EL84 fizzy,almost crackly distortion

[phsyconoodler]

I built a 36 watt lite and it has some nasty trailing distortion when cranked up a bit.It's not a cold solder joint or biasing issue and all the usual suspects like tubes and such have been eliminated.
The plate dissipation is 13 watts.
It sounds like an oscillation and it does lessen considerably when I upped the grid stopper resistors from 8.2k to 220k but it's still there.I could go larger on the grid stoppers but the highs are suffering from the increased stopper values.
Is this the dreaded EL84 fizz problem?
I'm using a separate pair of cathode resistors and caps for each pair of tubes so I'm wondering about the Paul Ruby fix and how one would implement it with separate R/C's ?
Any insight would be appreciated.I have built quite a few 36 watters and this is the first one with this issue.

[Willabe]

Is there anything different you did on this one? Parts, brands, types, wire, iron, layout?   

[rzenc]

nasty trailing distortion when cranked up

I would suggest layout/lead dress...Do you have shielded wires running to grids?

Since you said it got better using grid stoppers...have you tried them on ALL grids - including preamp tubes and specially at P.I. tube? It might sound extreme but really helps to tame high freaks...

Best Regards
R.

[phsyconoodler]

I am using different iron,a DSL50 OT on this build.
  The layout is the same,no shielded wires at all in this one.It's very quiet.
I really like the highs on this amp so I don't want to band-aid it too much with grid stoppers all over the place.I'll try using shielded wire to all the grids and see what happens.
 

[Willabe]

You've built a lot of amps and fixed many more, with great results.

So I would wonder if it's the OT, maybe try and swap in a different OT and see what happens if anything? Maybe that's a harsher sounding OT (with this circuit) or just a bad egg?    

[The_Gaz]

I dunno, I wouldn't blame it on the OT, but of course anything is possible. Small grid stops won't kill the highs (especially the sweet musical ones), and you could at least try to further isolate the problem.

Strum a cord and jiggle the preamp tubes - maybe a socket is bad?

If you're so sure it's not a bad joint, then maybe it's a component that has a 'bad lead', meaning it was stressed somehow and now is making intermittent contact.

Personally I would use an aural signal tracer or scope to find where the noise is creeping in. Are you able to see the noise on the scope?

[Willabe]

Well he said he's built a number of these and has had no problems.

I asked if he changed anything on this one and he said just the OT.

He's one of the best/most experienced builder/repair guys here, so....    

[phsyconoodler]

I believe it is the OT in this case.I have done some reading and it may be caused by unwanted high frequencies.The solution may be a snubber system that connects to the OT primaries.
  Nothing to lose trying it out I guess.
 It's just a high wattage resistor and capacitor connected between the plates.No diodes.

[tubenit]

Are you doing the high voltage on plates and lower voltage screens method on this amp also?

Fixed or cathode biased?

IF the conjunctive filter doesn't work for you,  maybe try these caps which have worked quite well for me.

With respect, tubenit

[phsyconoodler]

Hey Tubenit,
  Yes it has high voltage on the plates and lower screen voltages.It's cathode biased as well.
Dissipates 13 watts.
  I'm using a DSL50 OT which has me worried that it's just the OT. It rings and it sounds like a distortion on trailing notes.Nasty at times.The main voice of the amp is wonderful but too many band-aids makes for a dull tone.The highs were spectacular before I started upping the grid stoppers.
  I may try some of the stuff like the big Marshall's use,which is what you have done in your diagram.
Thanks  Tube!

[Heinz]

I'm pretty sure it is blocking distortion. If large grid stoppers didn't fix it you may want to try the zener diode method. Here's a good description & explanation:

http://www.18watt.com/storage/18-watter_buzz_info_311.pdf

[The_Gaz]

That article is referring to cross-over distortion. Just sayin'.

[DummyLoad]

> add degenerative feedback - try removing the cathode bypass cap.

> redesign LTPI for less output swing.

> higher value grid stoppers.

--DL

[jjasilli]

Have you tried global NFB, and ditto to Tubenit's post --  post PI suppressor cap, and maybe local NFB.  Global NFB + PI suppressor cap + 36K grid stoppers  has worked for me: Traynor EL84 Bassmate - the buzz is reduced to a bit of "glassy"-ness.  Noticeable at home, but not at jam sessions.

[phsyconoodler]

I've gone up to 220k grid stoppers and the loss of highs is unacceptable.I also have a lar-mar master volume in this so I'm not clear on how to install the Paul Ruby fix.
  I read an article in guitar player magazine about transformer 'ringing' and the sound it produces and it describes my issue to a 'T' .
  I'm going to try the method which simply involves a pair of 1.5k 5watt resistors,a pair of .01uf caps and a couple of 8.2meg resistors to form a filter between the primary plate taps.

[rzenc]

I've gone up to 220k grid stoppers and the loss of highs is unacceptable.

These @ each EL84 g1?
36W from Ceriatone Schematic/Layout calls for 470K from PI caps to ground. You said to be using larmar mv post pi... ASSuming 250KA pot + 2M2 resistors.... If so, you might be off max Rg1-k max. Mullard quotes 300K as limiting value...

Have you tried stoppers @ other grids? Maybe you have too much gain @ HF thru out the amp?
On a similar build, stoppers @ preamp grids helped a lot.

HTH
Best Regards

R.

[jojokeo]

 I read an article in guitar player magazine about transformer 'ringing' and the sound it produces and it describes my issue to a 'T' .
  I'm going to try the method which simply involves a pair of 1.5k 5watt resistors,a pair of .01uf caps and a couple of 8.2meg resistors to form a filter between the primary plate taps.

That GP mag article was about a year ago right? Which year/month was that? I was meaning to look that up a while back.

Almost forgot - conjunctive filtering also loses some high frequency respsonse and sound characteristics change a bit too. I'd be surprised if this fixes your issue.

These @ each EL84 g1? you might be off max Rg1-k max. Mullard quotes 300K as limiting value...

Isn't this cathode biased? 300k is for fixed bias. 1meg for cathode biased. I'd lower Pdiss down to 12w or less - I believe this can cause these issues too w/ el84s, they are a bit sensitive at times to certain things. Wondering if your issue is at all volume levels or only when it's pushed? I suspect the latter? I'm thinking OT as well... 

[tubenit]

This is what Heinz was referring to.

With respect, Tubenit

[proaudioguy]

For some reason? ...................... ProAudio posted on the WRONG thread here ................... so I moved his post to his own thread. 

Look at Reply #5 here (which is actually his post):

http://www.el34world.com/Forum/index.php?topic=13540.new#new

With respect, Tubenit

[RicharD]

+1
try removing the cathode bypass cap

[rzenc]

Isn't this cathode biased? 300k is for fixed bias. 1meg for cathode biased

Indeed!!Found a 1947 Philips data sheet where they show automatic bias and fixed max R_g-k

page 06
http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/EL84.pdf

For some ?? Mullard did not published this difference:

page03
http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL84.pdf

Thanks for pointing it out!

Best Regards
R.