Really, it made me want to build a new real amp, but brokeness in the too muchs foar that.

So instead, i decided to do some things to ye olde AC20 that i have been thinking aboot for a while now.

A refresher: The map is basically AC30 preamp, with various matchless and trainwreck things done to anger up its blood a bit. It also runs 6V6s instead of EL84s, because well, fuck EL84s, that's why. ACTUALLY, because of the transformers i used, it provides agreeable voltages and OT primary values of such a type that 6L6s are a straight up drop in. I should probably check the bias when i do that, but they sound good and don't redplate, so fuck it. Indeed, currently i have some old USA 6L6s that i found in my junktube pile. I think they were used, but suspect that they were only lightly used in one of my grandfather's hi-fi setups or perhaps some test equipment. In any case they sound nice and seem to offer a bit more gumption than the 6V6s i had in there, which i suspect might have begun to tire after the last three or so years of heavy abuse.

Right so, mostly voxtchless in topology. Anyhow, today i removed the following not-useful-to-me bits of it:

1. The Bright switch. Only really does anything at less than halfway on the volume/gain. Who the fuck would run a 15-20w voxy amp at less than halfway? No one, that is who. Simply not useful on this amp. This inefficiency is unacceptable and the switch has been removed.

2. The Cut control. Listen, i understand that Vox/matchless are a bit on the edgey side for lots of folks. But to me, they are just exactly the right amount of edgy. I would mess around a little with running the cut a little dark, but always always, would wind up with it max-bright in the end. It just sounds so much better to me for cleans, and so much more lively to me for dirts. So now it is gone too.

Interestingly, i am also running the matchless/trainwreck style master volume on this amp, a control which has notorious problems at the lower end of its spectrum. What is interesting rather, is that upon removal of the cut control, this MV seems to work a lot better? I will have to test it out louder through my normal loud cab to see for sure, but it used to be terribly and krunkly at anything less than 9:00 before, but now it doesn't get shitty until just before or at zero? Very interesting.

Anyhow, rather than leave this big hole in the front of the thing, i decided to add something back in:

3. A contour/shape/slope control. Basically this is nothing new, just a control in the place of the slope resistor of the tone stack. As examples, while Vox, Fender, Marshall, Hiwatt, Soldano, etc, pretty much all generally use in some amps or others, the same or almost the same tone stack design, and even many of the same values, there is some variability in what folks use for the slope resistors. Most Vox are 100k, as are some Hiwatt, some Fender, etc. Marshalls are generally 33k, 47k or 56k, as are some Fenders and some Hiwatts. Roughly, in conjunction with the rest of the tonestack the slope resistor sets kind of the center frequency of the midrange, sort of.

In any case, adjustable ones are popular in olde schoole marshalle mods and whatnot. And a proper and logical value to use is the 250k pot that used to be my Cut control. Thus, efficiency and convenience collided and so it came to pass that now i have a contour control in my map. I always meant to play with this circuit , but for some reason just haven't done it yet until now.

It is another one that i will have to play more with through my normal loud cab, but my initial impressions are 100% positive. At the minimum setting, it bumps the low end and mids a bunch, and with them, the gain gets a considerable boost. At the max setting, it scoops the mids big time, and with their removal goes a lot of distortion; it gets cleaner and scoopier, in a generally pleasant fashion. Basically, in between the two extremes, are tons of useful bits. About 11:00-1:00, is the general marshall values region, about 2:00 is normal/Vox. It all seems very useful, actually. I should have done this a long time ago.

Anyhow, next up, may very well be addressing the size of the headcab this thing is in. About two inches too wide and too deep, as well as about three inches too tall, it has struck me nearly from Day One, as a potentially unacceptable inefficiency. If i can find the correct flavour of scrap woods in my basement, i will aim to rectify it, once and for all.

I really have nothing to input, besides: This was interesting to read and I hope to hear some clips.

Sounds like a fun little tinker.
Cool thing I'll be trying soon in a similar(ish) circuit is adding a pentode/triode/ultralinear control to the EF86 for lots of flavours.

I am religiously opposed to the EF86, as i have become opposed to the EL84, for their fragility.

It is unfortunate, because i think they sound swell. Especially the EF86s.

Oh, one thing i forgot to mention about the amp, is that i am running a power MOSFET instead of a 12AX7 in the cathode follower. This was a perfect solution for the tiny chassis i have to work with, and the generally non-clipping nature of the CF in this circuit. Tiny little solid state, struck me as much more efficient than having a third preamp tube in there to do the job. I mean, i wouldn't have minded the extra gainstage for more fuzz porpoises, but i simply don't have the space for the supporting circuitry or the tube socket to do it. When i first built the amp, i simply left the CF out. It was okay, but adding the mosfet in later was vastly sonically superior, made a massive difference in how the tone stack functioned and how much distortion was no longer lost there. And i really enjoy an amp that makes as much noise as this one does, while only having two preamp tubes and two power tubes. Fantastically efficient, which if one had not already guessed, is this amp's word for the day, and my general guiding concept for it. [at some point, building a similar thing with a SE KT88 and something along the lines of a cathodyne phase inverter might be an interesting game. (maybe using one of those triple triode tubes like the old soldano hot mods had.)]

I may do some quickclips a bit later, though, i would expect them to be only a little less interesting than the writeup, particularly in the absence of "before" clips. And well, because two thirds of what i did was removing things that were inaudible anyhow.

this sound muchly like this to me:

أن يمارس الجنس مع هذا أن هذا الرجل يقول

But i agree it is swell

After spending some more time with it, i think that the contour control may very well be more useful to me than any fender/marshall/etc. type Mids control. In fact, i probably should have wired it backwards and just called it that.

I dont think the EF86 is inherently bad, only that people try and squeeze as much gain out of it which is dumb since its thrown away in an attenuation network anyway. I do want to dick with one at low gain but I've since decided that it doesn't fit with my current plan

They are notoriously difficult to get in a non-microphonic state, and they are exceedingly prone towards going into the same from even the vibrations of low wattage amps.

If you are going to run them, i suggest you go for the silicone/rubber vibration-isolated shockmount tube sockets, and then also use those damper rings on the tubes themselves.

A good example of what EF86s require to provide whatever reliability they are capable of, is to look at how the 65 Amps and Gabriel amps guys do in their maps. Even with that stuff, the tubes will go microphonic, just not quite as fast.

My own personal vandetta against them and the EL84s, is that i don't want to be replacing ANY tube in less than a year. Most people are willing to do that for the sound the tubes make, but i am not at all that specifically attached to those sounds.

It sounds it, from the description. I'd be interested in hearing clips of the thing in action with only the contour control being tweaked between settings to hear just how broad of a range the thing has.

Lately, i have been toying with the idea of removing all of the tone controls entirely.

I have a tone shift switch that in one position nearly removes the circuit from the amp, if the controls are zeroed. That is pretty much how i have been using it for the last week or so.

I really think that for this amp anyhow, i might just make it two knobs, Gain and Master Volume.

I have also been thinking that i want to build another amp sometime in the relatively near future. But aside from vaguely wanting to build a higher gainer, i don't really have any need to build one. I wonder if maybe i really just want a new speaker cabinet flavor to play with...

I like the notion of an amp with no tone controls at all, just: volume, dirt.

While the notion of something with just an on/off switch appeals as well, it's more as a middle finger than a practical object.

Yarr, i will tell you what though, i could probably get away with just four settings, so also removing the volume/gain knobs could *almost* work for me.

1. Low Volume(like bedroom), Low Gain(like 10:00)
2. Low Volume, High Gain(like dimed)
3. High Volume(like dimed), Low Gain(like 10:00)
4. High Volume, High Gain.

Sooo... In theory, i could replace the controls with two switches. Though, i don't know if really i would feel i was gaining much by the switch.

On the other hand, this amp is pretty good with the guitar volume gain reduction, so i suppose, i could just leave the gain maxxed all the time(i.e. removed) and just have a master volume only, maybe mount it somewhere odd on the amp, like on the back, or inside somewhere, just to allow some volume attenuation where required.



I will have to actually think about that, because it would actually come really close to working for my setup.

The only wrench in the works i could foresee, would be if i maybe wanted to run the thing cleanish with some pedals or something, then i might wish i had the gain knob there...

I do really like the switch idea. Clean/dirty; quiet/loud.

It would be fucking sweet if i could rehouse the whole thing in a dull grey metal box looking something like this:



Illuminated buttons and all.

Or indeed, like this:

This thread is now about things that amps should look like.