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horias2000

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I managed to perform a quick measurement with the feedback closed. Below you can see the frequency response (log frequency scale).

1647861714695.png


I'm also inserting the openloop response with log frequency scale, just for reference.

1647861762039.png



And I'm attaching the square wave response for 1kHz, 500Hz and 100Hz. The frequency response does look better than open loop but I still get a HF rolloff way too early. I'll look into that when I have more time.
 

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horias2000

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I removed C7 and the frequency response got a lot better (as expected).

1647875723503.png



So I now have -0.7dB at 20kHz and -3dB above 40kHz. I will add a smaller capacitor so that I will get -3dB at around 20kHz. I'm not sure what is the best practice here. Is it ok to have -3dB at 20kHz or I should go a bit higher than 20kHz?
As expected, the square wave response improved significantly (as seen below).

1kHz

rc_zobel_150nF_13R_1nF+1k_v1_plates_1kHz_square_closedloop_removed_22nF_detail.png



and 10kHz

rc_zobel_150nF_13R_1nF+1k_v1_plates_10kHz_square_closedloop_removed_22nF.png
 
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horias2000

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I did a quick performance evaluation of the amplifier and the results are shown below. I'm happy with this amount of distortion (0.035%) but the 50Hz noise (or oscillation) needs to be dealt with. So I will focus on that in the coming days/weeks. If I can manage to get rid of that (or at least reduce it significantly) I will be very happy with the performance. The measurement was done with 60mA quiescent current through the KT88 output tubes. This can be increased a bit (65 - 70 mA). So the THD will most probably get better. The measured frequency response looks really good as well (apart from the 50Hz issues). I plan to run the other module in ultra linear and compare the performance between the two modes (pentode vs ultra linear).

Tube_1kHz_THD_high_bias_200mV.png


tube_FR.png
 
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horias2000

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Just a quick update. I thought that the 50Hz could be coming from the ac filament supply/wires. I tried routing the wires as far away as possible from any sensitive parts and I also twisted the two wires together. Nothing, the 50Hz was still there. I tried using another 6.3V winding but with no improvement. I know that sometimes the 50Hz filament ac voltage can leak inside the small signal tubes. Therefore I used a regulated lab power supply for the two input tubes and BINGO! The 50Hz noise went down almost completely. I then looked at the spectrum and the frequency response again and this time it looks much better. I will most probably use DC for the low signal filaments or I will look online for tubes that do not exhibit this issue so much.

I must say that the performance of the amplifier looks to be very good in my opinion. Frequency response is really flat in the audio band and THD and THD+N look respectable in my opinion. With the 360Vdc supply I can get around 20W before clipping. I know this is not a lot but for my needs it's more than enough. More power can be achieved with a higher supply voltage.

This is for 1W output on 8OHM load

Tube_1kHz_THD_DC_filament_input_driver_optimal_bias_180mV_1W.png


This is for 5W output
Tube_1kHz_THD_DC_filament_input_driver_optimal_bias_180mV.png



tube_FR_DC_fialment.png
 

Remk

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That looks good. On a pcb you almost have the need to feed the heaters with DC.
Make shure they are not flooting; use 2 resistors or a pot betweed heaters and ground if not already.
 
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horias2000

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As I said earlier, the second power amplifier PCB will be setup to work in UL mode so that I can assess witch mode has better performance (UL or pentode). I managed to do this today and in order for it to work in UL mode I had to perform a few modifications.
  1. the grid voltage of the KT88 valves had to be lowered from around -18Vdc to around -40Vdc.
  2. the R-C compensation network that I placed on the anodes of V1 was modified to 1k + 330pF. This gave a -3dB frequency of around 40kHz
  3. the 100nF capacitor placed on the feedback input of V1 was increased to 200nF. This improved the LF response.

The closed loop frequency response is shown below. It looks good but in pentode mode it was a bit better. Not a big difference though.

1648567033674.png


The THD was the big surprise. It's clear that the 2nd harmonic is a lot bigger than it was in pentode mode. In pentode mode the THD was 0.017% (1W) and in ultra linear it is 0.096% at 1W. This is a big difference in my opinion. It gets even bigger at 5W, 0.038% vs 0.21%. Output power was about the same, 20W in UL mode and around 18W in pentode mode. Is this THD difference something normal? I was really expecting the THD in UL to be very similar to the THD in pentode mode but this is clearly not the case.

The conclusion is that I will modify the second module to work in pentode mode and I will use pentode mode for the final build. I will keep this thread updated with construction photos once I start.

1W output power
Tube_1kHz_THD__UL_1W.png


5W output power
Tube_1kHz_THD_UL.png
 
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horias2000

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I know it is a long time since I updated this thread but this doesn't mean that I wasn't working on the project. After all the measurements done on the boards, the next step was to order an enclosure and to start fitting everything inside it. I chose an enclosure from ModuShop, the Galaxy. I tried to keep the costs as low as possible while still looking good. I already spent a big budget on the project so far so I didn't want to go overboard with the enclosure. I had some minimal customizations done on the front and back panel. I also wanted to have the top cover customized but this can only be done on aluminium covers and the cost was to high. I chose a top cover from steel and I drilled it myself. I will try to give it a paint cover before installing it.

As I said in previous posts, I had to use DC for the filaments of the input and driver tubes as the 50Hz component was way to high. For this I will use a separate toroidal transformer and two separate LM317 regulators (one for each channel). Another important thing is to not leave the 6.3Vdc for the input/driver tubes floating. So the ground of the 6.3Vdc connects to the main ground of the amplifier.

I managed to install and wire the left channel and I also performed some quick measurements to make sure everything works just fine. I am again baffled by these measurements and how good these are considering that I use tubes :). Of course you get much better results from solid state amps but I have to say that these are not bad results in my opinion (0.02% THD at 1W).

I also chose to include the PHNO pre-amp inside the amp. That is the PCB in the upper right corner. Input switching will be done by relays and a rotary switch.

As per the advice I received, I also included a fan that is powered from the 6.3Vdc of the right channel. I'm not sure if this will suffice or not but I can always use a higher voltage to power it if this is needed. I really do not want this fan to be audible.

I hope to have the time to finalize all the wiring and actually listen to this things as I didn't have the chance to hear it yet :)
 

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SIY

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You're making me itch to get my next tube amp built.

If it's not too late... you might want to look at powering the heaters in a way to minimize common mode noise. There's a nice discussion of this in Morgan Jones's book. Here's how I did it in the raw supply of my little EL84 amp.
red_light_district_raw_supply.gif


Note that no DC heaters are used, but the amp is quite hum-free.
 
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horias2000

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I know what you mean :). I really want to finalize this build and listen to the amp. But when I will listen to it I'll be think of the next project :D. It's a continuous itch.

Interesting schematic. By the looks of it they use 12.6Vac for the filaments and the center tap of the transformer goes to the center tap of the EL84 filaments. The interesting part is the connection between the center taps to the resistive divider of B+ 2 and the 10uF capacitor.

As for my design, as far as I can see, the JJ KT-88 are not that problematic when it comes to hum. Having the 50Hz component at -100dB does not constitute a problem in my view :). But the ECC82 and ECC83 are definitely more problematic. I'm not sure which one is the the one causing the issue or if both are affected in the same way. Due to the construction of the case I chose (aluminium sides) I can easily cool down the two LM318s. I still have to perform a longer term temperature measurements but I do not think there will be any major issues.
 

SIY

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I know what you mean :). I really want to finalize this build and listen to the amp. But when I will listen to it I'll be think of the next project :D. It's a continuous itch.

Interesting schematic. By the looks of it they use 12.6Vac for the filaments and the center tap of the transformer goes to the center tap of the EL84 filaments. The interesting part is the connection between the center taps to the resistive divider of B+ 2 and the 10uF capacitor.

As for my design, as far as I can see, the JJ KT-88 are not that problematic when it comes to hum. Having the 50Hz component at -100dB does not constitute a problem in my view :). But the ECC82 and ECC83 are definitely more problematic. I'm not sure which one is the the one causing the issue or if both are affected in the same way. Due to the construction of the case I chose (aluminium sides) I can easily cool down the two LM318s. I still have to perform a longer term temperature measurements but I do not think there will be any major issues.
There's a few things going on there. If you want to remove both differential mode and common mode noise, both sides of the heater need to be driven and bypassed balanced. I didn't have a center tap on the 12V heater winding, so I artificially created one by series connecting the output tube 6V heaters to make a 12V heater. This allowed the 12V input stage heaters to be driven and bypassed balanced- generally, the hum from output tube heaters is pretty negligible, it's the input stage you need to worry about.

Second, the heater-to-cathode leakage can be greatly reduced by elevating the heater potential to be positive with respect to the cathode. In this circuit, the "virtual" CT is at AC ground (the 10u/100V cap ensures it) but DC is elevated by about 20% of the B+ voltage (the voltage divider formed by the 51k and 200k resistors).

If you want to be ultra fancy and use DC on the heaters, it will take two regulators, one for each side. Here's what I did for my MC phono preamp:

fig7-large.png

"H common" goes to the same kind of voltage divider that the CT goes to in the power amp schematic. The values I show here (100k and 33k) for the voltage divider reflect the difference in the B+ and won't be optimum for a power amp- you can just use the same 200k/51k as in the previous schematic.
 
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