Choke loading Anodes.

Max N · Post by **Max N** » Thu Jul 23, 2009 7:56 pm

Nice one Dave

Just a suggestion, for completeness you might want to also calculate the high frequency cut-off from the 76 to the 6EM7?

Dave the bass · Post by **Dave the bass** » Thu Jul 23, 2009 10:04 pm

Max N wrote:Nice one Dave
Just a suggestion, for completeness you might want to also calculate the high frequency cut-off from the 76 to the 6EM7?

Ooooo! G'warn then, how do I do that then Max? FWIW from the anode of the 76 I've got a 0.22uF cap to couple to the 1st stage of 6EM7 buuuuuut just after the cap there's now a 1M to ground as suggested by the pro's boys earlier. That's in parallel with the 220K Pot that acts as a volume control between the 76 pre and 6EM7 power, the 220K is what i use at work ('cos its there) but I've got a selection at home from 50K Alps (shunted) and a switched 100K attenuator too... Soooooo, I'm guessing its gonna be one of them RC calculations but I don't know how to arrange it TBH. Help!

@ Simon, the sound? Weeeeelllllllllll, you know that lush natural crystal sound that our Will and SteveS get with approx £31,234,789,362 quids worth of 1903 vintage DHT's through OB's...welll....its nuthin' like that at all

Its all right acherlee Simon, boosts the gain a bit, pushes the amp into distortion at an earlier volume knob setting just like I hoped it would. I've only heard it at work through my crappy school-era built 2 way's but it sounded nice, no hum or nasty's, I'll bring it home one day. At the mo it's just a learning project really not a keeper as such.

@ Nick and everyone who's advised and pushed me, many many thanks for your time and patience. I REALLY appreciate it. Fun and education at the same time.

Top Trumps.

DTB

Max N · Post by **Max N** » Fri Jul 24, 2009 8:44 am

OK, you've already calculated the series cap (coupling cap) which blocks low frequencies, so acts as a high pass filter.
There will also be some shunt capacitance somewhere which will shunt the high frequencies to ground, so acts as a low pass filter.
This will be the capacitance between the grid of the 6EM7 and ground.
The first tricky bit is that this capacitance also includes the 'Miller capacitance'. MJ explains Miller capacitance pretty well - all we need to know is the formula, which is:
Cmiller = (A + 1) x Cag

Cag is the capacitance from anode to grid from the datasheet for the 6EM7
A is the actual physical gain being realised in your particular circuit at your op point. (edit: I should have said, A is the gain of the 6EM7 stage, not the 76 stage...)

Once you've worked out Cmiller of the following stage, you need to add the capacitance from grid to cathode plus heater (also from your 6EM7 datasheet).

Ok, this gives you the total capacitance. (You could add a bit extra for strays, e.g. the capacitance of the interconnect)

Earlier, you worked out the output resistance of the previous stage to be 1278ohms

You use the same formula for a CR filter as you used before whether its high pass or low pass:
f-3db = 1 / (2 x PI x C x R)

Max N · Post by **Max N** » Fri Jul 24, 2009 8:47 am

One other thing - when deciding what volume pot to use, remember that a smaller value pot will need a bigger coupling cap - you could try re-calculating your required coupling cap for different value pots if you like...

Dave the bass · Post by **Dave the bass** » Wed Aug 12, 2009 9:22 pm

Hello, remember me?

Sorry for the delay, I was on Hols and skateboarding solid for 2 weeks then have been getting my schnizz together at work and over-seeing the skatepark extension for a while.

Anyway, back to business... to round off this thread in a way I'll attempt to solve the HF cut off when feeding the 76 pre into 6EM7.

Ol' Miller Cap again, so you say Cmiller = (A + 1) x Cag , OK lets have a go...

First up, capacitance betwixt Anode and grid of 6EM7 is 4.8pF according to data sheet.

Now then, Errrrrrrmmm....working out mu ('A' in Max's equation?) for 1st stage in 6EM7 amp as actually built, not from the data sheet yes?

I'm bit rusty again (Cornwall was wet)... Cct dia here -> http://www.the-planet.org/6EM7.html So, Anode voltage on 1st stage is 115V and there's 1.2V on the cathode. 210V is being dropped across the 120K anode resistor which means approx 1.7mA flowing through the first stage of 6EM7 amp I think.

I'm losing meself, its mu I need to work out innit not the O.P.

I'll have another bash tomorrow, sorry gents, I'm tired, really tired.

Sleepy head.

DTZzzzzzzzzzzzzzzz.

Post by **Nick** » Wed Aug 12, 2009 9:30 pm

Hi Dave, start again tomorrow. I guess skating is the same, do this a lot its easy, leave it alone before you have the mental version of muscle memory (and no I don't think that's just memory, you need to remember the process as well) then it needs doing over again.

Dave the bass · Post by **Dave the bass** » Thu Aug 13, 2009 10:08 am

Nick wrote:Hi Dave, start again tomorrow. I guess skating is the same, do this a lot its easy, leave it alone before you have the mental version of muscle memory (and no I don't think that's just memory, you need to remember the process as well) then it needs doing over again.

Kind of, if I don't skateboard for a week or so I loose a bit of balance and get a bit wobbly. I'm normally a bit wobbly (especially around the tum tum) and have never been particularly brilliant at skateboarding, I just like it a lot. It serves no purpose whatsoever in an adult life, where as in a childs life I believe its called 'playing'

Just had a coffee and sweet biscuit, the brain is set to maximum brightness level, lets go

I just read the thread back and think I've found the bit to find the gain of the stage at the actual built O.P.

Plot the load line, then measure a swing of volts on the grid + and - directions, drop a couple of lines down vertically then divide swing of grid volts into swing of anode volts yes?

That right?

DTB

Post by **Nick** » Thu Aug 13, 2009 10:48 am

Yep, that gives the gain.

Dave the bass · Post by **Dave the bass** » Thu Aug 13, 2009 10:57 am

Nick wrote:Yep, that gives the gain.

Phew! I haven't lost 'it' afterall.

OK, I've downloaded the curves for the little triode in 6EM7. I don't have my own 6EM7 amp to hand so I'm going to have to get info from the Kaufman cct diagram, hope thats cool.

Is it agreed that the OP is 115V @ 1.7mA like i worked out? -1.2V neg bias from the cell in the cathode.

DTB

Dave the bass · Post by **Dave the bass** » Thu Aug 13, 2009 2:49 pm

Finally! Sorry for delay

I calculate the actual gain to be approx 55. Quite fiddly to work out on the curves I thought so I zoomed up a copy of it.

DTB

Dave the bass · Post by **Dave the bass** » Thu Aug 13, 2009 8:16 pm

Hooray, peace at last from work, back home to do homework.

Cmiller = (A + 1) x Cag

Cmiller= (55+1)x 0.000,000,000,0048F = 0.000,000,000,2600F = 260pF (I think)

Then following on from what teacher Max says I need to add the 260pF to the grid to cathode capacitance on 1st stage triode which the data sheet says 2.2pF (actually data sheet says 'Grid to cathode and heater') = 262.2pF

So...

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

f-3db = 1/(2x3.14x0.000,000,000,262 x 1278) = 475200HZ = 475.2Khz

Answer HF -3dB cut-off is 475.2Khz Sir?

DTB

Post by **Nick** » Thu Aug 13, 2009 8:28 pm

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

Dave the bass · Post by **Dave the bass** » Thu Aug 13, 2009 8:29 pm

Nick wrote:Yep looks right to me. (assuming you don;t have a volume control between the two)

Ah! Errrrmm......ah errrrr...

Volume control goes in front of 76 pre to save me doing more maths

pre65 · Post by **pre65** » Thu Aug 13, 2009 8:34 pm

Dave the bass wrote:
Nick wrote:Yep looks right to me. (assuming you don;t have a volume control between the two)
Ah! Errrrmm......ah errrrr...

Volume control goes in front of 76 pre to save me doing more maths

Is there an "optimum" value for the volume pot in this simulation ?

Dave the bass · Post by **Dave the bass** » Thu Aug 13, 2009 9:24 pm

pre65 wrote: Is there an "optimum" value for the volume pot in this simulation ?

I dunno TBH Phil, as with most things in Electronics/life its a compromise I'm guessing.

@ Nick and Max, penny drop moment in the shower just now. HF cut off and capacitance to earth.... duh! Of course, I remember now from college, cap to earth gives the signal an easy path to earth, hence the reason fior looking into the inter electrode capacitances 'into' the valve of the next stage. Any big/unwanted capacitance here curtails HF, add in miller effect and bang goes your HF response... capacitance to ground = bad! We don't want much of it. Low pass filter.

Conversely, not enough capacitance in a SERIES with the signal and you're going to limit LF, hence bigger values when coupling. Too small and its bye bye LF response. High pass filter.

I wish I'd have remembered all that i was taught at college in the early 80's but I just never used any of it in my normal 9-5 duties.

DTB

audio-talk

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