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DC coupled single ended cathode follower 6SJ7/6V6 amp project

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I guess i’ll 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 can’t 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 i’ll be able to measure it’s 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 don’t have a computer that can run the tubecad programs and don’t 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, it’s been really instructive !

You don't necessarily need to find another tube. The easy solution is that you change the output stage from cathode follower to common cathode. It can be done in several ways. The simple one is RC coupling of input stage to the 6V6 and self-bias for the power tube with RC network. Can do triode or pentode with some feedback. If the gain of the input pentode is too much and don't want feedback you can connect it in triode mode as well. Gain will be similar to what you get from 6J5/6SN7 family.
 
The attached very early 6V6 data sheet shews triode operating specs. So in the RCA lab under ideal conditions the typical triode connected 6V6 manages one watt powered by a lab supply of 250V on the plate. Multiply that by something like 85% efficiency of a SE OPT, so less than a watt.

It does a little better with a higher voltage supply.

There are several modern power triodes used as vertical amplifiers in TV that will do quite a bit better. But in a CF cct needs a rather large drive.:)

A cunning devil cct would bootstrap the drive using a PP OPT. :yikes: But why bother, there are better ways.:D
 

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PRR

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The obvious solution to that circuit has already been posted by Tubelab. ....

"class H design with modulated supply rails"
"floating buck converters"

?? This looks like it was designed and assembled by a practicing cell-phone engineer and technician.

It is very not-1949, not very nostalgic (unless you remember burning your first 6336), and not real scrap-box friendly (unless you salvage at Motorola).

It's a great design!! But not what this thread is about.
 

PRR

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> very early 6V6 data sheet

Thanks.

We see around 5W in pentode, 1.65W in triode.

_IF_ we could drive a cathode follower HARD, we'd get the same powers at lower THD. (However making a driver deliver >125V peak at <5% THD requires heroic engineering.)
 

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"class H design with modulated supply rails"
"floating buck converters"

?? This looks like it was designed and assembled by a practicing cell-phone engineer and technician.

It is very not-1949, not very nostalgic (unless you remember burning your first 6336), and not real scrap-box friendly (unless you salvage at Motorola).

It's a great design!! But not what this thread is about.

Everyone has his opinion. I don't care about nostalgic stuff. My point is that he has gathered stuff and wants to use it.
 
It is very not-1949, not very nostalgic.......It's a great design!! But not what this thread is about.

The class H design line was not posted for the purpose of someone else's duplication. The information about the cathode follower amp it contains, and the "plain jane" stand alone cathode follower amp that was designed for it (also linked) is pertinent to the request for a two octal socket cathode follower design. The input and voltage gain stages are one octal bottle (6EM7) and the output tube could be just about any triode wired octal TV sweep tube. A 6CD6 ot 6DN6 would work.

I can’t find a suitable two valve all octal single ended schematic for a 6v6gt........I could also order my parts and populate the Tubelab SSE board that I have received two months ago. It may be more logical to put in use the big edcor OPTs

I don't think any of the 6V6 cathode follower circuits found with two octal tubes are going to work well on modern speakers and outperform a conventional design. A design 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.

The SSE works fine with 6V6's if you jumper across the 10K resistors in series with the CCS chips, and use a power transformer that makes about 275-0-275 volts. It however will only make about 2 watts in triode mode, and pure pentode using ANY tube will need feedback to sound good on most speakers.

If you already have the big Edcor OPT's (CXSE) it would be wise to use them with something that can take advantage of their size. I used the small XSE OPT's in my 6V6 based SSE with a cheap Allied 6K56VG power transformer. This combination makes a nice sounding 2 watt amp.
 
The class H design line was not posted for the purpose of someone else's duplication. The information about the cathode follower amp it contains, and the "plain jane" stand alone cathode follower amp that was designed for it (also linked) is pertinent to the request for a two octal socket cathode follower design. The input and voltage gain stages are one octal bottle (6EM7) and the output tube could be just about any triode wired octal TV sweep tube. A 6CD6 ot 6DN6 would work.



I don't think any of the 6V6 cathode follower circuits found with two octal tubes are going to work well on modern speakers and outperform a conventional design. A design 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.

The SSE works fine with 6V6's if you jumper across the 10K resistors in series with the CCS chips, and use a power transformer that makes about 275-0-275 volts. It however will only make about 2 watts in triode mode, and pure pentode using ANY tube will need feedback to sound good on most speakers.

If you already have the big Edcor OPT's (CXSE) it would be wise to use them with something that can take advantage of their size. I used the small XSE OPT's in my 6V6 based SSE with a cheap Allied 6K56VG power transformer. This combination makes a nice sounding 2 watt amp.

I intended to use the edcor cxse OPTs with my SSE board and 6550s. This thread’s purpose was only to find a simple/fun way to use some parts I have laying around, and maybe experiment with something different and learn along the way.

I have a Tek PT with 3 150V secondaries, 2 smallish SE 5k:8 OPTs (antique supply P-T31) and a reasonable stash of tubes. (Pretty much all common noval preamp tubes in different flavors and a few different sets of power tubes like el84, el34, 6v6, 6550...) from what I remember of your cathode follower design, wouldn’t I need OPTs with a 600 ohm primary ? I would gladly try your design if it’s possible to make it work with the transformers I already have, but if not, I would prefer to use a circuit that better fits those parts. Even if it’s a very interesting circuit ! I don’t mind buying some tubes I don’t already have though.

I haven’t thought of the fender circuit. It could be an interesting project too ! I’ve read that you test your amps by playing guitar through them. Could it be possible to build the amp in order to serve both purposes ? Is it just a matter of input/output connectors or there is something else that needs to be done ?

Edit : volume pot of the 5c1 is between the two stages. My guess is that the instrument needs a fixed input impedance, but I may be wrong. Could I use a switch or relay to bypass it, and put rca input connectors in parallel with the 1/4" jack, bypassing the 75k resistor in the input at the same time ?
 

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I wouldn't recommend building the Champ circuit as is. Grid leak bias is probably a bad idea in a hifi chain, your CDP output could probably exceed the input headroom and be clipped. The coupling caps are also on the small size as low bass is being intentionally filtered. Also, building it as drawn would result in enough hum to drive me crazy, as a little hum is no big deal in a guitar amp.
 
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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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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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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 !
 
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.
 
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.
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.
 
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.

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.

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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.
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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.

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.
 
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