Super Triode Connection Amplifier Build with 6BM8 and 6AK5

[DerFly]

Well, I've been lurking about here for a while, also been doing a bit of research on the net about first tube/valve amplifier projects.

I’d actually been researching simple morse transmitters using a 6BM8 and stumbled across quite a bit of info on STC amps using this tube. It seemed a good contender for a first project as I had several 6BM8 on hand, inexpensive output transformers are able to be used and construction doesn’t look too daunting.

As they say, fortune favours the brave (or stupid), so here we go with some thought, but probably not enough, already put into the project.

The original schematic available here (http://www2u.biglobe.ne.jp/~hu_amp/amput3e.htm) shows a requirement of 6.3v at 2A and a HV transformer of 220v output rated at 30VA.

Not wanting to invest too much in a project which may or may not work, I'm going to back-to-back a couple of cheap transformers for a simple power supply.

In the picture, the two respective transformers are 30 and 15VA. I should be able to take 6.3V at 1A out from the first and then step back up for 240V at about 60mA of HV.

I know this is only half of the power requirement so I guess the simple answer is to build two monoblocks!

Being newish to all this, I'd most appreciate any advise along the way. I'll also be posting questions as I certainly do not have all the answers yet.

This may well be a drawn out affair, so I hope you'll bear with me as this project takes its course.

[DerFly]

These are the output transformers I’ll use. They are 5k Ohm input with a tap for 2.5k and the output is multitap with 2, 4, 8 and 16 Ohms out. The original article above shows a transformer with 3.5k in/8 Ohm out but I’ve also seen a few schematics with two OPT’s per side.

This chap (http://www.atatan.com/~s-ito/amp/yakinori.html) is using two output transformers with specs 7k Ohm in/8 Ohm out with the inputs paralleled and the outputs in series.

As these were really cheap, I bought four so I guess I’ll use two each per monoblock. Not sure how I’ll wire these yet. Any suggestions? Has anyone else built one of these?

DerFly.

[SY]

Even after reading the explanation, I'm clueless about the "stopping diode." Could you elaborate on that? What happens when it reverse biases? How does it function when it's forward biased?

[tubelab.com]

These "magic diodes" keep showing up in various schematics. I tried a "stopper diode" in the plate lead of one of my sweep tube SE amps. The only thing that it stopped was good sound.

I noticed a definite change in the output signal when the amp was driven to clip with a strong bass note. What was previously a single cycle event with the top whacked off of a transient, now affected about 3 cycles and there was some funny ringing effects. Yes there is some definite speaker interaction here. It may sound good to some people with some music, but I didn't like it.

I have an EL-34 SE amp playing now. I will put in a diode and see what happens.

Disclaimer:

My experiments have been in triode or UL mode. The authors experiments were in pentode mode.

[Giaime]

I think we should distinguish the diode in the screen lead from the one in the plate lead.

Just from a rapid guess, the one in the plate lead looks very very suspect to me, the experience that George mentioned makes me think about interactions between the power supply and the power stage.

The diode in the screen path, however, puzzles me. Is the screen grid supposed to have a one-way current allowed, like the plate? So what's the effect of the diode?

I remember we already touched this subject:
Diode trick?

[SY]

So what's the effect of the diode?

Same as the plate diode: to convince the builder that he's done something different and clever.

[Brian Beck]

The more scientific answer is that they confer pixie dust.

[Giaime]

Originally posted by Brian Beck
The more scientific answer is that they confer pixie dust.

Now I'm really confused. I'm starting digging into the RDH4 to find a chapter about "pixie dust"

[SY]

Brian, that's twice today!

[tubelab.com]

Now I'm really confused. I'm starting digging into the RDH4 to find a chapter about "pixie dust

You need the "Disney Edition".