50 watt Marshall Super lead problem

[rfgordon]

I'm working on a 1974 Marshall 50 watt super lead that has, at some point in its life, been modded to run 6550s.

A couple of months ago I recapped the amp, it it was working fine.

Now it has a nasty hum--dark. like 120 Hz. All the caps check out fine, as do the tubes. Here's the thing:

The noise is only on one output tube socket. So if I turn the amp to play with a tube in one socket (tube is not the issue) the noise is there, and with only one in the other socket, there is silence.

I replaced the 150k grid load resistor, no change. Short of going in and replacing everything between that tube and the PI, any ideas?

The screens each have 1k resistors anchored to pin 6, btw. No grid stoppers on this amp.

[rfgordon]

One more thing.

The noise is there even with no 12ax7 in the PI socket.

So my choices of source are:

--bias
--ground
--screen
--coupling cap from PI
--??

[Roe]

first, there is no such thing as a 50w superlead.
second, check ground connections and electrolytic caps

[selloutrr]

It's a three prong power cord? Test continuity from ground to pin and look for any broken grounds.

[rfgordon]

My bad on the nomenclature. The sticker on the back says MK II.

Thank you all for your suggestions. However, filter problems would not manifest themselves on only one socket. I will check the cathode ground and the grid load resistor ground.

[Firestorm]

Leaky coupling cap to that socket?

[selloutrr]

I've found interesting things happen with poorly soldered grounds in tube amps. But let's assume all grounds are good.
Are u testing the parts in circuit or out?
Is there any traces of burn on the socket?

Since you isolated the noise and you are 100% positive it's not tube related output, pi, or preamp tube?. test back to the point the signal is shared by both sides. If you have doubts and the part if hand replace it with a know good one.

[selloutrr]

Also test the OT legs to make sure they are ok.

[Andy Le Blanc]

sometimes with old dried out phenolic it becomes conductive,
check the socket for carbon, it could fil. v leaking .

[hazy]

I'm working on a 1974 Marshall 50 watt super lead that has, at some point in its life, been modded to run 6550s.

A couple of months ago I recapped the amp, it it was working fine.

I have a similar problem - not yet resolved (thread on this page). Re-capped a 1978 50w MKII. A month or so later - deep hum caused by V2. I think mine is 60-cycle, but I could be wrong.

Good luck...wish I had more info.

Hazy

[rfgordon]

After messing about some more, and doing more careful tube substitution, it turns out that one of the 6550s is quiet, the other hums, no matter which socket it's in.

So now I wait for the owner to give me with word to order new tubes. He has some vintage RCAs in there, so it's too bad one of them went bad.

Unfortunately, this amp has been rather extensively modified by a less than excellent "tech" at some point in its history. In amps like this, it's hard to know which problems are caused by sketchy work or other causes.

[selloutrr]

The 80's were hard on amps lots of modification to get the trendy sound. It is a simple circuit to work on if you notice shorty work I've found in most cases it's quicker to fix on the spot then take the time to bitch about it. Besides if you are the last one in the amp anything from this point on is your work in the players mind.

The RCA's are a great sounding tube it's a shame one is bad but that amp gets most of it's tone from the preamps anyway since few places after the invention of the modern PA system give you the opertunity to turn to "11" output tube distortion is minimal.

give the guys at eurotubes a call and ask for a pair of tubes to compliment the superlead they will do you right!

[Structo]

Curious to what amp this actually is?

On Schematic Heaven they list a JMP Lead 50W and a JMC 800 1987 Lead 50w.

Is the Mark II the black flag model?

[Firestorm]

I think the "black flag" Marshalls are JTM plexis. Exquisitely expensive. If it's a '74, I'll bet it's a metalface JMP 1987 Lead(?). If it's got a Mark II sticker, it might have originally come with 6550s, in which case the output tube grid resistors should have been 150K instead of 220K.

If it was converted to 6550s and the resistors were left at 220K, that might damage even RCAs. 6550s can be pretty fussy about G1 circuit resistance. RCA manual shows a maximum of 50K (which everyone seems to have ignored). I've killed a few 6550s that way (not RCAs, thank God).

[jaysg]

I think mine is 60-cycle, but I could be wrong.

Good luck...wish I had more info.

60Hz is below the low E on a guitar. 120Hz is between Bb and B on the A string.
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Start by replacing the socket.