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 Post subject: TC15 Clean Output Power
PostPosted: Tue Nov 24, 2020 12:40 am 
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Has anyone measured the clean RMS output power of their TC15? I can only get about 8W RMS at clip point and am wondering if I have a problem. All voltages check out and all tubes are new. I'm using a 1KHz sine wave into an 8-ohm resistive load and measuring with a hand-held Oscilloscope. My other amps all test fine. Thanks.


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PostPosted: Tue May 11, 2021 9:49 pm 
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Bump. To Stephen or anyone in a position to test the output power of their TC15. Can you please test/check the RMS voltage across an 8-ohm resistive load with a 1KHz sine wave input. In my case, irrespective of the input channel I get about 8V rms clean & 9.6V rms at full clip. I need a benchmark to confirm if I have a problem or these voltages are normal (meaning the TC15 produces about 9W rms)! Thanks.


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PostPosted: Thu May 13, 2021 6:46 am 
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I'm a few weeks away from completing a TC15 but previous builds using Trinity iron I've got about 13 watts clean from an 18 watt build which is similar to another 5E3 build with 6V6. 1khz signal into 8 ohm resistor with a CRO across it. Have you changed anything with components in your build? Hope to give you some voltages when the build is finished.


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PostPosted: Fri May 14, 2021 5:23 pm 
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Nothing is changed from the supplied TC15 circuit and I've checked all voltages. I was using a small hand-held oscilloscope and I'm just waiting on an Tektronix Oscilloscope so I can have a proper look at the waveform and maybe hunt for a high-freq oscillation that's sucking power. The amp sounds pretty good/loud to my ear but that's not conclusive! Thanks for your reply.


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PostPosted: Tue May 18, 2021 4:40 am 
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Righto, have checked output with my Tektronix TDS200C and I get the same results. BUT.... I've discovered that I get more output when the speaker impedance switch is set to 4 ohms (which definitely connects the yellow OT wire which is supposed to be the 4 Ohm tap). Stephen, is it possible that the yellow OT wire is actually the 8-ohm tap?


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PostPosted: Tue May 18, 2021 6:26 pm 
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OK, with a constant 1 KHz sine wave into the amp's input and using my scope across an 8 ohm resistive load I get the highest output from the YELLOW OT wire. So I am assuming this is the 8 ohm tap (i.e. the closest Z match producing the most power in the load). The next highest is from the GREEN wire (I am assuming this is the 4 ohm tap) and the ORANGE wire gives the lowest output (so I am assuming this is the 16 ohm tap). So I've swapped the green and yellow OT wires on the impedance selector (4 & 8 selector positions). I've audibly tested the amp and all sounds good with the 8 ohm selector position now yielding the loudest audible volume into my 8 ohm speaker. So my conclusion is that the 4 ohm & 8 ohm wires on the OT have been reversed during manufacture (in terms of the color of the wires used). Oh, and when I drive the amp flat out into the 8-ohm resistive load I get just under 15W of power. And the clean (undistorted) power is about 9.5W.


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PostPosted: Thu Jul 22, 2021 3:10 pm 
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Hi,
I built my TC15 winter of 2019 (previously built the Tweed). I check back in here from time to time and saw this thread.
I can confirm the same behaviour; 8 ohm speaker louder when using the 4 ohm tap (yellow wire per schematic). 8 ohm tap next loudest, 16 ohm tap least loud.
Has there been any further discussion on this or feedback from Stephen?


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PostPosted: Sun Jul 25, 2021 6:36 pm 
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Maybe a silly question... Which primary setting did you test against, 5k or 8k? I would expect the best Z-matching for the 5K primary to occur with an 8ohm load on the 4ohm tap.

FWIW I've noticed similar behavior to what you described in my amp, built in March. I don't have a scope but maybe the 4 ohm tap sounds a little better than the 8ohm tap with an 8 ohm speaker, sometimes. The resistance, however, rises between the 4,8,and 16 ohm taps as expected and according to wire colors per the build guide.


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PostPosted: Mon Jul 26, 2021 8:33 pm 
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I am pretty sure I tested it on both with the same result.
I am away till next week but will try again when I get home.


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PostPosted: Wed Aug 04, 2021 1:51 pm 
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I double checked and the effect is the same in both the 5k and 8k modes. There is more output with 4 ohm selected with an 8 ohm speaker.
Maybe Stephen can chime in on this?


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PostPosted: Thu Sep 02, 2021 5:25 am 
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Just to add on my testing with a 1khz signal and an 8 ohm resistive load I got clean signal 8 or 9 watts on the 8K Munch channel and only 2.5 watts clean on the 5k crunch channel. Max volume it was about 14 watts and 5 watts respectively. I'm guessing if I plugged the 8 ohm load into the 4 ohm tap when using the 5K primary it would be better output. Have to scope it again and listen to the amp as well. When playing guitar through it sounded noticeably quieter on the 5K primary. Really like the amp either way as I was playing it today and the 8k primary is plenty loud for a home noodler. I'm lucky to have about 6 different styles of electric guitars and it really shows what they're like. Don't laugh but it's transparent to my ears. FWIW I'm using new JJ tubes through out and an old Mullard EF86 mesh was a teensy bit lower volume than the JJ EF806 but sounded very similar.


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PostPosted: Thu Sep 09, 2021 3:12 am 
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I've just finished building another TC15 for a friend. It has the same output "issues" as my own build did. That is, irrespective of the OT Primary tap I get the most power output into my 8-ohm RESISTIVE load using the 4-ohm OT secondary tap. Stephen, please chime in as I can't explain what's going! :hmmm:


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PostPosted: Thu Sep 09, 2021 3:17 am 
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zircontweezer wrote:
I'm a few weeks away from completing a TC15 but previous builds using Trinity iron I've got about 13 watts clean from an 18 watt build which is similar to another 5E3 build with 6V6. 1khz signal into 8 ohm resistor with a CRO across it. Have you changed anything with components in your build? Hope to give you some voltages when the build is finished.

No component changes.


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PostPosted: Fri Sep 17, 2021 10:45 am 
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It would be fairly easy to test for the output transformer turns ratio and then know for sure which tap is which.


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PostPosted: Sat Jan 22, 2022 4:10 am 
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I finally put my TC15 on the scope...

My kit is from March 2021, so it is possible something has changed since then. My amp checks out as I would expect. The highest output occurs with an 8 ohm load, on the 8 ohm secondary tap, with the 8K primary selected. My amp produces about 9 clean watts before onset of any pre-amp distortion. I measured, inaccurately, 15 watts output before onset of power tube distortion and 24 watts with everything cranked.

When using the 8K primary with an 8 ohm load:
Voltage on the 4 ohm secondary appears higher compared to the 8 ohm secondary when switching between the two. However, this higher voltage comes with more distortion. When the volume is adjusted to eliminate the distortion, the highest output occurs with the 8 ohm tap.

When using the 5K primary with an 8 ohm load:
Voltage is highest when the 4 ohm tap is selected, and is similar to the maximum output I measured on the 8K primary. When the 8 ohm secondary was used with the 5K primary, there was more distortion overall and lower output. This performed as expected given the load mismatch.

Power Output
The phase inverter on this amp overdrives way before the power tubes overdrive. It makes testing and interpreting power output difficult. Using a 1K sine wave and just looking at the output signal, I also got around 9 watts (8.76 watts) at the onset of any distortion. Obviously, there is a difference between measuring pre-amp distortion (which this amp is designed to generate at low volume as part of its tone) and power amp distortion (which tells us something about the limitations of the amp). To truly test the capabilities of the amp, you would need to feed each power tube a correctly phased sine wave with a strong enough signal to push the power tubes into overdrive. Then you could accurately measure the wattage the amp produces before the power tubes distort. I fudged things using FFT to look for changes in the pattern of harmonic distortion as I turned up the master volume. I measured just over 15 watts at the onset of power tube distortion on top of the already present pre-amp distortion. I measured about 24 watts maximum output with everything turned up. These measurements aren't accurate, as I measured peak-peak of a distorted waveform and extrapolated from there. The point is that the amp is capable of way more wattage than can be measured simply with a sine wave on the input and a scope across a dummy load. Subjectively, my TC15 is louder than my two other "15 Watt" 6V6 amps, both of which I've measured accurately at 15 watts clean power output.

Mike


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PostPosted: Mon Jul 04, 2022 3:53 am 
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I've verified the output with my Tektronix TDS200C and I receive the same results, so there. However, I've found that using a 4 ohm speaker impedance setting gives me a higher output (which definitely connects the yellow OT wire which is supposed to be the 4 Ohm tap). Is it feasible, Stephen, that the 8-ohm tap is actually the yellow OT wire?octordle


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