Tiny blues express combo / 12AU7 PP w. cathodyne PI

[El_Martin]

Hi!

Been foolin' around with a Tiny Blues Express for some time now. It ended up as a combo w. one 8".

See http://ampgarage.com/forum/viewtopic.php?t=3029 for Timos original documentation.
Slightly changed the layout. Two more resistors mounted on the turret board.

Since I decided to use a toroidal PT w. lower Voltages (200 V AC), I had to change some values.
I used a 100V PA transformer as OT. Primary load is 8k. Secondary outputs for 4/8/16 Ohms. Hammond has some nice OT for low power PP. Spend the extra 15 bucks for those Hammonds.

Here are the differences:

- tonestack treble capacitor 47 pF Silver Mica

- mid pot is 100k log. Gives a nice boost, when maxed. OK, treble&bass will be less effective.

- second triode 500 pF across Ra (less sizzle/buzz)

- after second triode and the coupling capacitor (2n2) there's 100k to ground. Tweak here!

- third triode has Rk=4k7 (10k original)

- 470k gridstopper before cathodyne
- cathodyne w. 56k - 1k - 56k
- 47pF (was 100pF) across cathodyne output

- output triods have 4k7 opposed to 5k6 gridstoppers
- 500 Ohm instead of 470 Ohm as Rk for the PP triodes

Power supply without sag R (original 180 Ohm)
chain: 47uF (220k/5W to ground, discharge) - 10k - 22uF - 4k7 - 22uF - 5k6 - 22uF

Voltages are: 273 270 265 254 [V]

Somehow all my builds end up as fully crammed lil boxes. Dunno why...
Lots of parts from the parts boxes. objet trouvé.

The little beast is still way too loud for bedroom level. Even w. the 8". Like its sound most w. Cel. 65 W (yep, that RF D-type speaker). ECC99 will be accepted as "power" tube.

Its very touch sensitive for an amp of this size (~1,5 W). Clean will be with lower volume. When cranked, there's a lot(!) of gain for three gain stages. For some more tweakin, I'd work on the PI and the output tubes. Maybe some capacitors here&there. Or some Zobel around the OT (like in some TW w. EL84).

Thanks for krash/joshs and UR12/danas valuable input.

Enjoy
Martin

[mcrracer]

Good Job! Very interesting cab treatment. Any clips? How do you think a 6SN7 tube would work for the power tube? I thought a 12AU7 as an output PP needed a primary impedance of 22k. Is yours really wired as 8k and it sounds good ? Thanks

[El_Martin]

TNX, mcrracer!

That finish is called "burn baby, burn"

6SN7 as output stage can be done. Requires some altered values.

See other postings regarding 12AU7 as power tube for reference. Sound is better into 8k primary load. Triodes dont sound exactly the same as pentodes. But speakers also sound different pummeled with that mighty 1,5 Watts

Currently the beast likes caps over guitar volume pots together w. humbuggies or SSH strats. Maybe I'll include switchable bright caps w. 3 positions: c1 off c2

Trainwrecks are a bit sensitive
Must admit, I like that. To a certain degree, that is.

Ciao
Martin

[flood]

Ha! I'm doing the same thing - converting my firefly to a 1.5W blues express type amp, using the cathodyne PI and 470R//47u (non-polarised) cathode resistor on the 12AU7 power stage. 8k2 grid stoppers though.

other changes: the tone stack is a bit different - using the Vox TS (because i only have 3 holes and can't modify this chassis much) perhaps with a 3 position bright switch or tone stack defeat switch, not decided on that one yet. no sag resistor either. PSU resistors are more like 22k/5.2k/4.7k. my cathodyne stage will be the one shown in merlin's book, with the diode to stop flashing over etc. DC heaters too. 22k OPT primary.

let's see how it turns out - i like the sound of the firefly while it lasted but i had just built it so badly (first build) that it was way too unstable. time to try something new

[Ian444]

Cool amp Martin! I also like the Blues Express circuit, and lowering that third triode Rk to 4K7/3K3/1K8 sounds good to my ears. Probably deviates from original TW design intent but I like it. A DC-coupled cathodyne with 12AT7/6N1P also works well for me, with 470K grid stopper on the splitter input grid (thanks Merlin Blencowe), third triode Ra = 150K, third triode grid leak = 820K instead of 150K. Very smooth, lots of gain, good clean and cranked/dirty sounds.

[M Fowler]

Great project Martin thanks for sharing.

The burnt wood reminds me of my wife's late Uncle Johnny he did his whole Sand Dune Saloon walls that way! Cowboy country.

[El_Martin]

TNX for the kind replies, guys!

Currently I inserted large gridstoppers for the output tubes as well. 100k instead of 4k7. More "big amp" like Not to much loss in high frequencies.
Lower Miller capacitance, I guess.
No more vomiting cows in the power stage. Smoother transition into distortion.

While working on it, I put in a on-off-on switch for the treble C in the tone stack. It parallels either a 300 pF silver mica or a 1nF ceramic cap.

Now I just like to put another 8" in the back panel.

Ciao
Martin

[flood]

servus martin,

thought i should post here and ask you for your assistance again - i've finished my build and could use a little help... here's my thread regarding my 12AU7 express problem:

http://ampgarage.com/forum/viewtopic.php?t=14650

i did make a couple of mods just a minute ago, switched the cap between V1B and V2A from 22n to 2n2 and the 100k grid-leak resistor is 168k now. put a 10k resistor in series from the "rise" pot to ground. i'm also trying your method of large gridstoppers - using 100k. will post back on the thread after trying these mods. thanks for any help!

[John-JCs]

I love the wodden covers... Very steampunkish

[El_Martin]

TNX, John

Somewhere in the back of my mind ("cheese and onions"...the rutles... ) there's an idea of having a small amp head with pp tubes in a V-shape ... like some Ducati motor. OK, I'm sometimes a little off-track.
Being German, I could as well go "boxer", like in the older BMW bikes. That'll be a real push pull configuration.

Ciao
Martin

[martin manning]

TNX, John

Somewhere in the back of my mind ("cheese and onions"...the rutles... ) there's an idea of having a small amp head with pp tubes in a V-shape ... like some Ducati motor. OK, I'm sometimes a little off-track.
Being German, I could as well go "boxer", like in the older BMW bikes. That'll be a real push pull configuration.

Ciao
Martin

Not too far off-track... it's a good analogy: The power valves are like the cylinders and crankshaft in an IC engine. Then there is a "gearbox," the OT, to match the impedance of the driven elements, those being the wheels or speaker motors.