Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on April 30, 2017, 02:42:41 pm

Title: 68 Marshall 50W plexi on the bench
Post by: TubeGeek on April 30, 2017, 02:42:41 pm
Hey guys, I have a cool amp on my bench! It's a 68 plexi.


I have attached a before and after pic of what I have done so far.  The owner wanted new caps installed and a rats nest above the power supply to be cleaned up. I am fairly pleased with the clean up job.


Couple questions I have though....


There are a couple of 0.22uF caps on the anodes of the rectifier diodes. What is their purpose and are they necessary?  I get the feeling they are there for transient noise suppression or something to that effect.


I have the standby wired up a little different than stock...is this ok?  The rectifier diodes cathodes connect to the 50/50 cap which then connects to the standby switch.  The other side of the standby switch connects to the B+ fuse and then onto the Ct/screens/choke.   RESOLVED.


Is there anything else you recommend for this amp?  I want to make sure I do good on it, since it's a bit of a gem or holy grail amp.
Title: Re: 68 Marshall 50W plexi on the bench
Post by: TIMBO on April 30, 2017, 04:06:46 pm
Hey mate, I think your right about the .022s
Is the amp's standby NOT wired as per the schematic?
Title: Re: 68 Marshall 50W plexi on the bench
Post by: DummyLoad on April 30, 2017, 04:16:43 pm
Is there anything else you recommend for this amp?

as little as possible. if there are any mods done, revert to stock. 

--pete
Title: Re: 68 Marshall 50W plexi on the bench
Post by: TubeGeek on April 30, 2017, 07:57:35 pm
Quote from: TIMBO on April 30, 2017, 04:06:46 pm
Hey mate, I think your right about the .022s
Is the amp's standby NOT wired as per the schematic?


The  standby switch was not wired as the schematic when it came to me. I suppose in my mind I was just following what was already wired in place. And now that you have clued me in I will wire it as per the schematic. I don't know why that didn't cross my mind before posting this!


Update: wired as per schematic now
Title: Re: 68 Marshall 50W plexi on the bench
Post by: TubeGeek on April 30, 2017, 08:00:44 pm
Quote from: DummyLoad on April 30, 2017, 04:16:43 pm
Is there anything else you recommend for this amp?

as little as possible. if there are any mods done, revert to stock. 

--pete


 Yes I agree. I even hesitated at working on the amp because I wasn't sure if I should. When I opened it up and saw the rats nest above the power supply I figured I'd at least give it a go and try to clean it up somewhat. It's not all stock, the output transformer has been previously replaced.  There were poor solder connections throughout and general poor attention to details.
Title: Re: 68 Marshall 50W plexi on the bench
Post by: purpletele on May 02, 2017, 02:01:49 am
Nice work, it looks a little sharper.

That's a cool amp!