YAPPAP (Yet Another Push-Pull Amplifier Project)

[arnoldc]

I really wanted to build a push pull amp for myself. I tried it once and I posted it here, the design, if you can ever call it, was just pasted TubeCAD results. It did put out a sound and Wayne spent some time with me privately optimizing it. Thanks again Wayne.

I know that there is another EL34 PP amp project where Wayne is again involed, but that talk of CCS, long tail with negative supply, and all that parts, just doesn't inspire me. I'm lazy

I was browsing the Lundahl site when I saw this LL1660S SE to PP interstage transformer. I haven't made an interstage project yet! Yes!

The LL1660S Alt-B connection has max of 18mA and I suppose my favorite driver tube at 10mA load *and* LED bias will just be perfect.

I drew up this schematic on our whiteboard and took a pic.

I think I might have screwed up on the output tubes' cathode.

Do you guys think this will work?

[lndm]

I'm looking at the tap on the driver side of the interstage xfmr. Aren't you shunting the B+ to ground there?

[Giaime]

Originally posted by lndm
I'm looking at the tap on the driver side of the interstage xfmr. Aren't you shunting the B+ to ground there?

Right. Big smoke involved there

Remove that connection between the primary of the interstage xfmr. and ground. IIRC that tap should be used when using a pentode as driver (to have some NFB, aka Ultralinear operation).

[arnoldc]

ooops!

I just based it on this diagram from Lundahl.

So I just leave pins 18, 13 connected but floating?

ps.

how do you draw a diagram corresponding to this Lundahl IT?

[DougL]

Pins 13 and 18 are the electrostatic shield.
They should be grounded.

In alt B, the primaries are in parallel, so there is no center tap on the "c" windings.

In other words, "That dog don't hunt"

HTH

Doug

[arnoldc]

Thanks Doug. I revisited the PDF and indeeed they're the shield, hence my schematic is wrong. I should have a dotted line between the windings that goes to ground.

[arnoldc]

I finally prototyped this project

Since I don't want to blow up my 417A/5842 tubes, I decided to use my Hitachi 6SN7 as the driver for the prototype, just to know whether this thing will sing or not. And it did

I wanted it to be a monoblock so I made separate power supplies in the same chassis. Here's what it looks like.

[arnoldc]

And this is the current schematic.

I'm using two orange LEDs to bias the paralleled 6SN7 driver. Currently my power transformer HV is low, as I haven't asked my winder to make a suitable one. The rectification is solid state and using a bridge W02M (warning! the W02M is rated *only* for 200V, do not do this even if I did ) to put out a B+ of around 280V. As I said, just want to know if this will put out a sound.

At this weird operating point, I'm surprised at the sound it does- smooth and heavy. The all important "tone" is good to pursue this project further. It does not sound like the ST-70 (note! personal bias here- I never liked the ST-70) or the push pull EL34 monoblock I was loaned before (sounds balsy and impressive at the start then becomes boring and fatiguing in the long run). I don't know if I should attribute the sonic to the LL1660S, as this is the first time I've heard this specific one.

Where would be the best place to put in a CCS?

[arnoldc]

Hmmm, since my ultimate driver is the 417A/5842, maybe a shunt regulator to feed that section...

CCS loaded VR150 + VR105 for 255 something volts...

[alexmoose]

how does using an inter-stage transformer differ from using a "traditional" inter stage? does one or the other out and out sound better?