Tube Time

[petemoore]

...er at least that's what I'm hoping.
 when I try going very high gain, which pretty much means medium volume it's ragged, lower vol. distortions do Ok, so I'm hoping that output tubes will help out there, I don't really know how impressed I am with the 4 Celestion Greenbacks [Marshall cab], as I haven't done direct A/Bing, I do know they are somewhat mismatched ohmage wise by listening to the individual speakers output levels.
 Anyway it's a Marshall MkII, EL34's in output sockets, is this amp self biasing, or do I need to re-bias to try different output tubes?
 I think there's Boogie tubes in there now, not known to be the best, IIRC it was rumored that Svetlanas for output on this one?
 Any suggestions as to tube brand in the EL34 line, or biasing info, a walk through would be nice, I've worked on Tube amps before, and understand the necessary safety rituals and possible consequences of not religiously following them.
 I have some GT El34's that I would like to put in there and compare.
 Anyone have opinions on the MkII Vintage-Reissues? I still need to mod channel ones way high end harshness also, the volume knob 'glitches' and the sweet spots can't be found, I suppose I need to taper it so that the pots range is modded and the glitch would be in a non used part of the range.
 For a high end rolloff cap here at channel ones input [would that be a decent spot to rolloff high end harshness, or should I follow further into the cct to shunt highs off to ground?], what is a safe minimum capacitor voltage rating?
 any help is appreciated !!!

[Jim Jones]

Hi Pete,

Yep, you'll have to adjust the bias when you make a tube swap.  A lot of guys desolder the wires going to ground from the power tube cathodes and install 1 ohm resistors.  That way you can measure the voltage drop across the resistor and that voltage will be equal to the cathode current in mA.  I have some biasing tables on my site you may find useful.  http://members.shaw.ca/house-of-jim  Look in the "Info" section.

The bright channel on your amp probably has a 500p bright cap.  I like switching those to 100p, that's the value used in the old organ model heads actually.  You still get a little chime at low volumes but it doesn't screw with the "feel of the taper" of the pot.  And it eliminates the "icepick-in-the-forehead' treble.

Jim

[AL]

Jim,

Nice site.  There's a lot of good info there. Thanks for the link.  

AL

[petemoore]

After diddling with the input caps. I believe I raises the value between input and wiper, also used a .0015 between the outer lugs of the Ch1 pot.
 Still a Marshall and all, but it's a different amp now.
 So I need to read some more now...

[petemoore]

I need some general reads, I still haven't found the Cathode pins.
 To actually Adjust the resistance...is that the litte 22k trimpot on the PCB?
 So to check that I'm on track...from the bias tables [hot med cool] for setting bias on el34's, I get a 1 ohm between the cathode and Gnd. then measure the voltage across that resistor and match that to the bias tables for EL34's ?

[Jim Jones]

Hi Pete,

Yep, that 22K trimmer is to adjust the negative grid voltage on the power tubes.

What I normally do is remove the old set of tubes and adjust that trimmer to give maximum negative voltage.  That way, when you pop in your new tubes they're running as "cold" as they possibly can.  When you take the amp off standby you want to measure the plate voltage on your EL34's  If you're using the tables I put together, go with the current settings that correspond with the nearest plate voltage.

When you adjust the bias the plate voltage will change too so you might have to "go back and forth" a few times between plate voltage measurements and bias adjustments to get where you want to be.

Some people like the sound of tubes biased a little hotter, if you don't notice much difference between a higher setting and a "cooler' one, go with the cooler setting - your tubes will last longer.  If your power tube plates are glowing red you've officially biased your amp too hot.  

Pins 1 & 8 are tied together on EL34's - just lift the ground connection and solder in a 1 ohm resistor between them and ground.  You're correct about measuring the voltage across that resistor.

Hope this helps some!

Jim

[Paul Marossy]

Yeah, and if they are running red, don't leave them like that! They will be destroyed in short order.  :cry:

[petemoore]

I appreciate all the help !!!
 I did 'nt get it the first time in, messing about in that area.
 This time it's 1000% better, at least in the high gain region THAT's what I haven't been able to get the whole time.
 Modding the front end, [I couldn't even make out the cap to wiper...no matter], the rolloff cap to ground just let me open the amp up like unbelievable [to me]. I was in denial about it, but compensating all the time [in retrospect..totally unsuccessfully], that one mod made all the difference in the world.
 I've had this amp since '96, just now is it filling the role it was intended for [IMO], they must figure Ppl are going to be trying it with a guitar plugged straight in or something because there was a monster in there on an invisible chain [go outside the limits and it gets zapped], that high end crap is for the birds.
 I haven't messed with the clean Boost settings, however I'm certain some of the high end content will be smaller, I doubt I'll be able to drill the face of the cute little head and put in a bright switch [I think it could use a bright switch].
 Absolutely Fabulous !!! If it weren't mine I'd give it a 10...lol !!! There's nothing wrong with this amp. Best dang Marshall Tone I ever owned, and I've owned a few.
 I was using the SD-1 from GM arts as the main Distortion, Guitar>Minibooster>Rangepig>ThunderChief>SD-1>Proverb>Amp, [Low Noise even with all that..!!} I did all the classic 70's test passages by Aerosmith, ZZTop, Steppenwolf Lynerd Skynerd etc. some Cream, Stones and Zep...can't believe the amp sounds this good, because it was so tardy before !!!
 So I'm tooting my horn here, and yours too guys, thanks for helping me find the tone...I define that as the goal of this community, and today I'm proud to say it has totally delivered ... Thank You, and you, and you!!!!!!!!!!!!!!
 Great Sound from a Great Place !!!