Double check my schematic? Please!

[steve-o]

Hi all,
Well, My Heyboer trannies are in; tube sockets, jacks, pots, etc. are all on the Chassis (that I took out out of my blues Jr.). But, this being my first build, I was hoping maybe some of you with more experience would take a look at my circuit before I start soldering stuff up.
What I started out with was a blues Jr. I know I could do worse, but after playing a couple of nicer combos, I noticed the blues Jr. had this "duck throwing up" tone that I couldn't take.
Here's what I'm shooting for: I wanted an amp with a fendery preamp section so that the clean tones lean that way. The power section is more in the vein of a Marshall 18watt TMB. Once the circuit was being drawn, it (unexpectedly) resembled a top hat ambassador. So, my layout is borrowing from that (and a club royale) as well. So, there's no reverb. but that knob from the blues Jr. is being used for a "cut" knob. I also have a power amp filtering switch down by the recto tube.
So, does anyone see any problems with this circuit? Something that's gonna' bite me later? Something that's gonna' explode on powering up? Real estate seems to be in short supply on this amp, but maybe it just seems that way 'cause this is my first build. I have one unused 1/2 of a 12ax7.
Thanks for your help. I can send pictures of what I've built so far if that'll help. Sorry this post is so long. Feel free to let me know if there's some sort of etiquette to this that I've missed...

[steve-o]

I didn't realize that you could put two attachments in one post. Here's the other image of the layout.
Plus, some of you may notice the initials in the bottom corner of the schematic. Those belong to our very own Dehughes!
He's been egging me on/helping me out. I love you, man!

[oldhousescott]

Steve-o,

I took the liberty of modifying your drawing. I tried to circle all the areas where I made changes.

I would parallel the first tube since you have that extra triode stage available. You need that 3.3M/100k voltage divider between the second and third stages to knock down the gain, otherwise you're into TWreck territory. With no negative feedback around the output, it might get crunchy kind of quick although the 47k tail resistor in the PI will help some.

Hope this helps.....

[steve-o]

Scott,
Thanks alot for your help! I'm glad I put this up before I just started soldering away.
Anyone else have any advice?
Also, does anyone have a take on the "limited space" issue? Is it any issue at all? I'm using the chassis out of my old blues Jr....

[dehughes]

I believe that the 1M on the input jacks should come before the 33k....that's my fault....I just re-checked my Emplex schematic today....

Those adjustments are cool. Easily reversible if it comes down to it...good catch on the 47K tail resistor...forgot the "K".

One thing: why a 500k Cut pot? Wouldn't a 250k give you more usable space, being as this type of control is mostly usable in the 1st third of the turn?

Oh, and the goal was Fendery clean at lower volumes with a gritty drivey crunch when the amp is pushed. Think Blackface Fender preamp with 18w Marshall power section....sorta... With that in mind, what do you all think should be moved/changed/removed?

[oldhousescott]

Matchless uses a 1M cut pot, the Tophat uses a 250k cut pot. I just split the difference. 250k will work fine.

The preamp section as modified is the Fender BF reverb channel (w/o the reverb). With the MV, you should be able to get a nice balance of crunch between the preamp and output stage.

If you want to stick closer to your original schematic, you'll need a grid to ground resistor on the third stage -- somewhere between 100k and 1M. I still believe you'll want a voltage divider here. You might try temp-ing in a 1M pot and adjusting to get the balance of clean and crunch you like, then carefully removing it and measuring the resistance on each side of the wiper, and substitute two resistors closest to those values as a voltage divider.

[dehughes]

Okay. Makes sense to me...

[steve-o]

Thanks for the help! What recto tube should I be running? Where should look to find out?
Also, where should I look (this being my first build) to find out how to wire up the tubes?
steve