gm70 driver tube tests
I've tested a few prospective driver tubes to see if i can improve my gm70 amp.
6e5p triode strap -what i'm using currently
D3a triode strap
6J9P (e810f) triode strap
EL84 triode strap
EF184 triode strap
all with g2 to anode via 300r resister and g3 to cathode
I made a test box yesterday so i could vary the bias voltage from -1 to -6 and the currant flowing thru the dn2540 from 5-28ma load was 100k//single dn2540 ht of 350v
I played with all the tubes to get the best thd figures within what the box could do. i need 64vrms for the -90v bias on the gm70, so i tried to get as close to this as i could but as i used a switched attenuator on my 4p1l preamp it was ~64vrms
tube ma g1 Va thd vrms out
6e5p 28 -3.5 180v 0.09 71:-0
D3a 17 -2.2 200 0.42 58
D3a 17 -2.2 200 0.47 80
6J9P 17 -2.0 166 0.18 66
6J9P 17 -2.5 166 0.67 95
6J9P 28 -2.2 208 0.066 67 just for fun 5.8w :-0
EF184 18 -2.16 207 0.38 68
EL84 was not much cop at these voltage swings but managed 0.004% at 2vrms, thats comparable to the 4P1L linestage i was using and my e-mu soundcard does 0.0009%
So on balance i think i'll stick with the 6E5p but i will bias it to 3.5v and a few more ma. wish the 6J9P anode diss was higher 0.06% at
~190v is impressive and 266v p-p swing from a 350v supply at 0.6%, what a tube.
hope all this is accurate and of help to gm70 builders etc
Great study Enzo. Is your amp a two stage? I plan on driving the GM70 with the 12GN7A. Can you test that one???:D:D:D:D:D:D
Yes 2 stage not inc preamp, but i need one of those as i listen mainly to LP, so i dont limit myself to <2vrms input. will try 4p1l sometime:-) I dont have any 12GN7A's but i'll look into them....
The most expensive tube was the el84, the rest were <£4 which i think is stunning considering how these tubes performed.
My gm70 measured 0.4% at 1w 5%at 15.5w (11k5 load 900v) so i'm looking to lower that, found a good operating point for the 6e5p just need to get one for the gm70's now.
p.s the 6e5p that went thru the wasing machine in my pocket measured ~15% higher thd!
Hi Enzo. When you say 11K5 load is that into the OPT? My OPTs are 8K and my B+ is just under 900v. How are the driver and output coupled? I am planning on choke loading the 12GN7A. Let me know if you want me to send you a 12GN7A. Dennis.
yes 11k5 lundahl 1620. as i remember the 1620 can be configured at a few different impeadences so i could try a lower load but 15 watts is fine for me and i get the damping factor. 8k seems like a good comprimise.
Im using rc coupling at the moment .1uf 1500v and 1uf 1200v pp caps and 100k grid leak, i wouldnt mind trying transformer coupling but the weight and expense put me off!
I'm using a mu follower with 6e5p's as the ccs, but i'll try other topoligys and see what i like. my wife doesnt understand when an amplifier is working why its not finished!
very kind offer of a 12gn7a, could i swap with 3Da and or EF184? i have a few of those
I have done almost similar tests a couple of years ago.
The main difference is that I had adjustable cathode resistor instead of adjustable bias. I also tried both bypassed and unbypassed cathode resistor.
My results with 6J9P and 6E5P are quite equal.
But I found some even more linear types: 6S45P, 6J11P and 6J52P.
For example 6S45P gave 100 Vrms with 0,34 % THD when Ub = 350 V, Ua = 200 V, Ik = 20 mA and RL = 100 k.
My conclusion of the tests I have done is that high gm-type tubes are most linear.
Typical 6SN7 was one of the worst.
Pete Millett has done some work on this subject High Gm driver pentodes
I feel a little sad, because I didn't manage to get it to sound with 6E5P as a driver, althought it seems a very linear tube on paper. The mids were somewhat harsh. But this tube has great potential, Ia and Pa. I remember I used 1k as a grid resistor, otherwise the tube easily oscillated. My amp is 6P45S output tube based SE.
Compared with other tubes, I have tested E180F, D3A, E280F and E810F.
E180F and E810F for now give me the best sound for my taste. D3A has the best gain, but it sound a little harsher for my ears than E180F. E280F is between E180F and D3A and has great stage and dynamics. I use CCS 10m45s cascoded as load, mu out. I love swapping these tubes sometimes.
All these tubes sound similar and ok compared to what 6E5P sounded to me - detailed, but harsh and intolerable. I use a 220R resistor from g2 to anode.
The grid stopper and g2 resistors have practically no effect to THD.
But the used anode load, cathode resistor and supply voltage have a lot.
And you do not mention about these essential things at all ??
How did you ensure that you used optimum conditions with 6E5P.
What were your THD values with 6E5P ?
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