Biasing a single ended output stage...

[bruno]

Hi,

I recently found some edcor transformer locally, the power transformer is a 275-0-275v (200ma) with a 5v and a 6.3v tap, which I reckon is more then enough to build a small amp.

Also the output transformer is a single ended 10w, 8k primary/16 ohm secondary ratio.

Given the 8k primary/16 ohm secondary I suppose my options are limited in terms of what tubes I can choose for my power amp stage. I initially thought about two 6v6s in parallel, but that would most likely suite a 4k/5k primary transformer better.


So my first question is , that I'm wondering whether a single 6V6 would work, or even a 6L6?


And my second question, is how would I go about biasing it?

thanks

[shooter]

let's "assume" 2 great speakers will sound better than 1 for now, that solves the 8K primary.
6V6 becomes the easy choice, so a "champ-ish" build


for bias I go with the data sheet "spec's", the example is for a 6K6 but works well for all tubes :)


the data has everything you need to "target" your build, once it's built and working, dialing it closer becomes pretty easy

[bruno]

Why the 2 speaker assumption, because of the 16 ohm secondary?

[HotBluePlates]

... 275-0-275v (200ma) with a 5v and a 6.3v tap, which I reckon is more then enough to build a small amp.

Also the output transformer is a single ended 10w, 8k primary/16 ohm secondary ratio.

... two 6v6s in parallel ... a single 6V6 would work, or even a 6L6?

If you have a 200mA PT, it sure would be nice to use 200mA.  If you do single-ended anything (less than 150-200mA idle current), you're severely under-utilizing the PT.  Like using a Super Reverb PT to build a Champ.

     - 275v * 1.414 = 389v.  Pencil "350v" for figuring.

     - 200mA RMS implies x1.414 = ~280mA peak

     - 6L6GC data sheet (Page 6 Top) shows it can manage ~200mA peak at 300v screen, 75v plate.  Or ~260mA peak with 350v screen.

     - 350v - 75v = 275v peak / 260mA peak = 1057Ω ---> "4kΩ plate-to-plate" if the amp winds up Class AB.

     - (275v peak x 260mA peak)/2 = ~36 watts output

[bruno]

So effectively I'm better off either buying a smaller power transformer, or I buy a PP output transformer.


The transformers I have are two edcors:


Edcor GXSE10-16-8K Output
Edcor XPWR007 Power transformer

[DummyLoad]

So effectively I'm better off either buying a smaller power transformer, or I buy a PP output transformer.

no, you're fine with what you have.


6L6/EL34 with 4K:8Ω load about 8-10W output power.
6V6 with 8K:16Ω load about 4-5W output power.

PT will handle either option.

--pete

[bruno]

ok so for the OT ratio if I reduce the secondary by half, then the primary should also see half of the impedance?


In that case I should also be able to use two 6V6s in parallel, right?

[HotBluePlates]

ok so for the OT ratio if I reduce the secondary by half, then the primary should also see half of the impedance?

Yes.

[bruno]

Thanks guys, that gives me something to work with.


I realise the PT is quite over spec'd but this was a set I sourced locally, so not much choice.


I'll probably make a simple amp with two 6v6s and maybe a tremolo of some sort.


much appreciated.

[AmberB]

I've built 2 amps that have a single ended 6L6 in them, I like the volume they can get to with a good speaker, and I like the way they sound.  The single 6L6 with a decent 12 inch speaker works really well.
I built both of my single 6L6 amps as heads instead of combos because I'm not good at building cabinets...

[bruno]

Yes I'm probably going to go with a 2 6V6 or 1 6L6 setup. Not going to do much in terms of tonestack, I looked into merlin's book and I'm going to use a treble control you'd find on a gibson ga30rvt and maybe a switch for a couple of differente coupling cap values. So really simple stuff.


It's a bit unfortunate that the PT is so big, but I should be ok with that.

[HotBluePlates]

... probably going to go with a 2 6V6 or 1 6L6 setup. ...

... how would I go about biasing it?

Single-ended = Class A = probably Biased at 100% dissipation @ idle.

Willing to stick to 6L6GC?  Idle at 30w.  We're under-loading the PT, so instead of 350v assume 390v (and round up to 400v).  30w / 400v = 75mA.

Go to the 6L6GC data sheet & look at either the bottom graph of page 7 or the top graph of page 8.  To get 75mA with 400v plate and screen, bias is something like -32.5v.  Bottom graph on page 8 suggests screen current is ~4mA.

75mA + 4mA = 79mA.  32.5v / 0.079A = 411Ω.  Round that up to 470-500Ω to to have a standard-value cathode resistor.


Or... Do the thing PRR used to recommend:  get a pack of 100Ω resistors.  Put maybe 6 of them in-series (however you want to temporarily do that) from cathode to ground.  You have a temporary 600Ω cathode resistor.  Check tube current.  Use a clip-on jumper wire to short-out one of the resistors for 500Ω total, and check tube current.

     -  Is 500Ω too low (tube current too high) but 600Ω too low?  Remove the jumper, and put a 100Ω in-parallel with another 100Ω for a 550Ω cathode resistor.  Or another 100Ω resistor in parallel with those for 533Ω.

     -  You could start with 7 or 8 or 10 100Ω resistors in-series, and inch upward on current.

     -  Keep in mind that individual tubes don't always land on the same plate or screen current for a given supply voltage & idle bias.  probably better to leave some wiggle room to accommodate the "overly-hot next tube" that gets plugged into your amp.

[shooter]

+1
I have a "box" of R's just for dialing in SE.  I use 2 meter's (plate and cathode), gator-clips, and a calculator, geek FUN 

[DummyLoad]

see attached - should be a good reference as a starting point for your plan. OT is the Allen TO11S which is a 4KΩ:8Ω/4Ω or 8KΩ:16Ω/8Ω


your B+ should be close to what's in the attached. buy an assortment of cathode resistor values: start with a 470R, 430R. 390R, 350R, 330R, 300R, 270R, 250R, etc.. all are available in 5W to 10W some even higher wattage. 10W is more than adequate here. as you can see i ended up with a 330R and i had 6.5W in my stockpile, so that's what i used.


--pete

[bruno]

thanks guys 

[bruno]

Sorry guys... one more question, I just noticed the OT I have is a SE but utralinear (it has a screen tap).


Is there anything I can do to take advantage of this, or am I better of just wiring it with the plate and B+ taps?


Thanks

[HotBluePlates]

... the OT ... is a SE ... utralinear

Is there anything I can do to take advantage of this ...

Connect the tap to the screen if you want to.  Or insulate it if you don't use it.

[bruno]

any benefict in using it? I'm assuming it will result in slightly lower output and a cleaner sounding amp right?

[shooter]

Marshall though so


and ya, you get "fuller" sound, not so much "clean" but it delays or curtails eventual current smack on the plate

[HotBluePlates]

any benefict in using it? ...

That's a personal value-judgment you'll have to make.  Which is why I phrased my response the way I did.   

... I'm assuming it will result in slightly lower output ...

Higher output actually, as the screen current now contributes to output power.  In pentode mode screen current drawn as the amp is played louder contributes to causing sag, but is otherwise "wasted."

... I'm assuming it will result in ... a cleaner sounding amp right?

The output stage will probably be a bit cleaner, as ultralinear mode is in-between pentode- & triode-mode operation of the output tubes.  Certainly the output tube(s) will distort differently, and the plate curves for the tube will be something between the curves shown for pentode mode & triode mode operation of the tube.

Again, you would have to try it if you want to, and form a value-judgment on whether you like ultralinear mode.

[AmberB]

Perhaps you could put in a switch to go from standard mode to ultralinear mode? 
Then you could have either...