Non-CF Clipper?

[sepulchre]

Okay, I was busy this weekend. I got the new faceplate: 'Volume', 'Tone', 'Drive'. Drive is the pot between stages one & 2, Volume is between stage 2 and the PI driver. As you can see by the schematic below I have reverted to the Blues Deluxe style tone knob with a slight mod (33K on the ground leg). It sounds good, but more about it later.

I played with the values of the V divider after stage 1 because frankly it doesn't really control the "Drive", it just acts like another volume control. Sure, there's a little more distortion when it's cranked but then the same is true, perhaps even more so, with the "Volume" control. In fact the Volume control Really takes it into distortion. So I'm wondering if the tone control is sucking the juice from stage one. Maybe it would be better placed after stage two. Maybe then the "Volume" would act more like its namesake and so would the "Drive".

As it is, the Platinum / Blonde switch does almost nothing - only a tiny boost in the signal. Unless I can cure this it will become a bright switch.

Btw, I got new 500A pots in and installed them. CTS; they seem hardier than the Alphas.

[matt h]

Yes, the "Drive" is just another "volume" control! I mean, in the same way the "volume" on a tweed deluxe (not blues deluxe. ahem.) is a volume pot. Your volume is more like, uh, a master volume. Really. They'll both work like volume controls, depending on how the other one is set.


The platinum/blonde switch wouldn't do much as it's in there. At most it's a 47k/270k voltage divider. So that's sorta like lopping 1/6th off the drive pot's signal. Sure, it might make the difference taking something off the edge, but it's not going to be a night/day difference. Didja try the other *cough cough touches nose to finger* thing we talked about?

This may seem elementary, but for cleans, max your "volume" and just barely crack open the "drive".

The c2/c2 confusion will make this comment awkward, but are the values reversed? Generally, you want the smaller value first in your chain. So of the three coupling caps, the current order in the schem is .01/.0032/0022. I'd expect .0022/.0032/.01 if anything. (progressively letting more bass through so it doesn't fart out in the first stages)

Are you generally happy with the bass response? I ask because the 10uF Ck on the power tubes seems tiny--but I also have been known to really like tiny in that position, so if you've tried some stuff and settled there, I get it... just want to ask to be sure. Putting that cap on your switch would be an easy (almost) half-power switch.

[sepulchre]

Yes, not "Blues" deluxe. Sorry - brain fart.

The switch goes between 47K/270K and 47K/4.7M so I would have thought there would be a bit more difference than it does. No, I have not tried other things we discussed (split load, right?) but I will as this is clearly not working. My hope was that I could get 'Drive' to overdrive stage two and thus control, well, overdrive. But I suppose I've gone about it all wrong.

Oh yeah, sorry for not fixing the C2 labels; I just quickly updated some values and put the latest voltages on the schematic. I should takes some time to clean it up. Now that I'm back at work I can do that. Time at home to work on the amp is at an extremely high premium so I leave the documentation for later.

I really don't know what I was thinking with the coupling values. I suppose they are reversed. What I have seen often is the same value throughout the signal path. So I'm considering .0022 for at least the first couple of stages.

The Ck on the power stage values is descendant from the original U45b design. Yeah, it's got plenty of bass. no problem there. But I don't want to put it on a switch. I don't want a half power switch, I want a switch that yields more/less distortion. If that can't be done then I'll make it a bright switch.

Right now it could sound worse but it's not really where I want it. I realize that this is a very simple circuit and room for variation is limited. It's very loud for the size, and acts a little like a Wreck in the sense that the Volume control goes from off to Loud in the first few degrees of turn. And from there it gets more and more distorted. The guitar's volume clean it up nicely and of course the drive control has its effect - just not what I'd like.

So, what do you think of moving the tone? Futile? Trouble for nothing? All the amps I've built before use a CF to push the TS later in the chain. No CF here but it seems like 1) later in the chain could use some taming, and 2) earlier could conversely use less drain.

[matt h]

As it is wired, the switch doesn't so much switch between 270k and 4.7M. Basically, you can think of the 4.7M as there only as a default/failure protection.

you're really switching between a 47k grid stopper by itself, vs a 47k/270k voltage divider (which, as stated before, is minimal attenuation).


If you want clean and dirt at the same volume level with the flip of a switch, this is *not* the amp topology to use. (you'd want to use something like the dumble ODS circuit--i can't think of an efficient way to do it using fewer than four triodes unless you want a really really limited clean and a very, very pre-set sounding overdrive)... Unless you're willing to build it a lot like a wreck and use the volume control on the guitar for your clean.

Generally speaking, "all the stages use the same coupling caps" is lazy/bad design... hahah not a rule of thumb to live by!

As far as moving the tone control later, you could. But the "Taming later" I think would be better accomplished by adjusting the voltage divider between stage 2 and 3. To test this theory, stick a 220k resistor after the coupling cap (c2, 2) and the volume pot.

Feel free to try out a "peaker" cap of a large-ish value either over that 220k, or over the 220k/220k that's in there. By large-ish, I mean like a .0027uF or thereabouts. (large for a peaker, not large for a signal path cap)

[sepulchre]

Yeah, "wrong topology" is what I feared. Ah well, I can get it to sound really good just with the guitar volume and playing technique. So yeah, more like a Wreck I 'reckon'. (groan)

Soo, reverse the coupling cap order, try a little something on Stage 2 to keep it from beating Stage 3 up so much, and maybe use the switch for a peaker cap on the top part of the 220K divider. Sound reasonable?

.0027 for a peaker cap seems a bit large to me, my experience with them being from a Marshall 2204. But I do have Several caps (just got an order in) so I'll give it a shot and adjust as needed.

[matt h]

Yeah, the standard Marshall value peaker is a 470pF. Feel free to go larger... beyond a certain point, the "peaking" is midrange enough that it really feels different. The much-larger peaker values have been used in number of hot-rodded marshall types. I'm sure someone who's got access to Pspice, or an amp and an oscope, or "the maths" can chime in and tell you what's goin gon--but I'll say the effect, as the cap gets bigger, is sorta counter-intuitive if you're thinking of a peaker as that thing that gives more treble.

[sepulchre]

Yes, larger cap, lower frequency. I'll play with it and see what I like.

[sepulchre]

Question: would there be any benefit from getting the screen voltage closer to the plates on the PA? Right now there's a 1.5K resistor feeding the screens but I could go lower.

[martin manning]

The treble "peaker" is really a bass- and mid-cutting shelf, but it's all relative ya know ;^)

The nw seen in Marshalls is usually a 470k/470k divider with a 470p. Since the resistors in your divider are half as big, the cap has to double (to 940p or ~1n) to keep the same response.

Raising the screen node voltage might be good for the preamp since it is operating at a relatively low voltage. Slap another 1k5 across the one that is there to try it out.

[Reeltarded]

I think I like the tone stack after the second stage and that gain works best in place of the divider before the third, where you could just switch in something at the top of that pot as an option.

Lots of loss in that tone and that loss never gets amplified... 'cause it's lost.



I'm a genius.

[sepulchre]

A big storm came through here so I couldn't get the work on the house I had lined up done. dang.

I changed the coupling caps, putting them in a more reasonable order of value. Then I added a LNFB to stage 2 - 4.7M. That helped a lot, not so overblown on the volume. Thank you Matt - the NFB really pulled stage two in line.

Then I switched the tone rig from stage 1 to stage 2. That Really sweetened up the volume control. It's not over the top at all now until everything is dimed, then that's what I'd expect. Also the Drive control now acts more like what I had in mind: it's a lot more about distortion control than merely another volume knob.

I don't think any of these changes would have done the trick on their own but together this amp's starting to get its sound. It's different from anything else I've built, or owned for that matter. Cleans can be best had with the use of the guitar's volume; it's difficult to get a clean sound with much volume when the gtr is cranked. In fact the guitar controls are very useful with it. I find myself turning all them knobs all the time to get various sounds and feels.

I'll make a clip tomorrow. Later tonight I'll post the schematic.

[matt h]

I am ridiculously excited for the coming clip!!!

[sepulchre]

Matt: I'll get one together tomorrow for sure.

Here's the current schematic. Note that the voltages may not be accurate since I have not remeasured since making the latest changes. Also, the Platinum / Blonde switch will be used for something other than how it is now rigged, maybe as a tone lift or to lift the bright cap on stage 1.

[sepulchre]

You "found" some interesting stuff at a site with your name? Yeah right.

[matt h]

We got spambotted. Hahdcore.