DSL 100: Changing Grid Blockers and Bias Resistors

[cudbucket]

My JCM2-60-00 issue 5 board sits on my bench and while I haven't had any issues with bias drift (Haven't had the amp very long but it is 16 years old), I'm considering pulling the grid blockers (220k in R7, R10, R66 and R70) and the bias resistors in R67, R68, R69 and R77 with those of more "appropriate" values.

For the grid blockers, I've read that they should be 5k6 1/2 watt, 5% Carbon Film. Some have even used 1 watters. For the bias resistors, same resistance values but bigger 1/w metal film.

My concern is the increased size of these resistors. Is it OK to use resistors this big? Maybe even lift them off the PCB a bit?

Thanks.
Dave

[Enzo]

Hi, welcome to the forum.

First, why not own it a bit longer first, and play it a lot. If it works well and sounds OK< why change it? Apparently it has successfully lasted 16 years this way.

But assuming you do it anyway, the power rating of the resistors doesn't matter. if you like big old resistors, then use them. It is like plugging a table lamp into a big 12 gauge extension cord. Won;t affect the lamp in the slightest.

One lifts resistors off the board to increase their ability to radiate heat. if your grid resistors need to radiate heat, you have far more problems than the size of the resistor. All of which is to say, they only might need sitting up off the board if they are too long for the spacing of teh holes on the board. The have no need of cooling.

[cudbucket]

Hi, welcome to the forum.

First, why not own it a bit longer first, and play it a lot. If it works well and sounds OK< why change it? Apparently it has successfully lasted 16 years this way.

But assuming you do it anyway, the power rating of the resistors doesn't matter. if you like big old resistors, then use them. It is like plugging a table lamp into a big 12 gauge extension cord. Won;t affect the lamp in the slightest.

One lifts resistors off the board to increase their ability to radiate heat. if your grid resistors need to radiate heat, you have far more problems than the size of the resistor. All of which is to say, they only might need sitting up off the board if they are too long for the spacing of teh holes on the board. The have no need of cooling.

Thank you Enzo! I'm glad you were the first to reply!

While I have the parts on order, I don't know that I will change them. I only just got the amp a couple of months ago and only played it a few times. The only reason I opened it was because while the amp functions perfectly, I do get an intermittent static coming from it. Comes and goes, both channels, with volume at zero or otherwise, with or without a guitar plugged in. The only time it would not make the sound would be: 1) on standby 2) when all power tubes were pulled. Changing preamp tubes had no effect.


So in an effort to have that fixed, I reached out to a tech or two. One very reputable tech in my area and a member of several Marshall forums. A full cap job was suggested and I learned it could be difficult to track these noises down.

Since this is not my main amp and I like the tone, I decided I would take the opportunity to learn as much as I could about working on THIS particular amp and see if I could find the issue myself. So I did a lot of reading for 3-4 weeks before I touched it.

When I was comfortable with the idea of draining the caps (I know my DSL does so on it's own but I built and use a bleeder wire as well), I went in.

So far I've removed C12 from the front board, swapped C46 for a 1vk on the tube board and changed almost every cap on the tube board. I've also performed the speaker jack mod and installed a Hammond 194G choke.

But a funny thing happened on my way to doing these mods: I read about the dreaded bias drift. There is one "expert" on one forum that insists even though my amp has survived 16 years, it is inevitable that my board will melt-down and that if I don't either change the board or isolate pin 5 on every power tube, yada, yada: I am in denial. The internet is such a double edged sword.

Luckily, I've been able to get useful info from many people like yourself. I must admit, I got a little solder iron happy and am only too ready to change out a part but so far, I haven't changed any values other than C46 and R71 for that choke.

My board looks spotless and shows know signs of having issues. But I think I've read so much about it that I'm afraid to zip it up without doing anything about it.

Anyway, I'm rambling. I do appreciate your response.

Thanks so much.
Dave Frattaroli

[Enzo]

Some people wisely suggest cap replacement jobs, others suggest a cap job for absolutely anything that goes wrong with an amp.

others may disagree, but in my experience, static noises are not usually caps.

You want to fix the noise? First...isolate the problem. First, find out if it is in the amp at all. I had an amp in my shop some years back, and about once every minute or so, it would have this increase in hum for 20 seconds. I spent a couple hours looking for the source fo that. I left it running on the bench and stepped out the front door of the shop for a breath of air, and happened to notice at the end of the block, every time the traffic light turned yellow one direction, my amp made the noise. yes, the noise was on the power line, and came from a noisy triac down at the corner in the traffic light. But could just as easily have been the drop forge across the highway, or the AC in the apartment above. I used to be a radio DXer, and the thermostat in my dad's fish tank heater sent a huge noise out electrically whenever it cycled. One trick is to tune a portable AM radio between stations and listen if it picks up the same noise pattern.

But if it is inside the amp, you have four power tubes. Remove just one, and listen to the amp. Now return it and remove the next one, listen. Go down teh row and eliminate one of the tubes sequentially. ANy make a difference when gone? We are testing for a noisy tube, but at the same time for a noisy socket and for noisy components around the tube. No help? Pull the two tubes on one end, power up, any help? It will hum more this way due to imbalance, but hum is a different noise you can ignore for the test. None of the power tube circuits seem involved? Pull the phase inverter tube. Noise stay or go?

And of course, the whack. Ball up your fist and whack the amp hard. Does that trigger a crackle or other reaction?

[cudbucket]

I actually had some dimmers in my house once that made my amps sound "off". I also had a fan that would come through my amps. When I first heard this particular noise, I immediately tested 2 other heads with the same guitar and cab and didn't hear it.

So far I've tried pulling each power tube one at a time, two outers only, two inners only, and then all four. I also changed all preamp tubes one at a time. The only time I didn't get noise was when all 4 power tubes were out.

The volume knob doesn't affect the level of the noise and neither does having a guitar plugged in.

Someone suggested at this point, check the plate resistors. I've made changes so I think now it's best if I put it back together and test it. Maybe I got lucky and it's gone. I fixed some dirty solder joints as well so who knows?

Thanks again Enzo.
Dave

[Enzo]

Not two inner and then outers, I want to hear pulled two lefties, then two righties. You are likely right, it won;t make a difference. But no reason not to check.

Pulling the phase inverter tube is done not because I think that tube is noisy. it is done to break the signal path. If the noise remains with that tube out, then it localizes it to the power tubes and OT. If pulling the PI stops the noise, then the noise is either in the PI stage or before it.


yes, environmental noise does not affect all amps, so the amp next to it not reacting doesn't mean it is not still environmental. Does it do it if you take the amp somewhere else, like across town?

WHAT plate resistors?

Plug the guitar into the FX return, and dial it to zero. That bypasses the preamp and also grounds teh FX return. Noise come or go?

See C28 at the input to the phase inverter? It is fed through CON11 on the left. Find that on the board and unplug it. Noise stay or go?

I am trying to systematically interrupt the signal path to find the point before and after the noise source. That is the central essence of troubleshooting.

[AmpFix]

Excellent advice from Enzo here. Isolate the problem in an incremental way. Changing parts without isolating the problem is pointless.

[cudbucket]

Excellent advice from Enzo here. Isolate the problem in an incremental way. Changing parts without isolating the problem is pointless.

Agreed. I figured I would mod first. See how it sounded. And then when I was happy with that, if it was still present. Hunt down the noise.

[cudbucket]

It works!

Put the board back into the chassis, popped in the tubes and tested it with my light-bulb current limiter. On standby, I had a dim glowing of the filament in the bulb (150W). Off standby and the it got brighter but still "dim". Let it sit for a couple of minutes and then plugged it straight into the wall.

Played for about 15-20 minutes and it sounds like it should. I want to say, if I notice any difference after changing all the caps on the main board, removing C12 from the pot board, changing C46 to 1kV and adding a Hammond 3H choke, that there might be a little less gain than I remember. It's more noticeable on the green channel but again, that's hard to say right now.

I have to re-bias now as it seems the bias has changed. It's no longer even and it's set higher than before. I will do that later.

Most importantly, the intermittent static is gone! Very happy right now.

Thanks to all who helped with info and support!

Dave