6CA7 UL Push Pull Amp Design - diyAudio
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Old 18th March 2012, 03:30 AM   #1
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Default 6CA7 UL Push Pull Amp Design

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.
Attached Images
File Type: jpg 6CA7ppschematicR3.jpg (170.1 KB, 399 views)
File Type: jpg 100HznoFB.jpg (248.0 KB, 370 views)
File Type: jpg 100HzwithFB.jpg (260.6 KB, 337 views)
File Type: jpg 2kHz.jpg (261.1 KB, 315 views)
File Type: jpg genoutput.jpg (245.1 KB, 303 views)

Last edited by scott17; 18th March 2012 at 03:45 AM. Reason: corrected choke spec - drawing error
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Old 18th March 2012, 03:50 AM   #2
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here is the corrected drawing
Attached Images
File Type: jpg 6CA7ppschematicR3.jpg (206.3 KB, 214 views)
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Old 18th March 2012, 04:03 AM   #3
tim614 is offline tim614  United States
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hi,
i have an old baldwin 40 PTX, i dont know the secondary resistence of it, how can i mearesure it?

i will be following this thread for sure.
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Old 18th March 2012, 05:45 AM   #4
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Hey, what is story with cathode bypass cap of the mu follower?

Art
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Old 18th March 2012, 06:01 AM   #5
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Quote:
audiohead: "Hey, what is story with cathode bypass cap of the mu follower?"
I was thinking the same thing. The cap AC grounded the cathode. . . it would defeat the NFB . . .
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Old 18th March 2012, 06:02 AM   #6
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Quote:
Originally Posted by audiohead View Post
Hey, what is story with cathode bypass cap of the mu follower?
Looks like he forgot to draw one more resistor in series.
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Old 18th March 2012, 06:30 AM   #7
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Quote:
Originally Posted by tim614 View Post
hi,
i have an old baldwin 40 PTX, i dont know the secondary resistence of it, how can i mearesure it?

i will be following this thread for sure.
Just measure the resistance with an ohmmeter from the center tap to both sides of the secondary.
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Old 18th March 2012, 06:33 AM   #8
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Quote:
Originally Posted by Wavebourn View Post
Looks like he forgot to draw one more resistor in series.
Nothing forgotten. The very minimal feedback clearly restores the low frequency component. Nothing more needed or intended.
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Old 18th March 2012, 11:14 AM   #9
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The grid voltage of cathodyne ( 65 V ) is not optimum. Should be some 80...85 V when Ub = 300 V.

Your NGFB does not work. The 22 uF capacitor at the cathode of mu follower is the reason.
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Old 18th March 2012, 11:18 AM   #10
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What are the diodes in series with the tube rectifier for?
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