• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Call it good enough?

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Would you call this good enough? These are the distortion numbers on my amp project, loosely based on the GEC 400 watt tube amp plans. I'm using 4 KT120's in parallel push pull, and instead of cathode followers driving the output tubes, I'm using paralleled 6SN7's with CCS plate loads. Output trans is Hammond 1650T. Measurements taken with Keithley 2015, at 1 Khz.

3 watts-.07% , 5 watts-.11%,
10 watts-.17%, 20 watts-.29%, 30 watts-.31%,
40 watts-.28%, 50 watts-.22%, 60 watts- .27%,
75 watts-.52% 90 watts-.61% and starting to clip
 
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DAAANG....finally put some music through this thing, test speaker is a Eminence 12" full range in a true infinite baffle, its in a very solid plywood wall of my shop with a whole adjoining room for it's enclosure. This thing is very listenable with some EQ. Enjoyable, foot tapping, surprising. Make me wonder if I have gone off-track with my home theater....

It has way more than enough power for the Eminence 12 of course. Bass is surprisingly good, sure sounds better than it looks on the scope when square wave testing...Mids are present and real, highs are pleasant but of course constrained by the speaker. Might have to throw a tweeter on.
 
Front end

Here's the front end of my amp project...look good?? The circuit I mean, not the cobbled drawing...LOL
Cathode bypass cap is actually 100uf Sanyo Oscon
This is driving 4 paralleled push pull KT-120s.
 

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Thanks, I wondered about the 10nf coupling caps myself. I'm going to change them to .1uf. Also thought about reducing that 450v , that was a holdover from the original GEC design. The distortion of the final 6SN7's kept decreasing as I increased the current sourced by the CCS's, but I stopped at 10mA (through both sections combined) due to CCS power dissipation. I think I will lower the B+ and try further increasing the current.
 
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I thought it might be advantageous since each 6SN7 is driving 2 KT-120's. But then I am no expert, just a "cut and paste" amateur, LOL Besides, I have 3 sockets in the front end. Just dropped the B+ for the front end down to 340 volts, changed the 1 meg grid leaks to 470K, and increased the coupling caps to .1uf. Distortion from the splitter/driver is .31 % at 40 volts out, P-P. Pretty much the same as before the changes. Splitter output is almost perfectly balanced, 17.74 RMS from one side, 17.77 from the other.
 
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10 ma ... cuz of power dissipation... (paraphrased)
Kind of an interesting observation (perhaps KodaBMX will pipe in)…

Using a CCS load to each of the pair of parallel'ed 6SN7s as phase-inverter amplification seems to introduce into the audio path whatever nonlinearities of compliance the CCS has with respect to its V-vs-I curve.

Not … of course … that that appears present in your distortion measures. But still … unlike a cap-bypassed CCS as a series-with-cathode auto-bias (where the cap squashes whatever I(v(t)) curve the particular CCS presents), the naked anode load CCS does just the opposite.

Still, I like it, since having sand based active components mixed with the vacuum devices is just fine in my book.

I kind of generically wonder tho' whether the CCS wouldn't have been better as the cathode auto-biasing device, with 100 µF / 35 V quality electrolytics bypassing, and a simple appropriately chosen metal film anode resistor (ohmmeter matched series chain) to “do the amplification”. Since you've implemented monoblocks, it wouldn't take much to modify one one way, and leave the other in your diagram's configuration.

Just saying,
GoatGuy ✓
 
PS: at 10 ma, the 830Ω R would drop 8.3 V. Looking that up on the 6SN7 datasheet, seems that it'd want an ΔV between A and K of 220 V, to get 5 ma per section. Thus the anode resistor with a B+ of 450 V would have to be

V = 220 + 8.3
V = 228

B - V = 222 V

I = 10 ma
I = 0.010 A

E = IR …
R = E/I

R = 222 ÷ 0.010
R = 22.2 kΩ

P = IE …and…
P = E²/R
P = 222² ÷ 22,200
P = 2.24 W​

So, choosing 4 ea, 5.6 kΩ, 1 watt resistors, sticking them in series …
then doing that again, and using a 3½ digit DVM to get a sense of resistance,
then choosing out of your parts bin a small value R to add to the lower resistance chain
Would deliver a couple of 4 watt capable precision-matched resistors for the anode gain load.

I'd love to hear your report on the apparent acoustic difference.

Sorry to be a pest.
Just saying,
GoatGuy ✓
 
Thanks, I need all the input I can get, I want to move beyond "cut and paste" to actual design. I used this Triode / Pentode Loadline Simulator v.1.0 (20161216 [url]www.trioda.com)[/url] to determine the cathode resistor value, but I may have used it improperly. Looks like my current CCS's may not lend themselves to this experiment though. They are from K and K Audio.

Switched from Sylvania "Bad Boys" to Baldwin/Sylvania "chrome domes" are they? They are shorter than the Bad Boys. Distortion increased from .31% to .36. Going to try some Russian 6H8C, although they pop, sizzle and crackle. Curios about the dist. though. hmmm....The Russians beat the Bad Boys, .24 % at 40 volts P-P
 
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