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6CY7 Stereo Amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5222
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Author:  Suncalc [ 24 May 2015, 22:12 ]
Post subject:  6CY7 Stereo Amp

So lately I have been working on two triode amps based on old television vertical deflection tubes. The first was my 6EM7 Vertical Amp discussed on this forum. Now I have finished the second one. It's a nice little low power stereo amp (1.3W/channel) based on the 6CY7. Here it is:
Attachment:
6CY7 Amp.jpg

Here is it undergoing testing shortly after it was completed.
Attachment:
6CY7 Amp Testing.jpg

And here is a shot of the insides.
Attachment:
6CY7 Amp Inside.jpg

The amp uses a Hammond power transformer and chokes and a pair of Edcor XSE10-8-5K output transformers. I have to say that it sounds really nice. There is just something about triode amps that nothing can beat. The circuit was first prototyped here.

As soon as I get some free time I'll type up project pages for both amps.

So what do people think?

UPDATE (31 Jul 2016) - Added project page: http://diyaudioprojects.com/Tubes/6CY7-SET-Amplifier/

Author:  Geek [ 25 May 2015, 00:02 ]
Post subject:  Re: 6CY7 Stereo Amp

Well done! :D

I love "useless TV tubes"; vertical outputs are fantastically linear. Looking great as well.

Author:  Suncalc [ 25 May 2015, 10:57 ]
Post subject:  Re: 6CY7 Stereo Amp

Just a little more data on the amp. The peak clean power output is 1.3 watts RMS per channel (as determined from watching an oscilloscope). The 3dB frequency response is ≈48 Hz to 30 KHz. A plot of the end to end frequency response is included below with the lines indication the stated bandwidth of just the Edcor output transformers.
Attachment:
Frequency Response.png
This plot is from measured data taken with a signal generator (Zout=600Ω) and a fixed resistive load of 8Ω. The test setup is pictured in the first post. Below is the as built schematic (nearly).
Attachment:
Schematic - as built (small).jpg
I say nearly because after finishing the amp and checking the plate dissipations, I found that the B+ voltages were running a little high. As such I added an appropriate dropping resistor in each B+ line to get the plate dissipations back under the maximum of 5.5 watts each. You can see the modification in this picture.
Attachment:
6CY7 Amp Resistors.jpg
They may not be in the most optimal position in the power supply, but with a nice constant current SET stage it works out just fine.

I still can't get over how good and how loud this amp sounds. It handles everything from classical, to blues, to choral music equally well. And the sound stage is awesome. I will never go back to building common power supply filters for both channels ever again.

Author:  Soundbrigade [ 25 May 2015, 12:21 ]
Post subject:  Re: 6CY7 Stereo Amp

There are no "useless" tubes (well, some maybe like diodes and some other). When all "normal" tubes have been tested, some experimenting with "odd" tubes can be rewarding.

Author:  Geek [ 25 May 2015, 14:56 ]
Post subject:  Re: 6CY7 Stereo Amp

A little NFB will flatten that right out, if you wish to do so.

Or even local anode FB and draw the cathode of the output stage through the secondary, de-saturating it a tad will drop that low end to close to 20Hz :D

Author:  Suncalc [ 25 May 2015, 16:28 ]
Post subject:  Re: 6CY7 Stereo Amp

Geek wrote:
A little NFB will flatten that right out, if you wish to do so.

Or even local anode FB and draw the cathode of the output stage through the secondary, de-saturating it a tad will drop that low end to close to 20Hz :D
Yes, but it will also greatly increase 3rd harmonic distortion which I cannot abide. Besides, it's only 4dB down at 40Hz and there is virtually nothing in music down that low. And even at 20Hz it's only ≈9dB down which is really not very much considering the variability in human hearing. I almost didn't include the frequency response characteristic because I knew that it would draw out the NFB comments.

I virtually never use NFB in my designs (with the exception of cathode degeneration) because I simply don't like the effects on the harmonic structure. I guess that's why I'm a SET (SE-UL) kind of guy. But to each, his own. :beerchug:

Author:  Suncalc [ 25 May 2015, 17:04 ]
Post subject:  Re: 6CY7 Stereo Amp

Ok, there is one more thing I really like about this amp. :blush: It's one of those "tube things" that either you care about or you don't. These are all small base 9 pin miniature tubes. The heaters for these tubes are 6.3W for the 6CA4 and 4.7W each for the 6CY7s, for a total of 15.7W. This is compared to 1.9W for a typical 12A*7. What this means is that this amp REALLY GLOWS! In a dim room the glow from the amp really stands out.

You can call it the "Tube Geek" factor if you want, but I really like it.
Attachment:
Tube glow.jpg

Author:  Les [ 25 May 2015, 20:09 ]
Post subject:  Re: 6CY7 Stereo Amp

Nice set (pun intended), Matt! :)
I can't say that I'm surprised about the performance of the 6CY7. Many good quality amps have been made using various H-drive VTs, so why not a V-drive VT?

:thumbsup:

I do really like the layout, the cabinet AND the nice warm glow!


:beerchug:

Author:  Geek [ 25 May 2015, 21:06 ]
Post subject:  Re: 6CY7 Stereo Amp

Suncalc wrote:
I virtually never use NFB in my designs (with the exception of cathode degeneration) because I simply don't like the effects on the harmonic structure. I guess that's why I'm a SET (SE-UL) kind of guy. But to each, his own. :beerchug:


One of the best reasons I've ever heard for zero NFB :beerchug:

That gorgeous pic.. into the wallpaper folder it goes :smoking:

Author:  Suncalc [ 25 May 2015, 22:05 ]
Post subject:  Re: 6CY7 Stereo Amp

Geek wrote:
That gorgeous pic.. into the wallpaper folder it goes :smoking:
Yeah... the 6V6 Lacewood has a nice glow as well...
Attachment:
lacewood_glow.jpg

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