Cathode follower into 6V6 on Champ

[BadCatCubII]

Hi Everybody! I have an amp I've been fiddling with for quite a while, it is the one I constantly have open and I'm tweaking this or that component and it has had lots of different iterations. Do you have one like that? It has been a great learning experience!

My latest idea comes from learning that the Steel String Singer uses a cathode follower into the power tubes to getting a pretty different sound, and reading Merlin's book talking about the Direct Coupled Cathode Follower (DCCF) adding a unique distortion. The idea is to use a switch to use the last tube before the power tube as either a standard gain stage or as a direct coupled cathode follower. Is that possible?

The amp is basically a champ with reverb, so it is single ended and I don't have to mess with a phase inverter or anything like that. I have the sound without the cathode follower dialed in to where I really like it. However, When i switch to the cathode follower I get almost zero output. I thought that maybe this was because of the negative feedback loop, since it is being disconnected from the cathode of the V6, but not from ground, but I get the same result even when the NFB loop is removed.

I am also very unsure of resistor values to use, and how the cathode resistors of the DCCF would interact with the grid leak resistors of the 6V6 since they would essentially be parallel resistors messing with all their values. For that reason, I used the same resistor (rather, two in series) for both duties. Let me know if this all makes sense, if this is possible, or if there are examples of this somewhere I don't know about. You all are the best!

[sluckey]

Well, you're switching from a GAIN stage to a NO-GAIN cathode follower. That's a big reason why the output is so low. Also, look at what you are doing to the grid of the 6V6 when in cathode follower mode. You're putting a big positive voltage on that grid! That's not good.

BTW, DCCF refers to a cathode follower whose grid is DC coupled to the plate of the previous stage. The cathode output usually must be cap coupled to the next stage to block the high cathode voltage from reaching the tender grid of the next stage.

[BadCatCubII]

Thanks, Sluckey! I get what you are saying about a GAIN stage vs. a NO GAIN stage. I also think I understand what you mean by a cathode follower whose grid is DC coupled to a plate of a previous stage. Can you help me understand how this schematic works differently from what I have going on? does it have to do with the bias? This way of using a cathode follower seems to contradict what you are saying, and maybe this is just because it is something I got from from the internet and isn't a workable schematic... I don't know.

One document is the full schematic, the other is just page three of it. Thanks!

[shooter]

confused;
the schematic in the OP is a single 6V6 Cathode biased
the one you have posted is a PP FIXED bias
they're like apples n grapefruit in "comparison"

[sluckey]

Can you help me understand how this schematic works differently from what I have going on? does it have to do with the bias? This way of using a cathode follower seems to contradict what you are saying

First off, did you not see the word "usually" in my statement? That implies that there are exceptions.

Yes, it has to do with the bias. On this latest schematic the cathode must be direct coupled in order to pass the negative bias voltage to the 6L6 grids. That's a good thing. But your circuit passes a big positive voltage to the 6V6 grid. That's a BIG difference. Like the difference between night and day, or a working amp and a non-working amp.   

[BadCatCubII]

Thanks, Sluckey and Shooter. Yes, I understand that the schematic I have drawn is completely different from the one i shared. I didn't mean to make an apples and grapefruit comparison as if this one should work like that one, but I am trying to understand how they contrast, what works and what doesn't and most importantly why. As someone with only the most basic understanding, it makes perfect sense to me that you wouldn't want a large positive DC voltage on the grid of a tube. That is actually part of why this whole cathode follower thing is interesting to me, it doesn't seem like it should work and that shows I don't understand.

So I appreciate your experience and patience. I hear what you are saying, Sluckey that my circuit doesn't work because my cathode follower passes a big positive voltage to the grid of the 6V6, while the other schematic passes a negative voltage. That makes a lot of sense, but why does the cathode follower of a 5F6-A work, when it sees +180 volts? Thanks, I'm not trying to be a pain, I swear. 

[sluckey]

why does the cathode follower of a 5F6-A work, when it sees +180 volts?

Glad you mentioned the 5F6A. That is a perfect example of a DCCF. Sure, the cathode has 180V. So what? The grid also has 180V because it is directly connected to the previous tube's plate. That's what makes it a DCCF. The reason it works is because the bias voltage on the tube (bias is voltage between grid and cathode) is about zero.

You may want to re-read what Merlin has to say about DCCF on the Valve Wizard's website.

[shooter]

^^^^
while you're reading, take note of his "improvements" , blocking diode, R values, they ARE worth implementing.

because my cathode follower passes a big positive voltage to the grid of the 6V6

in your 5F example, once the signal + big DC hits the TS, the DC is removed via those caps, so NO big VDC makes it "downstream"

[tubeswell]

There is no sonic improvement from driving a signal grid of a Single Ended 6V6 with a DC-coupled cathode follower - even if you sort out the preamp gain and the 6V6 bias* correctly


(*which is possible to do if you have a high enough negative voltage supply feeding the CF cathode).


For a single ended 6V6 amp, the best thing is to keep it simple and follow a tweed champ or tweed Princeton schematic.

[Jonas]

@shooter are you referring to Merlin's mod to help protect the tube during startup?

[shooter]

ya, i've implemented it in 3 or so DCCF's i've used.
somewhere else he talks about grid stop(?) R's that comes in real handy.  the pic below shows what happens when you "get it wrong"

There is no sonic improvement from driving a signal grid of a Single Ended 6V6 with a DC-coupled cathode follower

I'm forgetting some of the nuance since I haven't built in awhile, recall by using a DCCF as part of a "clipped/compression"
"signal conditioning sound" it added something "better" than using standard gain, gain - (clip/compression)
but I have zero problem being wrong

[Jonas]

thanks @shooter could anyone suggest a neon bulb part number for the mod? I have 0 experience with neon bulb so not sure which one is the ordinary bulb...

From the Valve Wizard site................"Another solution [diagram b] is to use an ordinary neon bulb (not the kind advertised as mains indicators as these have a resistor built in). The neon will strike if the grid-cathode voltage exceeds about 90V, which is probably ok for most valves, but maybe not for high-gm types like the ECC88 which have a very close grid-cathode spacing."

[sluckey]

ne-2

https://www.amazon.com/Jameco-Valuepro-NE-2-Terminal-65-120VAC/dp/B06WV8XJ6G/

[Jonas]

thank you sluckey

[BadCatCubII]

There is no sonic improvement from driving a signal grid of a Single Ended 6V6 with a DC-coupled cathode follower - even if you sort out the preamp gain and the 6V6 bias* correctly


(*which is possible to do if you have a high enough negative voltage supply feeding the CF cathode).


For a single ended 6V6 amp, the best thing is to keep it simple and follow a tweed champ or tweed Princeton schematic.

Thanks, Tubeswell. Can you help me understand what the sonic beneift is on an amp like the SSS, wnd why that doesn’t apply to a single ended amp?

[tubeswell]

There is no sonic improvement from driving a signal grid of a Single Ended 6V6 with a DC-coupled cathode follower - even if you sort out the preamp gain and the 6V6 bias* correctly


(*which is possible to do if you have a high enough negative voltage supply feeding the CF cathode).


For a single ended 6V6 amp, the best thing is to keep it simple and follow a tweed champ or tweed Princeton schematic.

Thanks, Tubeswell. Can you help me understand what the sonic beneift is on an amp like the SSS, wnd why that doesn’t apply to a single ended amp?

If the DC-coupled CF is in the preamp (like a 5F6A, where it is coupled to a preceding driver stage, and where there is also another gain (input) stage in front to boost the signal), there is a noticeable audible compression effect when the signal is driven hard due to the Cathode Follower ‘stealing’ current from the preceding stage (which increases with increased current through the preceding stage’s plate resistor). But (AFAICT) the OP was asking about removing one of the common-cathode (inverting) gain stages in a champ-style preamp, and turning it into CF to drive the 6V6 grid as an output stage in a SE amp - which is different. So, overall preamp gain will be reduced (compared to a champ preamp). But also, the CF will be driving the 6V6 grid, which has a relatively large grid-to-cathode voltage, and so while the impedance bridging from CF to output tube can’t get much more ideal, the resulting clean weak signal won’t do anything for guitar/amp toan. And it’s not necessary to go to those lengths for a 6V6 grid (which doesn’t sink much current anyway, compared to a bigger tube like a 6550, where using direct coupling from a CF is a good idea if you’re going to drive several parallel 6550’s). But YMMV. Try it and see.

[BadCatCubII]

Thanks, Tubeswell. I am the OP, and the adventure was a kind of "what if" exploration for the purpose of understanding why some amps are designed one way and others are designed other ways. So, your input is really helpful. The goal is just as much a deeper understanding as anything else, so even when I fail it is a win if I learn. Of course, its even better if I learn and it ends up working! 

I will let you know how it shakes out if anyone is interested.

[BadCatCubII]

Ok! This is where things are today. The amp is working, and I am calling the experiment a success (so far ) mostly because I learned a lot, thanks largely to you. To recap, the goal was to find out and experience why you would put a cathode follower right before the power tube, and if that could be done on a single ended amp like a champ.

I ended up using one of Merlin's cathode follower designs, and it is currently capacitor coupled to the 6V6—which isn't what I had in mind at first. I did this by changing my schematic to use the famous "one tube reverb" from a Tubenit thread, and used the extra triode for the cathode follower.

RESULTS: Since I can switch the cathode follower on and off, I can tell what having it in the circuit does. It is subtle, difference, and part of it is a bit of compressions. There is a noticeable tightening of the distortion, especially in the low end, and that is great. That is what I was hoping would happen, that I could dial back some of that wooly, loose champ distortion. That doesn't mean it is a clean amp now, but I do think that it sweetened up the distortion, and it has more note clarity as it breaks up. This is more apparent as you turn up the volume knob, and it seems the compression added increases too.

Next I am going to probably play with the mixing resistor to see if I can get a little more reverb and a little more room to move on the volume knob before it gets super crunchy. After that I might try to use grid bias on the power tube so that I can DC couple it to the preceding stage like I originally tried to do.

[shooter]

There is a noticeable tightening of the distortion, especially in the low end, and that is great.

as long as you're tweaking around;
"double up" (anything close) C19 and C23
see if that gets you a slightly bigger smile

[BadCatCubII]

Thanks for the tip, Shooter. C19 is the first cap after the 5Y3, and I thought that going higher than 32uF was ill advised on that one unless I want to burn out the rectifier. Is there any reason why you couldn't use bigger filter caps after the first one? It seems that I never see the filter caps get progressively bigger, so I just assumed that wasn't a thing.

[tubeswell]

Ok! This is where things are today. The amp is working, and I am calling the experiment a success (so far ) mostly because I learned a lot, thanks largely to you. To recap, the goal was to find out and experience why you would put a cathode follower right before the power tube, and if that could be done on a single ended amp like a champ.

I ended up using one of Merlin's cathode follower designs, and it is currently capacitor coupled to the 6V6—which isn't what I had in mind at first. I did this by changing my schematic to use the famous "one tube reverb" from a Tubenit thread, and used the extra triode for the cathode follower.

RESULTS: Since I can switch the cathode follower on and off, I can tell what having it in the circuit does. It is subtle, difference, and part of it is a bit of compressions. There is a noticeable tightening of the distortion, especially in the low end, and that is great. That is what I was hoping would happen, that I could dial back some of that wooly, loose champ distortion. That doesn't mean it is a clean amp now, but I do think that it sweetened up the distortion, and it has more note clarity as it breaks up. This is more apparent as you turn up the volume knob, and it seems the compression added increases too.

Next I am going to probably play with the mixing resistor to see if I can get a little more reverb and a little more room to move on the volume knob before it gets super crunchy. After that I might try to use grid bias on the power tube so that I can DC couple it to the preceding stage like I originally tried to do.

Thanks for posting your schematic- that is way different to what I thought you were wanting to achieve. It’s nothing like a champ at all.

[shooter]

C19 is the first cap after the 5Y3,

of the 18 or so "builds", I only used a tube rectifier once, so I didn't look.

[Willabe]

You might want to try putting the 500pF cap in, just before R21 1M Dwell pot, for the input signal to the verb driver stage, clear out the bottom end mud, low end rumble.

And add a power tube screen grid R of at least 470 ohms. Keep the R's lead as short as reasonably possible that goes on that screen grid pin.