ODS 75W!

[ElectronAvalanche]

Hi guys,

I finished it today (had half a day off):

Preamp:

Plain Vanilla values

220k,3k3,150k,2k2

0.1 and 0.05uF Tonestack

0.047uF Cl2

Overdrive:

220k,3k3,150k,2k2, 220k into trimmer (100k) -> 68k, 0.01uF and 0.005uF decoupling in OD, 100k pots

Footswitchable (still have some pops though): pre OD Master either 150k directly after decoupling OD2 or Marshall-Tonestack

Reverb:

as per Gary mods, not working yet. But I do get some hum when I turn the 250k Reverb pot.

Now to the special part:

PI as in Dumble, but Cathode Follower after PI as in Bassman 200W (see Tube Amp book for schematic). CF is cap coupled.

Output tubes: KT88. B+= 520V

Now to the sound:

Extremely nice MV sound. Even at miniscule volume you get full a bodied sound. This is due to the CF me thinks. Clean is extremely good, you get almost SSS like sounds.

Still there are some things to fix:

I get some nasty squealing with the OD maxed. Have to check my lead dress.

No Reverb..... need to recheck my ck.

I will post some pics when I get a digi camera.

The post PI CF is definitely something very special!

Electron

[oldmacman]

The cathode follower after the PI is definitely a good thing!

Which tube are you using for the CF?

Look at the block diagram for the 150W ODS to see how Dumble did it. He direct-coupled the drivers to the output tubes with +/- supplies on the CF a la SVT.

I will post a layout for my OD Reverb later to show how to do it with MOSFETs.

[ElectronAvalanche]

Hi Ben,

I am using a 12AT7. I get some 80V on the 12AT7 cathodes.


I would love to see the MOSFET version. How do you like the sound? Do you hear a difference between MOSFET and tubes?

I might use a 12AU7 for kicks some time. Maybe a 12AY7 (need to find one though..).

I have the CF cap coupled, since I did not want to bother with the high bias supply necessary when using a directly coupled CF. Then one should have at least -150V bias voltage.

I used 0.47uF coupling caps for the CF. The PI has 0.056uF on its anodes.

Just one tube changes the way the amp feels.

I am thinking hard about putting a CF in my other 100W ODS clone.

With an external Dumbleator, no Reverb necessary, one could get a total Killer amp using just 4 12AX7 and a quartett of 6L6 or a Duett of 6550/KT88.

Electron

[Bear]

With that topology, is there a substantial difference to full on SSS? There's a post about an EF-86-based preamp, but at least a couple of them are good old 12ax7's.

Bear

[oldmacman]

I can't tell any difference between tube and solid state. The MOSFET just provides a nice low-impedance drive to the power tube grids. It doesn't actually break up.

And yeah, it does a nice SSS.

Here's a clip (backing track from dogears):

http://www.duke.edu/~bhs3/sss.mp3

That's my OD Reverb with my friend's Squier Strat through it, with the clean channel cranked. Before the last chorus, I kicked in the PAB.

Standard 12AX7 ODS PI, 120K/130K plate resistors, .1uF coupling to the MOSFET drivers. Tubes are SED 6L6.

[dogears]

Great playing as usual Ben! Loved it. Amp sounds nice and warm!

Much better than my POD modelling take! LOL Check this out....

http://www.scottlernermusic.com/music/WurzelBlues.mp3

Here's a clip (backing track from dogears):

http://www.duke.edu/~bhs3/sss.mp3

That's my OD Reverb with my friend's Squier Strat through it, with the clean channel cranked. Before the last chorus, I kicked in the PAB.

[Icetech]

Man... i hate you guys sometimes:) I been playing my amp for bout 3 hours... pretty happy.. clean sounds ok with the strat.. OD sounds damn good with the SG.. then i come here and you put those clips.. BAH! i gotta start tweaking:)

BTW ben... is that the 3 tube reverb?

[oldmacman]

Yup, 3 tube reverb. I do it a little bit differently - I mix the dry OD signal with the wet clean signal.

Here's how I do the MOSFET source follower drivers. I don't think the negative voltage supply is enough - I might try 33K resistors instead of 47K.

[tonelab2]

Sweet Sweet sound, and great playing.....

[llemtt]

Ben it seems from your drawing that your CFs are cap coupled to the PI but DC coupled to the power tubes grids, is that right?

Teo

[oldmacman]

Yup.

[MarkB]

Ben it seems from your drawing that your CFs are cap coupled to the PI but DC coupled to the power tubes grids, is that right?

Teo

As I understand, the DC coupling to the power tube grids is the whole point. For a short but great discussion of this subject, read Randall Aiken's blocking distortion article:

http://aikenamps.com/BlockingDistortion.html

[groovtubin]

Yup.

[Funkalicousgroove]

do any of you have a schem for the post PI CF? or are you just using the SVT schem?

[Bob-I]

Sounds great. Smooth and clear.