Installing a choke in a Marshall dsl5cr

[Brodirt]

First time posting for a long time lurker.

Im planning a few mods for my Marshal DSL5CR, a choke install and a mercury output transformer.
The transformer is simple and straight forward but I cannot find schematics or a prior install posted on the net.
I have found a very good description of a choke install in a DLC5C but the updated R version is very different internally.
On the prior model the choke replace the R26 resistor, a very easy job; I was wondering if it was the same resistor on the current model and/or if not which it was.
Your help is appreciated.

[pompeiisneaks]

Since we don't have a schematic, and it would basically require we have it, it's not super easy to answer the question.

Do you have a schematic? Can you take pictures of the inside of the amp? Have you ever done this kind of work on an amp? Be careful, there are 4-500VDC in the caps if they're not drained and you can really hurt yourself if you're not careful.


~Phil

[Brodirt]

I dont have a schematic. Will try to post some interior shots shortly.
I have done similar work on stereo amps and am aware of the dangerous of capacitors (thank you though); I will discharge them even though the amp is said to be self-discharging.

Added photos (these are not mine, I found them online and compared with the photos of the choke install on the older unit; the two are very different.

[Brodirt]

This is a photo of the old model, centered on the R26 resistor to be replaced.

[thetragichero]

maybe nitpicky, but r26 should probably be a resistor

anyway, so you've got a big 5w power resistor. take resistance readings from each side to the power tube plate and screen pins to confirm that this is the dropping resistor to replace

edit: okay looks like it's a 12bh7 output tube, so check from each side of the resistor to output tube plates and phase inverter plates. we're kinda flying blind without a schematic

this might also be the cathode resistor as i'm assuming this is cathode biased. check continuity between ground and the power tube cathodes

basically use your meter to suss out what the resistors are

[Brodirt]

Thanks, I have "capacitors" in my mind regarding your comment, I will edit.
The amp is cathode biased and uses 12BH7.

[thetragichero]

with a pen and paper handy (to write down your findings. i don't trust my brain to not mis-remember things), a meter should help you figure out which big resistor is a dropping resistor and which is cathode resistor
really this kinda stuff is why i have a notebook to jot down findings, so if i encounter the same amp (or pedal) again i can reference my notes instead of reinventing the wheel (also helps for answering others' questions, can just flip around until i find it)

oh and make sure to post your results so that the next person can just search and find your work

[pompeiisneaks]

The problem with it being an older version is that it may not be R26, it may be in a different location, it may be a different value, etc. Seeing photos of the wrong amp only helps get a generic idea of what's to be done.

Seeing photos of yours will go a lot farther towards knowing what the 'right' answer is. As was mentioned by thetragichero, that resistor is probably still needed or something similar because it's likely a decently large value in yours, and unless the choke is exactly the same resistance value you won't get the right voltages down the B+ chain, and it could negatively impact the amp's tone, as now voltages will not be within expected ranges. I'm guessing I'm reading that right as a 1.5k resistor, but not sure as it seems like 5 band but in the 5 band chart the orange color on the left end has no tolerance value, so I'm confused. You should measure the resistance of your resistor that seems to fit the rough spot in the amp (could have moved it somewhere way different in the newer revision???)

~Phil

[Phil_S]

I did a search for the schematic and learned that Marshall is policing public posting of their IP. So, we aren't going to find it, nor would they allow us to keep it posted if they found it. However, the owner may be able to get one from Marshall by providing a serial number. I suggest it would probably be OK to post a snip of the power supply portion of the schematic that would guide a discussion on swapping a choke for a resistor. I'm thinking that may fall under the educational use exception. But I don't really know. You'd need to ask a high priced IP lawyer.

[populartsl]

R26 is 150R 3 watt resistor on the original drawing DSL5-60-02 and is connected straight after the reservoir cap.

[Speedypancake]

Is it OK to ask what you are hoping this mod will achieve?
I've built a few valve amps now, including the JCM800, and have been very pleased with the tone using a power resistor in place of a more expensive wound choke?