• 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.

6S41S Single-Ended Cathode Follower Amplifier

I thought that I should share my recent amplifier build:

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Single-ended 6S41S cathode follower amplifiers that deliver 7 W each. Here is the schematic:

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Running the output tube as cathode follower gives a very low output impedance for a single-ended amplifier. These amplifiers can drive difficult loads, but the concept has its cons. The output tubes need over 500 V peak to peak to deliver full output. The driver tube 6E5P-I is a small power tube with high amplification when connected in triode mode. It has a gain around 50 and can deliver quite a bit of current. By using an interstage transformer, I can get the required voltage swing without a very high supply voltage. Also, I step up the signal 1:2 with the interstage transformer and I also step up the signal with 1:2 with the input transformer.

The sound quality is great. It is most certainly the best-sounding amplifier that I have tried with my current speakers.
 
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The driver tube 6E5P-I is a small power tube with high amplification when connected in triode mode. It has a gain around 50 and can deliver quite a bit of current. By using an interstage transformer, I can get the required voltage swing without a very high supply voltage. Also, I step up the signal 1:2 with the interstage transformer and I also step up the signal with 1:2 with the input transformer.

The sound quality is great.

Have you measured the frequency response ?

You have used 6E5P as a triode, but with un-bypassed cathode resistor. This causes the output impedance to raise significantly.
According to Lundahl LL1671 specification (SE to SE Interst. 1:2) the primary inductance is 10 H (only).
http://www.lundahl.se/wp-content/uploads/datasheets/1671.pdf
This combined with your 6E5P stage makes poor bass end frequency response. At 20 Hz the response is -12 dB compared to 1 kHz.

However, if 120 ohms cathode resistor is bypassed with 470 µF capacitor, the response at 20 Hz is only -2 dB.

These results are got with LT Spice simulations.
 
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Arto, I did measure the frequency response. I don't remember the exact figures, but it was flat down to 40 Hz and had a reasonably small drop down to 30 Hz. The cathode resistor is actually bypassed. I am using a method described in Radiotron Designer's Handbook 4th Edition on page 538.

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Yes, Lundahl's datasheet mentions that the frequency response is +/- 1 dB between 40 Hz and 25 kHz. That also is in line with my measured frequency response. This is a penalty to pay for using the interstage transformer connected this way. But, it does allow me to get away with only two stages and a power supply with fairly moderate tension. My speakers fall off quite rapidly below 40 Hz and I don't perceive the bass response as any poorer than with other amplifiers that I have tried. On the contrary, the bass reproduction is really good - deep and tight.

Arto, leaving the effect of the inductor aside, does the two methods of decoupling the cathode resistor have any impact on the frequency response?
 
Thanks, project posts are my favorite! Lots of ideas come to be remembered with this project... I like the use of aluminum channel for the sides, if one likes the look of wooden sides it would be possible to rip some wood to fit inside the channel and now have the benefit of a totally shielded chassis and a woody, nice! Also I like repurposing TV Compactron damper tubes for tube rectifier, nice slow start and cheap. Also the topology is one not seen that often, hope it sounds good. Also the utter simplicity is great. Nice job dude!