The Janitor (on the drawing board)

[DummyLoad]

What is the middle 1K||33u doing? Seems pointless. Unless you intended to take the 1Meg to the other end of it??

fixed schematic for self biased cascode... now it makes more sense.

--DL

[RicharD]

Dummy Load drew it wrong.  The 1M grid leak is supposed to come from the other side of the 1K resistor, just like if it was a plain ol grounded cathode amp, except this time it's sitting atop the plate of another GCA.

[RicharD]

Here's Version 3.1 which is currently what I have up on the board.  I think I'm happy with it.  I finally got to use a 12SJ7.  I have a flippin case of them.  The tone stack was driving me crazy until I removed the bypass cap on the recovery stage.  I have used this tone stack before for a bass guitar preamp that everybody loves so I was confounded as to why it sounded so bad.  Too much recovery I guess.  Dummy Load dropped a "BAC" (big ass choke) on me and WOW what a difference that made.  10 pounds of "hum be gone" & "attack definition."  The final tweak on the output stage was a little global NFB which shaved off the last little bit of harshness.

[Tone Junkie]

Cool Butterylicious very interesting design.
Bill
                     

[octal]

That's funny. A project developed to use up 12au7s and experiment with a cascode ended up using neither. 

I do wonder if the voltage divider biased cascode might have performed better if it was set up differently. I'm just parroting Merlin (The Valve Wizard chap who had the sample 12au7 circuit I listed earlier in the thread) but supposedly depending on whether the load line passes above, below or through the knee of the curves can influence how 'pentod-y' a cascode sounds. I dunno. I probably will try his sample circuit at some point and will report back with results.

It sounds like overall you're pleased with the results though & that is what matters most of course!

Nathan

[RicharD]

12AU7's would totally work in place of the 6SN7's.  I jumped to the 6SN7 because of voltage concerns with the PI when it was still a Concertina.  By the time I went LTPI, the bread-board was already wired.

The cascode circuit was not a flop.  I was just shy of desired gain.  I slapped the 12SJ7 circuit in as a comparison and well.... you know your ears can make decisions.

My real battle was with the tone controls.  My original idea is not only costly but I was quick to settle on singular values.  I've used this James tone stack before in my Kelsey Bass project.  It was sandwiched a little differently.  Since I now had a shade more gain than necessary and a spare 6SN7, this seemed like a good idea.  The cathode follower is somewhat superfluous but doing no harm.  The following stage was originally a plate follower matched to recover losses from the tone stack.  Something was nasty, perhaps my feedback network was adversely affecting my treble control.  Ditching the 2 resistors and cathode bypass cap brought it right.

The icing on the cake is that big honkin' choke.  That made such a drastic improvement in noise and dynamic sensitivity.  The improvement in sensitivity may easily be a placebo effect from the improved noise floor.  Don't care.... I'll take it either way. 

[DummyLoad]

A project developed to use up 12au7s...

sub the 6SN7s with 12AU7 - doubt there would be any difference...




EDIT: is there  a parrot in here? - i see butterboy already mentions this...   

[DummyLoad]

....10 pounds of "hum be gone" & "attack definition."

if you ever build that amp, yer gonna get a hernia moving it.

[DummyLoad]

that thing sounds excellent! i had a chance to hear it for the first time since the12SJ7 and choke PS mod. very nice attack and punch with a very smooth tone, a tone stack that works very well with bass and reasonably well with guitar. it's whisper quiet with the DC filaments and stiff PS. with a guitar it's clean and loud and can roll off into a mean grit tone - if your ears can stand the volume. we measured about 60W RMS clean with about 75W RMS driven into overdrive. nice work butterman - that 12SJ7 first stage sounds fabulous.