[help] amp grid crossing?? crackling from dist pedals..

[ulysses]

hey guys

i read a post a while ago that discussed *grid crossing* or something similar that made a tube amp crackle with certain distortion pedals - yet those distortion pedals dont cause other amps to crackle at all..

i read a post by rg a long time ago discussing this but have been unable to find it.

can anyone point me in the right direction for more information on this topic?

cheers
ulysses

[jaytee]

There's some stuff about blocking distortion here http://www.aikenamps.com/ It might be the same thing you are talking about.

[ulysses]

yeah it could have been grid blocking.. thanks for the link.

cheers
ulysses

[ulysses]

perhaps someone can help me with my problem - rg if you have time perhaps you could read over my research.. 

QUOTE from http://www.aikenamps.com/BlockingDistortion.html

Increase the size of the so-called "grid stopper" resistor, which is usually a 1.5K - 5.6K resistor in series with the output tube grids, and is usually soldered directly to the grid pin as a parasitic oscillation prevention measure.  Increasing the size of this resistor will limit the amount of grid current that can be drawn and reduce the clamping effect quite a bit.

i checked my amp out and there is no grid stopper resistor on either of the output tube grids. its just a single wire soldered to it..

QUOTE from http://www.raw-sewage.net/jtm45ri_mods.html

Lord Valve's field reports indicate that using the Valve Arts (Chinese) KT-66 tubes in the JTM-45 (reissues and originals) can be problematic. The amp design is prone to parasitic oscillation that may kill your tube. The solution: put 5.6K swamp resistors in series with pin 5 on each power tube socket. First unsolder the wire connected to pin 5, then solder the 5.6K resistor directly to the pin socket. Re-connect the wire to the other end of the resistor. The goal is to make the leads on the resistor as short as possible.

i want to mod my bluesbreaker to kt-66's.. so i was going to add in the 5.6k resistor anyway.. do you think this will stop the grid blocking problem? what W rating should the resistor be? 1W?

you can see the 5.6k resistors have been added to this kt-66 modded bluesbreaker (green wire with heatshrink / orange wire with heatshrink)
http://soundsource.zoovy.com/product/MARSHALLBLUESBREAKER

QUOTE from http://www.raw-sewage.net/jtm45ri_mods.html

The standard modification is to change the power amp filter capacitor (the one closest to the GZ34 rectifier tube) to a 32x32uF/600V LCR capacitor. The other can (the preamp filter capacitors) should be changed to a 16x16uF/450V LCR capacitor. This also puts less stress on the rectifier tube, which should reduce the risk of failure.

can anyone recommend a good place to buy the LCR capacitors required for this mod?

can anyone recommend the easiest way to bias kt-66's in my bluesbreaker/jtm45? ive seen those tube socket addons that give you wires to connect to your multimeter. can i easily do the bias'ing without one of these devices? perhaps aligator clips to the tube socket inside the chassis? also - i cant seem to locate the trim pots on the circuit board to adjust the bias - if you look at the schem RV2 is for the tremolo circuit and RV1 is near the power transformer - does one trimpot do both power tubes?

you can see pictures of the board here: (already modded to the spec's im looking at)
http://soundsource.zoovy.com/product/MARSHALLBLUESBREAKER

here is the schem
http://www.raw-sewage.net/images/bb62ri-schm1.jpg

cheers
ulysses

[R.G.]

This is a bit of a mixed bag.

Your first post described a "crackle". When I hear that word, I think of the sound of someone slowly crumpling a stiff plastic potato chip bag. It's a bunch of intermittent pulses. That's not consistent with grid blocking, except in a remote, secondary way, which I'll get to later.

To me, the word crackle makes me think of an intermittent connection that makes and breaks; wah pot crackle is one such. Does your amp do this at any time and any volume except with the specific overdrive pedal? A bad solder joint or cracked conductor or cap in the amp might make that happen, as could a massive overdrive make a weak coupling capacitor break over intermittently.

When I think of grid blocking, I think of an amp driven to a blatting kind of sound by big inputs. What causes grid blocking is big signals make the grid conduct on positive transients, and that conduction gets stored in the coupling cap from the previous stage. The stored charge makes the grid more negative until it leaks away through the grid resistor. The tube is effectively biased nearly off until the charge leaks away, so the sound cuts out for a tiny fraction of a second. The grid actually acts like a tiny rectifier diode, making its own reverse bias.

The only way I mentally connect those is when blocking might make a cap break over and click. But that's got to be very rare - I've never seen it in the real world.

The real cure for grid blocking is to either never drive a grid more positive than the cathode, or if you insist on doing that, to drive it with a DC coupled follower. The follower can eat the excess electrons when the grid conducts without storing the charge. You might think that this just transfers the problem to the follower if it's a tube, but followers do not go into grid conduction except at truly huge signals, because the cathode of a tube used as a cathode follower... wait for it, OK... follows the grid, so it's really, really hard to drive a follower into grid conduction.

It's obviously not good to double the number of tubes you use, but if this is your real issue, you might want to find the stage that's going into grid conduction and causing the blocking and concentrate your work there.

As a commentary on the suggested fixes:
1. Reducing coupling cap size: good fix if it gets you there and does not lose you too much treble.
2. Reducing grid bias resistors: good fix if it gets you there and does not lower gain or bass response too much
3. Increase grid stopper resistor: Good for power tubes, not so good for preamp tubes because of the high values necessary and the fact that a triode clips much harder and buzzier if you drive it from an high impedance.
4. DC coupled follower: good fix, a slam dunk, but suffers from being complicated and expensive.
5. Limit the signal level: good fix, but complex to do for preamp stages. Good one for output stages.
6. Reduce the amount of negative feedback: OK-ish, but not for beginners to play with.
7. Problems in the preamp section; mostly what we're talking about here.

Randall is a really smart guy. We've shared a lot of ideas. He is solidly opposed to the ideas in my MOSFET Follies articles on principal though. His cut is that tube amps should be tube amps. I have a more eclectic outlook: if it works, do it. To me, MOSFET followers and clippers seem like the obvious ways to go about it.

[snoof]

Just a couple things to add;

When I hear "crackling" in tube amps, I always check the 100K(or whatever you have there) plate resistors in the preamp.

Adding too big of a grid resistor on the power tubes can reduce high end a bit, so smaller is better.

Be REAL careful when checking the bias, them's bigtime voltages in there!!  The Bias-rite type things that you mention would be the safest way to monitor it.

You can try partsexpress.com, hoffmanamps.com, tubesandmore.com for the can caps.

[ulysses]

thanks for the replies guys. i will read more and let you know how i go.

cheers
ulysses

[ulysses]

hey guys

well i finally did some more research on this after modding my amp.

the resistors across the power tubes are 470R and there is one resistor across the second preamp tube - it is 100k. they all measure fine.

this problem only occurs with certain pedals. you can hear an example of the hotcake in the "misc audio samples" section of my  the cracking fades as as you turn the volume up from 3-8 and goes away completly when the amp is cranked up to 9/10 - by that stage you really dont need a distortion pedal though.. i discovered this after aron mentioned that some distortion pedals seem to be rejected by certain amps if they have "power on tap".. when the amp cant deliver any more power, the problem goes away..

when i am not using any distortion pedals the amp sounds beautiful. no crackling at all. beautifully clean at low volumes.

this happens on my amp ('89 marshall bluesbreaker reissue modded to JTM45 specs (it had the same symptoms before and after the mods and new tubes)) and my mates 2000 vox ac30 reissue.. they have a GZ34 rectifier in common.. thats the only thing i can think of that might be causing this.. maybe a SS recifier will fix it?

rg, if you have a moment to reply i would appreciate and experience you have to share..

cheers
ulysses