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KOTriode wrote:

With 6C33C output run at 40W plate dissipation, this amp run much hotter than an 845 amp with output tube running at 100W.  I measured the hottest point on the bulb and it is about 380F. I guess the due to short bottle , the heat spread on the chassis make it hotter than the tall 845 bottle.  I have to increase fan speed under each tube to cool it down, while on bench , I make some measurement on the amp. Frequency response at 1W: 10Hz @ -1.37db, 59.1Khz @ -3db. 20Hz @ -0.45db, 20Khz @ -0.3db                                10W: 20Hz @ -1.1db, 20Khz @ -0.3db. This is excellent by SE standard and without negative feedback, better than any tube amp I ever built. Picture below is square wave performance @ 1W, 100Hz, 1Khz, 10 Khz. The bottom trace is the amp input, the top trace is the amp output. The last picture , the bottom trace show the grid input of the 6C33C at clipping , the top trace is the output of the amp, the 6C33C can't go into A2 as Romy mentioned.

KOTriode, a few things. The 40W plate dissipation is bit too low for both plates. The well heated 6C33C (at least 30 min) you can would drive at 50-55W. If you do not need power and 40W on plate is enough for you then use one anode on the 6C33C, it would be better for thermal operation of the tube and will be positively reflected on sound.

About the measurements. I personally discard any talk about frequency response if the measurements were no done at fool power. You got 10W but I think that with 40W plate dissipation you shell get around 12W. I “feel” that you might have a bit “small” transformer for 6C33C as fractional power I would anticipate better numbers then 20Hz @ -1.1db, 20Khz @ -0.3db. So, in order to speak with the same language I will present my SET frequency response acquisition guide. I am sure it will be nothing new for you or to anybody but for some reason very few people follow this directive and without the very literal follow on this sequence I feel any conversations about frequency response are worthless.

1) Turn amp and heat it for at least 1Hr

2) Connect you loudspeaker, play music and find what tap or combination of sections on your output transformer produce the more proper balance of harmonics within you playback.

3) Recalculate the ratio of your output transformer and find what impedance the output stage sees.

4) Substitute the speaker with 15-20W resistor of the value that would assure output tube to see the same load after the transformer. Let premise it was 12R for instance.

5) Drive 1000Hz into 12R and increase the input voltage until you see very first clipping, from here ad extra 5-10% of input voltage.

6) Measure the response of the driver and assure that driver is not clipping.

7) Using voltage and current on the output tube try to make the clipping symmetrical from top and bottom of the sinusoid. The top way is voltage, the bottom wave is current (or vice versa, I do not remember already). As the symmetrical will be find try to play with operation point more in order to get lessen symmetrical clipping at higher power.

8) If change of voltage and current will get rid of clipping then add more input voltage to drive the amp into clipping again.

9) Let say that the final numbers with perfect symmetrical clipping against the GIVEN IMPEDANCE would be 190V and 280mA winch gives 53W on plate. Now drive more or less input voltage in order to maintain the best symmetrical clipping as amp distort more or less. The objective is that output tube shall starve from voltage and from current identically.

10) Mark the found generator’s output voltage as the reference voltage.

11) Now, as the proper operation point against given load is found drop the input voltage up to the point as the clipping just smoothed out. Switch timing on the scope to faster to see the precise moment the amp stop distort. You need to find a few last mV where amp clips – this will be the true max power of the amp.

12)  Without touching the output voltage of your generator increase the frequency of your generator unit you see the very first clip. This would require you to adjust the timing of your scope.

13) From the position of your very first clip drop the generator’s output voltage to 3dB and drive frequency of your generator until you see the very first clipping again. Read the frequency from your generator display. This is the true upper frequency response of your amp at full power

14) Return the generator’s output voltage to the reference voltage and drive the frequency all the way down to the first clipping. Repeat the same ceremony for LF and find he true lower frequency response for your amp against given load.

15) Disregard your “hurt” ego as your frequency response will be not as great as the Morons claims out there. No you will not have 10Hz to 120kHz as some fools claims as you did measure your amp properly and at full power.

Generally an average 6C33C output stage with a proper transformer shell have 20Hz-23Hz at bottom and around 19kH -20kH atop. If you have some VERY good output transformer then you might get in full range operation something like 15-17Hz at about and 25kHz -30kHz. The 17Hz-23kHz at the very fill power of around 16W-17W are VERY good numbers for a full range SET.

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