SE EL34 Schematic - What do you think?

[Cassiel]

Somebody asked me if this one was good, I replied that the second triode looks like it's there for some kind of local feedback but I really don't understand how it works so I might be wrong. He's buying the iron so it's not too late to change to another schema if you guys think this one isn't good enough. All I know is that I wouldn't use the 12at7.

[DF96]

Some sort of guitar amp with variable feedback? Changing valves would affect loop gain so might need redesign.

[Cassiel]

Poor detective work DF96.

Next time you see a circuit with that particular look and with the word Tango written all over it you must know it comes from the Far East (likely Japan) and it's no guitar amp.

[DF96]

Sorry, never heard of Tango (apart from the orange fizzy drink).

I can't see the point of variable feedback, except for an FX box or instrument amp. Maybe I have misread the circuit, but it looks like you can have a small amount of negative feedback via the second triode anode (when pot at ground end) or lots of positive feedback when the pot is turned up.

[Cassiel]

Yes, the variable feedback doesn't look too hi-fi. Interesting circuit nevertheless.

[kevinkr]

This is a very odd circuit to be sure, unfortunate that the schematic does not come with a translation of the text that probably came with it originally. I am sure there is some sort of rationale for this design.

The second half of the 12AT7 is essentially in parallel to the grid and plate of the EL34 and probably contributes something to the overall output. I am not sure about the phase of any feedback that is occurring in the common portion of the 12AT7 cathode circuit - but it actually appears to be positive under some conditions. I could be very wrong about this as I don't really understand the function of the pot., at minimum setting I guess the cathode current would be dominated by the signal impressed on the plate by the output tube and that could result in negative feedback.. I would suspect this control affects both gain and damping factor to some degree. I'm pretty clueless, and won't have time to model it in spice for the forseeable future.

It does strike me as a rather interesting and odd design, and if I were to build it at all I would build it exactly as designed even down to using a 12AT7 as specified.

The other and safer option would be to build a discrete version of tubelab's simple SE design which incidentally also uses a 12AT7 last time I checked.

[dhaen]

This appears to be an experimental circuit for playing with feedback and output impedance to gauge the results.
Here are some referring pages from Google translate:
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[Miles Prower]

Originally Posted by DF96

Sorry, never heard of Tango (apart from the orange fizzy drink).

They make OPTs.

I can't see the point of variable feedback...

Two words: listener preference. I include variable gNFB in my designs. It does make a difference, depending on the style of music, and source quality. It works like Doug Self's proposed "Niceness" control.

With gNFB variable between none and 13dbv, with all the NFB dialed in, you're definitely headed towards a "solid statey" sound. With 20dbv, I can't tell the difference between hollow state and solid state. Been there; done that, and changed it pretty quickly.

Classical, soft rock (Carol King, Karen Carpenter) sounds better with lots of NFB, but hard rock, power metal, techno sounds subdued. Lesser gNFB adds the edge needed to bring this type pf music to life. Some source material such as inferior CDs made in the late 1980s also tend to sound better with more gNFB.