SE 101 for Dummies

[EWBrown]

Found this over at John Broskie's Tube Cad site, a basic "primer" for SET design and choosing that "magic" tube. Yellow_Light_Colorz_PDT_03


http://www.tubecad.com/2007/10/blog0122.htm


/ed B in NH

[EWBrown]

Note: my most recent updates / corrections are in blue font)

The RCA tube manual approaches these calculations a little differently:

To illustrate this, locate the RCA 2A3 data sheet and copy out the published plate curves - make several enlarged ccopies for ease of use.
These curves will also apply to 6B4G and 6A5G.

Given a 2A3 operating at a plate voltage of 250VDC, at a plate current of 60 mA:

Using their techniques, to "guesstimate" the grid bias voltage:

Start out by multiplying plate voltage X -0.68, and then divide the result by the mu of the tube:

250V X -0.68 = -170VDC; -170V / 4.2 = -40.5 V

Note: the figure of -0.68 is just a "fudge factor" chosen to get close to the initial correct value with the first guess Yellow_Light_Colorz_PDT_02

That grid voltage exceeds the 15 watts max PD load line, which indicates a -43.5V grid bias voltage for 250V and 60 mA.

Determining the Vmax, V min, I max and I min is the similar as described in JRB's method, so I'll skip the details, and cut to the chase...
The one difference is that Imin is determined by doubling the grid voltage, (in this case, from -43.5V to -87V), and using the plate voltage indicated on the curves. I max is 2X the I zero (60 ma in this case) where it intersects the VG = 0 curve.

V max = 390V, V min = 105V, difference = 285V
I max = 120 mA, I min = 15 mA, difference = 105 mA

The OPT primary impedance is calculated with an application of Ohm's Law, R= E/I, or in this case apply the delta betwen max and min E and I:

Impedance = (V max - Vmin) / (I max - I min)
Z = 285V / 0.105A = 2714 ohms

That is also the same as in JRB's method.

RCA's Power output calculation is a bit different:

Power out = ( (Vmax - Vmin) X (Imax - Imin) ) / 8

PO = (285 X .105) / 8

PO = 3.74W (into 2714 ohms)

THD % = (((Imax + Imin)/2) - I0 (I zero) )/Imax -Imin)

Using the above current figures for the 2A3:

THD% =( ((120+15)/2)-60) / (120-15) = 7.5/105 = 7.14% THD @ 3.74W with a 2714 ohm OPT primary. That's a bit "rough"... But in 1940s terms, is fully acceptable as was any THD below 10% :o

Now, let's try Paul Joppas 's formula for the "optimum" plate load (OPT) impedance:

RL = (V P-K/IK) - (2.4X RP) presume 800 ohms RP for the 2A3

RL = (250/0.045)-(2.4 X 800) = 5555 - 1920 = 3635 ohms. 3500 ohms is close enough, this isn't precision brain surgery, and just like with hand grenades and horse shoes, "close enough is good enough" Yellow_Light_Colorz_PDT_09

This will deliver somewhat lower power, closer to 3.0 to 3.2 W out, but the THD distortion will be a LOT lower, as well. Most will be 2HD which is good, and only a fraction of a % will be the less desirable 3HD.

So a James 6112HS with the 3.5K primary, will deliver power at the "sweet spot". 2.5K will deliver more power with a rougher, or as some call it, a "fatter" sound. The 6112HS allows for both tastes and styles.

Any 3K OPT like Transcendar, etc should also be excellent with the 2A3 or 6B4G, better than 2.5K, maybe not quite up to 3.5KUaing a 5K OPT (James 6113HS, 6115HS, or Magnequest RH-60)will deliver a lower power output but then the THD will be dramatically lower, which is a good trade-off.

Going back to the basic Ohm's Law presumptions, 250V / 60 mA would seem to equate to 4167 ohms, in a purely DC world, but we are working in complex reactive variables in series, essentially the tube's internal plate to cathode "AC" resistance, the impedance of the OPT primary and the R/C network of the 2A3's cathode resistor, the phase of the moon and the closest approaching cold front . Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_04 :o

(Last updated 6 OCT 2008)

/ed B in NH

[EWBrown]

Applying this to the A2293 triode (the Cliff Notes Version):

Scenario 1:
B+ = 150VDC, and I = 90 mA, vG will be -24VDC.

output Z = 154V / 166mA = 927 ohms

PO = (154V X 166mA) / 8 = 3.2 W

For cathode bias, raise B+ to 174V, and use a 270 ohm, 10W resistor.

-----------------------------------------

Scenario 2:
B+ = 175VDC, and I = 80 mA, vG will be -30VDC.

output Z = 190V / 140mA = 1357 ohms

PO = (190V X 140mA) / 8 = 3.3 W

For cathode bias, raise B+ to 205V, and use a 375 ohm, 10W resistor

-------------------------------------------

Scenario 3:
B+ = 135VDC, and I = 100 mA, vG will be -20VDC.

output Z = 108V / 170mA = 635 ohms

PO = (108V X 170mA) / 8 = 2.3 W

For cathode bias, raise B+ to 155V, and use a 200 ohm, 10W resistor

BTW, one of the NNETG'ers tried using A2293s and was quite disappointed with the results. Leave those for series or shunt voltage regulator service...

----------------------------------------------

Just having way too much "fun" with pencils, paper and rulers, no better way to occupy an otherwise uneventful lunch hour... Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_01

/ed B in NH

[EWBrown]

I tried replying to your PM, but for some arcane cyber-reason, it keeps getting bounced off into oblivion, so I'll just post the cliff notes version here:



THe BBS just "ate" my rather lengthy reply GRRRRR...........................


The 0.68 is just a "fudge factor" that RCA applies to the original "SWAG" for calculating the 2A3 values. THe general idea is to get you "close enough" for the initial calculations (somewhat like playing horse-shoes or tossing hand grenades) Yellow_Light_Colorz_PDT_07 :o

Now, one may look at 250VDC divided by 60 mA and come up with 4.167 K, but that would be fine in a purely DC world...

The 4.167K would have to include vectoring in the suggested 750 (or 780) ohms cathode resistor and the reactance of its bypass capacitor, the tube's internal plate to cathode (approx 800 ohms) "resistance" and the OPT's primary impedance, which would then more or less come out to more or less 4.16K. Probably more by chance than actual technical reasons :o Yellow_Light_Colorz_PDT_09

2.5K is the minimum"good" OPT impedance, 3K to 5K is better, and will deliver slightly less power but also signifcantly lower THD, as the impedance increases. I've had very nice results with 3.5K to 5K SE OPTs. The MQ "Robin Hood" RH-60s (5K, 60 mA continuous rated current) are excellent with 2A3s and 6B4Gs, and presumably 6A5Gs and triode strapped 6AV5GAs.

I updated the original posting to (hopefully) reduce or even eliminate any confusion of terms.

Now if I could manage to attach the original 2A3 curves to the posting, it would be even better Yellow_Light_Colorz_PDT_02 :o

/ed