Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on November 30, 2024, 10:46:39 pm

Title: Marshall 1987 schematic
Post by: fossilshark on November 30, 2024, 10:46:39 pm
Hi all, I decided to build myself a plexi and am working on the layout. I am just wondering why there is a 50uF capacitor connected to the EL34 cathodes, B+ and ground in reverse polarity? Wouldn't having a backwards capacitor across B+ and ground be an issue?
Title: Re: Marshall 1987 schematic
Post by: SEL49 on November 30, 2024, 10:58:03 pm
Obvious drawing error. #*it happens.
Title: Re: Marshall 1987 schematic
Post by: pdf64 on December 01, 2024, 04:45:55 am
Yes, the HT node decoupling cap for the screen grids has been drawn upside down. Well spotted in.
There's also an egregious design error in 70's 50 watters, ie the standby switches the bias as well as the HT.
When the standby switch is closed, the HT charges up instantly whereas the bias supply takes a few seconds to get big enough.
The result being the output valves take a beating every time standby is used.
The feed for the bias supply should be on the hot side of the standby switch, eg https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_4010_power.gif
Title: Re: Marshall 1987 schematic
Post by: Latole on December 01, 2024, 06:47:45 am
EL34 cathodes are at ground,
The 50 mfd you are talking about is connected to the EL34 grills, not cathodes.

Title: Re: Marshall 1987 schematic
Post by: fossilshark on December 01, 2024, 10:47:03 am
Quote from: Latole on December 01, 2024, 06:47:45 am
EL34 cathodes are at ground,
The 50 mfd you are talking about is connected to the EL34 grills, not cathodes.



I realized that right after making the post  :laugh:

Thank you guys for the info.
Title: Re: Marshall 1987 schematic
Post by: SEL49 on December 01, 2024, 11:27:23 am
Actually, it would not be incorrect to say the cap is connected to the screens AND the cathodes.
Title: Re: Marshall 1987 schematic
Post by: Latole on December 01, 2024, 12:29:14 pm
Quote from: SEL49 on December 01, 2024, 11:27:23 am
Actually, it would not be incorrect to say the cap is connected to the screens AND the cathodes.

Technically, it is connected to the ground.

It is no more connected to the cathodes than to the volumes, to the tones, to the jacks, to the output transformer, to the catode resistors of the preamps, and so on.
Title: Re: Marshall 1987 schematic
Post by: SEL49 on December 01, 2024, 01:33:24 pm
Of course it's not physically connected to the cathode but it is certainly electrically connected to the cathode. An electrical connection is no less valid than a physical connection.