Replacing Cap Cans on '74 MKII SL

rgalpin · Post by **rgalpin** » Thu May 05, 2005 1:12 pm

I'm replacing the cap cans on a '74 MKII Super Lead.

The main reason for this follows:

I use a bias setting tool that reads current being supplied to the tubes. On other amps I have used this tool on, I usually see current go back up on the bias tool when I shut the amp off. This, I believe is being released by the cap cans and is the normal state of affairs - the cap cans just doing their job - holding energy to be released later - hopefully, not into a human extremity!

Does the fact that I did not see the meter rise as usual when I turned the power off show that there is a problem with the cap cans?

And...

Is there anything to be considered with replacing them as far as using stock values or does changing the values have any affect on tone or perfmormance?

Flames1950 · Post by **Flames1950** » Thu May 05, 2005 2:31 pm

I guess I wouldn't worry about changing them unless you see or hear signs that they are failing. Visual signs would be a bulging or "blister" on the ends of the cap can, or any chalky deposits on the cans. Audible signs are ghost notes (where you hear additional out-of-tune notes underneath the note you play) or a humming that will get worse over time and that the volume controls do not affect. The tricky part is that there can be other causes of ghost notes and humming as well.
But it won't hurt to change them either, especially if it brings you peace of mind and confidence in the new amp. Just make sure they're drained of all voltage (the old jumper between the chassis and either of the first preamp tube plates will work fine. The plates have the blue wires going to them on a stock Marshall.)

rgalpin · Post by **rgalpin** » Thu May 05, 2005 4:01 pm

Ok. What about the values? How might they affect the tone or performance of the amp?

Does my explanation as to the current measurements shown on the bias tool when the amp is shut down make any sense and if so, is there anything to learn about the condition of the amp components from that?

Dax · Post by **Dax** » Thu May 05, 2005 4:08 pm

I have had this happen under different conditions, On my 70's Marshalls when i turn them off, i can play them out and hear the sound fade. On my JCM 800, when i turn it off, its dead. I always leave them on standby for about 10 seconds. What does this mean, do newer amps have better setups for automatic discharge? All caps are about 2 years old, JJ's.

Billy Batz · Post by **Billy Batz** » Thu May 05, 2005 5:17 pm

I think it has to do with where the standby is placed in the B+ chain.

The only filters that really effect the tone in a big way are the screens and the lower you go with them the more tubby and sluggish the amp becomes particularly in the bass since bas demands the most from the amp. Its something that I dont think sounds very good with stock post 73 type values like less NFB, 1.5" tranny, lower voltages etc..

NY Chief · Post by **NY Chief** » Thu May 05, 2005 7:03 pm

On the fairly new '59 RI I have been working on I have been draining the caps from cap "+" to chassis with a jumper. Just for fun I measure the voltage across the cap first. It drains very quickly without the jumper and immediately with the jumper (and little or no arc).