1st Distortotron Amp

[Justin Thomas]

Yay, it worked!

I whipped this up in an old Ampro projector chassis. I was trying to make my first "hi-gain" amp though I'm looking more for "Young Brothers" than "Santana." I've got something "workable" but it's too much. Too over the top & unmanageable. I'm thinking some strategically placed voltage dividers might help & am asking how to implement them here? Nobody is using pentodes for that or so few stages...

Put one after the 1st Volume control before the 2nd 5879 grid? Put one before the coupling cap into the PI?

I've got three holes for knobs but the tone controls are concentric so I have two Volume controls too putz with, and I'm also committed to ising the two 5879s in the amp because I just HAVE to bE wEiRd. I can tone-tweak later but for now would just like some guidance at the moment. Pot values? Dual pots like an Orange Tiny Terror? Different Master Volume? Where to stick voltage dividers? Switch to a cathodyne & add a gain stage or cathode rollower or something?

Jusrin

[Chuck H]

I've never designed with pentodes in the preamp. And you're going with pentodes for the whole thing prior to the PI. () If I have any commentary outside of my usual comfort zone I might say that the amplification staging seems good between the first two stages. Then the second preamp stage is feeding a tone stack so there will be some losses, though the standard TMB stack should be a decent impedance match there will still be A LOT of gain hitting the PI input. More than necessary for overdriving both the PI and power tubes. You do have a gain control before the PI so that is probably sufficient for controlling clipping aspects of the stages. But I might try adding a post PI master just to see what effect might be gained by getting more drive to PI stage relative to power tube clipping. You might be able to get a little more preamp clipping doing this, adding an element of compression and accented harmonics.

Your component values look smart for the goal in mind. Especially the tone stack. Maybe also experiment with a higher value for the fixed 10k mid dump resistor in the tone stack. With that 750p treble cap it may not make much difference, but it may fill out and thicken the upper mid tone. As long as the low end doesn't get muddy.

[35L6]

Screen bypass on V2 ?

[Justin Thomas]

Screen bypass on V2 ?

Thanks, I think I put it in there but forgot it on the schematic. I'll check it tonight!

Jusrin

[Justin Thomas]

Hi Again,

Consulted my amp & yes I've got the screen bypass on V2; forgot to draw it.

By PPIMV, are we talking the Trainwreck 2/Lar-Mar? I'd already tried a Crossline (TW3) & the TW4 is what's in it now... If so I think that'll involve ordering parts. I'll look through my stash of crappy pots & see if I got something that'll work to futz with. 'm also limited on knobs, so I'll hafta replace the current 2nd Volume with the MV.

As for a voltage divider to dump some signal, that's just a simple voltage divider with two resistors going into a grid, correct?

Jusrin

[Justin Thomas]

Had some ideas with pots so am going to see if my local shop has a couple dual and concentric pots. Will check back in hopefully on my next day off.

Jusrin

[Justin Thomas]

Okay, so I've tweaked a bit & it's a LOT better. Updated schematic attached, mostly in the tone stack & added the PPIMV. I took some inspiration from the Tiny Terror except I used concentric pots instead of a dual pot. The Master is a concentric with the shafts tack soldered together til I can order a dual pot.

Originally I had just added the MV & changed the teeble cap; it was really wooffy at most settings with the gain high & MV low. Now it's a lot better as far as controls being effective across their range.

I also had a .0033 cap across the PI outputs that I neglected to draw originally; I took that out.

Still a work in progress but much better!

Thanks Again,

Jusrin

[g1]

The Master is a concentric with the shafts tack soldered together til I can order a dual pot.

I'd suggest removing the solder and experimenting with slight offsets between the 2 sides. Maybe even more than slight.

[Justin Thomas]

G1,

If the knobs weren't so damn tiny I might do that! They're hard to grab with my bass player fingers.

Jusrin

[Chuck H]

I've done some fun experiments with the symmetry of PI drive to the power tubes. Most of what I learned was that if the drive is asymmetrical the tone is more loose and hairy and as it's dialed in to be more (or even perfectly) symmetrical it gets tight and crunchy. It's a really fun thing to play with if you get the chance And...

This is different from waveform distortions related to differences in duty cycle WIDTH. Which can be very different at changing drive levels with some amps. Like those Marshalls that sound like they have a built in touch wah. Sometimes called "swirl". It can be a very expressive effect. I can see it on the scope and even CAD simulations when it happens but I haven't learned how to control it for it's best contribution entirely yet. Anyway... Experiments are fun and sometimes lead to discovery if one is paying attention.

[Justin Thomas]

I'm thinking I might fire up the scope more often just to have fun. It's got a 1kV input threshold so ...

Maybe mucking about is in order later today.

Jusrin