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SEP using 6P30B and 5744WB/6C7B
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Old 11th February 2019, 05:05 PM   #1
mondogenerator is offline mondogenerator  England
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Default SEP using 6P30B and 5744WB/6S7B-V

Hi,

To start off I should state that this is my first attempt to design and build anything using vacuum tubes, and I have much to glean from the books I've bought.

I searched here and on the interweb in general and there isn't a huge amount of info regarding the 6P30B tube, a couple of threads here, a few of Wavebourn designs/you tube videos.

So, I've skipped forwards a couple of steps....

Being a first attempt at tube circuit design, I built a simple voltage amplifier, fully bypassed Rk, drew the loadlines for 150k Rload and Va of 2/3rds B+.

In testing using 6C7B or 5744WB (mu~ 70), all is well and I see OLG approaching 2/3rds Mu.

Great!
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I must be a child of good times... the scariest thing I've seen done thus far, has been connecting a cathode bypass capacitor reverse polarity...Then rushing to cut the HT!

Last edited by mondogenerator; 27th February 2019 at 08:08 AM. Reason: Wrong PN of devices used
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Old 11th February 2019, 05:50 PM   #2
mondogenerator is offline mondogenerator  England
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Default A power stage

Then, after reading around an awful lot, and going over the information again and again I found that TheGimp made a very helpful point in post #4 here:
Anyone have experience with transformers for a 6p30b-r PP Amp?

So I started to try an build a pentode power stage, using the data points mentioned, and I varied Vg2, Va, and Ik to arrive at something that just about stays within datasheet limits (I.e. for max watts)

I figured I should be able to make 25% efficiency, so perhaps1 Watt output, perhaps a shade more.

Imagine my horror when I barely made 0.5W using a 100v line matching transformer on 1W tap (IIRC about 3k)!!!

Then imagine my embarrassment when I realised I wired the 6P30B-R tube using the pin out of the 6P30B....

I still haven't bothered to ascertain what I was driving the tube with, which grid or other electrode, but somehow it still worked in an inefficient manner.

So...courtesy of auction sites, I had a old radio output transformer 7k:4R....studying all the information I had seen, this is still too high reflected Z, but its what i had to hand so I went back to the amp and hooked it up.

Much better! 0.8W.....limit of the sig gen output...
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I must be a child of good times... the scariest thing I've seen done thus far, has been connecting a cathode bypass capacitor reverse polarity...Then rushing to cut the HT!
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Old 11th February 2019, 06:22 PM   #3
mondogenerator is offline mondogenerator  England
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Default Triode stage to drive pentode

Taking the simple triode stage from before I initially ran with feedback to the grid to get around 30 V/V gain, found that I ended up running into soft clipping before the cut off clip occurred. Reducing the gain helped slightly, but of course required higher drive.

I then built an lower gain stage using 6C6B with a lower mu and better current capability.

This helped but not nearly as much as tweaking the Vg2 voltage, and adding plate feedback to control the gain.

This is where things get hazy. I merely added a large value feedback resistor following a blocking capacitor, and reduced it until I got a result which seemed good on the scope. I started around 1M and ended about 132k.

So I now have:

Va = 300V
Vg1 = 38 - 40V
Rleak= 150k
Va-k = ~260V
Vg2 = 250V (datasheet maximum for 6P30B)
Rg2 = 68k
Rk1 = 2k
Pdiss ~5.55 W
Po = 1.2 W (before clip on scope)

I've yet to redraw the loadlines and see where I'm at for either drive stage or output stage, so that is a work in progress.

Doubtless I'm exceeding the triode datasheet specs (in voltage) and I need to revisit that, and reduce them accordingly.

I will also cool the bias slightly to stay more in the range 4.5 to 5 W dissipation, and I'll post the revised schematic once I'm done.

Any advice or suggestions of improvement are welcome (once I post an up to date schematic so that there's something to criticise!)
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I must be a child of good times... the scariest thing I've seen done thus far, has been connecting a cathode bypass capacitor reverse polarity...Then rushing to cut the HT!

Last edited by mondogenerator; 11th February 2019 at 06:48 PM.
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Old 11th February 2019, 06:32 PM   #4
mondogenerator is offline mondogenerator  England
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Default Some pictures for those that like to see creative point to point antennae...

Oh I forgot...In my travels I tried small 3mm low current red LED for bias, and actually reply liked the effect this had, and the 1.7V bias was quite a nice point, (the cathode bias ~0.7V) driving ability was improved.

I really need to read up more on LED bias....
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File Type: jpg 20190201_113819-1224x1224.jpg (392.6 KB, 222 views)
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Old 12th February 2019, 09:32 AM   #5
jazbo8 is offline jazbo8
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SEP using 6P30B and 5744WB/6C7B
A picture is worth a thousand words, a schematic (with voltages) of what you have now would be helpful.
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Old 12th February 2019, 01:15 PM   #6
mondogenerator is offline mondogenerator  England
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Default As promised...schematic attached

Hi,

First off, here is a schematic of the circuit I have this far (attached) including measured electrode voltages (using 7k:4R OPT)

Drawing a schematic at stages in a build is always a good idea - I find most of my errors when re-visiting the schematic/re-drawing the schematic!

Secondly, I have noticed some errors and was already aware that some of my component values, selected by experimentation, may need re-calculating (or just calculating).

So....

1) I must look at the RC filter on the grid of both tubes, as so far I have only added a series resistance Rg, and no additional capacitor from grid to gnd.

2) The R element of the input RC isn't in the right place - it should be 'inside' the grid leak R, nearest the tube.

3) the Values of Rg and Rf for both tubes needs to be re-calculated (Rg in both tubes ended up being much smaller than ra, so either I need to compensate with capacitance, or alter the values of Rf and Rg to yield the same gain.

4) There is probably more wrong with my circuit....like the use of screen degeneration....

However, testing today with a transformer suitable for ECL86 (4k:8R), bias shifted slightly, Vg2 slightly up at 260V.

Ill need to review the design and lower this back to <250V.

MORE POWER! I'm now getting a max output before clipping of 1.5W into 8R
Attached Files
File Type: pdf 5744WB_6P30B.pdf (19.8 KB, 62 views)
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I must be a child of good times... the scariest thing I've seen done thus far, has been connecting a cathode bypass capacitor reverse polarity...Then rushing to cut the HT!

Last edited by mondogenerator; 12th February 2019 at 01:22 PM.
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Old 12th February 2019, 01:49 PM   #7
GoatGuy is offline GoatGuy  United States
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Several things…

From the 5744 tube datasheet (TungSol), and your design-point of having a B+ → V[sub]A[/aub] drop of about 100 volts (giving a VA of 200 V more or less), with a 33 kΩ load resistor and E=IR,

E = I R
100 = I 33,000
I = 0.00303 A
So, this logically will be going thru the cathode, too of the 5744. Thing is, what VK (to use the German K for Kathode, as C looks suspiciously like a Capacitor when one's eyes are squinting at 3 AM) is needed?

Go to the TungSol sheet. Find the curves, and locate the curve that best fits VA = 200 V and IA = 3.03 ma. Ah… there it is … about 2.0 volts on VK.

Well, that right there tells me that the 330 Ω RK resistor, dropping only 1 volt, is not the right size. You could employ Ohm's law easily enough, and see that a 620 Ω (nearest 5% value) would fit the bill. OR (if you keep reading these lovely weblogs), you'll also hit on the idea that an ordinary RED LED would do the job just as well. After all, the combo of a resistor + bypass capacitor is a teensy weensy power supply.

I'd go with the LED, myself.
Just make sure it is a common one. No ultra-super-bright thing.

PS: it isn't at all clear why you're feeding back some of U1's anode output back to its grid: hopes for negative feedback linearization? I wouldn't myself. But that's me.

PPS: I see that your measured VA is closer to 150 volts.
This makes more sense…

As it increases current to about 4.5 ma, which increases the VK to about 1.5 volts, which then brings it all into balance. See? Curves… wonderful curves…

No comment re: final stage. It looks nominally correct.

Your output transformer tho' doesn't likely have a 1 Ω primary. (the "1" in the drawing)

Just saying,
GoatGuy
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Old 12th February 2019, 03:14 PM   #8
mondogenerator is offline mondogenerator  England
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GoatGuy,

Thanks for the reply.

I should comment that the 5744 gain stage was originally designed via loadlines for a somewhat lower B+ (210V) and I neglected to rebias properly after increasing the B+ to 300V.

With Original B+ of 210V and Va of 2/3rds, the Q point intersects around the 0.7V mark, requiring 220R cathode resistor.

And you're quite right, the use of a low current red LED give bias around 1.7V and I liked it a lot!
I think I biased with 560R to get closer, then went back to LED.

I need to study more (or just insert a small resistor) to understand better how I can measure the DC current when using LED bias)
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Old 12th February 2019, 03:36 PM   #9
mondogenerator is offline mondogenerator  England
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Quote:
Originally Posted by GoatGuy View Post
Several things…

PS: it isn't at all clear why you're feeding back some of U1's anode output back to its grid: hopes for negative feedback linearization? I wouldn't myself. But that's me.
I'm a little confused by this comment...

Are you saying I shouldn't be using any local feedback around the triode?

Can you explain further?

Initially I ran the triode open loop (and so the pentode...), after degenerating the pentode because gain was rather high, then I went back to the triode and degenerated that too.

I've noticed that I made an error on the schematic, plate voltage, Va, is labelled twice.

To clarify, using red LED or 560/680R for cathode bias, yields plate voltage of 150V at Vgk of 1.7V

Plate voltage in current set up (330R cathode resistor) is 195V, with Vgk of 1V. Close to 2/3rds B+, but not the correct bias for centre biasing for max voltage swing.

Sorry for the confusion.

IIRC at either bias point, the triode still has headroom, Well after the pentode grid is driven far enough to clip the output.

I had systematically reduced gain in the pentode, to use more of that voltage overhead from the triode, but after several iterations, i think i found the best gain in the output stage.
I.e. reducing output stage gain to use a larger proportion of the triode voltage gain, didn't really work. Triode loading increased, and the headroom I had, disappeared. So I reverted to the best set up.
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I must be a child of good times... the scariest thing I've seen done thus far, has been connecting a cathode bypass capacitor reverse polarity...Then rushing to cut the HT!

Last edited by mondogenerator; 12th February 2019 at 03:53 PM.
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Old 12th February 2019, 06:05 PM   #10
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by mondogenerator View Post
GoatGuy, Thanks for the reply. I need to study more (or just insert a small resistor) to understand better how I can measure the DC current when using LED bias)
Easy enough: in DC conditions (i.e. no signal or as we say "quiescent"), just measure the volt drop across RA the anode resistor. It is a very close proxy for cathode current. Very close.

Also, with your values, the 1.7 VF voltage drop is almost perfectly suited for the front end stage.

Just saying,
GoatGuy
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