Princeton Reverb Mod

[Chuck H]

I happen to be working on a similar circuit for myself right now that incorporates a dual ganged pot. One gang is the regular volume control and the other is (sort of) like the circuit I've drawn up for you. The idea being that the amp will behave like a vintage design for gain and LF up to volume setting 5 and then ramp up quickly and trim LF for higher gain settings.

I'm using ltspice and sometimes I modify circuit values on Duncan TSC. I have a couple of other CAD type things I access sometimes too.

And I'm about to change the post yet again. So look out for that! Since my head is already in it I made another circuit improvement. But this is the last one, promise. I actually enjoy this part of the process. It's the soldering I can do without

[galaxiex]

Here you go

The green traces in the graph represents a stock PR second gain stage at the bottom trace (including the effect of the 10p cap) and a Trainwreck at the top. I used these as marker goals. The purple traces represent the circuit shown with the bottom being the 1M pot at full rotation one way and the top trace at full rotation the other.

An interesting thing happens by simply adding series resistance. You can change the frequency knee of the coupling cap due to circuit impedance. This actually allowed for closer approximation of both adjustments with less than .2dB loss at 82Hz in PR mode and a 16dB cut in TW mode much like the TW circuit.

On the same lines there are obviously other differences between your amp and a TW design. Like your third stage not being cold biased and un bypassed, the PI being a cathodyne rather than a LTP and the tone stack being very different. Not to mention plate voltages. But here you go anyway. This circuit has the advantage (?) of using a much more readily available 1M pot as well as representing the low TW AC loading in the higher gain mode while correcting to a higher AC load in PR mode.

Submitted for your consideration.

Sorry for the O/T... Chuck, what are you using to draw that nice looking schem?
Thanks!

[Chuck H]

Sorry for the O/T... Chuck, what are you using to draw that nice looking schem?
Thanks!

That's actually Fletcher Munson's original schem and I just modified it with paint. I use paint for my own schems too. I'd upload my symbols bitmap for all except that bitmap files are too large for the forum to post. Once you change them too gif or jpg they don't take transparency and other functions well. I've become a paint master.

[galaxiex]

That's actually Fletcher Munson's original schem and I just modified it with paint. I use paint for my own schems too. I'd upload my symbols bitmap for all except that bitmap files are too large for the forum to post. Once you change them too gif or jpg they don't take transparency and other functions well. I've become a paint master.

Paint?.... as in ancient old Windows Paint?
Wow.
We should call you Picasso or Rembrandt.

I did that once back in the bad old Windows 95 days... found it very tedious.
I did make a "component library" of symbols that I had drawn up. That helped.

Or did you mean Paint.net?
I use Paint.net to make board layouts.
Still tedious but having layers is fun.
Now learning DIYLC

Again, sorry for the O/T.

[Fletcher Munson]

And I'm about to change the post yet again. So look out for that! Since my head is already in it I made another circuit improvement. But this is the last one, promise. I actually enjoy this part of the process. It's the soldering I can do without

Looking good. Can't tell you how much I appreciate your (and everyone's) efforts. Just to be perfectly clear: In the latest version of the circuit, does the 33pf cap connect to the junction of the 1MA pot, 150k resistor, and the grid of the third gain stage? Or does it connect directly to the grid?

I also tend to prefer the computer work to the soldering work, but that may be because the office is air-conditioned and the shop is not.

[Chuck H]

Windows Paint. My old symbol files are from when they called it mspaint. It's actually pretty good when you get use to it. The graphics suck, yes. I don't see the need for high resolution though. Certainly the information a schematic is supposed to communicate gets across.

Things that help would be to have several useful blocks drawn up and saved. Example: Whole gain stages: triode graphic, grid leak cathode R with and without bypass cap, plate R, coupling cap. Volume control, tone stacks, basic power supplies, etc. I often need to make modifications to my blocks, but it's still faster than scratch drawing a transformer, AC plug, fuse, pilot, caps and resistors every time I want to draw a power supply. Other features like transparent overlay, cut/paste and the old click/drag make things go pretty fast once you have your blocks. I won't lie though. I bet it took not less than four hours to draw that schematic I posted. I don't think that's as much time as it sounds like though if you've ever drawn a schematic representing original designs. I've tried some of the schematic drawing programs and I thought the results looked clunky and they sometimes lock you into things like where you can put a component label, what route a lead must take or proximity of one thing to another. And sometimes they're glitchy and do things like post component lables on top of other stuff, etc. All of that is stuff that needs to be edited even for a schematic drawing program. I wasn't saving any time.

[galaxiex]

Windows Paint. My old symbol files are from when they called it mspaint. It's actually pretty good when you get use to it. The graphics suck, yes. I don't see the need for high resolution though. Certainly the information a schematic is supposed to communicate gets across.

Things that help would be to have several useful blocks drawn up and saved. Example: Whole gain stages: triode graphic, grid leak cathode R with and without bypass cap, plate R, coupling cap. Volume control, tone stacks, basic power supplies, etc. I often need to make modifications to my blocks, but it's still faster than scratch drawing a transformer, AC plug, fuse, pilot, caps and resistors every time I want to draw a power supply. Other features like transparent overlay, cut/paste and the old click/drag make things go pretty fast once you have your blocks. I won't lie though. I bet it took not less than four hours to draw that schematic I posted. I don't think that's as much time as it sounds like though if you've ever drawn a schematic representing original designs. I've tried some of the schematic drawing programs and I thought the results looked clunky and they sometimes lock you into things like where you can put a component label, what route a lead must take or proximity of one thing to another. And sometimes they're glitchy and do things like post component lables on top of other stuff, etc. All of that is stuff that needs to be edited even for a schematic drawing program. I wasn't saving any time.

Thanks!
Ok, last time with the O/T stuff... I promise.

I use ExpressSCH and have become used to it's quirks.
It doesn't look the greatest but it gets the job done.
Really, it doesn't look all that bad IMO.
Can create custom components and such.
I use the "snap to grid" OFF, keyboard shortcut a LOT!
Can position anything, anywhere.
Limited page size Legal, but can have multiple pages.
Still takes me quite awhile to draw a schem from scratch.
Never timed it but I do them in several sessions anyway.

Cheers!

[Chuck H]

In the latest version of the circuit, does the 33pf cap connect to the junction of the 1MA pot, 150k resistor, and the grid of the third gain stage? Or does it connect directly to the grid?

Well the schem pretty much spells out WHERE it is in the CIRCUIT. I suppose you can connect it anywhere you want to as long as it's still doing what it is in the schematic.?. One of the "improvements" to the circuit has nothing to do with circuit function, but ease of implementation. Notice that the .0033uf and the 33p cap can both be soldered to the pot (from either outer lug to the wiper). That way you don't need to make allowance on the board or float any components

[Steve A.]

That's actually Fletcher Munson's original schem and I just modified it with paint. I use paint for my own schems too. I'd upload my symbols bitmap for all except that bitmap files are too large for the forum to post. Once you change them too gif or jpg they don't take transparency and other functions well. I've become a paint master.

Some of the upload restrictions here for certain file types like PDF can be skirted by zipping them up. If the size is still excessive if you can get them to me via my box.net account I can share them with others. PM me for details. Heck even snail mail is an option...

Drawing and paint programs are a dime a dozen... personal libraries of electronic symbols can be priceless!

FWIW in modifying schematic drawings at The Amp Garage, et al, MS Paint still seems to be the tool of choice. What the heck, we love tubes going back to the 50's so why not programs that go back to the 90's...?

Steve A.

P.S. Here is your drawing cropped and shrunk to an 800 pixel width for in-line viewing on mobile devices... Kinda hard to read but it sure looks purdy!



.

[Fletcher Munson]

Well the schem pretty much spells out WHERE it is in the CIRCUIT. I suppose you can connect it anywhere you want to as long as it's still doing what it is in the schematic.?. One of the "improvements" to the circuit has nothing to do with circuit function, but ease of implementation. Notice that the .0033uf and the 33p cap can both be soldered to the pot (from either outer lug to the wiper). That way you don't need to make allowance on the board or float any components

Chuck, my question concerned the point where the lead going from the 33pf cap to the grid crosses the lead going from the 1M pot to the 150k resistor. On some schematics, crossing lines without a dot indicates no connection. Most of the old Fender schematics were drawn this way. Other schematic styles (like the Double Agent schem above) show a slight gap or a little semi-circle to indicate that crossing lines don't connect.

I know it should've been fairly obvious from the context, but I've never seen a volume or gain control configured in quite this way and wanted to be absolutely sure I understood what you were saying.



I love the old Fender schematics for their layout and clarity, and I like to follow that style. No paint or drawing applications here. They were all created in the days of pencils and rulers and drafting tables.

[Chuck H]

I understand. And taking things "in context" in this matter might be fuzzy. Yes, the crossover is contact if there is no gap or semi circle. The 33pf is strictly a "bright" cap across the 1M pot as represented in any of your "connection" diagrams above.

The reason it looks odd is because this isn't wired as a typical "volume" control. I added a "0" and "10" reference on the 1M pot in the schem to indicate gain so that you would be able to negotiate which lugs to select for "clockwise = more gain". I hope this clears it up.?.

[Fletcher Munson]

I understand. And taking things "in context" in this matter might be fuzzy. Yes, the crossover is contact if there is no gap or semi circle. The 33pf is strictly a "bright" cap across the 1M pot as represented in any of your "connection" diagrams above.

The reason it looks odd is because this isn't wired as a typical "volume" control. I added a "0" and "10" reference on the 1M pot in the schem to indicate gain so that you would be able to negotiate which lugs to select for "clockwise = more gain". I hope this clears it up.?

Absolutely. Many thanks.