Triode Pentode common anode mixer - Does this make any sense what so ever?

[G._Hoffman]

Does this make any sense?  I'm wanting to add a pentode input stage to a Dumble style amp, but want to have a triode stage available as well.  This is just a quick sketch - just a concept I'm trying to work out in my head, so please excuse the crudity of the model and the values are not to scale, as it were. 
I am torn between using two separate tubes or using a 6U8A or similar. 


Gabriel

[sluckey]

Needs work. As is, you have B+ tied to the tonestack.

[G._Hoffman]

Needs work. As is, you have B+ tied to the tonestack.

So I did.  That's an easy fix, however.  I'm more concerned about the basic idea.  Sharing the plate resistor between the two tubes, basically.

[G._Hoffman]

Looking more into the 6U8A, I'm liking the idea of using a 12AX7 and a EF806.  I can't THINK of an electronic reason I couldn't tie the plates together, but then, I'm not that knowledgeable about the subject.

[sluckey]

No reason this can't work. But it may take a bit of experimenting and tweaking. Would be a great project to put on a breadboard. Go for it. And let us know what works or doesn't work.  I'm listening.   

PS... Pentodes don't drive a TMB tonestack very well, so they say.

[G._Hoffman]

No reason this can't work. But it may take a bit of experimenting and tweaking. Would be a great project to put on a breadboard. Go for it. And let us know what works or doesn't work.  I'm listening.   

PS... Pentodes don't drive a TMB tonestack very well, so they say.

It just feels like dropping in a cathode follower or other tone stack driver would be a bigger change to the design than I'm after.  But it's a possibility.  That would simplify things - a couple mix resistors, and a cathode follower - pretty straightforward.  I could also just have a couple channels, which is a real possibility.  But if I can keep the parts count down, I would kinda like that.

Gabriel

[tubeswell]

Here's another idea using the same amount of tubes. Triode-pentode morph control with the CF sourcing the 'triode mode' as well as driving the following load (be it a tone stack or whatever) with a low impedance buffer

[Merlin]

No reason this can't work. But it may take a bit of experimenting and tweaking.

+1. It will work, but be prepared for tweaking.

An alternative option is two run the two tubes each with the own plater resistors and each with their own output coupling caps, joining together at the tone stack. That way you have a common-anode mixer from an AC point of view, but the DC bias conditions stay independent, which would make tweaking less frustrating!

[tubenit]

I've used 5879 and other pentodes in Dumble inspired builds both in V1 position and more often in the OD section.


Here is how I would approach your idea of a triode/pentode switching in V1. 


Having said that, I tried something similar to this in a HoSo56 build and was disappointed in it and removed the triode leaving just the pentode in V1. 


With respect, Tubenit

[tubenit]

And IF you're just looking for interesting ways to incorporate a pentode in a Dumblish inspired build, here are some other schematics to look at for ideas.  Except for the one that says DRAFT idea, I have built the other 3.  I drew up the DRAFT idea for you (& others) to look at this morning.


The Tweed BluesMeister OD Lite is one of my 3 current builds, and I love it.  It's more of a clean to boost than a clean to moderately overdriven tone.


The D'Mars ODS remains one of the most fun builds I have done. I felt like it had excellent contrast between truly clean and truly overdriven tones.  I do have some sound clips of it.       https://soundclick.com/share.cfm?id=11842172


As an FYI,  I can NOT tell any difference in tone using a mosfet in a CF position vs. a triode.   


with respect, Tubenit

[tubenit]

Here's another one I built with a 5879 in the clean and OD channel.  It was very chimey sounding and had a grind and sustain to it that was sorta Santana-ish in my thinking. It did not have much of a true clear clean tone like a Fender AB763 would.


I had fun with it for a while and then rebuilt it as I'm leaning more towards clean tones nowadays.



Tubenit

[G._Hoffman]

Thanks all for the suggestions.  Plenty to think about.
Gabirel

[acheld]

Question regarding the trim pot just prior to the OD stage in each of the builds:

To what level do you adjust its' value?  In the builds where I've used it, I've adjusted it so that the signal entering the OD channel is clean, but its' voltage remains highish.   I've assumed this is needed to prevent grid current limiting.   Is this correct?

[tubenit]

To what level do you adjust its' value?  In the builds where I've used it, I've adjusted it so that the signal entering the OD channel is clean, but its' voltage remains highish.   I've assumed this is needed to prevent grid current limiting.   Is this correct?

Trial and error and what sounds good to me.  I lean towards a cleaner OD tone that doesn't have alot of grit/grind to it.


with respect, Tubenit