Need advice on preamp project to mate with Dynaco Mark III

[thejaf]

Here's the situation: I was cleaning out my office closet, and "found" my old Dynaco Mark III power amp. My dad gave me this amp years ago, as it was kicking around his electronics repair shop, and it was used as the power for my first "real guitar amp" setup, as unfortunatley I started playing during the late 1980's rackmount era. Assorted preamps were utilized, can't remember them all, but once I bought my first real amp head (early 70's Marshall 50-watter), I stopped using the Dynaco power amp. Since my dad gave it to me, for sentimental reasons I never sold it. Before giving it to me for guitar use, he installed input volume pot with push-pull on-off switch.

So now, fastforward 15-years, and I'm itching to make a preamp project to mate with this old Mark III. I took it apart yesterday, replaced the broken push-pull pot (shaft had sheered off in one of the various house moves). It has all the old Mallory 600vdc 20uf electrolytic caps, but none appear leaking at all. Original "made in the UK" KT88 power tubes and 5AR4, Sylvania 6AN8. Last time I used it, was very quiet (no hiss) and worked flawlessly. I blew out the dust, but haven't fired it up yet. I don't own a variac, but with the 5AR4 will bringing up voltage slowly matter (since it's a slow powering rectifier tube)?

My main amps are either 50-watt AB763 head that I made last year for cleanish sounds, and Voodoo Amps hex 50-watt head for the dirtier sounding songs we play. Both heads put through Bogner 4x12 with V30's. I noticed BobW has an ODS inspired preamp on the Dumble Files section, maybe try that or go for something more aggressive. I really liked my Bogner Shiva when I owned it (circa 1999 model), so was leaning more towards something inspired by that circuit (smooth, fat, lots of warm mids, clear highs and tight low end).

Recommendations, thoughts, adivce would be well received!

Thanks!
Jeff

[jon]

Alembic f2b

Good enough for Gilmour, based on ab763.

[thejaf]

Thanks for the suggestion, I checked out that schematic. I'm looking for something more Marshallesque though (already have AB763 inspired amp).

Rummaging through my parts bin thus far, I have a small vented enclosure that I've chosen. Cut garolite board yesterday to fit, added two 9-pin sockets and wired up bridge rectifier and 10,000uf cap for DC heaters. Decided to use the 8-pin socket on the Mark III to power this preamp.

Question: I have a half dozen or so 10uf 350V radial e-caps I could use in the B+ supply on this board, as they are rather small and could cram a few in there. Will there be any noticable effect on the preamp sound using these as opposed to a pair of 50uf per the old Marshall power supply?

[cpollack]

[quote="jon"]Alembic f2b

Good enough for Gilmour, based on ab763.[/quote]

Yup. Do this and you'll be pretty close to a vintage Sunn-type amp. Nice combination.

[thejaf]

Well, I finished my preamp project about 2 hours before lost power on Friday (still out), but I was pleasantly surprised thus far. Guided by the available Shiva preamp info (schematic and voltages), used only the high gain channel as I had room only for two tubes. Turned out better than expected, and had way more gain than I remember. I learned a few things too about this circuit, and how it saturates with crunch while still being usable.

I'll post a schematic of my preamp when I get a chance. I'd like a little bit more low end (if possible), but can live with as-is. One last thing, I think the final 30% of the Shiva sound (more like feel) is in the power amp, filtering and feedback, which is not present in the separated preamp/power getup.

[thejaf]

Used it at band practic last night, played this preamp with Dynaco Mark III through Bogner 4x12 with V30s (I assume V30s, stock slant cabinet from late 1990s).

Really happy with it, turned out much better than expected. If I were to tweak it, it could use just a bit less gain and some added sparkle (just a touch).

I have attached the schematic as I remember it. One thing I realized while drawing it out this morning, when I was testing it I had a 470pf ceramic disc cap as the treble cap, and it sounded too grainy for my tastes. I searched through my parts bin and installed what I thought was a 432pf poly cap, but after research this morning I think it is a monobloc cap, just measured another one to verify it's spot on 432pf. Changing to this did clean up the high end quite a bit, FWIW.

I put in some old 12AX7s from my parts bin: V1 is an old USA long plate with "Baldwin Organ" in yellow silkscreen, while V2 (cathode follower) is a used Mullard with "Bogen" silkscreened. These tubes sound and function fine, so I'll leave them as-is.