Plexi 6V6

[sluckey]

I'm done with Mark Huss's Plexi 6V6 project. Thought I'd split off from the other thread. I've added a webpage to my site that has a bunch of pics and a pdf file that contains everything you need to build one of these. I had a lot of fun with this project. The biggest challenge was putting it in that little box.

     http://home.comcast.net/~seluckey/amps/6v6plexi/6v6plexi.htm

[kagliostro]

Idem (the same of my post on the other tread  )


Franco

[bkvance]

Very Nice Steve ,


I want to build this as my third effort, could you possibly help me with a build list so that I might have the correct components for it?


Thanks,
Keith

[JBP]

OK, I'm sold. Have'nt built anything in a couple of years and thought it was time for a new build. Checked my favorite amp site to see whats new and what do you know. I considered this build awhile ago but didn't have the skills/time to figure out how to do the layout. Thanks for your great work sluckey! I have most all of the components including a 12" chassis. The transformer set I intend on using comes out of an old 40's field recorder with proper voltages that drove a pair of 6v6's. My problem and question for the knowledgeable readers is the output transformer only has taps for 4 and 16 ohm. Is their a way to fake the 8 ohm tap and how do I treat the NFB? Thanks for the help/advice, Jeff

[sluckey]

could you possibly help me with a build list so that I might have the correct components for it?

Sure, I can give you some guidance. You already know which transformers I used. If I had to buy new transformers I'd get Hoffman's PT and OT for his 18W amp and I would use his small Fender choke. All resistors in my build are 1/2W metal film unless otherwise stated on the schematic. I used Mallory 150 series coupling caps, F&T dual cap cans, and Sprague or Illinois filter caps on the board. Hoffman stocks everything you need, including the blank chassis. He can even make a board and load it with turrets if you need this.

I suggest you print the schematic and use a highlighter to mark off components as you add them to your list. It's easy to do and the familiarity you gain from making your own list from the schematic will be very beneficial to you later on. I'll gladly answer any specific questions you may have about some components.

The transformer set I intend on using comes out of an old 40's field recorder with proper voltages that drove a pair of 6v6's. My problem and question for the knowledgeable readers is the output transformer only has taps for 4 and 16 ohm. Is their a way to fake the 8 ohm tap and how do I treat the NFB?

I don't know of any way to "fake" the 8Ω tap. Just use a 4Ω or 16Ω speaker. There's a lot of leeway on choosing NFB. Mark used a 100K resistor. I used a 22K resistor. It's a personal taste thing. Generally speaking, if the circuit remains the same, using a higher impedance speaker tap will result in more NFB.

[bkvance]

Thanks Steve,


I'm sure I will have plenty of questions.


Keith

[Fresh_Start]

That layout is a thing of beauty!

Cheers,

Chip

[MakerDP]

My problem and question for the knowledgeable readers is the output transformer only has taps for 4 and 16 ohm. Is their a way to fake the 8 ohm tap and how do I treat the NFB? Thanks for the help/advice, Jeff

Wire a 5k resistor or so into the PI coming off the wiper of a 100k trim pot. Adjust to taste and leave it there or measure total resistance and replace the pot and 5k resistor with a single resistor. You could also leave out the 5k resistor and experiment with not having any NFB at all.

This amp is almost identical to the AX84 October Club. It uses the 8R speaker tap -> 100k pot -> 22kR -> LTPI. The pot is mounted on the front panel for a "Variable Feedback" control. Users like it because they can have some control over the amount of power amp distortion.

Sluckey's layout is MUCH more compact though. I like it a lot. The AX84 version's layout is for a 16x8 chassis.

[DummyLoad]

slucky's uses 22k NFB off the 8ohm tap. sqrrt2*22k for the value if connected to 16ohm tap. if you connect to 4ohm tap .707*22k.

--pete

[jhadhar65]

Nice!!! I wanna hear this thing, but I didn't find any sound links anywhere? C'mon, man, don't hold out. =)

[kagliostro]

Ciao Pete

slucky's uses 22k NFB off the 8ohm tap. sqrrt2*22k for the value if connected to 16ohm tap. if you connect to 4ohm tap .707*22k

can you explain a bit more about the math ?

Grazie

Franco

[sluckey]

I ended up using 22K because while using Mark's 100K value, I could not determine the proper phasing for the NFB loop. There was so little NFB that I couldn't tell which was the correct phase. And the presence control did nothing that my ears could hear. I initially thought I had a wiring error in the NFB/presence circuit. After verifying the circuit was wired correctly I decided to start increasing the NFB by paralleling resistors across Mark's 100K. I started with a 100K, then 47K, then 33K and bingo! Now I could hear a difference between negative and positive phase on the NFB loop. So I permanently connected the secondary leads to the speaker jack and replaced the 100K with a 22K. And the presence pot now had an effect that I could hear.

Now if you have golden ears you may be able to determine proper NFB loop phasing with the 100K. I suspect that if many of these amps have been built there will be some that have the NFB loop phase wrong and they don't even know it!

[DummyLoad]

Ciao Pete

slucky's uses 22k NFB off the 8ohm tap. sqrrt2*22k for the value if connected to 16ohm tap. if you connect to 4ohm tap .707*22k

can you axplain a bit more about the math ?

Grazie

Franco

http://www.the12volt.com/ohm/ohmslaw.asp


--pete

[sluckey]

This morning I replaced the tired old speakers in my Sunn 2x12 cab with a Heritage G12H(55) and a Heritage G12H(75). This amp sounds amazing with this cab now! The Marshall growl is definitely there. Good, well defined bottom and not too glassy on the top end. Roll back the guitar volume for some good cleans too. Makes me think of the Ramada Inn in Warner Robins back in the mid '70s.

Now I'm headed back to the shop to see what else may like these new Celestions!   

[EL34]

Cool
Would like to hear something?


Even a crummy video camera or cell phone video?

[TubeGeek]

Nice project! I want to build one of these someday.

[kagliostro]

Steve, I noticed a thing



You used red and black wires on heaters and didn't twisted it .....

Are you using DC on heaters ? From the image didn't seems ....

Franco

[sluckey]

Baby did a bad, bad thang!   

The heaters are AC. I chose red/black because Marshall is fond of those colors for filaments. I didn't twist because the tubes were so close together that my stranded PTFE Teflon wire was just too slick to hold it's twisted shape together. I was only getting about two twists between tubes and the wires just fell apart before I could connect the ends to the sockets.

BTW, I just finished another amp project and made the same mistake again!      

[kagliostro]

Are you satisfied about hum level ?

Franco

p.s.: What is this last one, seems you have a pair of el84 plus 1 x 12a*7 as PI,  1 x 12a*7 (paralleled) an ef86, lot of curiosity about it 

[sluckey]

Are you satisfied about hum level ?

Absolutely.

p.s.: What is this last one, seems you have a pair of el84 plus 2 x 12a*7 and an ef86, lot of curiosity about it

That's correct. You've seen the circuit before, just not all at once. I'll post a link with the whole story later today.

[kagliostro]

Cool

Franco