Choke loading Anodes.

[Dave the bass]

Yep looks right to me. (assuming you don't have a volume control between the two)

In actual fact.... I'm not gonna be scared of the maths... <gasp from the audience>.... lets move the pot (no, make that 2 different pots 50K and 220K) to a location inbetween the 76 pre and the 6EM7 amp and see what that does to the HF response.

The rout of my 76 pre was 1278R and that was being fed into say, a 50K pot (Alps Blue with a shiney knob!) that rout is in parallel with pot isn't it?

So (I think) I need to add resistors in parallel I believe.

1/R = 1/1278 + 1/50,000 = 1246R eff rout with a 50K pot

and

for a 220K pot using the same formula above the rout = 1270R

So the -3dB HF cut off point for a 50K pot is...

f-3db = 1 / (2 x PI x C x R)

1/2 x 3.14 x 0.000,000,000,262 x 1246R = 487.776Khz

and for a 220K pot

1/2 x 3.14 x 0.000,000,000,262 x 1270R= 478.558 Khz.



The higher value the pot curtails HF response a bit.

Is that right above?

DTB

[Max N]

Nice one Dave - I guess 475kHz should be adequate

I had forgotten about the volume pot too. As you say, choosing the value for the pot would be a compromise - too low would require a bigger coupling cap, too high would curtail the HF.
If we took the 220K pot you use at work as an extreme case, then (according to MJ as usual cos I just looked it up), the worst case will be with the wiper at the halfway point. Then there will be 110K one way to ground and also 110K the other way to the source which we can take to be at AC ground. So 110K in parallel with 110K gives 55K.
So the HF cut-off would become.
f-3db = 1/(2x3.14x0.000,000,000,262 x 55000) = 11050HZ = 11Khz

I think that's right anyway.

Philip, I think one way to proceed would be to do a spreadsheet which calculated the LF and HF cutoffs for different pot values and coupling cap values.

[Dave the bass]

11Khz

Blimey, our super wide band amp just had the mega-dolby-button but turned on!

The reduction of HF with the big pesky pot has caught me out before, remember when i did a LW with the paralleled ECC83's at the front end? LW (and HF) deffo wasn't happy with the 220K pot up front. Nick worked out that the combined effect of Miller cap and a big pot chopped off a lot of HF info.

Live and learn, cheers for the help Max.

DTB

[Max N]

Hi Dave
I think we both posted our pot calculations at the same time, then after I hit the 'submit' button I had to go and do something else, so I've only just seen your version. And reading your version now makes me think that my version isn't right. I think actually the two impedances might be in series, so we should add the 55,000 from the pot to the 1270 from the 76 stage.....
Does that seem right?
Anyway, I didn't want you to think that my version was posted as a correction to your version.

[Dave the bass]

^^^ No worries Max, I thought it'd be resistances in parallel as the pot is 'across' rout of the 76. I see what you mean though about the mid-way position setting of the pot, I hadn't thought of that.

I'll put my hand up and ask Prof.
*Puts hand up REALLY high*

"Sir.... Sir...Sir"

DTB

[Max N]

I see what you mean though about the mid-way position setting of the pot, I hadn't thought of that.

I would never have thought of it either - in fact I've done the calculation in the past just using the pot value. But I cheated and swotted up before posting

[Nick]

AFAIK, as Max said, the wiper of the pot is seeing two paths to ground (at AC), one direct which is the wiper to ground though the "lower" part of the pot, the other is the "upper" part, that is in series with the output resistance of the driving stage.

But thinking a tad more (not easy on a Friday) the path back through the upper part and the previous stage output impedance, is also a bit more complex, as there is the coupling cap in the way as well so the upper path will have a frequency dependent part as well.

Of course I do have to mention that adding a capacitance reactance to a resistance like this is a bit naughty as there is the complex part of the reactance, so its not a direct addition, and there is a phase shift involved. But maybe I will back away from getting near AC theory in this thread (it is all out there though if you want to read about it)

[Dave the bass]

^^^Owwwwwwwwwwwwwwwwwwww Just when i thought I was getting somewhere

Were the calculations for the 220K and 50K pots OK Nick?

I didn't take into account any stray capacitance in the leads or owt.

FWIW, I took 2 Pots on hols with the Fonks and 6EM7 recently. Fed from the line-out of a Sony discman the shunted Alps although 'quieter' at full volume had a better HF response than the 220K conventional wired pot which was 'louder' but muted in HF.

DTB

[Nick]

Were the calculations for the 220K and 50K pots OK Nick?

Yes for the pot at full vol

I didn't take into account any stray capacitance in the leads or owt.

Knowing its there is normally enough and being able to work out when it becoes important

Normally the miller generated capacitance will be the significant part.

[Dave the bass]

Ta Nick.

OK, maybe the next step is ... using this thread as a guide.... maybe it's time to try and do a design of a simple single stage on my own using a different valve and HT. Still keeping it really simple natch but I've gotta learn somehow.

I wanna upgrade my Red 'L' to either 'ND' (New Driver, ha! Audio/Auto pun) or a green 'P'

DTB

[Nick]

Ta Nick.

OK, maybe the next step is ... using this thread as a guide.... maybe it's time to try and do a design of a simple single stage on my own using a different valve and HT. Still keeping it really simple natch but I've gotta learn somehow.

I wanna upgrade my Red 'L' to either 'ND' (New Driver, ha! Audio/Auto pun) or a green 'P'

DTB

Yep, build something .

Personnel I think you are past that point some time ago.

[Dave the bass]

Yep, build something .

I have built a fair bit in the few years I've known you lot y'know!!!

Personnel I think you are past that point some time ago.

Nah, disagree, personally I REALLY needed to get over some mahoosive personal maths hurdles and basic thermionic principles before I loose that red 'L' for good.

Getting there, thanks to you and everyone else that proffered help/guidance and advice, really appreciate it.

Onwards!

New thread.

DTB

[Dave the bass]

Remember this epic thread? I do but then I suppose I would wouldn't I...

Amazingly, lets bring it back on-topic!

Originally this was started off when I (foolishly) thought I could just bung a choke on the anode of any driver valve and that's it, fame fortune loose wimmins and powders, but no. What actually happened was an acute LF drop-off and a journey into transconductance and loadlines which was/is brill. Thank you gents.

Now then, one of the upshots of this thread was being told how important (in some applications) a low value of ra is if you want to start choke loading your anode. The 1st stage triode in 6EM7 if you remember is approx 40K, as I found out to my cost this was way too high a contender for choke loading and could only get a passable sound with 2 x 200H chokes in series on each channel. Lesson leant and a lot more too besides.

I bought a few 6DN7's the other day as the first little triode has a much lower ra, about 9K IIRC and is thus more suited to chokey loading. As a sperryment I simply plugged in the 6DN7's after checking the pin outs are the same, they are.

3 observations came out of this little experiment.

1. It works, nothing cooked.
2. Flippin' heck, what a springy, full and lovely sound. If this is Choke-loading then yes please i'll have some more Sir.
3. The HT is almost up to 400V from about 325V!

HT with 6EM7 was about 325V, after slipping 6DN7 there's less current being pulled from the PSU I'm guessing hence the rise in HT.

I'm guessing its down to the fact the O.P's are totally way off now WRT to 6DN7, I'm guessing the 2nd stage (being more current hungry) is operating close to cut-off due it being bunged into a cct it wasn't designed for.

Now I think I can learn from this (and apply what i learn from here and applying it to the 6EW7 thread) by working out just where I'm operating 6DN7 right now. Good idea?

FWIW 6EM7 was exactly as per the Barker/Kaufman cct and worked beautifully.

DTB

[andrew Ivimey]

hmmmmmmmmmmmmmm tricky! I keep coming back to 6DN7; indeed I can be cleaning my teeth and I remember e.g. I haven't tried CCS on the driver anode (though I suppose I've tried everything else) and yet this evening I was searching out the components for a 6ew7 driving something tricky a little bit beyond an 801a. great valves though I didn't enjoy my bARKER-kAUFMAN.

these valves are PERFECT for learning and enjoying.

[Mike H]

Ah yes it's one o' them TV vertical deflection thingies with 'dissimilar triodes'. Which kind of implies it was designed to work with coils.


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