FinallY !!

[Deisel401]

Finally smartened up and purchased a JJ 5751 per MattFigs advice. This is the 2nd biggest tone change I've experienced in the last 3 or 4 years. The 1st being a Plexi/SL+ module Scott Souter built for me. Most amazing Marshall tones I've ever heard !! Anyway, can't believe how clear and punchy this single tube made my RM20. Totally kicks *** !! Can't imagine the JAN 5751 being significantly "better", but have read a lot of posts that claim it is. I do have a question for all of you about biasing; I bias like everyone else, I assume. All controls at zero. I set my EH EL84's between 19 and 20 ma's. I like it a bit hot. After letting the tubes stabilize for 15 or 20 minutes,I set all knobs to taste and play loud and quiet while watching my Multimeter. I'm seeing the bias fluctuate between 19ma, and up to 50 or 60 ma's. Is this normal, or an indicator that something is wrong?

 

[webrthomson]

Bias should be set with no signal flow at all (and on some amps the volume set to zero - can't remember about Randall?s) - the minute you play the bias will shoot up as per your readings.

Not sure if they are in the expected range for a RM20/22 or not but I would guess so - bigger tubes (EL34/6L6) certainly jump to 3/4 times the sitting level.

Cue - Rob at this point, with the real science ;-)

Nice feedback about the 5751, I have been thinking about that for a while too.

 

[Jaded Faith]

Setting the bias should always be done without a signal plugged into the input and with a load attached at the speaker out. It's the exact same concept as setting the idle on a car engine. You are determining where it sits when at rest on the gas/input.


The manual for your amp states 15-20mV as an acceptable range. In my experience, the 20/22 are best biased a bit cold, regardless of your typical preferences. The power supply is not very robust and there's not much in the way of headroom so hotter settings are very compressed.

As for the meter jumping as you play, that's exactly what should happen. It's like watching the Tach jump when you hit the gas pedal. It's nothing to see the value go up 6 times your bias setting if the amp is turned up to about 1/2 on the Master and chugging some palm muted chords.

 

[eyeball987]

I have a JAN GE 5751 in the PI spot of a RM100 and love it. I keep 4 or 5 around at all times.

 

[Deisel401]

Thank you Rob. Sounds like you just saved me a lot of trouble down the road. I've read about folks changing their transformers to Mercury Magnets. Perhaps the less than stellar stock transformer is the reason.

 

[Jaded Faith]

You're quite welcome. I often get asked about tranny upgrades and my general answer for the MTS line is don't do it. Particularly those who use modded modules, as there can become too much of a good thing. I have had MM in my own amps and installed them for others, but I like the stock tone better personally. I often see those who upgrade start to spiral, looking for darker speakers, darker tubes, etc. They try to undo the changes the tranny made and create all these other issues!

 

[Mattfig]

I have one RM100 with Mercury Magnetics and it's the best sounding one I have....How much better? That's all opinion...I would say it brings a smoother feel to the amp itself...Certain modules in particular really shine through that head - SG Afterlife being the biggest benefactor....

 

[Jaded Faith]

I have one RM100 with Mercury Magnetics and it's the best sounding one I have....How much better? That's all opinion...I would say it brings a smoother feel to the amp itself...Certain modules in particular really shine through that head - SG Afterlife being the biggest benefactor....

How's your high end hearing testing out these days! :lol: I kid, I kid.....

I think the one thing I forgot to mention was the cost to benefit ratio in this application is also kind of poor all said after parts and labor.

 

[Hamner1]

Where are you guys finding the JJ 5751 tubes?

 

[Jaded Faith]

I buy them through one of my parts distributors, but a quick Goggle search of "JJ 5751" shows them at all the regular consumer options.

 

[eyeball987]

Some tube pron. Just because:

 

[rblyn]

Thats probably one of the few things I havent tried on my RM100's or RM20. (putting in a 5751 tube into v1) So does this give you more headroom? I would assume less gain too- Is the different that much? I think I may have to give it a shot- Anyone have input on the JJ's vs the Jans? (JJ's are a bit cheaper!)

 

[Deisel401]

That's def tube porn !!! Thanks buddy . . . . And rblyn . . . I haven't purchased the JAN 5751 yet, due to funds. But I just got a JJ last week. To my ears there's a very noticeable difference. I wouldn't say less gain. Just smoother, tighter, the highs are no longer harsh in any way, and just better. And maybe it's my imagination, but I'm quite sure my Presence knob is more responsive. Got it at Tube Depot for under $14.00. DEF worth it. If the JAN tube is better than this, I'm gonna have to break down and get a couple soon.

 

[Mattfig]

Quite honestly I notice zero difference in brands in the PI position....5751 of any brand that is in good working condition should do the trick...

 

[eyeball987]

Quite honestly I notice zero difference in brands in the PI position....5751 of any brand that is in good working condition should do the trick...

I would guess that my findings would be the same in the PI. I have just never had any other 5751 than the JAN GE's.

rblyn: in the V1 spot of an amp or module, a 5751 will lower the gain, give more range on the gain knob (headroom), and I would guess have an audible difference between tube types. I have put one in the first position of a RM100 and a module before and had varying degrees of success for me depending on the module.

 

[Jaded Faith]

I have traditionally used the JAN 5751 because it was the only good option out there. I would buy 6-12 at a time and always have then in the shop in stock. Once I was turned on to the JJ offering, it's all I buy now. I am with Matt here, I can't tell any difference in the PI position versus the JAN which costs twice as much.

It's best to think of it's impact as adding clarity and definition to your notes and particularly within distorted chords. Don't think of it strictly like a gain reduction technique as in a V1 swap.

 

[Deisel401]

One more thing I wanted to mention, with the JJ 5751 in the PI spot, i noticed no difference when changing the tube in the Input Gain spot. I switched from a JJ ECC81 to a Mullard 12AX7 for Input Gain, and heard no change. I read a post from Rob , if I understood correctly, you really don't want a low gain tube in that spot. Could be just my ears.

 

[Hamner1]

I definately need to get a couple for the RT2/50.
But how would you compare it to a 12AU7 in the V1? I put one of those in the RM4 before so the cleans would actually be clean, but it seemed like the High Gain mods lacked any life. Once a 12AX7 replaced the 12AU7 the high gainers were back to breathing fire.

 

[kc2eeb]

A 12AT7 would also be one to try, the Fender standard PI tube.