Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Madison on May 14, 2010, 09:06:00 pm
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Crackling or shorting out sound.
This is a pretty typical Plexi build.
I can run 6L6s and 6V6s fine.
I try EL34s and they crackle/shorting sound at higher volumes when hitting a chord sustained.
I have tried two different sets of EL34s.
These are all JJ tubes.BIAS set@ 35mA. All matched within 2-3mA
Unfortunately I don't have another brand of EL34s in hand now.
I was wondering it if is my screen resistors?
I see that they are 5.6K on a plexi and 1.5K for a 5f6a
I have 1.5Ks in there now.Change their values?
OT is 6.6K
Only other thing, presence pot is a 10K because I didn't have a 5K.
Any ideas?
Thanks a lot.
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5.6K grid resistors / 1K 5Watt screen resistors
If you see a white flash in any of the power tubes break them and throw them away / They are shorting internally and can cost you a nice OT
Is it possable that your PT can not handel the extra current draw of EL34 tubes?
10K is fine for the presence pot
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My bad.
Right now I have 1.5K grid resistors/ 1K screen.
No flashes afaik.
I took the second pair of el34s out of a known good working amp and they had the same symptoms.
PT power, good point.
This is a leftover ceriatone I had sitting for a long time.
290-0-290.
It was originally for a 36 watt.
I would think it has the juice to do the job?
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Madison,
I think the flash he was referring to was the "silver" flashing, not arcing. but as your other tubes worked the same (and were known good) it's moot.
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> 6L6s and 6V6s fine.
I try EL34s
Pin 1 on EL34 should go to ground or cathode.
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>>Pin 1 on EL34 should go to ground or cathode.
Pins 1 and 8 tied together to 1 ohm resistor to ground
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After checking again this problem is much more apparent in UL mode.(UL/Pentode switch)
BTW.
Is there any rule as to how long the wires from pin 1/8 to the bias test points should be?
Mine sort of snake around a bit under the OT secondary wires.
Also, I have tried adjusting the NFB resistor with no luck.
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This sounds somewhat like an intermittent connection.
At one time, JJ had problems with the diameter of their tube pins. They were too small and wouldn't always make good contact with the socket. You might try putting on an oven mitt and pushing lightly on the top of the EL34's to see if you can induce the noise. I stress "lightly". But you can also tell if they slide into the socket a little too easily.
If that doesn't get it, I'd be looking around the chassis for loose connections or cold solder joints.
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Bloody blocking distortion!
I can't believe it..........my luck.
I lowered the coupling caps just before the power tube input grids in steps from .022 all the way down to .0022.
Maybe a little thinner but still damn loud and good enough for R&R.
Thanks for all the advice!
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Put those coupling caps back the way they were and increase the grid resistors ON the power tube sockets. Should stop the problem and do less to the tone.
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http://www.aikenamps.com/BlockingDistortion.html
Look ~~1/2 way down that page @ "Solutions"
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I was looking at that page.
I went up to 5.6K on the grid resistors before changing the caps.
Okay, I'll replace the caps and try to go higher on the resistors.
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I'd say lowering the grid reference/bias feed resistors will have a bigger impact than increasing grid stopper resistors. The trade-off is lowering the cap value or the grid reference resistors shave off bass. Increasing the grid stops might shave off highs.
If it were me and I was to increase the grid stopper resistors, I'd start with 100k to each grid and see that the blocking distortion stopped. If it works and the highs are fine to my ear, I'd stop there. If highs were attenuated too much, halve the resistors approximately and use a 56k to each grid; taste test again. Each iteration would be looking for a definite solution to the grid blocking while working towards the sound I wanted.
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I had this issue on a recent build. It sounded great, but low on power, using an old pair of Mullard EL34's I got at a swap meet. When I put in new JJ EL34's it was a lot louder, but had a harsh tone and sick crackling sound. All my measurements were good. I soldered 22k grid resistors to the socket pins in line with the grid signal wires and it responded beautifully. No more crackling, and it warmed up the tone. I suspect haveing no resistors there initially gave rise to parasytics or blocking distortion. This stabilized it.
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Okay, crackle be gone.
I went back to the .022 caps.
I tried 100K but the crackle was still there.
300K seemed good but I finally went to around 400K with the resistors to be safe.
Seems going over 500K starts to attenuate the highs to a "noticeable" level.
I also have a bright switch on this amp so that takes care of a high boost.
It sounds very good.
Any technical problems with 400k?
Thanks a lot.
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The only effect it will have is the one you detected by ear: high series resistance interacting with the tube's miller capacitance to roll off highs. If you don't hear a problem, then there's no problem with 400k.
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Ah ha!
ze old "trust your ears" bit!
Uh, I can do that.
Thanks man.