bass amp project

[John]

Okay, it's done, finally. SilverGun did a great job making the cab for me, and I won't even talk about the deal he gave me since I don't want to put him on the spot. Build pics, gut shots and finished project are here.


https://www.facebook.com/pg/Loafin.Shed/photos/?tab=album&album_id=577450432448577


My stuff probably won't ever be the works of art that a lot of you post. But, it does sound good.    Thanks everyone on this great forum!

[TIMBO]

Looks GREAT!!!!

[sluckey]

Looks good to me. Nice bias setup.

[SILVERGUN]

Ahhhh, she's a Bute!! 
 
Nice work sir, and it's my pleasure to be a little part of it....hope I sped you up a little.
 
Your second installment payment is late, though,,, so, sadly, I'm gonna have to foreclose on the farm.

[John]

Oh, I'm sorry sir, did I forget to mention you're 7th in line? 

[John]

I did have a question about the PI circuit. I copied it exactly and it works fine, no issues there. I just wondered if this is the "concertina" type, and also wondered about the sort-of-high part count on it.


Schematic here:


http://el34world.com/charts/Schematics/files/ampeg/Ampeg_b15nf_portaflex.pdf

[sluckey]

That's called a paraphase inverter. See this link...

    http://www.valvewizard.co.uk/paraphase.html

Concertina phase inverter is the same as cathodyne phase inverter, is the same as split load phase inverter.

[PRR]

Sluckey types faster. To add to his reply:

> if this is the "concertina"

No; it is "floating paraphase".

Simple paraphase takes the one tube, divides the signal down, and feeds a second tube. If the divider loss and tube gain are equal, you get equal-but-opposite outputs.

The floating paraphase seeks to reduce inequality by running second tube at high gain and connecting unity-gain inverter NFB around it. Note the 470K and 510K are equal. Well, if V3b had infinite gain and infinite input Z they would be. The ~~8% difference makes it closer to final gain of unity.

> sort-of-high part count

I make it as one R and one C. There are more complicated ways to do this. There are ways to do it 2-bits cheaper. Many ways to skin cats.

Like many/most plans, this one is fine up to clipping. How it reacts in OVER-drive is a different matter, and maybe not easy to suss-out in advance. But the B-15 has been super-popular for years. And it is 95% the same as many other plans. You could convert to simple paraphase sort-of easy (easier if not for the negative bias on the finals), or to long-tail with some more stuff below the common cathode resistor (OT phase-swap may be needed).

[Ed_Chambley]

Nice work John. I really like this circuit, my favorite for bass. Also the 6sl7 tube has been finding a home in a lot of my stuff lately and that sir is all your fault.

[John]

PRR, thanks for that.
Ed, I do love those tubes. I think they make a Champ type amp sound so so sweet.
Sluckey, that bias setup is all you.

[Ed_Chambley]

PRR, thanks for that.
Ed, I do love those tubes. I think they make a Champ type amp sound so so sweet.
Sluckey, that bias setup is all you.

Yes buddy, I am working on a 6sj7 to 6SL7 to el34 ss rect brow face stack that is K driven which is unusual for me. Very promising so far. I am waiting on a PT to arrive to complete. I need to up the voltage to the 430 th 450. The 6sj7 is a mix tube. And also the EL 34 is a demanding thing.


Boy is it fat and the 10 Warehouse veteran speaker that is 50 bucks is amazing.


Funny thing is I started with only making a souped up 5f2.


I was kidding you, but it really was your schematic you shared with me that changed a few things I commonly now do.


I always build with a 200ohm balance pot and now I consider using Octals I have had around for years. Seems as if you bias the 6sl7 in V1 at -2 and bypass with 6.8uf and apply 180'v the second and third harmonic is very strong and the signal complexity is very nice,to me.


See what you did. I sound like a nerd.


I love the amp and I have lusted after a flip top for years and will buy one if I ever see one in collectors condition, As you know I could really use another amp.


Sorry for floating around the topic, just wanted to let you know you have influenced my building and also my ears.

[John]

Seems as if you bias the 6sl7 in V1 at -2 and bypass with 6.8uf and apply 180'v the second and third harmonic is very strong and the signal complexity is very nice,to me.

I am going to keep that in mind! I think that is sort of close to what I do, but that would only be a happy accident, no planning on my part.


I am unbelievably humbled that I have influenced anyone, except for maybe "don't do it like John did!!". lol