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-   -   6CA7 UL Push Pull Amp Design (http://www.diyaudio.com/forums/tubes-valves/209065-6ca7-ul-push-pull-amp-design.html)

scott17 18th March 2012 03:30 AM

6CA7 UL Push Pull Amp Design
 
5 Attachment(s)
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.

scott17 18th March 2012 03:50 AM

1 Attachment(s)
here is the corrected drawing

tim614 18th March 2012 04:03 AM

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.

audiohead 18th March 2012 05:45 AM

Hey, what is story with cathode bypass cap of the mu follower?

Art

directdriver 18th March 2012 06:01 AM

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 . . .

Wavebourn 18th March 2012 06:02 AM

Quote:

Originally Posted by audiohead (Post 2950542)
Hey, what is story with cathode bypass cap of the mu follower?

Looks like he forgot to draw one more resistor in series.

scott17 18th March 2012 06:30 AM

Quote:

Originally Posted by tim614 (Post 2950497)
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.

scott17 18th March 2012 06:33 AM

Quote:

Originally Posted by Wavebourn (Post 2950549)
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.

artosalo 18th March 2012 11:14 AM

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.

Globulator 18th March 2012 11:18 AM

What are the diodes in series with the tube rectifier for?


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