Questions on Doug's Tube Preamp Pedal project

[markmalin]

This may have to be answered by the man himself but ... if anyone's built this project, I had a few simple questions.  My plan is to build this, then start playing with mods.  Attached is Doug's schematic. 
A couple quick questions...


1.  On the 4th gain stage (V2B) plate there is no plate resistor.  I think I just answered my own question...this is a "cathode follower" circuit...just curious why a cathode follower was chosen rather than a standard style gain stage like V1


2.  How did you mount the torroid to the chassis?


Also, I'm curious - if anyone else has built this, I'd love to see what you came up with.  Seems like a fun project - my end goal is to maybe mod it to look like a different style pre-amp circuit, or maybe add a D-style OD circuit using V2 rather than the cathode follower.


Cheers
Mark

[shooter]

why a cathode follower

They are typically used for impedance "matching".  I typically use them as part of a DCCCF "combo"

here's the reading;
http://www.valvewizard.co.uk/dccf.html
never built the pe but have studied it and gotten good ideas on voltage dividers 'tween stages, for signal and tone shaping

[markmalin]

Thanks, Shooter - I appreciate the link. I'll read up some more.

Mark

[sluckey]

Looks like the toroidal PT mounts with a single screw through the center.

[tubenit]

Mark,

I don't see any reason why you can't use V2-b as a gain stage vs. cathode follower and design the thing in a D-Style topology?    AND ...... IF you need a cathode follower for impedence reasons, then you could simply use a mosfet CF connected to V2-b.   And still have a D-style tone stack between V1a & V1-b.

Just a thought to ponder.

With respect, Tubenit

[markmalin]

Looks like the toroidal PT mounts with a single screw through the center.

Right - I see Doug had it mounted with the toroid top toward the side of the chassis.  The screw hole doesn't go through, it's threaded, which is great, but that's on the bottom (pin side).  I wondered if he just drilled it out?

[markmalin]

Mark,

I don't see any reason why you can't use V2-b as a gain stage vs. cathode follower and design the thing in a D-Style topology?    AND ...... IF you need a cathode follower for impedence reasons, then you could simply use a mosfet CF connected to V2-b.   And still have a D-style tone stack between V1a & V1-b.

Just a thought to ponder.

With respect, Tubenit

HA!  That's exactly what I was wanting to do! :)  I'd like to build a "D" in a box - the pre-amp section basically.  The D circuit pre-amp uses 2 tubes in front of the PI, so I got looking at Doug's design and wondered how hard it would be to hack something together for the fun of it.  But I don't know enough about the impedance mismatch issues.  I may have to ask more questions when I get to that stage.


My plan is to build Doug's design first, get it working, then let the hacking begin...


Mark

[sluckey]

I wondered if he just drilled it out?

Send him a pm. You'll have a definitive answer (rather than our guessin') before the days end.

[PRR]

> why a cathode follower was chosen

The whole thing was drawn from knowledge of the Fender 5F6a Bassman. Also early Marshall copycats. And maybe half the large amps made today.

Leo lowered the tonestack values for more action. This didn't help until the tone driver stage was made low-impedance, a cathode follower.

Doug dropped the 5F6a's mix network, seems to have inserted an added gain-stage, and shimmed V2A for low gain (otherwise gain would be obscene). We know it works, so do it like he drew it, *then* go crazy with changes. (I'm not sure what wants changing.)

[markmalin]

Thanks for the input, PRR.  Always appreciated :)
I appreciate the background, actually.  I'll get this running nicely and see if I can come up with some ways to tweak it w/out breaking it.  Should be fun.

> why a cathode follower was chosen

The whole thing was drawn from knowledge of the Fender 5F6a Bassman. Also early Marshall copycats. And maybe half the large amps made today.

Leo lowered the tonestack values for more action. This didn't help until the tone driver stage was made low-impedance, a cathode follower.

Doug dropped the 5F6a's mix network, seems to have inserted an added gain-stage, and shimmed V2A for low gain (otherwise gain would be obscene). We know it works, so do it like he drew it, *then* go crazy with changes. (I'm not sure what wants changing.)

[tubenit]

This is sort of what I was thinking of

With respect, Tubenit

[markmalin]

This is sort of what I was thinking of

With respect, Tubenit

Tubenit...you're too good to me :P   Seriously, you guys are the best - I appreciate the support of the Hoffman Amps forum more than you'all can imagine :)


Mark

[markmalin]

I wanted to follow up on this thread.  Rev I was Doug's schematic exactly, which sounded great.  Rev II is using Doug's original power supply, but the pre-amp design is effectively a D* style #183 preamp.  This is sounding pretty Dumbly.  Rev III is currently underway, which will use a Fender Reverb power transformer...which will probably produce a bunch of questions to come... I need to figure out how to get the right voltages at the plates, so I'm looking at Fender's original schematic for their reverb unit.


For my D pre-amp I am working off of a printed out #183 schematic covered with whiteout and pencil traces...once this is up and running correctly I can post a real schematic, so sorry for not having that available yet.


Here's some noodling on Rev I - Doug's design:

Here's me noodling on Rev II (D-schematic version).  Please ignore the old guy playing...

[tubenit]

Mark,

That sound really nice!  Great job.  Looks good & the revision one does have some D-flavor to it, IMO.  Nice playing also.

THANKS for sharing your success!   

I think IF I were going to do a D-style preamp, I'd be tempted to either put in an active FX loop ……….. OR use a mosfet CF with the last gain stage.  Just a thought,  not saying you should do that.

With respect, Tubenit

[markmalin]

Thanks.  An effects loop would be kind of cool, though this is basically everything from the input to the place where an effects loop would be...so theoretically you could plug your guitar into the input, plug the output into the Power Amp In (Return) and you'd effectively HAVE a D'style amp with your power amp.