Author Topic: single-ended cathode follower output stage  (Read 3409 times)

Offline Mars

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single-ended cathode follower output stage
« on: May 27, 2015, 07:52:51 PM »
Hi all tube amplifier aficionados :mates:

I have always wondered what this amplifier would sound like:

http://diyaudioprojects.com/Technical/Papers/Direct-Coupled-6V6-Cathode-Follower-Amplifier/

It uses this circuit:


I decided to start with this one instead:


Has anyone built this circuit?
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Offline Mars

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Re: single-ended cathode follower output stage
« Reply #1 on: May 28, 2015, 09:34:40 AM »
The critical element to this amplifier is the 250 ohm DCR of the primary in the output transformer. I recently stripped a Sansui SM12M amplifier, and when I measured the DCR of the primary, it was 250 ohms! Immediately I knew that these OPT's might work well in the Bates circuit.



The Sansui uses EL84's in single ended mode in the output stage. I decided to build the above amplifier with EL84's and a readily available pentode like the 6AG5. (I have quite a few JAN 6AG5's in my tube stash waiting to sing)
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Offline Mars

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Re: single-ended cathode follower output stage
« Reply #2 on: June 05, 2015, 11:54:36 AM »
I assembled the circuit. Here's the first incarnation:



Pre-amp 6AG5 wiring: I considered different anode/screen grid values, and settled on the the values for which I had resistors in the box :BWAHAHAH:
The resistance coupled chart in my RCA handbook suggests values and tabulates the gain for the combination. I first went for max gain, but that was a bit too much. The 220K/680K combination works well with my sound card and pre-amp. I might up the grid leak resistor for the EL84 to 1 Mega-ohm for slightly more gain.

I love the fact that there is no cathode resistor/bypass capacitor in this amplifier and that the power supply is quite simple.


Now a question. Is this a pentode or a triode output stage?
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Offline Eejit

Re: single-ended cathode follower output stage
« Reply #3 on: June 05, 2015, 12:08:28 PM »
.........a Single Ended Pentode (SEP) maybe? I can see what filled the holes in my chassis.  Those transformers sound wonderful in the Sansui, does your amp deliver what is expected?

Offline valvesound

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Re: single-ended cathode follower output stage
« Reply #4 on: June 05, 2015, 01:02:36 PM »
looking forward to read what your thoughts are on the result (sound)
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Offline Mars

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Re: single-ended cathode follower output stage
« Reply #5 on: June 05, 2015, 07:28:34 PM »
The sound is fantastic :notworthy: It is way up there :groovy: Drives my Mission bookshelves and three way vintage speakers with ease. I am going to explore this topology a bit deeper. I think, without having an amp to compare, that it sounds better than anode loaded SE. I have to admit that the 6AG5 surprised me too. Lying in the box, they did not look like the formidable tubes they are!
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Offline handsome

Re: single-ended cathode follower output stage
« Reply #6 on: June 05, 2015, 08:06:29 PM »
EL84 screen and anode are connected ergo it is in triode, 6AG5 has a separate screen supply thus working as a pentode

Offline noob

Re: single-ended cathode follower output stage
« Reply #7 on: March 02, 2019, 10:04:42 AM »
The sound is fantastic :notworthy: It is way up there :groovy: Drives my Mission bookshelves and three way vintage speakers with ease. I am going to explore this topology a bit deeper. I think, without having an amp to compare, that it sounds better than anode loaded SE. I have to admit that the 6AG5 surprised me too. Lying in the box, they did not look like the formidable tubes they are!
Hallo Mars, Hallo Everybody!  :)

I am very much interested in this CF amp and I am thinking of making one.
My experience with cathode followers is limited to a preamp output stage.
I mean: I really have no idea how a CF power amp could work    :-[  :roll:
Any kind of help or suggestion will be highly appreciated   :D

My main concern is power output, maybe it is really too low,
have you ever measured it?

In the RC coupled version this amp should swing very few Volts on the output tube cathode,
the schematic does not tells how many, but my guess is somewhere between 20 and 30 Volts.
so my guess is that power should be well below 1/2 Watt  :thinking:

Maybe the DC coupled version has some more power?
Cathode is +85V from ground, does this mean that - in teory - it can swing 85V peak to peak?


Offline Mars

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Re: single-ended cathode follower output stage
« Reply #8 on: March 06, 2019, 11:00:17 AM »
Hi Noob

I would recommend you give this design a try. The low output impedance is appealing to the ear. That makes up for the low power. Do you have a SE OPT with 250 ohms primary resistance?
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