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KT120 PPP Issues/Improvements

Hi, sorry for the delay, I only got to measure last night. Changes to the driver operating point increased the total available power output, but the clipping point remained the same no matter what was changed.

60w 8 ohm tap into 8 ohm load, and 36w 8 ohm tap into a 4 ohm load for @Kay Pirinha.

Given the above and previous information, would it be a correct assumption that the output transformer is the limiting factor in max clean power?

I forgot to measure the 4 ohm tap into an 8 ohm load which should have helped in the investigation. That should be something like a 1.2k primary which should put out more than the current max of 60w.
 
60w 8 ohm tap into 8 ohm load, and 36w 8 ohm tap into a 4 ohm load for @Kay Pirinha.

Given the above and previous information, would it be a correct assumption that the output transformer is the limiting factor in max clean power?

I forgot to measure the 4 ohm tap into an 8 ohm load which should have helped in the investigation. That should be something like a 1.2k primary which should put out more than the current max of 60w.
This tells me that my assumption the primary impedance might be too high wasn't correct.

Connecting a 4 Ω load to the 8 Ω tap will double the primary impedance, not halve it. Let's see what happens...

Best regards!
 
120W into 3K means that the output stage needs to deliver 600 V r.m.s (425 V-peak per side). Impossible to do with 430V supply. I have never used the KT120 but I think you need at least 620-650V anode voltage. I call it bad design.
IMHO, it is easier and cheaper to get 120W of good quality in UL mode just buying another power transformer, possibly some capacitors (or do series with bleeders for higher voltage) and make a proper power supply.
If want to stick to 430V supply and get some 80W+ then need to connect the the KT120 in tetrode mode, biasing each tube at 50-60mA.
If happy with 50-60W and still want to use 4xKT120 per channel then could try triode mode and no feedback. In triode mode if bias at 90-100mA per tube, it's basically class A amp. However, gain might be too much...Pick your poison!:)
 
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If I understood correctly, output transformer 8 ohm tap, load 4 ohm. The reflected primary impedance will halve.
Yes, you're right, of course. My mistake, as I muddled it up while writing. Sorry about that. What I wanted to say is that connecting an 8 Ω load to the 4 Ω tap (see #22) will double the plate to plate load.

Best regards!
 
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The other bad part of the circuit is the driver. What's the point of running the 6SN7 at 80V anode voltage and less than 2 mA per tube?
From 430V supply, even a good regulated supply could be around 360-370V.
Then 68K for parallel tubes is exaggerated. 18-22K (i.e. 36-44K per tube) is more like it. With about 330V supply as shown, 22K common anode resistor and 680R+1.5K in place of 1K+2.2K at the cathodes, it should settle around 8.5 mA for the two triodes and 130V anode voltage. The bias would be -4V or just a bit higher and that might be just enough....
 
120W into 3K means that the output stage needs to deliver 600 V r.m.s (425 V-peak per side). Impossible to do with 430V supply.
This exactly was my thinking before I posted #16. More output power with limited voltage means more current by lowering the load impedance, rather than by doubling the tubes count. Anyway, in #22 he said that halving the primary impedance would almost halve the output power, at least according to his measurements. Hence, there's a current limitation also, or his measurements were erroneous.

Best regards!
 
@Kay Pirinha Maybe 600V are too much and 510-520V are enough with 3K, looking at what the KT88 can do. But it surely needs more HT for 120W output power. If the plate load gets too low, distortion will increase significantly and available outpower will decrase a lot. Not suprising that with 1.5K plate-to-plate he gets less power. Tubes amps are constant power generators. One can maximize output power over a somewhat lower impedance than nominal (like max Pout into 5-6R and get less into 8R) but it would require some 2-2.5K into 5-6R. Not lower impedance.....
 
Yes, usual AF power tetrodes and pentodes feel much more comfortable with higher plate supply voltages. The amplifier's efficiency would increase also. Perhaps lowering plate load impedance will work with tubes with high cathode perveance, such as sweep tubes?

Best regards!
 
120W into 3K means that the output stage needs to deliver 600 V r.m.s (425 V-peak per side). Impossible to do with 430V supply. I have never used the KT120 but I think you need at least 620-650V anode voltage. I call it bad design.


My current fixed bias p-pp amp using 4x 6550 Svet winged C with 450V B+, each tube set 70mA quies (125mA full load); I get exactly 115W o/p with 2K A-A prim imp with 40% screen taps. Re-doing the loadlines, it would appear the optimum A-A load for this tube in parallel paired operation is lower, approx 1.5KA-A with same screen tapes, but at 2KA-A, distortion is also lower as is available power.
With the same 2KA-A conditions I will get 130W o/p using EH KT90´s (lower internal impedances) but I know my transformers were based on using KT88´s. (460V B+ = 65W o/p)
On another aspect
Svetlana data sheet for 6550C (black winged C logo) strangely doesn´t provide max values of grid resistance for fixed bias, however the other C version suggests 200Kohms. The original coke shaped Tungsols reissue suggests a wicked 50K ohms max, requiring a tough slam job for the driver stage. For those HiFi operators swapping tubes, take note, the same types of tubes are not always compatible.

KT120´s seems a pricy overkill when other tubes would equally provide excellent performance with a given B+. The other constructional aspect is I don´t like stacking electrolytic caps to make up voltages. It takes up valuable space and poorer ESR values. Parallel pairs for me is a very efficienct solution.

Bench B
:)
 
On another aspect
120W with 2K plate-to-plate means 82% of voltage output required by 3K. So with 450V it is possible. I likely exaggerated with 600V supply but 510-520V are surely required for 3K load.
However, I think that the output power is not the main concern as there is not much practical difference (i.e. listening mode) between 60W and 120W. If one has to settle for lower power it's more about unnecessary waste of electrical power. The driver stage looks really poor with the 6SN7 working at very low plate voltage and small current. As someone pointed out, it might have been designed for the 6SL7. For the SN7 a few resistors need to be changed.
 
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