I took the plunge today into the world of tube amps of sorts by way of a Marshall 9005 rack mount power amp.
Said amp is a 50 watt per channel, EL34 based pushpull design, with two 12AX7s per channel (one as driver, one as phase splitter, of course!).
Either of you guys (Eli, or Steve) want to take a crack as advising me on modding this bad boy? I haven't tube rolled yet (all tubes are stock Marshall ones, so pretty old even though they test OK!). I have the schematics (thanks to Dr. tube's website!).
While it doesn't sound that bad as a HI-FI amp (not its real intention, and I won't pretend they are!), it shows serious promise if tweaked out! Sound is a little on the "blurry" side, but the imaging is acually really good. Bottom and top end extension aren't the greatest either (output irons, my guess).
Let the solder flux fly!!!!
If you are considering replacing the O/P "iron", take a look at what your "local" winder (Hammond) has to offer. The MagneQuest ST70 replacement O/P trafos are better than Hammond, but they cost a good deal more.
Take a look at increasing the EL34 idle current. Maximum power O/P will be reduced, but moving the EL34 operating point closer to class "A" will reduce cross over distortion.
50 watts is a cake walk for a pair of EL34s. I have some Bogen KO100s doing 90 watts on a pair at 800 v plate 425 on the screens or so.
I can send you a schematic so you can see what I'm working with. Drop me a line directly, so I can reply with it it you like.
I understand the "optomistic" power rating, but remember, this is first and foremost a guitar power amp. So that rating could be obtained quite possible against a much higher distortion rating (hint: it actually can drive the DQ-10s to relative high 80dB range in my listening room!).
let me know, I'm taking any advice currently before taking it apart!
Get your Strat or Gibby out and air the baby out. Try it through a half-stack (4 x 12 inch Celestion loaded would be best).
Try some Hendrix riffs, maybe a little EC Blues, and some AC/DC or LZ power stuff. Then, you'll get a feel for what needs working on (if anything).
I'd played the thing for a good week or two---at volume, before even thinking about tube substitutions. What you have may be just dandy, as is. And that's the best way to go.
BTW---Marshall PT's always run hot. I wouldn't worry too much. If you go to over-sized PT's & OPT's---you'll lose a lot of that early crunchy "Marshall tone."
If the thing needs power tubes---JJ E34L comes very close to the Mullard tone. Front end tubes aren't that critical, in my opinion, as long as the voicing is cool. You can chase after THAT TONE, by subbing out expensive Mullard ECC81 and ECC83 tubes (in my opinion, the best in Marshalls). Or go after the coupling caps with SBE 716P Orange Drops or even Jensen oil-filled caps.
But, only do this after you've out some miles onto the unit. Try several different guitars. They make a big difference, especially with Marshalls. The pickups can make or break a tone.
I think that "Dman" intends to convert the amp to HIFI duty. That's the reason for considering new "iron".
Yeah, well if Yngwie Malmsteen can do it, why can't I!?!!
Just kidding, yes I would love to mod this thing for Hi-Fi use.
Drop me a line, so I can send you the schems if interested at seeing what makes R'n'R tick!
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