audio-talk

Steve, if we get Christmas out of the way maybe I could bring my 300Bs up to your place for a comparison? I would like to hear your speakers too.

Ray P wrote: ↑Wed Dec 05, 2018 10:26 pm Yes Steve, they're not commonly available but I still paid less than I would for even a a pair of EH 300B Golds. I will keep an eye open for another pair in due course, maybe even some NOS.

Fortunately the 6C33C and 13E1 amps share most of the parts so I have options to build both should I feel so inclined. The main thing I need for a 13E1 build is a transformer for the output stage B+ (more volts and amps than the one I have for the 6C33C can deliver) but I can probably make use of the toroids I offered to lend you a few posts back. 135VAC * 1.414 = 190VDC, minus 20V or so for the CLCLC filtering, gives a B+ of circa 170V; each secondary is rated for 1100mA so they'll be good for a B+ current of 500mA, which would mean 85W dissipation - maybe pushing things a little?

I don't think they'll be bothered overmuch at 85W, if mine are anything to go by. I had them cooking yesterday, when I was playing with the CCS adjust resistor to give 140V/600mA (84W diss) They didn't appear to care and nothing even hinted at drifting. It was the CCS I was more worried about.

Ray P wrote: ↑Wed Dec 05, 2018 10:28 pm Steve, if we get Christmas out of the way maybe I could bring my 300Bs up to your place for a comparison? I would like to hear your speakers too.

Sounds like a plan to me.
Just let me know nearer the time, and we can sort something.

Cheers Steve. I'll look to book a day off in January and get back to you.

Oh, and good luck with the decorating.

Funnily enough I was thinking only yesterday, if I was a manufacturer wanting to produce a valve OTL amp, first I'd think about developing a single o/p tube to do it with. I am curious why no-one did it in the past, that I know of, probably because of cost.

I don't think it's the cost, Mike. It's because OTL needs a lot of output current and only few tubes can deliver high current/low impedance. And suitable tubes for OTL operation normally have very low mu. Nothing is really "efficient" with OTLs.

The only thing a developer can do in order to make it work for speakers with 8R or less impedance and less sensitivity is to add tubes and run them parallel. That gets the output impedance down before even adding NFB and adds current for driving the load. Problem is that two or more tubes never behave exactly the same. This is only possible with autobias that constantly matches the dynamic tube parameters.
There is a practical way how to dynamically bias two or more tubes within the OTL (more or less) even without using autobias but it needs a little patience and as many DMMs/adapter sockets as there are tubes in the amp.

A single output tube is the best choice and the better sound proves that if tubes don't work 100% in line with each other the sound gets a little "bright" and less precise with less solid foundation in the lower mids. The drawback of using a single tube is that it's an enormous stress for the tube to drive constantly changing impedances at high output power. Increasing the current looks like a solution but in the long run it will wear the tube down very fast despite the fact that it can handle it regarding max dissipation. If the tubes are cheap it doesn't matter of course.

The 6C33 is pretty ideal in many ways . The only drawback is the 60W limit. A beam tetrode (like the 13E1) in triode mode has a lot more headroom regarding max power dissipation which is a clear advantage over the 6C33. But it will be stressful for the tube to drive loads with higher current although it will be able to handle it regarding power dissipation. A fast acting reliable protection circuit is highly recommended in case something goes wrong.

All these things make single tube OTLs not a very profitable business if one wants to sell them for making money. But for music lovers it's a dream come true if they are willing to use speakers with high sensitivity.

Big list of questions there. First why would you want to commercially develop an OTL, efficiency around 1%, when the normal was a a pentode PP output stage using a existing and well known valve. After that, I would think the 6080 and 13E1 and the like was as good as it probably could get, the requirement for a OTL is just about the same as that for a series regulator. As to what it would cost to design a valve from scratch no idea, but no way it would make sense financially.

That would be why then.

Took a break at tea time, from building a set of cupboards and shelves into the alcoves, either side of the chimney breast.
The choke is now properly wired into the input stage PSU, and shoehorned onto the edge of the board. An extra 800uF has been added to the last cap of the driver stage power supply. The hum is now only audible with an ear to the speaker.

The 3300uF output capacitors have been bypassed with a 3.3uF Mundorf Evo, silver/gold foil in oil caps. Cost me a bleed'n fortune and they've lain idle for most of the time I've had them, so I thought "why not?"
They might as well do something useful.



We'll see how it goes later this evening with some music.

you'll be running out of tag strip sometime soon I imagine.

can I interest you in a couple of packets(free gratis of course). I might get some brownie points before Christmas if I can shift a bit more of that junk(sic).

ed wrote: ↑Thu Dec 06, 2018 8:18 pm you'll be running out of tag strip sometime soon I imagine.

can I interest you in a couple of packets(free gratis of course). I might get some brownie points before Christmas if I can shift a bit more of that junk(sic).

Yep, I'm interested. When it goes in a box, more tagstrips will come in very handy.

OK,
Now listening to music on the amp. The high quality bypasses on the electrolytic output capacitors have opened out the soundstaging and cleaned the windows into the studio by a useful amount. The presentation is "just there" with every musical strand, easy to follow, all the time, with congestion when the music gets busy, conspicuous by its absence.

As an example, "Wanna Be Startin' Somethin' the opening track to Michael Jackson's ubiquitous 'Thriller' album is bristling with multitracked effects, with a wicked synth bassline, which drives along beautifully and comes over quite unlike anything I've heard from this track before. Everything is locked to that bassline with metronomic precision, PRaT and jump-factor in spades, with whipcrack dynamics, but presented with a deliciously liquid efforlessness that has to be heard to be believed. And Jackson's voice sounds more human than I've ever before heard it on this album. The spitty, splashy, nasty mess I had to put up with from my old Flat Earth system, on this track is completely absent and just proves what a godawful mess the Flat Earth bollocks, made of playing music.

What this amp does with voices is revelatory to my tired old ears. Nat King Cole, Sinatra, Dean Martin, Sammy Davis Jr, Matt Monro, Petula Clark, Sandie Shaw, et al, appear between the speakers as if to sing just for you. It's so darned intimate it makes you feel quite self-conscious until you snap yourself out of it.

I love this amp. It may have efficiency figures on the low side of appalling and be hopelessly impractical on anything but high sensitivity speakers with almost resistive flat impedance characterisics, which of course, rules out 99.999% of the speakers in existence.

For a practical OTL, you need to look, for example, at Transcendent Sound's offerings, but like Wolfgang has said, a single tube OTL, amplifier if you can get it on the right speakers, will give a musical experience that is one of, if not the best there is. Nothing I've heard from this amp would cause me to disagree with him.

I'm just glad that Ray chose to put his 6C33C project up on here, as I would never otherwise in a million years, have thought of digging out my 13E1s. I'd have missed out on something very special.

Thanks for the write-up Steve and really pleased that you're enjoying the music so much. What you describe is similar to my own experience with the 300B SE-OTL, especially the voices and the imaging, it made my spine tingle when I first experienced it.

And to think you were ready to give up on it just a week or so ago!

Ray P wrote: ↑Fri Dec 07, 2018 7:14 am Thanks for the write-up Steve and really pleased that you're enjoying the music so much. What you describe is similar to my own experience with the 300B SE-OTL, especially the voices and the imaging, it made my spine tingle when I first experienced it.

And to think you were ready to give up on it just a week or so ago!

Yes, funny how things turn out. On paper, it didn't look too promising, but I'm glad I saw it through.