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PostPosted: Fri Nov 21, 2008 7:08 pm 
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In my ongoing quest for ****Tube Amp Enlightenment****, (cue angelic choir & backlit fog effects) I've been working up a hypothetical amp schematic. If anyone out there has the time and inclination to take a look, I'd appreciate any opinions, guidance, advice, suggestions, criticism, and even ridicule (of the amp design, preferably). It started out as a 5f2a and evolved. There are elements from several amp designs I've found "out there". The tone circuit is from the "Big Muff" design in the Duncan Tone Stack Calculator....I went the cathode-follower route due to the low Z source used there. Also, I thought a 4th gain stage seemed excessive. The hypothetical trannys are the Allen TP25 and TO11C.

http://www.burntmonkey.net/chimp/

Anyone?

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PostPosted: Sat Nov 22, 2008 1:18 am 
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Interesting, I'll give it some thought.

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PostPosted: Sat Nov 29, 2008 12:06 pm 
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...helloooo?......helloooo?......helloooo?......helloooo?...
...anybody?......anybody?......anybody?......anybody?...

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PostPosted: Sun Nov 30, 2008 1:07 pm 
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I tried to open that link, and all I get is a vertical line down my screen... :?

(Running Firefox 2.x on Linux BTW)

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PostPosted: Sun Nov 30, 2008 4:44 pm 
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Image

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PostPosted: Sun Nov 30, 2008 4:47 pm 
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I like the idea of the extra OD. I know Phil has looked at this so will let him start it off. Along similar lines as the Tweedle I plan to build.

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PostPosted: Mon Dec 01, 2008 12:46 am 
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Very cool. Thanks much for posting the schematic. I'm anxiously awaiting the feedback.....bad pun not intended. :P

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PostPosted: Mon Dec 01, 2008 8:37 am 
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You may not need the extra pre amp stage before the tone control. What was your design intent there?

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PostPosted: Mon Dec 01, 2008 10:32 am 
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coco wrote:
You may not need the extra pre amp stage before the tone control. What was your design intent there?


I was wanting to minimize insertion-loss......but also it's because other cathode-follower designs I had seen were set up that way. kind of a b0b0-see, b0b0-do rationale. There's quite a bit of that in this design.

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PostPosted: Mon Dec 01, 2008 11:43 am 
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Check out Phil's p1extreme design on AX84.com No extra tube required. this is the path I am currently following with Phil.

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PostPosted: Mon Dec 01, 2008 12:42 pm 
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coco wrote:
Check out Phil's p1extreme design on AX84.com No extra tube required. this is the path I am currently following with Phil.

Yeah, I've spent quite a bit of time going thru the schematics and other info at AX84....very helpful. I may go that route in the end. My purpose in working up my own design is to try to apply the bits and pieces of info I've gathered and, thereby, identify and correct some of the knowledge gaps and misconceptions. The idea behind the extra gain stage actually came from the AX87 Hi-Octane design, as well as from an old Allen class-A design that actually uses both sides of the additional preamp tube as gain stages (yikes!) and from another builder's class-A design (currently in production) that uses the additional tube as a 3rd gain stage and a cathode-follower for their (James-style) tone stack....although I now see that their design has the CF-tone stack before the final gain stage.

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PostPosted: Tue Dec 02, 2008 12:31 pm 
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OK, I've finally found some time to look at this design. :) A few comments:
- Overall it looks good, and should work well. Congratulations on not using high values of cathode resistors in the preamp, especially after the first stage, which will help avoid fizzy tones.
- IIRC the Big Muff tone stack is quite lossy, and should work more efficiently when driven from the low impedance of a CF. The CF also adds some nice soft clipping, providing some thick Marshally crunch to the amp's tone. So that's good too. :D
- You've got a DC path from the cathode of V2B through to the grid of the 6V6 :shock: You need to add a 0.022uF between pin 8 and the tone control.
- I would recommend increasing the 220k resistor at the 6V6 grid to 1M, to increase power tube drive. A 1M grid reference in parallel with the 1M pot gives you effectively 500k, which is about perfect for a cathode-biased 6V6. Having a tone stack in front of the 6V6 means you have to overcome all the losses in the stack when driving the power tube. With a regular James stack BTW those would have been real high - hence the recovery stage. Otherwise it's usually preferable in higher gain preamps to have the CF and tone stack at the end of the preamp.
- Also consider using a larger cathode cap at the 6V6, since low values can make it sound rough when overdriven. 100uF is a popular value in cathode-biased amps. Or try a huge cap in the 1000uF to 2200uF range to really stiffen up the power amp for high-gain playing.
- I'm not sure what voltages you've got in your preamp, but aim for around 300V+ at the last two stages, to help drive the 6V6 (ideally running at around 350V). Also then consider reducing the 3rd stage's 1k cathode resistor to a more Marshally 820 ohms, to keep the stage warmish-biased at the higher rail voltage.
- The 22k resistor in the heater/elevation bleed, needs a cap across it, or you will get increased buzz in your amp. Something like 16uF shuld be good.
- Since this is a Class A SE amp, that vacuum state rectifier will have practically zero benefit in terms of sag and compression. Unless you simply want to use the tube for cool factor, you would be better using solid state rectifiers, preferably fast ones such as UF4007s. This would also allow you to improve the power supply filtering with bigger caps. Just a suggestion....
- Since your preamp is quite similar to the High Octane's, I would recommend also inserting an additional gain control on a ganged 500k pot between the 2nd and 3rd preamp stages. This helps the HO manage its gain much more effectively, so it can go all the way between clean and dirty.
- Do you really need/want the dual Hi/Lo input jacks? If not, I suggest reducing to one jack, and using a 10k input resistor instead of 68k. This will help the preamp not to pick up buzz from the power supply.


HTH.....

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PostPosted: Tue Dec 02, 2008 6:03 pm 
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:D WOW!...What an extremely helpful response! I will be incorporating most (maybe all) of the fixes and suggested tweaks. As you suspected, cool-factor was the primary motivation behind the tube rectifier....plus the fact that I have an empty chassis with an available octal-size hole. :) Hmmmm....superior reliability & filtering vs tube-mojo....tough call. The chassis is also already drilled for two inputs, which is the only reason I went that way in the schematic.....easy enough to plug 'em and re-drill, though. The dual ganged gain pot is a VERY cool idea.

Thanks much for your time and expertise!

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PostPosted: Tue Dec 16, 2008 1:56 pm 
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:oops: Took me long enough to get around to this. Projectus Interruptus, you know? I've made the corrections and mods to the schematic...I THINK I got 'em all. If anyone's interested, I've posted a JPG in addition to the HTML file this time.

http://www.burntmonkey.net/chimp/

Thanks again for the help/guidance/edumacation!

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PostPosted: Tue Dec 16, 2008 7:43 pm 
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Once again I just get a blank page with a vertical line up the middle.... :?

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PostPosted: Tue Dec 16, 2008 8:06 pm 
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zaphod wrote:
Once again I just get a blank page with a vertical line up the middle.... :?

:damntech:
.....hmmmm.....how 'bout this?

http://www.burntmonkey.net/chimp/Chimp0.9.jpg

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PostPosted: Thu Dec 18, 2008 1:51 pm 
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Here's an alternate link to the schematic:

http://picasaweb.google.com/FaaVog/Chimp#5281188191313332994


Image

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PostPosted: Thu Dec 18, 2008 6:55 pm 
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b0b0 wrote:
.....hmmmm.....how 'bout this?

http://www.burntmonkey.net/chimp/Chimp0.9.jpg

That's more like it! :D

OK, the schematic is looking pretty good now. Just you need C11 to be between R13 and R12, or the stage won't work. Also you don't need 25V caps on the cathodes of the first three stages. Anything 6V or above will do fine.

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PostPosted: Tue Dec 30, 2008 2:17 pm 
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Okay....latest version of the schematic is at:

http://www.burntmonkey.net./chimp/Chimp0.92.jpg

I've started on the layout and, once again, begin to realize just how little I know.
...this may take a while.....

Zaphod- Thanks again for all the help. MUCH appreciated!

Image

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PostPosted: Thu Jan 01, 2009 4:49 pm 
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Also worth noting.....the schematic is still evolving. Among other things, I'm still trying to work out the B+ voltages. I was originally considering a 310-0-310 PT thru a tube rectifier. If I'm thinking correctly, this now works out to ~400DCv since there's no voltage drop across the diodes.

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