• 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.

6CA7 UL Push Pull Amp Design

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Just what we need, another 6CA7/EL34 push pull amp. I apologize in advance.

Here is my version of an ultra-linear 6CA7 push-pull amp. The input stage is a classic mu follower with the output taken from the lower triode to allow for quasi direct coupling to the phase inverter. Taking the output from the lower triode may make this more recognized as a common-cathode stage with a CCS load.

I had posted this unfinished design in another thread with questions and the responses were helpful. I originally built this with an under-sized PT so I could not bias the 6CA7s properly. This was built with the proper size power xfmr.

The first stage is direct coupled to the cathodyne phase inverter via a step network/voltage divider using a 220nF cap across the top resistor, for a -3dB point of 2.2Hz.

The RC compensation network prior to the voltage divider controls ringing at the output. Feedback to the input stage restores symmetry to a 100Hz test square wave.

The cathodyne phase inverter directly drives the 6CA7 cathode-biased, class A push-pull outout stage. The output of the cathodyne is 120V P-P/42V RMS right before clipping. Plenty to drive the 6CA7s. The 6CA7s are biased at 56mA, about 21 watts dissipation.

The 6CA7 output stage clips well before the driver or the phase inverter. I measured 22.5W RMS across an 8 ohm dummy load at the onset of clipping with a 1kHz sine wave. Input sensitivity is 1V RMS. Hum and noise at the output is about 1mV. Dead quiet.

The amp sounds very good. Excellent bass response. Very well-balanced mids and highs. Crystal clear sounding to me. I have no way to perform harmonic distortion measurements at this time. I have read that the Right Mark Audio Analyzer will do those measurements, so as soon as I learn how to use it I will post the results.

I have attached a few square wave photos. These are taken at 2.83V across an 8 ohm dummy load. The first is 100Hz without feedback. The second is 100Hz with feedback. The feedback clearly squares up the wave. The third is 2kHz. The fourth shows the actual square wave output of my function generator at 2kHz. Pretty bad, I know. I either need to repair or replace that unit.

edit : The specs for the power supply choke shown on the schematic are wrong. The choke is rated for 250mA.

edit: I just noticed that the diodes are shown backwards. They should be connected cathode to the 5AR4 plate. They are correct in my build. My drawing program has been doing some funky things lately, like when I modify a document created on my Linux machine, and then work on it on my Windows 7 machine. Things seem to get changed or flipped like in this case.
 

Attachments

  • 6CA7ppschematicR3.jpg
    6CA7ppschematicR3.jpg
    170.1 KB · Views: 572
  • 100HznoFB.jpg
    100HznoFB.jpg
    248 KB · Views: 510
  • 100HzwithFB.jpg
    100HzwithFB.jpg
    260.6 KB · Views: 436
  • 2kHz.jpg
    2kHz.jpg
    261.1 KB · Views: 392
  • genoutput.jpg
    genoutput.jpg
    245.1 KB · Views: 419
Last edited:
Nothing forgotten. The very minimal feedback clearly restores the low frequency component. Nothing more needed or intended.

Then you don't need the compensation network or the cap across the feedback resistor. All of those components are there in a normal design to ensure closed-loop HF stability.

You might add some reactance to your dummy load (or use a speaker) to see what happens to those square waves.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.