low voltage single ended el84

[cminor9]

Hiya folks, it's been a while!

Been getting the itch to tinker with some parts I have salvaged. I'd like to build a low-power single ended el84 amp.

I have a power transformer that will give me a B+ of about 190 VDC. Say I throw that at the plate of an el84 and want to cathode bias that tube. I have been looking at datasheets and this page (http://www.tedweber.com/webervst/tubes1/calcbias.htm) to try to figure out how to bias that tube in such a situation. I'm kind of stuck.

I could just try stuff, but I'd rather approach this with some intelligence. Can someone point me in the right direction of how to calculate the resistor I need to bias that tube?

I don't have any other numbers, because I don't have anything built yet.

[Dave H]

What is the impedance of your output transformer?

[eschertron]

Welcome back! I figured you had blasted yourself into the future with no way to return...

I use NickB's calculator here: bmamps.com

[cminor9]

My OT can be anything I want, I haven't bought it yet!

[cminor9]

Welcome back! I figured you had blasted yourself into the future with no way to return...

I use NickB's calculator here: bmamps.com

I went to the future, but I came back. The beer is better here.

That calculator looks useful. I plugged in some numbers, but let me see if I understand them. I need to better understand how to use those graphs.

[J M Fahey]

Weber calculator only simplifies dissipation math but says nothing about biasing.

Nick B´s one is way better, but apparently uses some tube models unaccessible to us.
And we still must settle on some idle current all by ourselves.

I´d do it by hand but please post one EL84 datasheet so we all use the same graphs.

That said, I think datasheets "should" include at least one example of single EL84 running about 190/200V Dc supply, simply because that would be the power amp "recipe" for a cheap European Radio or TV or record player.

EDIT: Plan 9 (from outer Space)

Start with "somewhat similar" single ended 250V +V datasheet example, as in, say, JJ datasheet, and "tweak it until it works with 190V"

Actually build the d*mn thing , the amplifier that is, using the datasheet suggested transformer (which you might already have pulled from some corpse), cathode resistor, the full Monty, and it WILL work , 190V is not *that* far away from 250V, just won´t be optimized.

Then drive it just to clipping, I´m quite certain that since idle current is not optimized and it´s very important in Class A amps, clipping will be quite unsymmetrical, meaning either top or bottom peak will clip way before the other one.
Then replace suggested 135 ohm cathode resistor with two different values, higher and lower, say 100 ohm or 180 ohm and retest.
One of them will carry you closer to desirable symmetrical clipping, meaning new idle current will be halfway between zero and maximum possible.

It might already be close enough or, worst case, adjustable by slightly varying Rk until waveform looks fine (clips symmetrical):

State of the art "seat of the pants" design

[eschertron]

Based on my playing around with NickB's calculator (5 minutes tops, no way "due diligence") I found myself looking at about 200R cathode resistor and a 5k or 6k OT (probably a champ OT would be close enough) for maybe 2W at clipping. If I was building it, that would be my starting point. IIRC that gave me about 30ma for idle diss.

[Leo_Gnardo]

I went to the future, but I came back. The beer is better here.

Uh oh, that does not bode well for the future does it. After hearing a report about craft brewers starting to offer "lite" options, looks like you may well be right.

About the cathode resistor. I was expecting the true experts to sound off - and we've certainly heard from one - Juan - who's thinking along the same line I was. Eschertron too. Those early European hi fi sets that sported a pair of EL84's running about 275V on the plates and sharing a common 130 ohm cathode resistor. Same deal in some old small guitar amps. Extrapolating roughly from that, I was going to suggest a 200 ohm Rk for you, plus or minus 10%. As Enzo might say, "it's a guitar amp, no need for precision here." As long as you're not threatening to melt down your EL84's plate, all is good. And might I add it's good to see you back.

OT thoughts. Sure you can get a cheap & dirty Champ replacement. But they are very limited in low frequency response, dropping off precipitously below 400 Hz. Granted, with only 2 watts or so, why stress about lows. BUT if you'd like a more solid low end snoop around for a bigger & more 'spensive single ended OT. Last year I had a nice Laney SE amp in for repair with a bad OT, and luckily had a Ceria SE OT left over from a mod I'd done a couple years previous and it worked a charm. Plush tone at bedroom volume levels, quite a nice concept.

[Helmholtz]

EL84s were used in European radios but typically at Vp around 250V and all the datasheet examples I found were for Vp=250.
I found one chart for Vs=210V in the datasheet of the long life version E84L (has increased plate dissipation of 13.5W):



Nick's calculator works fine but requires some iterative trial and error. I got useful data for Vp=Vs=185V, plate load 3.5k, grid bias -4V. Results: Pout=4.05W, Rk=78R, Ip at idle 51mA.

(In the real circuit the DCR of the OT primary will reduce available plate voltage and output power.)

[Dave H]

The EL84 data sheet doesn't have plots for a Va and Vs of 190V so some interpolation is necessary.

I'm guessing the peak voltage available will only be about 160V. If the OT is 5k then peak (bias) current is 160/5 = 32mA. Drawing an imaginary curve on the graph for Vg2 =180V gives Vg1 at -5V for an Ia of 30mA, Ik will be about 35mA so Rk = 5/0.035 = 143 ohms. I'd start with 150 ohms and adjust from there as Juan said above.

Three different methods giving near enough the same result. It must be right

I used this data sheet EL84.pdf

[eschertron]

While I'm reading this discussion, I'm wondering if the application calls for loudest, cleanest power (near 100% idle dissipation, lower Zpri) or some potentially gratifying distortion at 'home' levels (colder bias, higher Zpri, lower output power). Either option is really a resistor value or an impedance match away. An assortment of power resistors (and maybe speaker cabs/loads) for testing should be sufficient to settle this. We're in the ballpark, it's time to, well, you know...

Three different methods giving near enough the same result. It must be right

[Helmholtz]

Of course, there is nothing wrong with close to 100% dissipation in a class A stage. And with cathode bias, average plate dissipation actually decreases at high output.

[eschertron]

Of course. But while we're optimizing the power stage, we might want to know what we're optimizing it for. (sorry about the bad grammar)

[Helmholtz]

Of course. But while we're optimizing the power stage, we might want to know what we're optimizing it for. (sorry about the bad grammar)

Sure, I completely understood. You raised a legitimate question (to the OP). I think the proposals/examples cover most of the reasonable options and any interpolation between should work.

[cminor9]

Some things never change, what a wealth of info!

I'm wondering if the application calls for loudest, cleanest power ... or some potentially gratifying distortion at 'home' levels

I am definitely not looking for the loudest cleanest power. Ideally, I'd get some nice greasy distortion when I slam the amp but when I back off the guitar volume it cleans up a bit. I'd use it for recording, living room playing, or small gigs where I play with a vocalist and another guitar.


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?

Actually build the d*mn thing , the amplifier that is, using the datasheet suggested transformer

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.