5751's

[dude]

Are USA made 5751's subject more toward microphonics than a 12Ax7?


Or, tubes noises like static or intermittent sounds usually after or at the end of a distorted sustaining chord.
 
al

[DummyLoad]

5751 is a high reliability industrial tube. has a tougher heater that draws more current than a 12AX7. so yes, should be less microphonic. less gain though. 


--pete

[dude]

Thanks, I was looking to cut the gain a bit. Perhaps this noise I hear is not the 5751.


It's driving me crazy, this noise I hear in the middle and at the end of a sustaining power chord with the gain up.


It's sounds like a frizzle at the end of a sustaining power chord, like a static frizzle that decays in and out in the end of a distorted power chord. The amp sounds great just this annoying "frizzle" is the best way to describe it.


To me sounds like a tube, I have changed the first preamp tube several times with others but it's not that. I have the Paul Ruby mod (diodes and Zeners) I did years ago, from memory this cured some type of frizzle I had at the time, I think.

Anyway, can anyone lead me in the right direction? I have checked all the solder connections and any loose socket pins, all tight.


The el84's are Dutch made old used stock Phillips that sound killer but have many hours on them... does this sound like a power tube thing?


Update: it's not the speaker tried another, new power tubes it's much better but still there (I think), not as bad though, seems to be a lot of blow-by even when no instrument is plugged in and gain is over half up (normal?).


I'm thinking it might be "string rattle" tried different guitars and sometimes I can't hear it. Seems to be worst with my strat which always buzzes a bit just never let it bother me.


Biggest difference was new power tubes, I need to get away for a while, let my ears fresh up a bit, ha. 

[DummyLoad]

try removing the bypass cap from the output tubes and compare.


--pete

[shooter]

it might be "string rattle"

I only have 1 cheapy strat, I chased a cracked "saddle" on the fat E string for awhile, sounded similar to your description

[tubenit]

Do a search on the "enhance cap" which is typically a 120p to 390p range cap across the LTPI entrance resistor. 

I use this on all my LTPI amps and find it helpful with no discernible loss of high end.  It seems to eliminate the "hash" on the high end and smooth it out for me.

Very easy mod to try out.  You can carefully use insulated alligator clipped wires and try different value caps across that resistor.

with respect, Tubenit

[dude]

Thanks for the info on the "enhance cap" in the PI. I also found some interesting info on the PI tube here:


http://el34world.com/Forum/index.php?topic=20918.0


I put all new 12AX7, and a good 5751 in V1, Amp sounded great no frizzle. The power tube change had no effect (after hours of listening and playing my ears become fatigued, and after a long break things sounded different).


Reading the link above, Tubenit and others had some interesting points about what the effects of the PI tube, some suggestions a 12AT7 or even a 5751. The amp I'm working on is a "18 watt Trinity Plexi MK II" , basically an 18 watt Marshall lead, browned a bit. I can post the schematic but not sure if Trinity would allow it...? I'm sure it's available with a Google.


Perhaps the PI tube I wasn't changing was old with many hours on it...? Hence the frizzle...?  I'm going to try a 12AT7 in the PI. I had a friend come over and he didn't hear the slight frizzle but that was after I changed all the 12AX's. 


Thanks,
al


Update:

I'm finding the frizzle or harshness only comes with the preamp gain up past two thirds or the volume almost pegged. I'm thinking that to have to the gain and distortion set so high that PI is over loading. Perhaps a 12AT7 in the PI might be the answer or Tubenit's enhancing cap.

Either way, IMO now an 18 watt is not meant to be pegged, the distortion becomes ratty and over loaded. The amp sounds great with preamp gain no higher than half or a bit more depending on the volume setting, you can't dime these amps or get close to it. The grid leak resistors on the power tubes were lowered to 100K from 470K per the schematic (similar to Jason's PI mod), better cleans and able to get higher on the gain before the frizzle. A voltage divider (470K, 470K)for the grid on V2 is in the schematic.

I'll try the 12AT7 in the PI and see if I can get the gain up more before the frizzle and harshness comes and post.

[dude]

try removing the bypass cap from the output tubes and compare.


--pete

You're not talking about the cathode biasing cap on the output tubes?


al

[tubenit]

Perhaps a 12AT7 in the PI might be the answer or Tubenit's enhancing cap.

I hope you will try both.  The enhance cap has made a significant audible tone difference in smoothing out the highs in my amps. Perhaps it will be a useful mod for you?

With respect, Tubenit

[dude]

Perhaps a 12AT7 in the PI might be the answer or Tubenit's enhancing cap.

I hope you will try both.  The enhance cap has made a significant audible tone difference in smoothing out the highs in my amps. Perhaps it will be a useful mod for you?

With respect, Tubenit

You have several places for changes, in your schematics in that old link but I'll stick with just the enhancing cap in the PI.   Are you talking about this cap:


http://el34world.com/Forum/index.php?action=dlattach;topic=20918.0;attach=61097;image


These are some other changes you mentioned:


http://el34world.com/Forum/index.php?action=dlattach;topic=20918.0;attach=61099;image


Does it make a difference which plate R, gets the cap in the PI? My schematic has two 100K plate R's. I do remember an unbalance can produce better harmonics...? Changing the one to 82K, really help harmonic?


But I will try the cap, a simple mod as soon as I open the amp. BTW, the 12AT7 in the PI lower the overall volume of the amp just a bit seemed to cut a little gain some too. I like the 12AX better, but the frizze can be heard just a bit with the 12Ax and not so much with the 12AT. I'll add the cap, start with a 221pp ceramic.


Another 12AX in the PI help a bunch, I should toss the old one.  Maybe the cap will seal the deal, thanks, I'll post results.


Thanks,al

[tubenit]

Put it on the (entrance)  plate resistor just like in the 61097 image.  I didn't find much help trying it on the other LTPI plate resistor.

With respect, Tubenit

[dude]

I added the enhancing cap and it does seem to level out the frizzle I was hearing and  smooth out the top end.


With the new 12AX7 in the PI and the cap,  I only start to hear the frizzle when the preamp gain is way up which is not often as the amp's gain is way over the top pass 2 o'clock.


I find mixing the volume and gain around 1 or 2 o'clock, gives a nice distorted tone that's not over the top. Using the volume further up the dial gives a much better distortion than turning up the gain from that point. I'm happy but I'm wondering if the old USA EL84's I'm using are closely matched...?


Thanks for the info,
al