 

[sluckey]

The Value for the Cathode resistor on the phase splitter is bugging me.  I've seen schematics that use 470 OHM and some that use 475 K Ohm.  That seems like quite a big difference to me. Any thoughts?

475KΩ is extreme. Can you post a link to a schematic that uses this value?

[LooseChange]

Looking forward to more pitures. Tell me how you will be keeping it cool.

[nandrewjackson]

Sluckey:   BIG BIG oops on my part.  The 475 that I thought was K was actually an R after the 475.  I guess that's another way of notating values? ( 475R ) ? 

DrGonzo;  thanks for the input.  I was unsure mostly of the volume when it's wired as variable R, instead of  the usual voltage divider with the swiper being output.  I guess I'll go with common values and see how my voltages turn out, then go from there.

LooseChange;  I'm still not sure about cooling.  I know there will be lots of vent slits cut in the top, bottom, and sides of the enclosure.  I'm debating an internal fan, or possibly rigging up a fan inside my rack box, but not inside this unit.  I haven't decided.

[LooseChange]

Do you have the rack mount chassis? How many U is it?

[nandrewjackson]

I will Fab the chassis.  I get the pieces cut and/or bent at a metal shop that's local.  I then weld, drill, paint myself.

3U is just big enough for the Transformers.  The powertubes will be horizontal.  I'm not sure if the board with the preamp tubes and caps will be horizontal or vertical.

It will have to be about 12" depth to contain everything.

[LooseChange]

Fabricating your own rackmount chassis is cool. I buy mine.
Have you seen my rack preamps? They may give you a hint or two. I've been wanting to do a rack 50w power amp.

[nandrewjackson]

Yeah, man, your stuff looks real good.  Clean, Pro, very well made. 
That twin reverb pre is top-notch.
I got the materials for my preamp for about $23 from that shop. "Summerlot Engineering" (local to me) they do work for a lot of the Farms, Factories, and Contractors around here, but they don't mind small jobs like me coming in and wanting some steel.
my stuff turns out pretty rough, but it's for personal consumption, and I'm not very concerned about the presentation.



that's my preamp.  Functional, and somewhat durable.  I haven't drop-tested it like Carvin does with their poweramps, but so far after a couple gigs in my rackbox, it's doing fine. 

[LooseChange]

Now that is very cool. The color is perfect. I love seeing how others use creative methods to house and label their projects.
I tend to go foe the commercial look. Probably a bit on the generic side.

[nandrewjackson]

Thanks.

18ga front panel.  22 ga. on the rest.  Rustoleum "Key Lime Green" on top of primer.  I had the controls all labeled with Sharpie.    That's been sanded out and repainted, gonna go to a local printshop and have some decals made for lettering.

I don't think yours look "generic".  I'd love to have the skills to put something out like that.

[Willabe]

I think even with a fan there's not enough openings in the chassis to draw in cool air and push out the hot air.

Maybe if you had a small fan in the chassis side wall on 1 end and a screened opening about the same size as the fan on the opposite side chassis wall while leaving the top panel slits it will be plenty then?


                  Brad           

[nandrewjackson]

thanks for the suggestions, it's definitely on my mind.

[nandrewjackson]

I finally got this beast working.  my first poweramp build.  I had the bias supply circuit all messed up at first, but that's been redone and it's running.  Voltages looked good so I fired it up.

B+ (HT) at standby is 517 VDC.  Then on down the rail goes 516 , 491, and 481.

At the anodes of the pre/PI I am getting, stage one (279), and the PI left (362) and right (374).

As for the powertubes (6L6GC) anode 517, screen grid 516, and my bias is at -51.
I measured the "current" (voltage across 1 Ohm cathode Resistor) and was getting 29.x to 30 for each tube. my multimeter wouldn't stop on any one value, hence 29.X

I had to change the bias supply range resistors to be able to get enough swing to handle -34 (EL34) to -51 (6L6).  I originally had 10 K resistors on either side of the 50K bias pot, but I had to go to a 1K on the diode side and a 5K on the ground side. This allows for a wider range of bias.

I haven't played the amp properly with a 4-12 cab and effects and everything yet, as you can see it's not 100% finished.  I'm waiting on a light up rocker switch, those are both non lighted switches.  I also need to install the fans and their powersupply, paint and reassemble.

The presence control seems to work pretty good, but it might get tweeked after a full run of the guitar rig.

[LooseChange]

Nice job!

[nandrewjackson]

I've got fans for the unit.  They aren't installed in the picture, but they are in there now.  Well, they still don't have their powersupply, but they are installed.

12VDC fans, basically computer fans.  One in the back, pulling air into the unit, one in the front, pushing air out.

I do agree that I made the PowerTubes too close together, but I'll do some testing with the lid on and fans running, and check with the ol' temp guage before I play an 8 hour set at Wembley Arena. 

The unit is DEAD QUIET.  I'm really amazed and delighted about that.  With it all hooked up to a cab and guitar and preamp, and the volume all the way up, very very very very quiet.
***Hopefully*** when the fans get up and running, they won't add too much noise.  I still haven't decided if I will gut a 12VDC wallwart and install it in the chassis to powerup when the mains is turned on, or if I will put a receptacle on the back to plug in the wallwart?