Radiotron 4 Treble Tone Control - I don't understand how it works in detail

[kagliostro]

I've seen an amp schematic due to our friend TIMBO that uses an intriguing Treble Tone Control



To me that Tone Control is interesting because, with the help of a pair of friends, I'm planning a small amp (2W power - only one tube - Compactron 6AF11 tube - 2 triodes plus 1 pentode - triode 1 gain 68 - triode 2 gain 41)

As the triodes of this tube has not a lot of gain I don't want to use a standard ToneStack to avoid signal loss; I've seen the Treble Control used by our friend TIMBO and after a bit of research I've find that the circuit comes from Radiotron 4, I think that in that position the loss in signal is very low respect the action of the circuit

I tried to understand how that Tone Control acts, but if to me is way more easy to undertand a Tone Control placed between Power Tube and OT like this that rensamble the VOX Treble Cut

Radiotron 4 Fig 15.31 Pag 655


---

I only can undertand the principle of functioning of the following Tone Control (a filter send back to the circuit a part of the frequencies in an NFB loop, this cuts or increase the content of those frequencies on the output), but I'm not able to understand how the filter acts in detail, the fact that one extremitie of the filter is connected to the plate is not difficult to be understand, why the opposite side of the filter is connected to G2 and also the reason of that voltage divider are a mistery to me 

Radiotron 4 Fig 15.35B Pag 698


Can someone give me help on understanding ?

Many Thanks

Franco

[DummyLoad]

tone control works the way any other does. note that load requirements of the OT dictate RC vaules... knowing and calculating for the source resistance is paramount in making the treble bleed TS work properly. use a more conventional TS in the preamp section... just my 2cents.

since you mentioned the 6AF11 - i was doodling early on in our quarantine because was terrified of getting COVD19, so stayed home for just under 2 weeks straight. i came up with the attached.

it's a fender tweed-ish circuit. have no idea if it'll work as i have not breadboarded or built the circuit into a chassis. presume that it will somewhat, but suspect that it will howl and squeal like a pig being butchered. have not proofed the chassis wiring - lost interest. odd pinning of the 6AF11 elements don't make for very audio friendly layout.   

--pete   

[kagliostro]

Ciao Pete

In the Radiotron there is wrote that the Tone Control is planned in a 5K OT primary load

My 6AF11 will have a load of around 6K, so I'm not very far from there, but as you say, may be the values of the Tone Control components must be tried

--

Thanks for sharing your nice PP job, some time ago there was someone who tried a PP amp based on the same tube, but if I'm not wrong, the results were not optimal

https://el34world.com/Forum/index.php?topic=15489.msg149480#msg149480



I think your plan will work, way less gain involved than in the upper schematic

Thanks also for the .vds file

--

About layout, for my SE with 6AF11 I've draw 3 different versions (just going to finish the third) for the same schematic and was my intention to submit them here on the forum as to have opinions about

Franco

[PRR]

> why the opposite side of the filter is connected to G2 and also the reason of that voltage divider are a mistery to me

Not "connected to G2", but connected to AC Ground (a well-bypassed B+). Ignore pot and capacitors. A fraction of the plate swing is fed-back to 6V6 grid via the 220k plate resistor. It is important that the driver is a pentode; a triode woul absorb most of the feedback. With just the divider we have flat feedback at all frequencies. By adding small capacitors we can change the feedback and gain at high frequencies.

[kagliostro]

Thanks PRR

Now I understand more about that arrangement

It is important that the driver is a pentode; a triode woul absorb most of the feedback.

Oh, so it is inusable on my amp where the driver is a triode 

What about a reduction in value of the 220K (.25MΩ) resistor as to increase the NFB level ?

--

Or .... I can try to have a different approach, may be the Fig 15.31 Tone Control ?

Radiotron 4 Fig 15.31 Pag 655


Previously I was thinking to use something like that but between the Driver and the Power Tube, didn't know it was possible to have it connected between the Power Tube and the OT

Franco

[PRR]

I would just copy the Champ. The AA Champ is two triodes with tone and volume between. You can extend it to Bass-Mid-Treb to have many knobs on a small amp. With the lower-gain triodes it will not be a "sensitive roof-lifter" but may play well.

[kagliostro]

Ciao PRR

I'm still wondering what to do about Tone Controls for that amp ......

Franco