2w Sub Miniature Express

[deiseldave]

Upload crapped out. See following.

[deiseldave]

...Regarding speakers, I was modeling some smaller speakers in Eminence Designer, and found that a ported Eminence LA6-CBMR shows a pretty decent low end profile, while not being extreme in the top end. It also provides a respectable 97.8db @1W@1M sensitivity. That's what I'm thinking of trying next.

If you want me to share some box plans for that particular driver, I would be happy to. Just let me know.

I have already built a ported box with the Alpha 6A, and it is worked out pretty cool. But the LA6-CBMR is 4.5db hotter, handles 50 more watts, and is lighter. Obviously the wattage handling capabilities are much higher than needed, but I figure that will give you a voice coil that can handle heavily distorted signal without blinking an eye.
Just some thoughts.
BTW, If you make any boards up, I'll buy one from you.

Thanks,

Dave

Dave I'd love to see those speaker box designs... if you don't mind sharing.

rj

I've attached the Alpha 6A cab I built. Also, as I stated in a previous email, I was full of crap on the LA6_CBMR specs. Not sure what I was looking at, but for the hell of it, I also modeled an LA8_CNMB in the same size box as the Alpha. I think I'll try that driver next.

[UR12]

The Lil' Devil is alive I finished up the mockup for the prototype this evening and the board works great! I checked the voltages, put the tubes in it , adjusted the bias and let it rip. Fired up first time. It was late and I couldn't crank it. Believe it or not this little devil is loud. I just couldn't believe those puney pentodes.

Here are a couple of pics of the amp. Just keep in mind it's not beautiful, its functional. Something to play with till I make sure everything is where I want it. The finished board will be double sided. This is one that I etched and is only single sided. All of the orange jumpers on the bottom of the board will be top traces on the manufactured boards. I don't have the power /standby/off switch wired up and it is missing a pilot lamp while I try and figure out which one I want to use.

[El_Martin]

Congrats, Dana!

Can't wait to hear some sounds

Ciao
Martin

[UR12]

Congrats, Dana!

Can't wait to hear some sounds

Ciao
Martin

I'll try and get some clips made as soon as I can. After hearing this thing with my 4x12 w/ greenbacks last night, I might just go with a 8 or 10" speaker instead of the 2 6". Too many descisions

[drhulsey]

... Just keep in mind it's not beautiful...

Sure looks beautiful to me What's not to like

[sst4270]

Dana,

That thing looks fantastic! I can't wait to hear how it sounds!
I also can't wait to build one. Definitely keep me in mind if decide to do a board run.

Steve

[RJ Guitars]

Dana,

Very cool amp... keep me posted if you decide to go into production on the boards... would love to build one.

rj

[stevo]

Hi folks, this is my first post in this forum, El_Martin tipped me off to this thread. As background, I've been working on a 5f11 with nearly the same tube complement (6112-6021-2x5902)

Dana, what bias voltage are you using? What value did you use for the slope resistor in the bias circuit?

Also, it looks like you are using taps 3 & 6 on the 125A; is this correct? are you using an 8 ohm or 4 ohm cab?


thx

Steven

[UR12]

Hi folks, this is my first post in this forum, El_Martin tipped me off to this thread. As background, I've been working on a 5f11 with nearly the same tube complement (6112-6021-2x5902)

Dana, what bias voltage are you using? What value did you use for the slope resistor in the bias circuit?

Also, it looks like you are using taps 3 & 6 on the 125A; is this correct? are you using an 8 ohm or 4 ohm cab?


thx

Steven

Believe it or not, I started out with about -15v bias, figuring that would be plenty for a tube running at 191v plate voltage. For the 5902, that wasn’t even in the ballpark. I used the scope method and cranked the bias up to about -40v to get crossover distortion on the scope and slowly turned the bias down until I got the crossover distortion to go away and ended up with between -30 and -31v bias. I was a little shocked by this bias voltage but I guess that is where these tubes like to run. The amp has lots of gain and sounds good at this setting. I haven’t had time to do any current measurements yet. I had to modify the 220k resistor in the bias circuit (used in the original Express) to a 68k to get the voltage range I needed on the bias pot.

The OT is using taps 3 and 6. I am driving an 8 ohm cab and this puts the primary impedance to 6.8k. The specs IIRC on the 5902 call for a 3k load and this is a good fit for 2 in a PP configuration. I would also like to try it at 5.6k but the 6.8k sounds pretty good to me.

I am also looking at changing the 100k resistor in the NFB circuit to something like 27k – 33k to get a little more NFB. Everything else is the same values as found in the real Express.

Best of luck with your 5F11!!

[stevo]

Thanks, I had landed in the same place for the OT taps. I haven't worked much on the output stage yet, but I'm up to the (cathodyne) PI now - it's half right (the anode half) but the cathode part goes into cutoff.

One last question; we're using the same NT (well, 369AX); what are you using for your dropping resistor values in the PS?

steven

[UR12]

Thanks, I had landed in the same place for the OT taps. I haven't worked much on the output stage yet, but I'm up to the (cathodyne) PI now - it's half right (the anode half) but the cathode part goes into cutoff.

One last question; we're using the same NT (well, 369AX); what are you using for your dropping resistor values in the PS?

steven

Instead of a choke I am using a 1k 7w resistor for the screen nodes. Then a 20k (PI) and the rest are 10k

[stevo]

Thx

steven

[UR12]

I ended up rebuilding the amp this weekend. I had some problems with oscillation and decided that I wanted to make a double sided board as close to what I would be sending off to have made as possible. I added a ground plane to the bottom copper, added a couple of caps on the input to cut down on some DC grid leak that is aparently part of the 6112s design and I changed the 100k NFB resistor to a 33k resistor. I now have an amp that is behaving properly. It's funny the things you have to deal with that you don't see on paper

Here are a couple of pics of the new board. I placed a pack of cigarettes in the pic to give you an idea of the scale of this thing

[Structo]

Man that looks great!
Very professional looking!
I'd love to hear some clips of this amp.