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DC coupled single ended cathode follower 6SJ7/6V6 amp project
DC coupled single ended cathode follower 6SJ7/6V6 amp project
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Old 19th July 2019, 01:29 PM   #31
pcan is offline pcan  Italy
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
The full issue of Radio News (issue 11 1949) and Audio Engineering (june 1949) are currently available for download as pdf files from the americanaudiohistory.com website. The audio engineering article is a single page one. This article has been published at least on a further magazine because I've found another scan on the web with different impagination but the exact same text and schematic; I have been unable to recognize the magazine. I plan to build in the near future a small amplifier with cathode follower output stage, and this Bates schematics is often quoted as example. It does have a few seriuous drawbacks, however. It has been obviously designed with a trial and error approach, using the parts that the author had on its drawer. This means that it probably works, unlike other designs from magazines, but it is equally probable that it may work better after optimizations and checks on a circuit simulator. The 6SJ7 / VT116 really looks to have been grabbed from a scrapped WW2 communication receiver. I have at least a dozen metal VT-116 and this tube has the important advantage to be really cheap, but I don't see a compelling reason to use it on a modern Hi-Fi build; I tried it on a test circuit with the standard component values listed on the databook and I was not impressed. As small output tube, 6V6 is ok but a tube with higher gm such as EL84 / 6BQ5 will ease the driver tube job. It would be interesting to compare on the simulator the original 6SJ7-6V6 tubes to something more recent at the same power level, such as EF86-EL84. The Tubelab cathode follower amp with sweep tubes is a far more reliable and documented design and I am certain that it work as described, but it does not meet my current need to have something small on power and size. This is why I currently have the Bates schematic around on my desk again.
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Old 19th July 2019, 01:40 PM   #32
Bas Horneman is offline Bas Horneman  Netherlands
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What is the function of the 2 diodes and 2 electrolytics in the primary circuit?
DC filter.
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Old 19th July 2019, 02:48 PM   #33
Dinolobe is offline Dinolobe  Canada
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Quote:
Originally Posted by pcan View Post
The full issue of Radio News (issue 11 1949) and Audio Engineering (june 1949) are currently available for download as pdf files from the americanaudiohistory.com website. The audio engineering article is a single page one. This article has been published at least on a further magazine because I've found another scan on the web with different impagination but the exact same text and schematic; I have been unable to recognize the magazine. I plan to build in the near future a small amplifier with cathode follower output stage, and this Bates schematics is often quoted as example. It does have a few seriuous drawbacks, however. It has been obviously designed with a trial and error approach, using the parts that the author had on its drawer. This means that it probably works, unlike other designs from magazines, but it is equally probable that it may work better after optimizations and checks on a circuit simulator. The 6SJ7 / VT116 really looks to have been grabbed from a scrapped WW2 communication receiver. I have at least a dozen metal VT-116 and this tube has the important advantage to be really cheap, but I don't see a compelling reason to use it on a modern Hi-Fi build; I tried it on a test circuit with the standard component values listed on the databook and I was not impressed. As small output tube, 6V6 is ok but a tube with higher gm such as EL84 / 6BQ5 will ease the driver tube job. It would be interesting to compare on the simulator the original 6SJ7-6V6 tubes to something more recent at the same power level, such as EF86-EL84. The Tubelab cathode follower amp with sweep tubes is a far more reliable and documented design and I am certain that it work as described, but it does not meet my current need to have something small on power and size. This is why I currently have the Bates schematic around on my desk again.
We seem to have similar goals with this circuit. I have thought of modifying it to use an ef86 driver and el84 output stage, but since I wanted to keep the dc coupling at this moment, I decided not to do it. But now that I learned more about the limitations of the dc coupled version, it becomes tempting to try to modify the rc coupled version.

Something else that could be done would be to modify the mullard 3 watt amplifierís output stage into a cathode follower one. I havenít checked for the component values, but it could be a matter of adjusting some resistor values, removing the NFB loop an putting the opt on the cathode instead of the anode. The bias of the el84 might not be good with the 250 ohm DCR of the OPTs primary though.
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Old 19th July 2019, 03:17 PM   #34
DF96 is offline DF96  England
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You always need to remember that a cathode follower output stage needs much higher signal voltages from the previous stage. In some cases there will be inadequate gain for this, sometimes inadequate voltage swing, and almost always increased distortion. Hence such an amplifier needs careful design from the start, not merely "modifying" an existing quite different circuit.
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Old 19th July 2019, 03:31 PM   #35
Dinolobe is offline Dinolobe  Canada
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Originally Posted by DF96 View Post
You always need to remember that a cathode follower output stage needs much higher signal voltages from the previous stage. In some cases there will be inadequate gain for this, sometimes inadequate voltage swing, and almost always increased distortion. Hence such an amplifier needs careful design from the start, not merely "modifying" an existing quite different circuit.
I was mostly referring to the mullard circuit because it has the same two valves that I would like to use. But as you said, there would probably be nothing left from the mullard circuit at the end of the modifications. I just thought that it would be easier to start with a circuit that uses the same valves. Do you think that the Bates (rc coupled) circuit would "work" with the valves replaced by the ef86/el84 as a starting point, and that the components values could be adjusted afterwards ?

Btw, I have found this thread. The guy built the rc coupled version using an EL84 output and a 6AG5 driver. He surprisingly claim to be able to drive "vintage 3 way speakers" with it. Looks like that, without being perfect, the kind of modifications I’d like to do could be done.

single-ended cathode follower output stage

Last edited by Dinolobe; 19th July 2019 at 03:55 PM.
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Old 20th July 2019, 10:06 PM   #36
PRR is offline PRR  United States
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As for Bates: a then-new RCA 45 player, a 12-inch Jensen, a small livingroom, and unknown taste in dynamics.... maybe he could "astonish" himself. I've been "impressed" by some quite modest amplifiers in a similar situation.

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Originally Posted by Dinolobe View Post
Maybe I could also ask you guys what it would take to make the output stage work as itís intended.........
An ideal device as a power amp, choke/transformer-loaded, 250V supply, will swing +/-250V (500Vpp) around the idle point.

Cathode follower is "unity gain". So the driver has to make 250V peak or 500Vpp.

6V6 Triode won't swing full supply voltage. But also won't reach unity gain. So for stick-in-sand figuring, we still need 250V peak.

As DF96 and others say-- this won't happen by happenstance.

A good rule-of-thumb for a resistance-loaded driver is: peak output at 5%THD is near 20% of supply. Or the other way: supply must be > 5X the peak out needed.

5X 250V is 1,250V supply.

We "can" cheat and bootstrap the driver (above) since we have a cathode follower. However all distortion in the CF boots-back to the driver; not a great cheat. (BJTs make better "CF"s which is why many BJT audio power amps use bootstrapping effectively.)

> has many different HV secondaries

OK, so spitball a >1,250V few-mA supply. Driver plate must sit 1/3-1/2 of supply, 400V-600V.
Ignore the fact that few small tubes enjoy such voltages (we can always stuff a EL34).

With pentode driver, 1,300V supply, 650V on plate, driving AC coupled 6L6 with 355V supply, 6k load, I'm simming 4 Watts at 22% THD. 2 Watts at 8%THD. There's an unresolved kink, which may be a model flaw, though when I isolate the cathode follower alone in DC Sweep it's dead linear.
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Old 21st July 2019, 02:03 AM   #37
45 is offline 45  Italy
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The obvious solution to that circuit has already been posted by Tubelab.

If one wants it simpler it is possible but RC coupling, CSS for the gain stage and independent voltage supply (if the one for the output stage is not enough) are necessary.

The Bates' circuit is just wrong. Forget it, throw it in the bin!!! You will only waste your time and money. If you are happy with less than 0.5W output power there are better and cheaper solutions....it's really not worth the trouble as is in the original article.
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Old 21st July 2019, 12:50 PM   #38
Dinolobe is offline Dinolobe  Canada
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I guess iíll have to find another ocatal/two valves circuit with 5k OPTs to fill my enclosure, wich has already been drilled. The other version of the circuit, the RC coupled one, was still tempting, but the power figures are so low for such an investment in time. 1250v is just wrong for such a low power output !!

Since my chassis is ready to receive this circuit topology, and because I canít find a suitable two valve all octal single ended schematic for a 6v6gt, I think that I will assemble the rc coupled version of the bates amplifier. That will satisfy my curiosity and iíll be able to measure itís output power in real world.

The other option would be to tame myself and take the time to design something else that would fit the bill. I donít have a computer that can run the tubecad programs and donít know how to use LTspice so I would have to do it "by hand", unless you guys know of a circuit that I could use. Thanks to everyone who answered this thread, itís been really instructive !
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Old 21st July 2019, 04:29 PM   #39
leadbelly is offline leadbelly  Canada
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6V6 SE2

6J7 is just precursor to 6SJ7.

The 0.02 uF should be up sized, or even omitted.
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Last edited by leadbelly; 21st July 2019 at 04:32 PM.
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Old 21st July 2019, 07:09 PM   #40
jhstewart9 is offline jhstewart9  Canada
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For me the bottom line of this cct max output would be One Watt on a good day. It is triode connected no matter how we look at it. 4.5W is the spec for a pentode connected 6V6 on a 250V supply. Better jack that up a few volts to account for the cathode bias resister & OPT primary R losses. So a supply of 275V might do it.

But never for the triode connexion.

Some of the ccts published in that era are not exactly as claimed. Need to be careful. I wuz there!! Few people had any kind of reliable test equipment. I was lucky, I worked in an R&D lab.
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