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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 23rd July 2019, 03:25 AM   #51
PRR is offline PRR  United States
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Quote:
Originally Posted by Tubelab_com View Post
...similar to the Fender Champ 5C1 with some added plate to plate feedback should work far better. Convert the 6SJ7 to cathode bias and add a resistor between the two plates for feedback......
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Last edited by PRR; 23rd July 2019 at 03:44 AM.
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Old 23rd July 2019, 04:01 AM   #52
jhstewart9 is offline jhstewart9  Canada
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Unfortunately the plate to plate NFB makes the output stage more susceptible to power supply hum & noise.
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Old 23rd July 2019, 12:00 PM   #53
Dinolobe is online now Dinolobe  Canada
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Is there two versions of the 5c1 or you copied the cathode resistor/cap from the 6v6 and pasted it to the 6sj7?

I’d like to ask which value you would recommend for the input and coupling caps ? The 0.02uf on the input seems too small, and I don’t want it to roll of the low end too early. Something at least 0.1uf seems more reasonable, or even bigger.

Also, the B+ is not specified and I definitely won’t use the power supply from the original schematic. I was thinking of 250v at the plate of the 6v6, so 250V + the voltage drop of the output transformer’s primary would be my B+ at the output of the psu. What do you think ?

Thnaks agin !
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Last edited by Dinolobe; 23rd July 2019 at 12:10 PM. Reason: Added details/questions
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Old 23rd July 2019, 12:33 PM   #54
rongon is offline rongon  United States
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
I think PRR's local feedback drawing is a good start.

The 6SJ7 should not be run with grid-leak bias (contact potential bias), but should have a proper cathode load resistor to put a positive voltage of about 2 or 3V at the 6SJ7 cathode (grid at ground or 0V equals -2 or -3V grid bias, aka 'cathode bias' or 'self bias').

I whipped up a simulated circuit that is basically PRR's idea (pentode voltage amp RC-coupled to pentode output stage, local feedback from plate to grid of output pentode). I can post a schematic this evening when I get home from work.

Question:
Does the 6SJ7 work with very low plate+screen grid current (like 1mA combined plate+screen)? Or should it run closer to the data sheet example 3mA plate current and 1mA screen current, with closer to 100V at plate and screen?
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Old 23rd July 2019, 02:20 PM   #55
Dinolobe is online now Dinolobe  Canada
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I would really like to see your schematic. Can I ask you what you use to do your simulations ?

And for your question, what would be the point of running the 6sj7 at such a low current? The curves on the datasheet shows approximately 3ma at -3v on the grid, from 100V and up. My reflex would be to follow that, but you might have a good reason to do something else !
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Old 23rd July 2019, 02:51 PM   #56
jdrouin is offline jdrouin  United States
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I breadboarded the Bates circuit a few months ago. It was interesting, but very quiet. My guess is it put out less than half a watt at most, which correlates to PRR's calculation early in the thread.

Eventually I breadboarded a conventional RC coupled 6SJ7 into a triode-strapped 7C5 (loctal 6V6GT), using resistor values and operating points recommended in the manufacturers' datasheets. One of the best SE amps I've ever heard.
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Old 23rd July 2019, 03:32 PM   #57
Dinolobe is online now Dinolobe  Canada
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Good to know ! Thanks for your input !
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Old 23rd July 2019, 04:12 PM   #58
Tubelab_com is offline Tubelab_com  United States
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Quote:
I wouldn't recommend building the Champ circuit as is. Grid leak bias is probably a bad idea
I wouldn't either, that's just where I started in the early 60's. Grid leak bias IS a bad idea, even in a guitar amp if it is hit hard with a distortion pedal. I found out that it's possible to generate blocking distortion in the input stage when I made my first germanium Tone Bender pedal clone in the mid 60's.

I stated in post #47 that the input stage should be converted to cathode bias, and plate to plate feedback should be added to improve the damping factor. It is a given that the power supply in a 1940's vintage guitar amp would need some help, but big fat low ESR electrolytics were not yet invented in the late 40's.

I have not been able to draw up a diagram or even dig into this since my wife is in the hospital again. The schematic posted by Rongon in post #49 is pretty much what's needed, but the resistor values will need some tweaking to match the chosen tube set. I would also use a two resistor voltage divider on the input tube's screen to make a stiffer supply.
Quote:
what would be the point of running the 6sj7 at such a low current?
The current through a pentode and the value of the plate load resistor are the main factors in determining the stage gain. Tweaking up a pentode gain stage is an art in balancing the screen voltage, cathode resistor, and plate load resistor. I often stick three pots in the circuit and spend several days turning knobs in a guitar amp since the overload characteristics are a primary design criteria. This is not important in a HiFi amp since distortion is unwanted.

I would tweak each stage independently to get them working, measure the plate voltage on the 6SJ7, then raise it's load resistance as you add plate to plate feedback to get back to the same plate voltage. The balance of these two resistors determines the gain, distortion, and damping factor of the overall amp. Tweak to match your speakers with a familiar music source.

Need more overall gain? The 6AC7 is a pentode with more Gm. It's more like the 6EJ7 than the 6SJ7. Different resistor values and a bit more current will be needed.
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Old 23rd July 2019, 05:33 PM   #59
leadbelly is offline leadbelly  Canada
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I know George, I wasn't challenging your post, just the OP's posting of the original Champ.

I still think grid leak bias would be cool for a guitar amp, say for something like a portable buskering amp to be used without pedals.
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Old 23rd July 2019, 06:07 PM   #60
rongon is offline rongon  United States
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
Quote:
Originally Posted by Dinolobe View Post
I would really like to see your schematic. Can I ask you what you use to do your simulations ?
LTspice. The few circuits I've put together after first simulating them have turned out fairly close to the simulations, at least as far as DC conditions go. AC conditions (i.e., gain and distortion) are not always as expected. But they've at least been in the ballpark.

Quote:
Originally Posted by Dinolobe View Post
And for your question, what would be the point of running the 6sj7 at such a low current? The curves on the datasheet shows approximately 3ma at -3v on the grid, from 100V and up. My reflex would be to follow that, but you might have a good reason to do something else !
I was looking at the old pentode-based Fender Champ schematic and reacting to the very large value plate and screen load resistors I saw there.

Click the image to open in full size.

The 250k ohm plate resistor means it would drop 250V with only 1mA across it. With B+ of 350V this would leave only 100V at the 6SJ7 plate. 2M screen load resistor is also a larger value than I'm used to seeing there.

I was thinking the plate voltage should be more like 150V. With 3mA plate current, that would mean something like a 33k plate resistor to drop 100V, leaving 150V at the 6SJ7 plate. The screen grid load resistor might be something like 220k(?).

Also, in a 'hi-fi' amp the 6SJ7 will be cathode biased instead of grid leak (contact potential) biased.
--
Quote:
I would tweak each stage independently to get them working, measure the plate voltage on the 6SJ7, then raise it's load resistance as you add plate to plate feedback to get back to the same plate voltage. The balance of these two resistors determines the gain, distortion, and damping factor of the overall amp. Tweak to match your speakers with a familiar music source.
Thanks George! Nice recipe for us less knowledgeable folks. I'll put this to use for sure.

Quote:
Need more overall gain? The 6AC7 is a pentode with more Gm. It's more like the 6EJ7 than the 6SJ7. Different resistor values and a bit more current will be needed.
Yes indeed! 6AC7 is a good one, and keeps the metal tube vintage thing going.

Last edited by rongon; 23rd July 2019 at 06:17 PM.
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