Marshall 1959hw Super Lead: Volume Drop And Supersonic Frequency !@#?!

[theWalrus]

I have a Marshall 1959HW super lead and I like to crank it: every single knob on 8 or higher.

An issue recently came up: volume dropped dramatically while playing and it sounded strange; a "bees in a jar" output, but not like when the output transformer (OT) goes; it has a replaced OT from the factory.

I took it to a technician I trust and I'm sure he's working on it, but I just wonder if you guys ever had that issue come about. He had the amp connected to a scope and the frequency being amplified is really high; he says it's amplifying frequencies too high to hear, like bats screaming.

My tech tried changing the tubes, but the problem is still there. Is that the OT or possibly some other part? I love that amp.

Thanks!

 

[jgab]

Did you try eliminating potential issues like your guitar, cables, speakers cabinet? Does it happen after while of playing, or immediately after you fire it up?

 

[theWalrus]

Did you try eliminating potential issues like your guitar, cables, speakers cabinet? Does it happen after while of playing, or immediately after you fire it up?

Yes: I plugged right into the amp. The cab and the guitar are fine. I kept on playing with a different head into the same setup.

The tech saw those high frequency oscillations in his oscilloscope, so it's the amp. Looks like it's amplifying frequencies beyond what people hear; very strange. I just hope it's not the output transformer. My previous cab had old wiring, falling apart, and when the OT goes, it's not very musical.

 

[neikeel]

If you are caning it all the time and your tech is seeing oscillations then you may need to add
5k6 swamps on each grid of the outputs (HW) only have 1k5 on outers.
Wonder if something else is failing like an output coupler?

 

[theWalrus]

Hey, thank you. I'll pass on those ideas to check to the tech. I appreciate it.

 

[ampmadscientist]

I have a Marshall 1959HW super lead and I like to crank it: every single knob on 8 or higher.

An issue recently came up: volume dropped dramatically while playing and it sounded strange; a "bees in a jar" output, but not like when the output transformer (OT) goes; it has a replaced OT from the factory.

I took it to a technician I trust and I'm sure he's working on it, but I just wonder if you guys ever had that issue come about. He had the amp connected to a scope and the frequency being amplified is really high; he says it's amplifying frequencies too high to hear, like bats screaming.

My tech tried changing the tubes, but the problem is still there. Is that the OT or possibly some other part? I love that amp.

Thanks!

What I know about it is: the hum balance control can blow the amp up...(if the amp has this on the board)
and should be removed, replaced w/ 2X 100 ohm resistors.
What happens is that the tiny little pot overheats and fails. When it fails the heaters are no longer balanced to the chassis, and the amp makes a really loud squealing / buzzing noise.

It could also be that HT is arcing to the filaments, causing the pot to blow.
But suspect that this happens when the speaker load is the wrong impedance....don't know what causes that for certain.

The control does absolutely nothing to reduce filament hum. (if the amp has this)
The filament buzzing can be reduced by changing the filament wire layout...

High frequency oscillation>
install 7 pf 1000V cap between plate and grid of first pre-amp stage. This stops the HF oscillations.
OK, there is several other methods but that cap is one of the factory Marshall methods.

Clarification: this HF oscillation is normal for this amp design. It does it when you crank the volume up high.
The HF oscillation (in an otherwise working amp) pretty much always comes from V1, first preamp tube.

You can pick out a certain tube, that oscillates less, but that will wind up being a lower gain tube also.
The cap method makes it easier for the amp to operate with just "any" good preamp tube....and it's not so touchy.