low voltage single ended el84

[loudthud]

With only 190V B+, a 5K primary will get you about 2 Watt. You should get one of those "Universal" OTs so you can adjust the primary impedance down to something below 2K where maybe you can coax 5W out of this thing.

[Leo_Gnardo]

Someone else recommended a transformer, I was kind of eyeing this one: https://www.tubesandmore.com/product...w-single-ended I want that primary Z for the el84, right?

That's a really good idea and what I'll most likely do. Flying by the seat of my pants is fun. This project is the first one where I'm not following some schematic, I'm just taking all that I've learned along the way and trying to see what comes out. I'm probably going to pair this with a tiny speaker, one of those 6" or 8" really nice alnico speakers from Weber.

One of the reviewers said that OT had unusually good bass response. Which is a huge surprise given its size & price, assuming the reviewer is giving us the truth. If that's the case why not let it play thru a larger speaker? Or at least have an extension cab with a larger speaker so you can make use of the bass response available.

[Helmholtz]

or small gigs where I play with a vocalist and another guitar.

This sounds like a good reason for optimizing output power as bedroom level won't suffice. Meaning low Zpri (2.5..3k) and low Rk (68R). Realistically you won't get more than 3W anyway after the OT.

[pdf64]

Fixed bias to the grid may work out simpler than faffing around with cathode resistor values

[J M Fahey]

Fixed bias to the grid may work out simpler than faffing around with cathode resistor values

Well, yes

To begin with your "effective" +B goes higher since you are not wasting Volts across a cathode resistor and to boot bias is easily adjustable, so you can drive your amp *just* to clipping and adjust for symmetry.

[Helmholtz]

To begin with your "effective" +B goes higher since you are not wasting Volts across a cathode resistor

..by maybe 4V. (The OT primary DCR will "eat" up to 15V at idle and about twice as much at full power.) Easy adjusting is certainly a benefit.

[cminor9]

With only 190V B+, a 5K primary will get you about 2 Watt.

If that's the case why not let it play thru a larger speaker? Or at least have an extension cab with a larger speaker so you can make use of the bass response available.

This sounds like a good reason for optimizing output power as bedroom level won't suffice.

These are good suggestions, just not really in line with my goals for the project. I left some context off my original post so as not to bore y'all with the details but that might have been a mistake on my part. But basically, 1-3 watts through a small speaker and loud bedroom levels are ideal here.

I have many low powered amps and cabs I've built over the years. I can plug this little guy into a cab if I want to move more air. I am talking about maybe using this in situations where even a 5f1 is too loud, like with an acoustic guitar and a singer who is singing through just enough PA so she doesn't have to strain. If it gets loud enough to swamp the acoustic guitar that's ok, but I'd like to have at least some usable tones in that ballpark. If I only use it for recording and living room jamming, that's ok too.

Every build I do, it seems like I always justify bigger and bigger speakers. On the 5f1, I just couldn't bring myself to use an 8" so I used a 10" speaker. I'd really like to hold the line here. I'll try a 6", but if I really hate it I'll make the cab large enough to fit an 8".

I have some nice black walnut and cherry laying around, and this will be a woodworking project just as much as it will be an electronics project. I'm want something 12"x12" or even smaller, and going for an antique radio vibe. I'll glaze and stain the wood, cut dovetails, etc, so something smaller is a lot more manageable. I see myself putting it on the credenza in my living room and not hauling it around to just any gig. This one place we play is a *tiny* room, though, and it might be fun to use it there.

As for more info... I am pretty open to how it sounds. It'd be a bonus to come up with something funky and unique. I was going to use that power transformer, a SS rectifier, and a 47uF/250v solen I have on the shelf for filtering. The preamp will be pretty standard 12A_7 fare, a volume and tone knob, and a recovery stage with the other half of the 12A_7. So that leaves the power amp. I have some noval sockets laying around, so I'd just as soon use those and the el84 for low power. So I'm really just trying to design a power stage for that setup. I know that sounds an awful lot like a valve Jr or something.

There was some talk of fixed bias vs cathode bias. I was leaning toward cathode bias just because it's easier and I don't have any negative voltage from the transformer unless I build a rectifier that'll get me some. But I am open to options.

I guess I just don't know as much about how to design the power amp. So if I go to fixed bias, how would I get some negative voltage to apply to the grids? What would be the advantage there?

[cminor9]

It seems like I have some gaps in my knowledge because I don't really understand the advice I'm getting. I'm trying to fill them in. I didn't know how to ask the question I wanted to ask. I was pretty much trying to figure out how to design the power amp.

I did some digging and found this thread here: https://music-electronics-forum.com/...ad.php?t=48879. That person seems to be building something similar to what I want.

It linked to the same page eschertron linked me to: http://bmamps.com/ivds.html but I still didn't know what to do with that.

So I read this: https://robrobinette.com/Drawing_Tube_Load_Lines.htm and this: https://wtfamps.com/load-line-calculations/ Those seem to fill in some gaps for me.

I think I plotted a load line for the el84, but I don't know what to do with that? I know my PT voltage, and I know the tube I want to use. How do I know what values to fill in on http://bmamps.com/ivds.html?

[olddawg]

Buy a used Valve Jr off of CL for $80. I gigged with one for years. Hint.. use a compressor..

[pdf64]

Merlin’s page on output stage design may help? http://www.valvewizard.co.uk/se.html

[J M Fahey]

Donīt reinvent the wheel

Build a Champ or Gibson GA5 or VOX AC4 or basic AX84 or similar which will start working (which is fine in my book ), then tweak to adapt it to 190V +B instead of datasheet suggested 250V .

Shortest path to build "just one".

PS: or the "Champ with tone controls" ... is it the Princeton?

[cminor9]

Donīt reinvent the wheel

Build a Champ or Gibson GA5 or VOX AC4 or basic AX84 or similar which will start working (which is fine in my book ), then tweak to adapt it to 190V +B instead of datasheet suggested 250V .

Shortest path to build "just one".

PS: or the "Champ with tone controls" ... is it the Princeton?

yeah, that might be what I have to do here. I really do like the idea of figuring it out as I go, but then I started trying to geek out

[cminor9]

Merlin’s page on output stage design may help? http://www.valvewizard.co.uk/se.html

I've been reading and re-reading that that, trying to digest it. I'm not quite there yet, but I'd like to try and learn more instead of painting by numbers. I'll probably just end up taking J M Fahey's suggestion and start with a known model, but I'd also like to understand what I'm doing.

Thanks for bearing with me, folks.

[J M Fahey]

Both are very valuable goals, but in interest of practicity pick one first

Also the time scale is different: starting with known and tweaking is a short path with relatively little unknowns, and satisfaction guaranteed in, say, within a week.

Learning the full deal may take from Months to Years.

Compare it to learning a foreign Language.

[Tony Bones]

A single ended output stage is not hard. Just copy the basic circuit of any similar amp. There's really one one good way to do it. The GA5 is as good an example as any. The only real unknowns are the load impedance that will work for you, and the required bias.

So here's the recipe:

1. Buy a universal OT that will allow you to try different loads and choose the one that sounds best to you.

2. Build the circuit using your PT and OT.

3. Adjust the cathode bias resistor to get 65mA or so cathode current. That's less than 80% max dissipation considering screen current, so you can safely go higher (or lower) than 65mA.

4. Play the amp. Try different taps on the OT to see how they sound.

5. (optional) Try adjusting the cathode bias resistor to see how it changes the sound.

6. Report back here to tell us how happy you are.