SE 6V6 with pentode in preamp

[j-stylez]

Hello, thanks for stopping by in this projects

I have a 6au6 tube lying around and also some parts from a small amplifier that I converted from SE to push pull.

Out of interest, I want to build another SE 6V6 amp with these parts, with a pentode in the preamp. I have come up with a first draft of a schematic, and also with some questions for anyone interested in this project. I know this is not conventional, but I want to try out these concepts: bootstrapped triode with cathode follower and pentode gain stage.

This is the schematic I came up with: https://drive.google.com/file/d/1ZGD...w?usp=drivesdk
(Fuse and Power switch to be added)

B+ is ~260V

Questions:
1. Can I inject negative feedback at the cathode of the 6au6, or would that not work?

2. With the cathode follower driving the tone Stack, Do I still have to worry about the pentode being loaded down by the tone Stack? Or is that only an issue when the tone Stack is placed post pentode?

3. Do I need the last filter stage in my power supply or do you think that is overkill?

Any help is appreciated. Cheers!

[eschertron]

Random responses, at random...
I think you'll want the grid leak for the 6V6 smaller than 1M. Check out a data sheet.

I've got an odd dozen of some low-power pentodes, with the warning that they're susceptible to noise from the heaters if not DC. So I haven't done anything with them yet, but would like to try something like what you've drawn up! I have the 6AC7 not the 6ca7 (EL34)

If you're after a bone-clean preamp and a 6V6 that can get really grungy, I think you're on the right track. I'm not sold on the idea of a CF after the input stage though. By the time you've gone through the james (?) tonestack, you'll have precious little signal left. That's where the noise floor of whatever pentode you've selected come into play.

Pretty random, right?

[j-stylez]

Hi eschertron, thanks for your thoughts. The maximum grid leak stated on the data sheet for 6v6 is 500k, so I figured that together with a 1m volume pot, the 1m grid leak resistor is ok, as there is a parallel path to ground. Is my logic off?

[j-stylez]

So if I want to actually get the pentode to overdrive I could use two triode gain stages and a low loss one knob tone control, to hit the pentode with a strong signal?

I think Ill try it with the cathode follower just out of curiosity, if that doesnt go well I could also use a FET for the cathode follower and use the Extra triode for another gain stage somewhere Else

[mozz]

Never heard of pentodes needing DC on the heaters. Millions of radios didn't need it. As to the 6ac7, let me know if you get it working decent for guitar , gonna bet 90% of them are gonna be microphonic.

[eschertron]

Hi eschertron, thanks for your thoughts. The maximum grid leak stated on the data sheet for 6v6 is 500k, so I figured that together with a 1m volume pot, the 1m grid leak resistor is ok, as there is a parallel path to ground. Is my logic off?

I did not look at a datasheet, which is why I suggested that you do so. IIRC I've seen 220k..250k on the grid. That's what raised my question.

I'd use a program such as Duncan's tone stack calculator http://www.duncanamps.com/tsc/ to see what attenuation to expect from the network. Then compare that to the gain in the input. [aside] I didn't see that you have the input bootstrapped. Cool. Do you get another 6dB gain out of that? [/aside] What bias do you expect to generate on the preamp pentode? That'll determine the nominal input headroom (Vpp = 2x the bias) at the pentode. To slam it, you'll want 2x, 4x ... ?x more signal at the grid. That's the strong signal goal for that stage.

I have used a MosFet follower for the same reason. If you don't want a CF for tone, a fet is a good option IMHO.

[eschertron]

Never heard of pentodes needing DC on the heaters. Millions of radios didn't need it. As to the 6ac7, let me know if you get it working decent for guitar , gonna bet 90% of them are gonna be microphonic.

I haven't even built a test jig yet. But the stated function is for RF amplification, as mentioned in the executive summary, and they warn about heater noise at audio. 6AC7

[mozz]

Maybe that particular one pentode needs dc on the heaters, i thought you were generalizing. More trouble than it's worth to use a 6ac7. Other metal tubes are much more friendly, 6sj7, 6sq7, 6j7, 6sc7.

[eschertron]

Maybe that particular one pentode needs dc on the heaters, i thought you were generalizing. More trouble than it's worth to use a 6ac7. Other metal tubes are much more friendly, 6sj7, 6sq7, 6j7, 6sc7.

You are probably right about "more trouble.." I picked up a few sleeves of these things (in what looks like the original packaging) at a swap meet for 40 cents each. If they turn out to be good for nothing at audio freqs then I learned something.. even if it's not what I wanted to learn