20 year old Marshall - regular maintenance

[esandes]

My old 1987 has a buzz to it like a single coil buzz. It happens at all volumes and using any of the four inputs. I changed the tubes and it didn't help. The power tubes are matched.

What could be the issue? I am planning on replacing the electrolytic caps.

[JMP/HBE]

Re: 20 year old Marshall - regular maintenance

Do you have a 3 prong plug ? Do you have modern house wiring with true earth ground ?

Could be filter caps, could be other things.

Do your guitars all have good grounded pickups.

Does the amp buzz with other guitars too ?

[esandes]

Re: 20 year old Marshall - regular maintenance

Yes three prong plug. I use the same plug in and cable for each of my heads. My newer heads are dead quiet with humbuckers.

[LLL]

Re: 20 year old Marshall - regular maintenance

I am planning on replacing the electrolytic caps.

Although typically a "buzz" indicates poor grounding, bad/old can caps (as you mentioned there) can create similar symptoms.

[esandes]

Re: 20 year old Marshall - regular maintenance

I just read in another forum that someone capped their Marshall and it's dead quiet now.

Marshallforum

http://www.marshallforum.com/threads/filter-cap-experts-reformed-caps-jcm-800-what-should-i-expect.99153/#post-1662197

[esandes]

Re: 20 year old Marshall - regular maintenance

My tech said the caps are fine. The amp was grossly underbiased and the tubes were running hot causing that hiss. It was running like a class B rather than A/B.

He said the original 1987s have even more treble in channel 1 and it would be easy to mod it to the original spec. Tempting.

[Joey Voltage]

Re: 20 year old Marshall - regular maintenance

My tech said the caps are fine. The amp was grossly underbiased and the tubes were running hot causing that hiss. It was running like a class B rather than A/B.

He said the original 1987s have even more treble in channel 1 and it would be easy to mod it to the original spec. Tempting.

It would have been running closer to class A in that scenario. I’m surprised you didn’t see those valves red plate.

[JMP/HBE]

Re: 20 year old Marshall - regular maintenance

It would have been running closer to class A in that scenario. I’m surprised you didn’t see those valves red plate.

Thats what i was thinking ...

[esandes]

Re: 20 year old Marshall - regular maintenance

It did smoke once.

Help me understand because my tech is quite experienced. The way he explained it was underbiasing increases the voltage to the tubes but cuts the current compared with overbiasing which does the opposite. Underbiasing results in more power output such as 30W per EL34. I thought class A amps produce relatively low power output which would be like an overbiased A/B.

[Lake Placid Blues]

Re: 20 year old Marshall - regular maintenance

.....

[Joey Voltage]

Re: 20 year old Marshall - regular maintenance

It did smoke once.

Help me understand because my tech is quite experienced. The way he explained it was underbiasing increases the voltage to the tubes but cuts the current compared with overbiasing which does the opposite. Underbiasing results in more power output such as 30W per EL34. I thought class A amps produce relatively low power output which would be like an overbiased A/B.

Most people get these terms mixed up. The terms underbiased and overbiased are not used to denote the amount of idle plate current, they are in direct reference to the bias voltage applied to the grid, which in turn affects plate current. Since the bias voltage is negative in quantity the terms under and over will have the opposite meaning than what is intuitive. If something is overbiased it means the grid voltage is more negative in quantity with respect to cathode, which will decrease plate current and result in a colder bias. Underbiased means the grid voltage is less negative in quantity, resulting in more current through the valve and a “hotter” bias. Biasing hotter or underbiasing will move you closer towards class a, causing the valve to dissipate more at idle, whereas overbiasing will bias the valve colder and take you closer towards class b.

You definitely were underbiased (hotter) if it was that noisy. Typically a colder bias is quieter than a warmer one for a few reasons. Less idle plate current is one, and the larger crossover notch of the colder bias will have a crude noise gate effect


Neither per se results directly into more or less power output on their own, there are other factors that need to be addressed.

[esandes]

Re: 20 year old Marshall - regular maintenance

The amp was very boomy being underbiased BTW.

He said underbiasing decreases current and increases voltage in the tubes.

I just hope that hiss is gone. I like the presence and EQ all at 10 and that treble channel on high input. If that hiss is still there albeit to a smaller degree than before what are my options to cut it back or eliminate it? New caps? I bet the new 1987x are as quiet as my 2203x and 2555x.

[Joey Voltage]

Re: 20 year old Marshall - regular maintenance

He just has the terminology mixed up. A lot of people do it. Overbiased = more -vg = less plate current. Underbiased = less -vg = more plate current. More plate current moves you closer to class A, less towards class B.

Hiss comes from a number of places, the caps are not the most likely. Failing filter caps mostly result in hum, and loss of frequency response. There are things you can do to minimize it but it really depends on how far you want to go.

[LLL]

Re: 20 year old Marshall - regular maintenance

The amp was very boomy being underbiased BTW.

He said underbiasing decreases current and increases voltage in the tubes.

I just hope that hiss is gone. I like the presence and EQ all at 10 and that treble channel on high input. If that hiss is still there albeit to a smaller degree than before what are my options to cut it back or eliminate it? New caps? I bet the new 1987x are as quiet as my 2203x and 2555x.

Out of curiousity, what volume do you run the amp at?

[esandes]

Re: 20 year old Marshall - regular maintenance

Out of curiousity, what volume do you run the amp at?

It's a plexi so even at band volumes it's at 1 or less. I used to rehearse alone with it around 4 in the studio. Nowadays It's at talking volume-just a hair above fully turned down. When turned down completely there still volume but it cuts some tone out.