help with custom preamp

[llama]

  Hi.  It's been a while since I've posted.  I've been slaving over a new amp build that I've been playing around with for about a year.  It's a rebuild from an old HP Oscillator. The face plate limits me to 4 pots (5 if i go dual concentric on one).  I've already built the 5e3-ish power amp section, plus the driver (Driver, Cathodine PI, 6V6 P-P, 8K OT).  With a guitar plugged straight in, the amp sounds clear and warm. 
 
 I want to finish this one, but I keep running in circles as far as the rest of the preamp goes.  The preamp sockets already have heaters wired for a 12a_7 in v1 and a 5879  in V2 and that’s the direction I want to go.  My last build was an AX84 October with paralleled 12ax7, like a plexi.  I really like the sound of the October with a 5879 replacing the second tube (driver/CF) for a great distorted tone,  But since I'm limited in controls and space I keep spinning in circles in my own madness.
 
 I recently searched upon a brilliant solution, by JoJokeo, for a Parallel to Cascade switch, which helped to stoke the flames of mad science, in the tradition of Silvergun.  I have a few 4PST switches laying around so using JoJokeo's drawing as a start, I worked up the idea for a BF Deluxe tone placement in the Cascade mode.  I'm hoping for two uniquely different, yet somewhat familiar voicings. 
Position 1 - ||Triodes->TS, …
Position 2 – Triode 1, TS, Triode 2, …
 
 I've poured over the design with my 10 year-old and I (we) think that the switch should work -without too much calamity.  The schematic up to this point, is attached to this post for your review. 
The second set of drivers is from is from my imagination and I assume it will problematic, but…I gotta see what you all think.
The 5E3-style driver triode, PI, and Power section I built are based on Doug’s 5e3, and changed suggested by Merlin to limit ugly distortion.  I want to use a 5879 with gain controls to “bypass or parallel the 12ax7 Driver.  I was hoping for a Deluxe type of sound with the pentode rolled all the way down, and some really kickass grit rolling it up.
Will either section make sense to build?
I know I will need some additional coupling caps and mixing resistors.  I hope I’m closer to a solution..
 

[tubenit]

I personally would never want to put a switch directly on the plate pins such as you show on 5879 V1. 

With respect, Tubenit

[Willabe]

I personally would never want to put a switch directly on the plate pins such as you show on 5879 V1.

Agree!

Why chance putting the hi B+ plate dcv on a switch? Much better to switch the grid or at least after the plate coupling cap.


                   Brad       

[llama]

I personally would never want to put a switch directly on the plate pins such as you show on 5879 V1.

Agree!

Why chance putting the hi B+ plate dcv on a switch? Much better to switch the grid or at least after the plate coupling cap.
                   Brad   

OK Thanks guys.  I dind't include the coupling caps, nor grid leaks, because i wasn't sure where best to use them yet. I suppose that the first triode(V1a) is in a more dire situation due to the Plate load resistor being switched.  I'll change the DWG to show the Coupling caps in  for V1 -12ax7

Am I just chasing rainbows here, or is this something that's worth a shot?

BTW here's a shot of the power section in the Oscillator chassis.

[kagliostro]

As to have inspiration about switching, give a look to this schematic



--

I was looking to the power section, and ......... do you think there will be enough ventilation ? The box is nice, but seems very compact

K

[llama]

Thanks Kagliostro,  I'll look over your alternative when i can freely do so.

As far as heat, so far i'm running the those 3 tubes + 5y3 Rec.  with no voltage dividers to take the signal down, and the chassis cover on, I've played for over an hour with a drummer, in a normal temp room and the whole outer cover felt way cooler than I expected, even near the tubes.

The outer case is fully vented and I'm running 6L6's @60% with ~300Vdc on the plates, which may help control heat output...

I'll do some more testing with the temp and use my laser temp sensor, to report back  What's a high temp I should begin to worry about ?

Will it get too much hotter with two more noval tubes running?  We'll find out.

BTW here's a the switch behind the plate coupling caps...and moved load resistor on V1a.

[llama]

Kagliostro, Thanks for the new idea.  That does exactly the same as i was looking with half the switch.

It does still put the b+ and the switch though.   I'm taking it form Tubenit and Willabe that that's dangerours and /or a bad idea.

has any one here tried that?

Again i'm just playing the part of "what if" mad science, so I can deal with straight talk about how stupid of an idea something is.

-Scot

[Willabe]

Fender amps with a standby switch had the B+ across them, so you can put B+ on a switch. Guys still do it and I do too. (Remember these are large bodied switches with higher voltage/current ratings.)

But, if you can avoid it why not?

A preamp switching arrangement especially with a relay I would try and avoid any plate/B+ switching. Most likely there are other points in the circuit to use for switching points.

IIRC, guys have done a cascade/parallel switching for the 1st 2 triode's at the input jacks. 



                      Brad     

[llama]

Thanks Willabe.

 I'll try to follow your advice and find a better points for switching.  I did find somethinf over at the ax84 Forum that points to an October build with such a switch (DPDT) -shown below (don't get confused with a second DPDT swich in that dwg for Moderm/Vintage, as its labeled.
It does avoid the B+ issues...

I did not find any references to the parallel/cascade from input jacks as you mentioned.  Does any have that info, if its available?

[Willabe]

Yeah, that should work.

I'm not sure you need 2 volumes, you can try 2 mixing R's (~270K?) then go to a single volume pot. (Like in a Fender with 2 input channels.)


                Brad        

[kagliostro]

I did not find any references to the parallel/cascade from input jacks

do you mean something like this ?





K

[llama]

Kagliostro, Are those from Merlin's first ed Preamp book?  I have the 2nd ed.  The drawings have that Merlin feel to them.

Yes that's closer, but still uses a switch to make the change.  I'm going to try the circuit i posted last to see how it works.

Often times I rack my mind for weeks to "come up" with a new, great idea (in my own mind, that is) and then i find that someone had come up with a far superior solution years ago.  I guess with so many people working interdependently most of the new ideas are gone... but i keep looking...

Thanks again for all your help, and Willabe too.

I'll start to collect what i need to solder it up.

-Scot

[llama]

While I'm wiring the input up,  Does the second part of my original post where I try to parallel a 5879 stage with the existing 5e3 driver stage make sense- or hogwash?

They are in the same phase, at least...  My goal with this is to have a cleanish/vintage tone with the 5870 attenuated fully, and a rip saw <tea> party with the 5879 at full tilt.  Kind of like a dry/wet mix.

I have modified the remainder of the 5e3 to take the abuse as per Merlin's advice for his 2nd ed. Preamp book.

I know i could use a DPDT switch to switch it out, but i like the idea of the blend.

-Scot

[llama]

Silvergun tells me repeatedly that i don't really ask questions for you all to answer, instead i kind of throw a schematic up and say "here it is"...

So, in the spirit of my amp mentor...The questions:

What tone differences can I expect between the knob changes in pre gain vs the post pentode gain? 

    I suppose the pre depends on if the first stage can drive the pentode in to cutoff - so, I imagine then, the biasing of the pentode is important to the pre gain's tone affect, and the overall tone of that circuit as well.?

What, if anything, should i add to the 5e3 driver stage to accommodate this change? 
and BTW. This stage is likely to be after a tone stack...
Schematic in previous post.

-Thanks all, Scot

[sluckey]

I can't answer your questions but may I suggest this?

[llama]

may I suggest this?

Simple and elegant. Thats probaly why i didn't think of it.   

Thank you Slucky!!

[llama]

I can't answer your questions but may I suggest this?

I'm going to have to order a long shaft potentiometer for the Blend pot, (Chassis req.) I was thinking 500K linear would work (like a two 270R resistor mixer).
But i'm going to test it out first.  I have a few lower value long shaft pots in hand.  can I use 100K for temp?  should I add a 200K resistor to each end leg?  -Scot

[llama]

Even though the Cascade/parallel switch intrigues me, I jumped ahead to building the || 5879 as per Sluckey's blend pot. I used a 1 Meg "B" pot with a .047 OIP cap on each leg. 

After running it the first time, however, the sounds out of either side the the blend was "Tooting out" <trying to clean up my act> under a heavier hand.  I re traced and checked my voltages.  I saw close to a 90Vdc drop across the second Filter resistor.

I swapped preamp, and PI tubes and found that a 12aU7 worked heartily with out any sound issues at all.  Sounds great now

Looking back, I realize that I never added an additional capacitor to the second resistor node, as the 5e3 has a similar setup, where two tubes share one node.  That's what i get for building the two ends of the chain before the middle...

I'm assuming that the extra current draw from two paralleled stages is drawing too much off from that node and the 33uf cap cannot fulfill the load.  I believe that the 12aU7 is a better current handler than the x7. -  So that's why it works???

I'm going to add that "extra" cap to aid in the supply.

Any better descriptions and/or recommendations are welcome.

It would seem that I'm learning something here.  Thanks  everyone.