"Fiddy" 50-watt build log

[xtian]

Fine, I admit to being a masochist: I am deviating from existing layouts and schematics, which leads to Questions, Problems, and Hair Pulling. But hopefully ends in Solutions and pride in a job well done. We'll see.

I'm launching another "beaks and feet" build, meaning I'm using parts from the bin, donated transformers, a cheap Weber chassis, and a pine head box I'll build myself. The last one, 18 Bone Monkey, came out really well, and is a favorite of mine.

I'm aiming at a 50-watt, EL34 powered, Plexi Lead 1987, parallel/cascading mashup. Because the Weber chassis is weird, I'm modifying the layout to fit, and I have been looking at Hiwatt builds for inspiration.

My layout so far is attached. I use Adobe Illustrator, in case anyone wants the parts, which I've drawn to scale.

[Colossal]

Looks good. I did a 6V6 Plexi with a parallel/cascade switch and it was a great sounding amp. I like your alternate layout where V1 and V2 are up front. It will make for less risk of oscillation in cascade mode with the runs being much shorter and more direct. The zone around the mixing resistors can be microphonic in traditional builds (in cascade mode) so the way you've shown it is nice and discreet. No need for long shielded runs; should sound really good.

[xtian]

Refined layout, proofread, and staked turret board.

[topbrent]

Nice layout idea!

Is that being built on the 6MX chassis from Weber?

[img:500:233]https://taweber.powweb.com/store/chassis/6mxch.jpg[/img]
4 octal, 2-9 pin socket holes. .090" aluminum chassis
1 Channel, 2 Inputs, Nine 3/8" control holes, PWR, STBY, Pilot Lamp

Generic 50 to 100 watt head chassis. Same size as a standard '45 chassis. Standup PT and OT with large grommet holes to fit just about any transformer wiring.
PT and OT foot mounting holes are slotted. Also includes mounting for choke. Mounting holes provided for 2-1/2" by 10" fiber board.
Output tube socket holes are positioned to accept 6L6's, 6V6's, EL34's, KT66's, and KT88's.

*

[M Fowler]

I used that chassis for my Matchless Lightning build not the best for what I needed but it worked.

Mark

[xtian]

Yes, that's the Weber 6Mx chassis. Only $30 plus shipping!

Got all the parts mounted today; took a lot of work drilling, milling, and Skilling. Check out the custom PT brackets! Yah!

[xtian]

Got the board all soldered up. BUT I accidentally ordered a bunch of 0.0022uF caps, instead of 0.022uF. Damn. Now I have to wait for replacements.

[xtian]

Board mounted.

[Structo]

Got the board all soldered up. BUT I accidentally ordered a bunch of 0.0022uF caps, instead of 0.022uF. Damn. Now I have to wait for replacements.

I hate when that happens.
I got a bunch of extra parts like that.

Nice looking build!

[xtian]

All parts in, all wired up. Have not proofed yet. Gotta rest my brain hole.

[Teleguy61]

Nice job.
I have a couple of those 6MX chassis hanging around, waiting for a project
to develop. Couldn't resist the price.
I built a D'Lite into a Natalie chassis, similar size as 6MX, and it is
very crowded, so there's a limit to what you can do.
Good luck with it.
Love the attitude-hmmmm, let's try this.

[xtian]

Fiddy lives! Breathes fire! Almost literally!

Tubes are running very hot. One (crappy old mismatched one) started to red plate.

I've got 460vdc plate voltage, and 60mV across the 1ohm resistor at the cathodes. Bias calc tells me I want about 35mV. But I cannot turn the current down any further with my bias adjustment pot.

So, I'm looking at the 47K resistor in series with the bias pot. I think I want to INCREASE the value of this resistor in order to allow the bias voltage to be more negative, thus DECREASING the current through the power tubes. Did I get that right?


Here is my voltage chart:

Code: Select all

     1    2    3    6    7    8
V1 169    0 1.07  210    0  1.8
V2 180    0 1.11  286  180  181
V3 222   44 68.5  220 43.6 68.5

    1+8    3    4     5
V4 .045  465  461 -40.5
V5 .060  465  458 -40.6

B+ = 468
wall AC = 120

[martin manning]

Ya you got that right, or you could decrease the resistor right off the HT.

[Phil_S]

The simplest thing to try is to tack in another resistor in parallel to the one off the HT before the diode. Once you see the result of that, even if it's not right, you can pretty well guess from there a value that will produce the result you want.

[xtian]

Nailed it. Had to try a couple different values, but 75K is right on the button. A matched pair of Mesa Boogie EL34s now easily biases at 32ma.

So finally I got to play a little, quietly. First: WOW! Awesome! This build is extremely quiet. No hum, barely any hiss. LarMar master is excellent.

I put in a parallel/cascade switch (basically the "one wire mod"). In normal (stock) mode, opening both Volume pots and turning the Master way down to bedroom levels, the gain is what I'd call HIGH gain. And with the cascade switch thrown, the gain is literally overwhelming--I have to turn the Vol pots down to about 75% to avoid runaway oscillation and blocking distortion.

Will have to wait until house is empty tomorrow to see how much clean headroom is available. Will report.

Color me extremely self satisfied.