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 Post subject: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 3:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Arlington, TX, USA
I wanted to try a single ended 6L6 amplifier design using shunt (otherwise known as Schade) feedback. This circuit does not include the output transformer in the feedback loop, so it doesn't cause stability problems with the amplifier. This type of feedback circuit really needs the high plate impedance and high gain of a pentode as a driver, so I used the EF86 since it's the only currently produced small signal pentode available.. The 1MEG resistor is the feedback resistor in this design. Also, I used an output transformer with a screen tap to further linearize the amplifier. I get about 8 watts output at clipping (one peak only since it clips at zero grid voltage on the 6L6). Subjective sound in my opinion is excellent, and with decently efficient speakers has plenty of bass even with the smaller output transformers I used. The power supply uses solid state rectifiers and I used the unused 5V heater winding to provide DC heater power for the EF86 tubes through a voltage doubler and regulator. Since this circuit uses a screen grid tap it really isn't suitable for earlier 6L6 types even though you'd likely get away with using them (this is due to the lower screen grid voltage rating).


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File comment: schematic
6L6SE.jpg
6L6SE.jpg [ 115.27 KiB | Viewed 1124 times ]
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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 3:20 pm 
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Love it! Low parts count high performing circuits have a special place in my heart. Thanks for sharing this.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 3:26 pm 
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Joined: Aug Thu 20, 2015 3:09 pm
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Location: Albion, CA, USA, 95410
Just when I had pretty much decided to go push pull... Gee, thanks! :?

What output transformers did you use?


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 4:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Arlington, TX, USA
I used the Edcor GSXE series output transformers. The part number is on the schematic. I chose the 6 ohm secondary as a compromise between 4 and 8 ohm loads. These transformers cost about half of their top of the line and don't give up much in the way of performance. Yes, there is a certain amount of core saturation distortion at low frequencies and high power, but I don't notice it using normal program material. I suppose one could use a better transformer but it would cost a lot more for a small performance gain.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 4:42 pm 
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Location: Albion, CA, USA, 95410
Oh duh. Sorry, I didn't see that. Do you have a schematic for your power supply? I hear that single ended amplifier supplies need to be pretty clean.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 5:17 pm 
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Location: Arlington, TX, USA
I'll have to draw one up later. I do have a bit of low level hum, but it sounds and measures to be mainly 60Hz, and is still there with the EF86 tube pulled, so I guess it's due to AC on the 6L6 heater. I suppose I could have used filter chokes, but that would have been extra weight, bulk, and expense, and I wanted everything to fit on the 12" x 8" chassis box. The hum really isn't that bad though.

The reason SE power supplies need to have low ripple is because there is no hum cancellation from the other output tube used in a push pull design.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 7:43 pm 
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Erich Loepke wrote:
I'll have to draw one up later. I do have a bit of low level hum, but it sounds and measures to be mainly 60Hz, and is still there with the EF86 tube pulled, so I guess it's due to AC on the 6L6 heater. I suppose I could have used filter chokes, but that would have been extra weight, bulk, and expense, and I wanted everything to fit on the 12" x 8" chassis box. The hum really isn't that bad though.

The reason SE power supplies need to have low ripple is because there is no hum cancellation from the other output tube used in a push pull design.
If I were looking for the source of hum the first thing I'd check is the fixed bias voltage. Any hum there get's amplified by the 6L6, so it has to be a lot more clean than B+.

We just went through tracing a heater hum problem over in the "Another Little Prince- tube preamp" topic and the solution was to raise heater voltage with a resistor divider off B+. But that was on the RIAA (phono) preamp that's dealing with mV level inputs. That makes me suspicious that heater hum could be the cause in your power output tubes where you're looking at 50-60 mA of idle current. But, if it is, raising the heater voltage would surely solve it.

Btw, you'd only need to raise it by 12VDC. The RIAA amp used 12V tubes and so needed double that. Not that it's all that critical because the 6L6 can take up to 200V (including AC peak). It's just that the raised DC voltage should be higher than the AC peak of the heater.

Woops. I jut noticed early in the thread you're getting the EF86 heater DC off the same 6.3 VAC transformer. Same winding? That probably won't work, as is, because of screwing up the EF86 heater circuit.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 8:07 pm 
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Location: Arlington, TX, USA
The rectified heater voltage is from the otherwise unused 5V winding normally used for the rectifier tube running through a voltage doubler and regulator. I'd have to check if I tied the 6.3V winding to ground... I'm pretty sure I did through the center tap, but it's been about a month since I built this amplifier. I'll look into doing what you suggest and raise the 6L6 heater voltage above ground and see if that helps. The bias supply is filtered both before and after the adjustment pots and shows no ripple at all that I can see on a scope set to 2mv/div and a 1x probe, so I doubt that's it.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jun Fri 07, 2019 8:14 pm 
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Erich Loepke wrote:
The rectified heater voltage is from the otherwise unused 5V winding normally used for the rectifier tube running through a voltage doubler and regulator. I'd have to check if I tied the 6.3V winding to ground... I'm pretty sure I did through the center tap, but it's been about a month since I built this amplifier. I'll look into doing what you suggest and raise the 6L6 heater voltage above ground and see if that helps. The bias supply is filtered both before and after the adjustment pots and shows no ripple at all that I can see on a scope set to 2mv/div and a 1x probe, so I doubt that's it.
Ah, so it is a different winding. Okay, then it should work.

From what you say your bias voltage sounds fine.

Btw, the heater has to be reference to 'something' so if the CT isn't already tied to ground that could cause it too.

Oh, I forgot to mention. The filtered voltage in front of your heater regulator might be just barely high enough to use.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jul Fri 12, 2019 8:42 pm 
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An easy thing to try is change the routing of the wiring for the heaters. It should all run close to the chassis while the wiring carrying the signal is kept high and well clear of the heater pins. Just a thought.
If that doesn't help, look into changing the grounding to just a couple of terminals, star grounding, sorry if I'm insulting you with basics. :lol:

Twisted pairs are good, because that allows phase cancelling between the two wires. Separate them from the other twisted pairs.

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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jul Sun 14, 2019 2:19 am 
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What speakers?

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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jul Sun 14, 2019 9:20 pm 
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Location: Rochester NY USA
If the EF86 heater circuit doesn't have a ground, it should - DC plus or minus side should be fine.

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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jul Mon 15, 2019 2:25 pm 
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Location: Arlington, TX, USA
I played around with this a bit last night. Seems like the parts have all settled in, there's a bit less hum and the EF86 voltages are more normal. The hum is now more 120Hz, and is about 20mV P-P, audible at close range with no signal. It looks to be power supply ripple since the hum at the output and the power supply have the same waveform. Guess that's why some people use CLCLCLCLC... power supplies with SE amplifiers. I'm going to do a PC board layout for the power supply and use a zener string and pass transistor to get the ripple down.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jul Mon 15, 2019 10:18 pm 
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I don't think this is your problem, but I built a tube amp that hummed until I physically located the power transformer away from the tubes. Since then all my tube amps have a separate power supply connected by an umbilical cord. I figure it can't hurt.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jul Tue 16, 2019 1:02 am 
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Erich Loepke wrote:
I played around with this a bit last night. Seems like the parts have all settled in, there's a bit less hum and the EF86 voltages are more normal. The hum is now more 120Hz, and is about 20mV P-P, audible at close range with no signal. It looks to be power supply ripple since the hum at the output and the power supply have the same waveform. Guess that's why some people use CLCLCLCLC... power supplies with SE amplifiers. I'm going to do a PC board layout for the power supply and use a zener string and pass transistor to get the ripple down.
I've used this circuit with excellent luck.
Attachment:
MOSFET filter.jpg
MOSFET filter.jpg [ 81.33 KiB | Viewed 721 times ]


Advantage is it gives great filtering with minimum voltage drop as it 'seeks' (and follows) the 'natural' B+. As such it needs less heat dissipation than a zener regulated supply, which must be sized to be less than the minimum B+ at minimum line voltage to stay in regulation (or else all filtering is lost).

Basically, it's a cap multiplier.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jul Tue 16, 2019 3:12 pm 
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Location: Arlington, TX, USA
Thanks for posting that circuit. I never thought to use a MOSFET; looks like it would work really well. Will try to simulate in LTSpice later today if I get time. I have the power supply already simulated, so I just need to add in this circuit and do the comparison.


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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Jul Tue 16, 2019 8:46 pm 
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Hi Erich:
Do you have a schematic of the power supply?

BTW...
I built a somewhat similar, single-ended 6V6 amp driven by 12SL7 (or a 12AX7 .. (there is an extra 9-pin socket added there in parallel w/the 12SL7 octal socket)
"Plug and play" either driver tube desired.

I feed the audio input from my Mp3 player

It also has flexibility with 2 extra 7pin sockets to allow for other 6V6 type compatible output tubes (6AQ5 /12AQ5)

To switch between 6v and 12v output tubes I employ a simple A/B switch arrangement in the filament wiring allowing for either 6volt or the 12volt output tubes.
(filaments in series for 6v or parallel for 12v)


Image

I used a voltage doubler power supply and some quite large 820uf filter caps in the B+ ... but it works just as nicely with 470uf ..... (so the 820uf are overkill)
Note: Instead of a 12v filament transformer, I simply used about a 12-13uf (non-polarized film) voltage-dropper cap for the 12v AC filament supply.

For cathode-bias I used an LM317 strapped as a (46ma) constant-current source in each output tube cathode

I really love the performance of this amp.. and it is dead quiet... no hum

You may like looking at it and comparing the two circuits since they are basically the same idea.
Image

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 Post subject: Re: Single ended 6L6 amplifier
PostPosted: Aug Sun 04, 2019 10:26 pm 
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Location: Royal oak MI
That's a very nice amp you have there!


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