JCM 800 2204 1988

[David Burner]

Update! The HT fuse blew after one of the power tubes failed. Seems like everything else is fine after replacing both the fuse and pair of tubes. Did not change the caps as they were measuring fine.

Trying to work out a replacement or refund from the tubedepot as the tubes were brand new.

 

[Chris-in-LA]

Update! The HT fuse blew after one of the power tubes failed. Seems like everything else is fine after replacing both the fuse and pair of tubes. Did not change the caps as they were measuring fine.

Trying to work out a replacement or refund from the tubedepot as the tubes were brand new.

I think I had some preamp tubes go bad and they took them back and tested them as bad and replaced them without question. Send them an email

 

[David Burner]

Got a very quick response from tubedepot -- they already shipped a replacement tube, no questions asked. Great service btw! They even asked what the rating written on my tubes was so they can send me a matching one.

 

[scozz]

Got a very quick response from tubedepot -- they already shipped a replacement tube, no questions asked. Great service btw! They even asked what the rating written on my tubes was so they can send me a matching one.

I buy all my tubes from Tubedepot. Their prices are as low or lower than The Tube Stores, and shipping charges are a quite a bit lower.

Nice that it was an easy fix David.

 

[David Burner]

I agree! First time buying from them, but I'm really pleased. Definitely recommend em if they carry what you are looking for. Also worth mentioning that I didn't get charged duties on my shipment to Canada, which was a nice bonus as my order was for over $200, I also got some tubes from Doug Preston that I couldn't find at the tubedepot. It's good to have options

 

[Jethro Rocker]

Good news and does show the most likely culprit is the weakest link even brand new tubes. HT fuse going usually indicates a power tube.
Good deal.

 

[Greg70]

Those two resistors that are replaced are both 100K Ohm (brown, black, yellow). The original schematic shows them as that same value. I don't know why anyone would specifically replace them. Maybe they did a mod and didn't like the sound so they changed them back? Most mods that I have seen alter these values closer to the input stage.

 

[David Burner]

Those two resistors that are replaced are both 100K Ohm (brown, black, yellow). The original schematic shows them as that same value. I don't know why anyone would specifically replace them. Maybe they did a mod and didn't like the sound so they changed them back? Most mods that I have seen alter these values closer to the input stage.

View attachment 84449

Wow thank you so much for checking all that Greg! So if I understand correctly your guess is that they changed it as a mod and put back the original value ones which is what you can see there now?

 

[Greg70]

Wow thank you so much for checking all that Greg! So if I understand correctly your guess is that they changed it as a mod and put back the original value ones which is what you can see there now?

Yeah, my original comment wasn't real clear. They definitely look like they were messed with, as you originally pointed out. The values of the resistors that are currently in there are correct to the original schematic so they are fine the way they are. Resistors are not an item that you typically replace as part of maintenance, like you would a capacitor or a pot. The only logical explanation, IMO, is some sort of mod BUT usually it's the resistors in the first gain stage that are changed, not these guys. It's certainly strange when everything else in that amp looks bone stock and original.

If you ever want to verify something or look into a mod, the Marshall labeling system makes it super easy to modify and check things. The numbers in the schematic match the numbers on the circuit board. It also makes it really easy for people on a message board to exchange ideas since we can refer to an electrical component by number in the circuit.

 

[1956tvmodel]

I have the same amp form 1985. Great amp. I had the 2 4x12 cabs loaded with the same version of those speakers. I sold them for 4 Greenbacks and 4 Creambacks just to change it up after playing the 75's for 30 years. I also just changed the filter and bias caps on the amp. At some point that might be a good idea on yours too.

 

[Trapland]

Put in in the “I don’t automatically change caps” category too. Often they work fine for another decade or so, it all depends on the amps history of use. If the last owner kept it home and played it regularly they could be fine.

Blown HT fuse could be tubes/mismatch or both. Probably not from filter caps, (‘except in bias supply).