Anyone willing to waste a bit of their time for my benefit?

[G._Hoffman]

I'd very much appreciate it.


I THINK I'm done with this schematic (although there are places where the exact values will change with experiment, and I'm fairly certain I'm going to have to drastically change the dropping resistors in the power supply, but I just can't bare the idea of all that math right now!), but if someone (or ones) could look it over for any gross and horrible errors, I would certainly be thankful.  The point is to learn, and I've done that quite well...well, I've certainly learned something.  Perhaps you could tell me if I've done it well?


Thanks in advance,


Gabriel

[triode]

It is early in the morning, here is my 30 second look observation...
 
I question the validity of C10.  It is not necessary, unless you are going to tell me that
you blow out bias diodes all the time for some reason and are using it as a (high pass)
filter, which makes little sense as you want the 60-120hz from the secondary of the
power transformer.

Don't use a ground lift.

There is nothing wrong with your tone stack, but a wise old amp builder once told me,
as you put the TS later and later in the circuit, after more and more gain, you find that
sometimes it does not impact the circuit as much as you would like. Maybe (and maybe not)
you should be ready to move the mid control further "up" the chain.

Your standby switch may make more noise than you would like, be prepared to strap some
resistors across it.

I can not help you with the PIC in the tremolo this early in the morning. :)

I don't see a filament supply on the transformer, I assume you left it off for brevity.

HTH some.

[G._Hoffman]

I suppose I should give a bit of context.

First of all, I've been reading Merlin's books, so in broad terms I got the basic concepts at least mostly from his books.  

I really like the sound of parallel gain stages, and I really wanted to try a preamp Pentode, but not using the Pentode (which Merlin says likes to be driven a bit more than most guitar pickups can pull off) as the first gains stage - so, I decided on the Triode and Pentode inputs.  Each with two gain stages, one of them containing the EF86.  (Which, I suppose, I should note on the Schematic.  V2 is an EF86.)  

The PI and the Power amp are very similar to the 5F6A or the AB763.  I've always used EL84's in the past, and I wanted to try some 6L6's, and that is just where they ended up.

My power transformer is a Toroid from Antec - the AN-2T300.  It turns out to be a bit more than I needed, but it will be just fine!

The PIC in there is Electric Druid's TapLFO program.  I've used it once or twice before, and it works very nice.  I like it.  Speaking of the Tremolo, I'm planing to use the VTL5C3 for the LED/LDR - any problems with that?  It looks appropriate.  I'm also thinking of changing how that is switched, since right now turning the tremolo off leaves the LED ON.  

Oh, and the Bass and Treble controls, though electronically separate, will be on stacked pots, so just one knob for each.

I haven't done the physical layout yet, but I'd like to at least get that started this weekend, so any hints you can give would be great!  (And please remember, this is as much about my education as it is about having a great amp at the end...though that would be pretty awesome too!)


Gabriel

[tubenit]

Gabriel,

Interesting build! I have limited knowledge of electronics & build almost all cathode biased amps, so some of the schematic is completely beyond my understanding.

I don't see anything problematic building it as you have drawn it.

However, if I were building it, I would consider changing a few things:

- My guess is that a 250k or 500k pot may suit your needs better than the 1M pot following the pentode. (based on my
  experience with 5879 pentode & 6Au6/5654)

- I would also want a pot prior to the pentode or at least a trim pot. The pentodes really have to be tweaked right to
  to sound smooth, in my opinion. In other words, I find a pot prior to and after the pentodes to be rather critical to the
  tone I want.  Far more so, then with 12A_7 tubes.

- I would use 33k instead of 68k input resistors

- I quit using 22uf & 25uf cathode caps years ago. Typically, I use 5uf or 2.2uf caps. I think the 22uf caps can leave a
  higher gain amp sounding VERY muddy based on my experience. If you look at Dumble or BadCat amps with lots of gain
  stages, they use pretty low value cathode caps.

- I am not sure about the standby switch as I am not familiar with that.

Given the size of the schematic you posted & how much is going on in the schematic,  I have shrunk a couple of versions of it for viewing it in larger sections.  Just trying to help with that.

With respect, Tubenit

[G._Hoffman]

It is early in the morning, here is my 30 second look observation...
 
I question the validity of C10.  It is not necessary, unless you are going to tell me that
you blow out bias diodes all the time for some reason and are using it as a (high pass)
filter, which makes little sense as you want the 60-120hz from the secondary of the
power transformer.

It's a capacitor coupled bias supply, since the bridge rectifier would short out with a traditional bias supply.  (At least, that is the way I understand it, and Merlin's book recommends this for bias supplies in bridge rectifier amps.)  

Don't use a ground lift.

this thing will likely be going in a rack someday.  It is ONLY a signal ground lift, and would only ever be used if I was getting a ground loop with another piece of gear.  The CHASSIS will always remain tied safely to earth.  Much better that way.

There is nothing wrong with your tone stack, but a wise old amp builder once told me,
as you put the TS later and later in the circuit, after more and more gain, you find that
sometimes it does not impact the circuit as much as you would like. Maybe (and maybe not)
you should be ready to move the mid control further "up" the chain.

 

I used these tone controls in my last amp, and I really like that the Mid control is a lot more functional than in a traditional FMV style tone stack.  And I like that they are NOT interactive.  I'll probably have to change some values along the way, though.  Also, the Mid control seems to want a fair bit of current to work well, so I like having it after the cathode follower, and giving each tone control its own gain stage must help them to be less interactive, which is something I (as a, mostly former, professional sound guy) HATE about FMV style tone stacks!  I know other people seem to like it, but it just annoys me.

Your standby switch may make more noise than you would like, be prepared to strap some
resistors across it.

I can not help you with the PIC in the tremolo this early in the morning. :)

I don't see a filament supply on the transformer, I assume you left it off for brevity.

HTH some.

Thanks.  I kind of figured that with the "Standby" switch.  It's a Mute switch, of course, but that seems to be the only real function of most standby switches anyway!  The "Standby" label is a private joke for myself.

I'm OK with the PIC, since I know that works fine (I might need to change the load resistor on the LED/LDR, though - that needs some experimentation).

And yes, I left off the heaters because I just didn't need to take up that much extra space on the sheet.  The PT has two 4A 6.3V windings, so one of them will be enough for the heaters, and the other will supply the PIC oscillator.

Thanks for your look, though.  It is very much appreciated!


Gabriel

[G._Hoffman]

However, if I were building it, I would consider changing a few things:

- My guess is that a 250k or 500k pot may suit your needs better than the 1M pot following the pentode. (based on my
  experience with 5879 pentode & 6Au6/5654)

- I would also want a pot prior to the pentode or at least a trim pot. The pentodes really have to be tweaked right to
  to sound smooth, in my opinion. In other words, I find a pot prior to and after the pentodes to be rather critical to the
  tone I want.  Far more so, then with 12A_7 tubes.

I'll was thinking of trying that.  I was just going to fiddle around with different values for the voltage divider, but at the very least a trimmer would be a faster way to find the right values!

- I would use 33k instead of 68k input resistors

I'll try it both ways, and see if I can hear a difference.  Thanks.

- I quit using 22uf & 25uf cathode caps years ago. Typically, I use 5uf or 2.2uf caps. I think the 22uf caps can leave a
  higher gain amp sounding VERY muddy based on my experience. If you look at Dumble or BadCat amps with lots of gain stages, they use pretty low value cathode caps.

All cathode bypass caps are up in the air until I've tried a bunch of different values.  Those will be the FIRST pass.  That is far from meaning they will be the last.  

That being said, in my last amp I started with some very small caps in the first stage, and it was painfully brittle and bright.  Using larger bypass caps in the initial stages, but keeping the latter stages very bright, has given me a really nice bright but smooth sound in that amp, and since this amp's preamp is similar (except for the complications caused by the Pentode) I figured it was still a good place to start.  But I'll give it a try.

- I am not sure about the standby switch as I am not familiar with that.

A standby switch is, for most players I know, just a mute switch.  That's why it is in quotation marks.

Given the size of the schematic you posted & how much is going on in the schematic,  I have shrunk a couple of versions of it for viewing it in larger sections.  Just trying to help with that.

With respect, Tubenit

 

Sorry about the size.  I've got a largish format printer, and printing these thing on a bigger sheet of paper makes them easier for me to read.  Exporting it to a GIF just kind of ended up that size!

I'll definitely try some of those changes you suggested.  (The paper version is, after all, never intended as more than a place to start!)  I've also never built a fixed bias amp before (or, for that mater, one with global negative feedback!).  This stuff is all new to me, and those challenges are the whole point.  At least, they are for me.



Gabriel

[sluckey]

I'm also thinking of changing how that is switched, since right now turning the tremolo off leaves the LED ON.

Yes. And that means the LED inside U3 will be on also and that will cause the LDR to be at minimum resistance which will severely attenuate your guitar signal. The easiest way to fix this is to move the switch to the base of Q1.

Or, you may consider connecting the LDR in series with the signal path rather than shunt. Sunn did this with their old tremolo circuits. I'd just move the switch.

[G._Hoffman]

I'm also thinking of changing how that is switched, since right now turning the tremolo off leaves the LED ON.

Yes. And that means the LED inside U3 will be on also and that will cause the LDR to be at minimum resistance which will severely attenuate your guitar signal. The easiest way to fix this is to move the switch to the base of Q1.

Or, you may consider connecting the LDR in series with the signal path rather than shunt. Sunn did this with their old tremolo circuits. I'd just move the switch.

Goodness gracious me!

Yup, I guess that wasn't as well though out as it could have been.



Gabriel

[RicharD]

V3 you have your sections A & B mislabeled.  6,7,8 is section 1, & 1,2,3 is section 2.  It doesn't really matter until you get into tubes with dissimilar triodes such as the 12DW7.  (I'm nit picking.... sorry).  So...R39 and R36 form a local NFB loop which switched the triode between a grounded cathode amp and a plate follower.  As a grounded cathode, it has a gain of about 50.  When in PF mode, the gain is set by Rfb/Rin.  40M/68k=  588  I don't think that NFB loop will do anything.  That 68k Rin might be eating Hi frequency.  It certainly will of you bring Rfb into a range where it's doing something.  I'd change R36 to a 22k and make R39 a 1M to shave just the edge off (1M/22k=45) or make it a 760k if you wanna kill about -3dB.

Funky cool circuit.  Can you change the wave form of the LFO?

[G._Hoffman]

V3 you have your sections A & B mislabeled.  6,7,8 is section 1, & 1,2,3 is section 2.  It doesn't really matter until you get into tubes with dissimilar triodes such as the 12DW7.  (I'm nit picking.... sorry).  So...R39 and R36 form a local NFB loop which switched the triode between a grounded cathode amp and a plate follower.  As a grounded cathode, it has a gain of about 50.  When in PF mode, the gain is set by Rfb/Rin.  40M/68k=  588  I don't think that NFB loop will do anything.  That 68k Rin might be eating Hi frequency.  It certainly will of you bring Rfb into a range where it's doing something.  I'd change R36 to a 22k and make R39 a 1M to shave just the edge off (1M/22k=45) or make it a 760k if you wanna kill about -3dB.

Funky cool circuit.  Can you change the wave form of the LFO?

The Negative feedback stuff is another area where experimentation is planned, but I'll try your suggestions. 

Yes, you can change the wave form of the LFO.  Its a very cool little program, and since it is freeware for non-commercial use, it makes life much easier.  It also saved me from having to program my own, since it has basically every feature I wanted when I was thinking about it.  All the better, since I would have no clue how to go about doing it, and I would be spending years just trying to learn!



Gabriel