Hello I am seeking a bit of help.
I came across this schematic for a amplifier utilising sub-miniture tubes and plan to omit the power amp stage and have a pretty cool tube preamp in a box.

So the problem: I don't understand tubes all to well, I'm pretty confident I can follow a recipe but as is working with higher voltage I don't fancy taking risks playing around with things.

My line of thought is that I cut out the last two tubes and connect the output of the preamp to the transformer, wrong right I have no idea, I see the output transformer connect to the high voltage by the center tap is that right? Also what kinda voltage would I expect to get from the secondary coil because my aim is to use this as a normal preamp pedal.

Quote from: Blurry blur on October 23, 2024, 06:47:45 PMHello I am seeking a bit of help.
I came across this schematic for a amplifier utilising sub-miniture tubes and plan to omit the power amp stage and have a pretty cool tube preamp in a box.

So the problem: I don't understand tubes all to well, I'm pretty confident I can follow a recipe but as is working with higher voltage I don't fancy taking risks playing around with things.

My line of thought is that I cut out the last two tubes and connect the output of the preamp to the transformer, wrong right I have no idea, I see the output transformer connect to the high voltage by the center tap is that right? Also what kinda voltage would I expect to get from the secondary coil because my aim is to use this as a normal preamp pedal.

AFAIK, tubes are sorta, kinda like JFETs. Triodes at least. Grid is like gate, usually at ground potential, cathode is source and plate/anode is drain¹. You can sorta, kinda turn a tube circuit into a JFET circuit (different voltages, different behaviour but similar logic).

¹: I *hate* that conventional current is the other way round. This always messes with my head when I have to remember what is the anode and what is the cathode.

Yes I know I could make a simpler jfet version as many others have but my aim is to have real tubes in play. Upon further inspection of other tube preamp it seems they do not have output tranformed and just take the out from the cathode of the last tube section, would that be correct for this circuit also?

Quote from: Blurry blur on October 24, 2024, 07:03:10 AMYes I know I could make a simpler jfet version as many others have but my aim is to have real tubes in play. Upon further inspection of other tube preamp it seems they do not have output tranformed and just take the out from the cathode of the last tube section, would that be correct for this circuit also?

I wasn't suggesting you convert the circuit to JFETs. You have to rework the values and it won't sound the same anyway. That said, you don't need an output transformer for preamps, you only need one to convey actual power (both current and voltage) into a load (impedance matching), but preamps (and solid state power amplifiers) are impedance bridged, not matched. You will need a high-impedance input power amplifier after it, but that is not a big deal. Since the amp is at least two dual triodes anyway (I mean, that's what makes sense), add in a cathode follower at the end so that you can plug the preamp into a 10kohm input impedance.

By the way, I would have suggested you tried out a Nutube (basically a low voltage dual triode based on a vacuum fluorescent display, and you can drive one with a 12V power supply - maximum anode voltage is stated at 80V, although the datasheet then shows curves with anode voltage up to 200V?!), but they make *no* sense at the price they are sold at. As well as having very little gain, so you basically need to pair it with a solid state device.

Thank you very much that helps alot. I'll be sure to check out nu-tubes but the "low gain" factor doesn't seem very appealing

The low gain is not the biggest issue (although it is much lower than most or all tubes: the amplification factor is 14 compared to 70 for one of the two subminiature tubes you have in your posted design, and 100 for a regular 12AX7/ECC83. Transconductance for the Nutube is stated as dimensionless "54"). That basically means that you won't be able to use it the same way you would use the usual tube, and it is essentially just a fancy waveshaper. You stick it in series with another amplifier and it distorts ("tube tone"). That's it. Or maybe you use one dual triode in sort of a Darlington configuration to replace one single triode. That would be fine, if stupid (but, to paraphrase Lev Trotsky, "everyone has the right to be stupid on occasion"), if they didn't cost like €30 *each* (if you buy them as a couple). Stupid quickly becomes stupid *and* unforgivable.

¹ : "...but some abuse the privilege".

So just to be clear if I take an output from the cathode of the last 6021 section put a coupling cap in-between that and the actual ourput then ground the drain in the same way as the rest of the tubes it should work as a preamp pedal? I'm planning to run it into my effects loop return of my amp

By doing that, deleting C9 and making R14 equal to R15, you turn it into a phase splitter (like V2b)..

P.S.
V1b "Drain" (grid) is already grounded via R13 & VR1..

V2b is a phase splitter. You don't need a phase splitter. Wire V2b as a cathode follower (like a source follower... a buffer).

https://www.valvewizard.co.uk/accf.html

Just so you know, the overall output is phase inverted with respect to the input. It doesn't matter, unless you plan to mix the preamp's output with another preamp which doesn't phase invert.

Once I found out where I could get miniature radio tubes for amplification and thought that it was a great success.
But I was told in one forum specialized in tubes that with these small tubes the harness is even more complicated and larger, and there is nothing advantageous in them.
I don't know if this is true, but my interest in this then disappeared)).

What I ment was do I do the same for Vb2's drain as is on Vb1 with r22 c15?

No, just... follow the link, will ya? 

https://www.valvewizard.co.uk/accf.html

I mean... V1a is an amplifier with gain. (You have the volume pot here, with a parallel cap which will work out to an attenuating bass shelf, which is often what you want in a guitar amp). V2a is another amplifier with (less) gain. V1b is yet another amplifier with gain (but you need this to recover gain loss due to the passive tonestack). Two pots for bass and treble. (Are you sure about that Baxandall tonestack? It's a good design but not necessarily functional for guitar). You don't need more gain, you need low output impedance. So V2b as a cathode follower is best.

Ok thank you I'll do my homework now lol

What do you think?

How did you choose those values? (They're wrong  ).

Quote from: fryingpan on October 24, 2024, 01:24:30 PMHow did you choose those values? (They're wrong  ).

Wrong how??

It IS over-complicated-- direct coupling can work.

From another circuit 
But if I used the valve wizard article to do the math could this work? And would a grid stopper be needed?