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PostPosted: Mon Dec 09, 2013 3:47 am 
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Mark II

Joined: Tue Oct 08, 2013 2:29 pm
Posts: 61
Hey everyone, hopefully someone can help. I've been searching literally all day and have to go to bed now.
I have parts ready to install into my Ace to make it adjustable bias but I can't find the bias resistor in the amp to replace.
The instructions and schem that came with the Granger kit is for a rectifier (6L6) amp with an 82K and a 22K but I think the Stiletto Ace's EL34s have a different bias arrangement?

Can anyone who has installed an adjustable bias mod into a Stiletto (ace preferably) PLEASE give me a hane or shoot me a link or photo?
Please?
Thanks you.


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PostPosted: Mon Dec 09, 2013 1:02 pm 
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Mark II

Joined: Tue Oct 08, 2013 2:29 pm
Posts: 61
OK...a little more detective work.
I went back and checked my email correspondence with Granger Amplification.
I had told them I needed parts for a Mesa Stiletto and they recommended the 8.2K resistor with a 50K trimpot.
The schem that came with the parts was for a rectifier so said to replace the 22K or 82K resistor.
Well, the Stiletto, being an EL34 amp is different.

In the Granger email they said to replace R82 (10K) which is in parallel with the (R83) 47K resistor. The (wrong) schem shows the bias resistors in series with two 47uF 100V caps.
OK so I've located the 10K (brown, black, black, red) over near the 47uF caps but there aint no 47K anywhere nearby. The resistor cluster in that location obstructs the view of the R#s on the pcb of course. Not that you can really go by the pcb's numbering since there are probably several versions/revisions?

SO...here are a couple of pics. One shows a larger gutshot and the other shows the caps and resistors (and the 10K brn, blk, blk, red) in question.
Can anyone confirm that this IS the 10K I need to replace with the 8.2K/50Ktrimpot?
HURRRRRY!!!!
Mama is getting tired of looking at the bench...which is actually the rec-room floor. It's littered with literally 1500 Ge ttransistors being tested and matched, FIVE Shure mics being repaired (grrrrrrr), a pile of amps and effects...and the Stiletto all over the place :-(.

Pics:
Image
Image

Thanks in advance for any help you can donate.


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PostPosted: Mon Dec 09, 2013 1:06 pm 
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Mark II

Joined: Fri Jun 11, 2010 10:46 am
Posts: 91
Its R 83 a 39 K resistor


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PostPosted: Mon Dec 09, 2013 4:37 pm 
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Mark II

Joined: Tue Oct 08, 2013 2:29 pm
Posts: 61
Thanks for the reply GordyBaby.
That's what I thought too and what makes the most sense. AND...there's a 39.2K right there (orange,white,red,red) to the left of the 10K.

Dude at Granger said replacing the 10K (which is in parallel with R83 47K...of which there IS NO 47K in that location/area...it's the 39.2K) with the 8.2K resistor and 50Ktrimpot will give me a net range of about 7K-26K (I guess he means in parallel with the 47K?...which isn't there) . The actual measured resistance sweep of the 8.2K and 50K trimpot is actually 8.1K-56.9K.

SO...I guess those 2 resistors ARE the ones I'm dealing with? Is it then just a question of do I want to replace the 10K or the 39K with the little trimpot board?
Seems like ...as GordyBaby says...makes a lot more sense to replace the 39.2K fixed bias resistor. Don't think I'd ever want to put 57K (56.9K) of bias resistance where the 10K was? WOULD I?

The more Resistance, the less current right? And let me think for a second...less current on the screens isn't what I want is it?
God...now I'm confusing myself :-(

So replace the 10K or 39K with this 8K-57K doo-dad?
Thanks again GB


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PostPosted: Tue Dec 10, 2013 1:23 pm 
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Mark II

Joined: Fri Jun 11, 2010 10:46 am
Posts: 91
I purchased the Mansfield bias mod and followed his instruction to the letter and have never had a problem.


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PostPosted: Tue Dec 10, 2013 3:13 pm 
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Mark II

Joined: Tue Oct 08, 2013 2:29 pm
Posts: 61
Yeah. I kinda wish Mansfield was still up and running. Evidently his kits came with proper instructions for a Stiletto.
Mine is from Granger.

So...if anyone KNOWS whether it's better to put a 8.2Kresistor/50Ktrimpot in series in place of the 39.2K as Gordybaby suggests and as you'd think would be right....or in place of the 10K as Granger who built the kit say to do (in an old email...not in the instructions, which are for a 6L6 Recto)?
Please?
Anyone?
Pretty Please?


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PostPosted: Wed Dec 11, 2013 11:05 pm 
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Mark II

Joined: Tue Oct 08, 2013 2:29 pm
Posts: 61
Really? No one?
bump


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PostPosted: Thu Dec 12, 2013 6:17 am 
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blaren wrote:
Really? No one?
bump

Your question has already been answered.

Gordybaby wrote:
Its R 83 a 39 K resistor


Dom

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PostPosted: Thu Dec 12, 2013 11:46 am 
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Mark II

Joined: Tue Oct 08, 2013 2:29 pm
Posts: 61
Yeah you'd think so. I swapped the 39K and I'm not getting any measurable range from the trimpot. I wouldn't be so needy if I had any resistors but living where I do, ALL I have access to are 1/4w.
Since the manufacturer of the kit says to replace the 10K (their resistor and trimpot might be different than the Mansfield one everyone incl Gordybaby used), I'm thinking it might have been the way to go. In my mind and being an old Marshall guy, the 39K made a lot more sense.
If anyone is CERTAIN that with an 8.7K resistor/50K trimpot you want to replace the 39.2K, then I have other issues. If it should have been the 10K...well...I have to find a new 39K resistor and start again.

NewDay Bump.


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PostPosted: Thu Dec 12, 2013 12:33 pm 
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With the pot (50k) and resistor (8.7k) in series the available resitance range is a minimum of 8.7kohms and a max of 58.7kohms, perfect to replace a 39k resistor (39k would be just past mid-point). If you replaced the 10k you would not get the proper adjustment range.

I would suspect you have a poor solder joint, or maybe compromised a nearby joint or you may have damaged the PCB trace underneath from excessive heat.

Where are you measuring that you have no range?

Dom

_________________
'07 Roadster Voodoo modded
'08 Stiletto Deuce II
'10 Mark V
'01 Recto 4X12 Slant
'04 Recto 4X12 Slant/Straight
'74 Les Paul Custom
'95 American Strat
'12 Les Paul Signature T
Slinky's & Jazz III's
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PostPosted: Fri Dec 13, 2013 11:11 am 
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Mark II

Joined: Tue Oct 08, 2013 2:29 pm
Posts: 61
Thanks.
Yeah that's why I did the 39. Just made all the sense.
It wasn't giving me a reading before either. Well like barely reading.
Board is fine, traces are fine, used heatsinks so as to not put heat into the board.
Meter probably needs a spanking.
Thanks again.


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PostPosted: Sun Feb 28, 2016 9:12 am 
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Mark III

Joined: Sat Jul 01, 2006 1:23 pm
Posts: 264
I have a question on this, if anyone is still reading these posts.

Last night I confirmed that the current to the power tubes changes, with channel one, depending on if you have the mode set to a clean mode,(fat clean, tite clean), or crunch. Having a EL34 in there that pulls 36ma in channel two, pulls over 50ma in channel one-clean mode.

There are two resistors in parallel, R82(10k) & R83(39k).

What feeds the Clean mode to increase the current?

Is the another resistor that can be replace to lower the bias to the Clean mode?


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PostPosted: Sun Feb 28, 2016 10:47 am 
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Mark III

Joined: Tue Dec 22, 2015 12:15 pm
Posts: 283
I'd be surprised if they used different Power amp bias to act with selectable pre amp modes.

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PostPosted: Sun Feb 28, 2016 11:23 am 
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Mark III

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Nicklotsaguitars wrote:
I'd be surprised if they used different Power amp bias to act with selectable pre amp modes.


I just measured this with a Weber BiasRite meter.

If you think about it, it would be pretty easy to use a parallel resistor that gets added to the existing bias circuit when in a clean mode that would allow more bias current.


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PostPosted: Sun Feb 28, 2016 3:42 pm 
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Mark III

Joined: Tue Dec 22, 2015 12:15 pm
Posts: 283
Would you want even the momentary break in contact as that switch operates. Acting upon the bias circuit?
Sorry, I just don't see it myself.
Mesa rely pretty much on pre-amp circuits for amp tonality, it seems to me. Now OK, I can't say that about every model. But looking at the way Randal Smith thinks about it. It seems pre-amp for tone shaping, power amp for horsepower. It would also make sense that is where he looks to bolster durability. It's rare to hear of pre-amp circuits developing faults. Well rare in comparison to how often power amp sections do. Even on cold biassed amps.

As I typed this, I had a voice in the back of my head going, "you don't know for sure Nick". So while I don't see it as a worthwhile risk. I'd in fairness have to admit that someone has probably done it.

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