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In Reply to: RE: Input tube suggestions? posted by Genp on July 11, 2016 at 13:28:37
The 6EM7 is a dual triode. What is the large-triode side doing? The answer to your question depends a lot on that.
Follow Ups:
I thought it would be better to show what I have in mind.
Keeping both sections of 6EM7, it would look like this.
Section 1: around 1mA, 200V, -2V
Section2: 42mA, 160V, -24V
300B: 60mA, 350V, -74V
Interstage transformers are designed for low rp tubes (600-900 ohm optimum. Ok from 300 to 1200 ) and from 40 to 80mA. Inductance around 31H (I over 30mA) . They can swing 85Vrms. DCR 152R/153R.
Don't know if I'll put a resistor at secondary of IT.
Power supply not designed yet (as you can see).
Maybe I forgot details but I'm not really a designer...Suggestions are welcome, thanks
Hi,
That looks good.. You may want to try the 6BM8/ECL82 triode-pentode in a similar scheme using an IT with the pentode section triode strapped.. You may get away with unbypassed cathodes for the high mu triode sections too.. IRRC, Cyrus Brenneman used the same input-driver for one of his SE amps but choke loaded instead of IT's.. Good luck.
6bl7 or 6bx7. JH
The way the high mu section of the 6EM7 is set up now, it has an in-circuit gain of 51. By eliminating the cathode bypass cap the gain will drop to 25. Not only will this achieve a gain reduction by 6dB, but it should also quite nicely reduce distortion from the degenerative current feedback. You will be eliminating a capacitor that's in the signal path while getting your reduced gain along with improved distortion in the process, a nice win-win IMO.
Even if you go with an additional input tube such as the suggested 6SN7/6J5, it would be very simple to try what I've described here. It would only require a couple solder connections and at the very least it should be a good education.
Removing capacitor will reduce gain (NFB at cathode) and increase Zout.
If I have: 40k + (68+1) x 2k = 178k
Then new Z = 178k II 100k = 64k
New gain = (68 x 100k) /(100k x 178k) = 38dB
Am I right?
I admittedly got a little lazy and used that same operating point but on the very similar 6SL7 instead, since I could obtain the parameters for that tube much easier. Following that, I went back and had a look at the 6EM7 datasheet and for your set up as shown, the OP is at Va = 152V, Vg = -2V, ra = 50K and mu = 64. I didn't check your math, but keep in mind that the impedance looking into the cathode is in parallel with Rk and I don't think that you took that into account. In any case, I've developed an excel spreadsheet that calculates Zo, gain, etc., so with the aforementioned ra and mu values that correspond to this specific OP, the numbers become:Zout -- bypassed = 33.3K
A -- bypassed = 42.7Zout -- unbypassed = 64.3K
A -- unpypassed = 22.9As you can see, the input section of the EM7 is hardly high gain in the unbypassed config. Going with say a 6SN7 that's going to yield a mu of ~17 will not make that much of a difference, but of course that will have to be a judgement call on your part. Just let us know what happens and if you happen to try both schemes it'll be all the more interesting.
Edits: 07/13/16 07/13/16
~!
The Mind has No Firewall~ U.S. Army War College.
On a side note, based on your desired operating point for the 300B while taking into account that the tube is cathode biased, the plate voltage will need to be at 422V if you want 350V through the tube (422V-72V = 350V). The 72 Volts are dropped at the cathode resistor (1.2k x .06A = 72V). Since you'll probably be dropping another 20 or so volts at the transformer primary, you should be designing the power supply for a B+ of around 445V
That's what I hoped. In this case, a 6DJ8 or similarly voltage-limited tube would be no problem, since the first stage only has to drive some 15-20vRMS to get the full 85vRMS out of the interstage.
Hi.
Maybe the 6922/6EM7(L-µ section) combo could be interesting.
(Keep in mind 6EM7 H-µ section with an unbypassed cathode resistor)
This is how it looks:
6EM7 Low-µ
operating point: 160V, 42mA, -24V
plate diss.= 160V x 0,042= 6.7W
rp= 900, S= 5.6 mA/V, µ= 5
Rk= 24V/42mA= 571....560R
6922/E88CC (according Telefunken datasheet)
operating point: 90V, 6mA, -2.2V
rp around 4k, S= 8mA/V, µ= 30.5
plate dissipation=(90V x 6mA)= 0.54W
drawing line from 250V leads to 9.3mA...
...giving plate resistor of 27k
Rk= 2.2V/6mA = 367...360R
Gain= (µ x Ra)/(Ra+rp), Ra=22.9k (27k II 150k)
= 26
The 39µF cap should be Solen, Axon or similar
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