Non-CF Clipper?

[matt h]

Is the schematic still not fully accurate? (Lack of decoupling resistor in power supply, anode/cathode/coupling values of first two stages, etc) I'm glad to see voltages on there though! You're definitely going to want to drop the value of R15 (the 22k) Heck, you might even want to drop R16 to 1k, and then use individual screen resistors if you still need more drop. I suspect the slightly higher voltages you get for the preamp will give you a bit more clarity.

[sepulchre]

Yeah, I apparently missed changing those values though they are changed in the amp.

Note: I have changed those values so the schematic is now accurate.

Martin pointed at the Gibson GA-5t to look at the power supply. It only has the one 22K dropper between the screen and the preamp stages. I guess I should have a dropping R for each filter cap. I was just following Gibson's lead . . . sort of. Also, I have not yet gone into the tone control but that's in the plan.

When you say I should lower R15 I understand. But I was also pointed at this from Merlin Blencowe: http://www.valvewizard.co.uk/fullwave.html
About 2/3 down the page he talks about minimum limiting resistance in valve rectifiers. Maybe I'm not grasping the concept completely.

I would welcome a bit more voltage on the preamp stages. It tends to distort very rapidly when the volumes are being turned up. When I said in the OP that I wanted more distortion (I called it gain) I didn't mean to throw the clean out to get it. In fact, I'm in the process of looking at voltage dividers between the the 1st & 2nd stage, and between the 2nd stage and the PI driver. These, I'm hoping, would bring the control's ranges into a more useful state. It would be better not to have to do so - the board is fairly populated already and I'd have to rig something up to do it. I need to focus on getting rid of the HUM at this point anyway.

So, decoupling resistors. One to separate the PI driver from the second stage? Others? I'm really in the dark here. I'm sure I need to read some treatise on the subject. I do have Merlin's Power Supply book but it's been a year or so since I cracked it. Plus the fact that he stopped publishing it due to errors and typos does not bode well for it.

[matt h]

My crippling arthritis prevents me from writing the treatise on power supplies you need. hahaha

Short version: you need a power supply node (cap) and a decoupling resistor for (rule of thumb) every two gain stages provided they are out of phase. You've got three inverting preamp gain stages in a row, right? This means the first and third are IN phase with each other and therefore need to be on separate nodes.

The value of the decoupling resistor (I prefer this term to "dropping resistor") is chosen not only to drop voltage but to prevent one node's signal-induced "wiggle" (rather than ripple) from spreading to other stages. This is what caused the motorboating you head before.

The Rser feature has to do with, basically, everything between the transformer's center tap and your first filter cap. This means the actual secondary winding of the PT's resistance is part of the "minimum series resistance". Most people, for better or worse, ignore this entirely. I'm not telling you to do it, but if you chose to, you're in the majority.

Other "dropping" or "decoupling" resistors later in the power supply don't apply to that feature.

R15 is a) a decoupling resistor and b) a dropping resistor. As long as proper de-coupling is achieved, the value of that resistor is selected mostly for dropping. If you really, really wanted you screens to sit at this low a voltage, that value is fine. It's good practice for screens to be lower than the plate, but that seems excessive to me.

Since you already have volume pots in there, you don't need additional voltage dividers if space is a consideration. If, for instance, you have a 500k pot, just fly a 500k resistor TO the top lug of the pot after the coupling cap. When the volume pot is "maxed", that is now effectively a 500k/500k voltage divider. Get it?

You can play with the values of the fixed resistor, I just used the same value as the pot for ease of concept.

Another option, and a very awesome one, is to use a split-load plate resistor. I think you mentioned you have a switch available, that's a great place to do it. It's better, of course, to use an extra coupling cap for the mid-point tap and then only switch the AC, but your voltages aren't crazy. Consider like a 150k/50k pair of resistors for the 200k plate. When the signal is taken from the normal position, the effective plate load is still 200k. When the signal is taken from the mid-point, it's effectively reduced by a quarter (the 150k/50k plate resistors act as a 50k/150k voltage divider!) while still presenting the full 200k load, so the loadline stays the same and the tonality is consistent.

[sepulchre]

After looking at a bunch of other designs I took a shot in the dark and came up with this. Hopefully it doesn't violate any "Minimum Resistance" limits. I can adjust R15 either way with more wattage capacity - 5K, 10K or whatever. I'm not sure how many 1K I have on hand that are over 1/2 watt, one at 5 watts that I know of for the screens. Btw, isn't R16 the screens adjustment, not R15?

One thing I haven't mentioned: this little sucker is Way Louder than I expected. Also, since adjusting Ra, Rk & Ck on the first two stages the tone has improved 100%. It's still really noisy though; not so bad at low volumes but you don't have to turn it up much at all for the hum monster to raise its ugly head. Besides the PS decoupling I'm looking at the heater supply. Ampgeek's suggestion of raising the CT voltage may be of use. Currently, I'm not using the heater CT at all, which I suppose could be a factor itself. (am I being really Duh here?)

[matt h]

You posted that schem as I was typing.

The new schematic is also a problem! the power supply nodes ar ein the wrong places.

[sepulchre]

I see that. R20 & R2 should be backed up a "notch", right? So that R21 goes to the second stage and R20 goes to the PI driver?

[matt h]

Yes, from the schematics as I'm reading them now, R16 is what drops the voltage to your screens. because of the way the schem is drawn (not how I was expecting) I was misreading R15 as your screen dropper. That was my bad.

As the schematic currently stands:

C11 is your plate supply
R16 and C10 are your screen supply.
R15 is missing a filter cap and is/should be your PI (driver and splitter) supply. Move the 10uF C4 here.
R20 and C4 (this should now be the OTHER c4, the 4.7uF value) this will supply BOTH of the triodes in the preamp.

R21 should be deleted.

You definitely need to ground your heater CT. That's a major party-foul.

[sepulchre]

Sorry about my numbering scheme screw ups. Comes from copying parts.

I think I'm getting it now. Is this better?

[matt h]

Yes, that's the ticket!

Build this, ground your heater CT (whether you ground it to, uh, ground, or ground it at your power-tubes cathodes probably won't matter a ton, but I'd shoot for the cathodes even for the modest DC reference if for no other reason than the cathode of your concertina , while not high, is higher than the other stages by a good bit.)

Edit to include: I keep meaning to say, "damn straight it's gonna be loud." A 10W amp is only half as loud as 100W, and that's still enough to shake walls with an efficient cab moving some air.

[sepulchre]

Thanks a million, Matt. Tomorrow when I get home (I'm at work now) I'll make these changes and see how it goes. There was a time when I would have been able to recite all this off the cuff, but that's been long while and I've had too much other stuff going on.

So thanks again, man. And I'll try the split load switch. I've read about it before but never had the opportunity to try it, so this is good.

I'll post results tomorrow.

[matt h]

I think you'll enjoy the split load. They can be subtle or not so subtle depending on where you want it. And if you ever end up having the room for the second coupling cap thing, the other cool thing is that it lets you tailor the low end to the amount of signal. So in the full-on position, you can use a smaller coupling cap (bass limiting for higher gain, prevents blocking distortion, better "high gain" sounds), and in the tapped setting (for cleaner playing) you can use a larger coupling cap value to give you more booty.

I hear you on "up to other things". The last few years of my life have been, well... uh... a story nobody believes when I tell them, they then furious google stuff, and go "oh... you... weren't joking..." Yeah, let's just say amp-building wasn't part of that.

[sepulchre]

You seem to have retained the crucial stuff in spite of the distractions. That's more than I can say. But I'm not doing too bad considering all that's happened (health issues). Plus I have remodeled and out into the yard, etc. and taken up some other hobbies (I make knives & daggers & letter openers), so nothing anyone would want to google, just stuff.

The coupling cap idea sounds like just the thing I want here. Plus these are the things I like to do: playing with values and tweaking things until I get it just right. Lots of fun ahead.

This had me wondering if I could pull it off. I'm in your debt, pal.

[matt h]

Ooooh! Debt! Yay!

Seriously though, no real need to thank me. I love knowing more awesome non-cloned amps are getting made.

I'm gonna shoot you a PM.

[roberto]

Yes, that's a nice feature. I've posted it somewhere around two years ago, I guess on a modification read.

[Reeltarded]

Hey, Matt, no matter what that newb dickhole said in his maiden butthole thread, you rock, lots.