Impleminting global negative feedback on an input pentode screen resistor

[Williamblake]

Howdie!

So i am building a practice amp that is too loud to run in my apartment but what can you do (build a speaker attenuator) and at the stage i am at i would really appreciate input on implementing global negative feedback. I took the Vox AC4 and the parts i have and build an amp with EL95 output without any screen or cathode bypass capacitors, yet, and it runs fine, has no hum (yeah!), keeps me playing (aka sounds good but too basy) and has all the right voltages, too (or so i think). It does not yet have screen or cathode bypass and it does not have negative feedback either, yet. But i would like a knob to turn this amp from clean to mean and a knob to control high end rolloff as well. Lacking screen or bypass capacitors it has a ton of negative feedback already but i would like to get this on a dial. I got the ideas from this forum (hail to all Vox AC4 threads) and

http://www.valveradio.net/audio/t-bridge-tone-control.html

and the screen resistor feedback picture attached. I would really appreciate some input to where to start with the feedback loop values and anything else, too, of course. I put some question marks on parts not yet imlemented. Probably in all the wrong places.

So the question-marks components are not in the circuit i tried out, or to be more frankly, anything that is screen- or cathode-bypass or nfb related is not in the circuit, yet. But it works and sounds like an amp. The amp as it is behaves nicely to the guitar volume control and breaks up when to loud.

Thanks so far, this forum and it`s members tought me a lot. Sorry to order european, though.

Regards
Jens.

[Latole]

Tu parle français ?

[Williamblake]

Tu parle français ?

J'amarais.

[kagliostro]

... and a knob to control high end ...

Few days ago I was asking something similar

https://el34world.com/Forum/index.php?topic=25975.msg282503#msg282503

Unfortunately my amp use triodes as preamp section, not pentode, and, as PRR say, the kind of Tone Control I was considering didn't work well with triodes, but you have a pentode as preamp tube

---

Looking more close to your schematic you use a 6N2P tube on which a CF is used to drive the power tube, why don't insert there (between CF and Power Tube) a common Tone Control like those we see in many amps ?

Franco

[Williamblake]

All i really want from a tone control is to have the amp not sounding too dull but without earbleeding, too. So having a tone control that lets you select the corner frequency sounds like a perfect fit. In a feedback loop this might be even more powerfull. I also like the Boneray/Merlin tone control. But this started as a no tonecontrol amp and instead of being satisfied i wondered what this nfb can do. And i thought i could cut corners and have somebody throw in some values to start with. I will alligatorclip this out, couldnt even find a radio that uses this kind of feedback. There seem to be quite a lot of transformers with a dedicated feedback wiring,  though. I am still wondering where they are coming from.

[Williamblake]

Yes, the way your treble control works got me starded, controlling screen bypass. And on the other hand controlling nfb bypass... I thought this  could make a dial from clean to mean. Also i cant sleep with the secondary being wasted.

// ... i failed to see your tone control is at the output stage, dont know if screen bypass is affecting gain there, too. Dont understand it but surely will keep the picture, thanks.

[Williamblake]

So i added a pilot light and tried out the nfb-loop like attached and it gives a huge decrease in volume. No reason to drill holes for pots so far but it seems to apply feedback. The amp at the moment sounds not bad if played clean but beyond that it`s a brick to the head. So i will add some bypass capacitors play with the nfb and see where this takes me.

[Williamblake]

So i took the values from the radio tone control and put in bypass caps and the controls behave like expected only it doesnt`t go from clean to mean but from mean to shred. I have to turn the guitar to three or something to get some clean sounds. If you turn the negative feedback down the tone control obviously has no effect at all. Thought i wouldn`t have to attenuate the output that much but this thing can get really loud.

[Williamblake]

So in a sober moment i ditched that whole nfb-screen-resistor subject as it would probably not be what i wanted and too far above my head anyway and went back to what i wanted this amp to become. A simple amp with decent overdrive to sound good good and to learn from. Learning how to get the gain stages set right that would be. I also backed from the idea of not having any knobs at all and now i have a gain and a tone control. I think i also learned that going to a 3W output tube does still not give you a bedroom amp to crank without speaker attenuation and will be looking on ebay for cheap EL91s thinking that this will probably still be too loud but could be a way of using cheap OTs from radios with the 8Ohm Celestion Speaker i have. This and all the information on the the interwebs is more than enough to get me sidetracked but anyways.

To cut a too long story longer:  I put a tone contol after the cathode follower as suggested and will go on from there. I do not have 1Meg-linear pots now so i didn`t take the tone control from the book but from a schematic i downloaded somewhere and happily used before.

[kagliostro]

... will be looking on ebay for cheap EL91s thinking that this will probably still be too loud ...

An alternative can be the 6AU6 Tube



Franco

[Williamblake]

After trying out some tone controls that didn`t work i took the top-boost circuit and will call it a day. Sorry bout the bad timing but you get the idea.

[Williamblake]

So i played this and changed this a bit, tried some coupling capacitors after V1 and sticked to the original 5nF but changed the tone control to not give so much base below ca 100Hz. The base control now behaves a little strange but i think in a usefull way. I guess my ES335 copy with Schaller Golden 50s has a lot of base and i have base control (CR) on the guitar, it is still more than enough base available. I did want to add more brightness for some pickup combinations that are a little dull but sound better bright (north and south of each coil in series which i liked a lot with a different amp) so i put in the "Merlin bootstrapping cap" and tried to give this some brightness boost this way which i think works but i have only tested this with a speaker attenuator so i might be off here, might even be this was all in my head and changing this cap has a different impact on sound than i think it does but i went with 8nF. I also played with the treble cap and will increase this to 80pF so it passes some "lower highs" too and also lets this be a more versatile treble control, it does not have too many good seettings across it`s range with 50pF.
After all this does not seem to be an amp for playing single notes but to play clean or distorted strumming preferably with humbuckers.

[Williamblake]

This is what it looks light now. I do like the tone control and the second gain stage. I had read you would use a 820 Ohm cathode resistor there and i did that and changend it to have V2 and V3 breakup at the same point and i dont know if this really happened but it sounds fine with me.

[Bieworm]

This is what it looks light now. I do like the tone control and the second gain stage. I had read you would use a 820 Ohm cathode resistor there and i did that and changend it to have V2 and V3 breakup at the same point and i dont know if this really happened but it sounds fine with me.

Always love seeing people having some crazy ideas!!! Great!!!

[Williamblake]

The tone control really has a broad and useable range avoiding tlo much bass and the amp responds to guitar tone control nicely.

[Williamblake]

Tried to give this some late eighties vibe.

[PRR]

If you remember the 1960s, you probably weren't there... but you can *still* buy Dymo Emboss label stuff.

https://www.amazon.com/s?k=Dymo+Embossing+Label+Maker

[Williamblake]

But the tape needs a clean surface to stick to.

[Williamblake]

Looking into the amp and verifying the schematic i noticed i did put an 820R resistor on the second gain stage's cathode.

I was checking back on this after is was breadboarding something else noticing 1k5 gave little gain and sounded like a bad distortion pedal. I was pondering on that resistor quite a while and i was very happy to be able to verify a statement made by PRR anything but 820R would make no sense. So a lower bias resistor allows for more headroom and dynamic? Because it is about the possible voltage swing between the cathode followers grid and plate?

[Williamblake]

I built another amp trying to stick to this schematic but this time adding the feedback tone control. I still have to completely ruin the faceplate other than that I am pretty happy with it.

[Williamblake]

pictures & shematic

[kagliostro]

To fix those small scratches on the faceplate you can take two different paths

One, more difficult, to make the faceplate mirror again using this type of products



An easier one is to make the faceplate frosted using a 3M ScotchBrite wheel (or similar)



Franco