GIBSON67

Hello, my first time to post anything but here goes. I am having problems with
my Marshall 4 inputer, 50 watt combo. Here's how it happened...I have been trying to find a good 12 speaker that fits and can handle 50 British watts, so I got a Celestion 200 watt PA speaker to try. Well the speaker was labeled 8 ohms and it measured 12 ohms(16 ohms). Of course I didn't find that until after the blow up. When I cranked it it smoked and blew the fuse, and shorted across 2 pins of my power tube. Shit! Well, ok, I discovered the speaker problem and got new tubes, replaced the fuse and tried again. But it happened again after getting it up to only 3 on my volume, shit! I metered the screen resistors and they measured 1k as normal. But I now need to replace my power tube sockets(I think) and the resistors across the power tubes. Help, does it sound like I blew the tranny? Where's a good place to get similar sockets? Need advice, please.

BTW, it is a modded 4210 20th Anniversay 1x12 Combo in White that someone changed into an all hand wired four banger...and I have never had any problems with it since I got it back in 1999.

timstoel

Unfortunately, there is a very good chance you did blow the output transformer... I have the sockets for sale at newoldsounds.com, and I also have the output transformer if you decide you need one.

If you need more help, just ask

Tim

__________________
http://www.newoldsounds.com - Marshall parts, tubes, speakers and more

GIBSON67

Thanks, Tim. I guess I am going to have to take it in and get it checked.

Is there a way to know whether or not the transformer is good?

PaoloJM

Often a blown transformer is accompanied by an aweful smell. Have you noticed this?

Here's a simple test you can do with a multimeter.

1. Power the amp down as normal, and discharge your caps.
2. Remove the valves, not essential but they may influence the results.
3. Take your DMM and measure the DC resistance between both plate connections on the output transformer primary.
4. You should be reading around 50 to 150 ohms here.
5. Repeat for the secondary. You should be reading in the region of .8 to 6 ohm for the different connections.
6. Measure the resistance between one of the plate connections and the chassis. Repeat for the other. This should read a high value, 500k +.
7. Repeat for the secondary.
8. Measure the resistance from one of the primary plates to a secondary connection. Again this should a very high value, say 500k +.

If you read a very low value < .3 ohms on any of these, then you have a problem.
If you read a value above 500ohms on the primary connection then you have a problem.
If it's passes it's probably ok.
A qualified techy will be able to test it fully but this will give you some indication of whether it's blown or not.

__________________
www.youtube.com/paolojm

69 plexi

I might as well post this problem with my head in this thread also.I have a 69 100 watt 1959 SLP that is driving me nuts.When all 4 tubes are in their socket,the one outer tube glows bright red and blows the fuse.When I take out the tube that glows bright red and the other outer tube(now amp is knocked down to 50 watts)the amp plays fine with no signs of burning,overheating,muddy sound,etc.I know its not a good idea to play an amp that is designed for 100 watts with the outer tubes taken out to knock it down to 50 watts for a period of time before more problems occur.Can anybody chime in and give me an idea of the cause of this problem?Thanx

69 plexi

Any ideas on this problem?I need help from the pros on here before I breakdown and have to take it to the almighty amp tech down the street.

Pete240885

Plexi, unless your problem is of a similar nature its impolite (not to mention confusing somtimes) to hi-jack threads, however start your own thread and give me the link via my visitor/personal messages and i'lll be willing to help

GIBSON67

Thanks, PaoloJM. I understand your post but I don't think I am good enough to try and then my soldering skills aren't the best either, so I will be taking it in to get it checked out. I will post their findings when I get her back. Again, thanks.

Tim

Pete240885

Did you ever get this fixed?

GIBSON67

Hey, I finally got my baby back from the shop. It had 2 bad diodes and needed the tube sockets replaced. She's running better than ever. Thanks for all the replies and all the help. BTW, I was very happy when I found out that it was not trannies.

Tim