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6N2P/6CW5 design with toroidal outputs
6N2P/6CW5 design with toroidal outputs
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Old 17th September 2018, 07:27 PM   #1
Lingwendil is offline Lingwendil  United States
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6N2P/6CW5 design with toroidal outputs
Default 6N2P/6CW5 design with toroidal outputs

Well, since this has been in daily use for the last couple months I figured it was time to declare it a success, and post it up to show off

Its a variation of my favorite topology, the unbypassed cathode voltage amp, step-network biased concertina, and garter bias. Very similar to my flea amplifier, with a few tweaks. I wanted to go for an all-noval design this time, so went with the wonderful and affordable 6N2P for the front end, and went with rather boring but effective values to set it up. Unbypassed cathode lowers gain and distortion, and the distortion there is will end up primarily 2H from past experimentation with this set of operating points. A little feedback to the cathode is a nice way to tighten everything up

The output stage is where things get a little unusual. For the first time in a few years, I'm actually going with a pentode output stage. Since garter bias works so well I decided to try it out on here, and it works pretty damn well, even on pentodes that are reasonably matched. I went with the 6CW5/EL86 for this use, with a nod to trying out the 6P43P-E later on (I have 21 of them, may as well )

I went with the Antek AS-0505 power toroid for my output transformers, which works out to 4232 ohms, with both primaries and secondaries respectively in series (for the relevant maths, 115+115=230, 230/10=23, 23x23=529, 529x8 ohms=4232 ohms) which should give pretty good linearity for these affordable and lovely, overlooked tubes. I'm running them a little conservatively at 32mA per cathode, for about 9.2 watts of combined plate/screen dissipation, but will likely increase this soon now that I know this setup works well. Of the tubes on hand, I find the Polam EL86 to be particularly nice.

Interesting to note, that if you connected the secondaries in parallel, and used 4 ohm speakers, these transformers would work well for EL84 or 6V6/6P1P as well, showing a primary impedance of 8464 ohms. Some minor changes to the screen supply would be needed to give a higher range of voltage. Easy, add another zener in the string

For power, an Antek AS-1T230 - 100VA toroid gets rectified through a 1N4007 bridge, fed to a pair of IRF820 mosfet filters, one of which uses a string of five 1N5262 50 volt zeners, with a pot/resistor in series feedig the mosfet gate for adjustment range. This gives an effective way of setting the screen voltage. The output tubes get their plate supply straight from the first reservoir cap, and the additional unregulated but smoothed mosfet filter feeds the preamp stage.

All resistors are basic type stuff, concertina resistors matched, and the garter resistors are 10W non-inductive bulk lot wirewounds, selected to within a couple ohms of eachother. All coupling caps are siemens poly films. Most everything I had onhand other than a few power caps, but it's all basic grade stuff for the most part. Used some leftover hammered texture paint for the affordable 12x8x2.5" BUD industries aluminum chassis. I went ahead and sprung for a fancy swooky purple power button LED this time, though


How does it sound? Well, very, very nice, if I do say

I'm not one for audiphool jargon, but compared to my flea amplifier it is noticeably different, owing to that pentode sound. Overall it is much more powerful, of course, but it's super clean and accurate, not too clinical or sterile. Sounds like a nice EL84 pentode amp, but with a little more mojo that one seems to find with low impedance tubes. This thing makes thunderous bass, and has a wonderful midrange/high end. Nice general-use amplifier.

Total iron cost, $34.50 power, $17.50x2 for output, plus $13.50 shipping, makes for $83 total. Not bad


Pardon the terrible picture and sloppy schematics!
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File Type: jpg IMG_20180917_103430.jpg (572.8 KB, 704 views)
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Last edited by Lingwendil; 17th September 2018 at 08:20 PM.
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Old 18th September 2018, 03:35 AM   #2
kodabmx is offline kodabmx  Canada
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Nice one, buddy!

Quite similar to an amp I built using 6F12P and 6P15P
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Old 18th September 2018, 01:23 PM   #3
mondogenerator is online now mondogenerator  England
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Hi,

Novice question about your schematic:

What is the purpose of the paralleled RC network on the grid input to the 2nd triode stage?

Is it merely to form a potential divider with the grid leak resistor, I.e. to lose some voltage from the first stage?
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Old 18th September 2018, 04:14 PM   #4
Fenris is offline Fenris  United States
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mondogerator,

The RC network is a voltage divider that drops the DC voltage by half to get the second tridode where it needs to be DC wise to act as a concertina. The capacitor shunts the AC and allows it to bypass the upper resistor on the voltage divider to restore all of the AC gain of the first stage.
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Old 23rd September 2018, 09:43 PM   #5
Lingwendil is offline Lingwendil  United States
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6N2P/6CW5 design with toroidal outputs
Sorry, yes. It is to reduce the DC voltage at the second triode grid, while preserving the AC signal level.

In this particular case, the first triode is set up to have ~60-65% of the supply voltage at its' plate, and the divider brings this down to approximately 25% of the supply voltage in order to bias the cathodyne at a good operating point.

Also, a little error, the 1M and 1M5 resistors should be switched on the schematic.
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Old 24th September 2018, 02:50 AM   #6
Tubelab_com is offline Tubelab_com  United States
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I also made a similar amp. It was a guitar amp, but the output stage was tested for power and frequency response before the preamp stage was built. I torture my HiFi amps by playing guitar through them, and likewise I test the amp stage of my guitar amps by playing music through them. This one sounded quite nice in glorious mono and the bass was surprisingly deep and solid considering the use of a similar Antek toroid for the OPT.

The amp ran 45B5 / UL84 output tubes. These are 6CW5's with a 45 volt 100 mA heater. The power supply used a Triad 100 VA isolation transformer. B+ was derived from a voltage doubler and ran about 340 volts. The screen supply was derived from a FW bridge on the same secondary for about 170 volts. The OPT was an AN-509 with both 9 volt secondaries wired in parallel providing a 5200 ohm load. Power at clipping was 22 watts, and the THD was around 3% at 20 watts. A bit more power could be had with a lower load impedance, but I have the Antek already so I tried it. I also tested the amp with a real OPT rated at 3300 ohms. It made 26 watts at clip and under 2% THD at 20 watts......but broke the $100 budget for the amp.

The 6CW5 is pretty good little tube and I have squeezed 30 watts from a pair in my SPP amp boards.
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Old 15th October 2018, 07:14 PM   #7
Francois G is offline Francois G  United States
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Nice job with low-cost tubes and iron! I will try it.

What are the voltages on you B+, Preamp, and screen, in case I can use some of my accumulate transformers? Thanks.
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Old 15th October 2018, 07:31 PM   #8
Lingwendil is offline Lingwendil  United States
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6N2P/6CW5 design with toroidal outputs
Main supply voltage is around ~320-315 volts at the first cap and rectifier, the fixed mosfet ripple filter knocks that to ~300 or so, and this feeds the preamp circuitry. The screen supply is through the separate adjustable mosfet ripple filter/regulator, and is around ~200 or so, depending on tubes and what sort of screen voltage they want, with maybe 75-100 volts of adjustment. The output tube plates take their supply right from the bridge rectifier/cap, before the filters. Any 230V secondary should give you a pretty close raw supply if going for silicon for your rectification, and all the voltages aren't too fussy.

The mosfet filters sure do work well, it's dead silent with no music playing, on my 91db efficient speakers. I'll never buy a choke again with how cheap these are to implement

There is a minor error in the schematic, the voltage divider at the concertina grid should be 1M5 up top, 1M on bottom, opposite to what I've drawn. You could also make these both equal at anything from 470k~1M each without hurting anything really.

You could leave out the screen regulator circuitry, and triode connect the outputs too, for a nice 8-10 watts out, but this would of course require some work figuring out new cathode resistor values, or reducing the supply voltage to the outputs slightly. Whatever you need to get dissipation in a safe range, really.

Pretty versatile design, really. One could easily run EL84, 6V6/6P1P, 6P43P-E, or even sweep tubes like the 6AV5GA or similar with changes to cathode resistance, output transformer load impedance, and screen regulation voltage range, as well as triode connect things too. Lots of room to play around. As built with the 6CW5/EL86- pentode connected, figure abut 20-25 watts output.
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Last edited by Lingwendil; 15th October 2018 at 07:34 PM.
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Old 16th October 2018, 02:12 AM   #9
Tubelab_com is offline Tubelab_com  United States
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Quote:
and triode connect the outputs too
The 6CW5 and its higher filament voltage brethren really don't like 300 volts on the screen grid. It may work for a while if run at full tilt all the time, but G2 will glow itself to death when the amp is idle. Max spec is 200 volts, and I keep it lower than that if bending the plate voltage spec a bit. Max plate voltage spec is 250 volts, but 340 works fine if the screen is at 170 volts. I have been to 400 volts without issue with G2 in the 130 volt range. 400 volts is the limit of the small power supply I had at the time of all the 6CW5 experiments.

If you want to triode this amp, stick EL84 / 6BQ5's in it.
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Old 16th October 2018, 05:24 AM   #10
Lingwendil is offline Lingwendil  United States
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6N2P/6CW5 design with toroidal outputs
Yeah, if going triode connected, I would bring the supply voltage down to ~240-250 or so, since the garter resistance will eat up 30-40 volts or so it will be in a safe range.
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