JCM800 6550 tube substitution and fuses

[misterc57]

Here is the hum/noise recorded ( WAV file )

Recorded with guitar plugged into HI input (guitar vol at zero).

This noise is present at any preamp level or volume level (above zero) on the high gain input. Recorded with PreAmp at 10, Volume at 10.

http://www.noalibirocks.com/000/2204HI.WAV

[misterc57]

any advice?

Anyone have any thoughts on this noise or how to get rid of it?

Thank you. Mark

[52 Bill]

Anyone have any thoughts on this noise or how to get rid of it?

Have you tried changing the pre-amp tubes?

[misterc57]

power tube change has helped

I changed the three pre-amp tubes and this did not alter the noise.

I changed the power tubes to another matched pair (that I am pretty sure were new), and this has helped.

The only noise I now have is a hiss sound. Which I think may be normal (can anyone comment on that?).

The hiss is very amplified when plugged into the high input, but it is ok on the low input.

This amp is so loud through the low input, and insanely louder when using the high input. I cannot imagine having to use anything but the low input.

My voltages have come down slightly with the tube swap...

V5
pin 3: 427 VDC
pin 4: 422 VDC
pin 5: -39.7 VDC

V4
pin 3: 424.5 VDC
pin 4: 419 VDC
pin 5: -39.7 VDC

I think I may be good at this point!!!

Thank you to all for all the help! Mark

[misterc57]

? @ 1M log pots

I have been investigating the possibility of cleaning up the HISS sound.

All tubes are new. HISS was present before and after tube swap.

In checking all my pots I found that the three 1M pots do not measure nearly to 1M resistance.

At the extreme turn (set at zero) of the pots I get the following readings

Bass pot: 683K

preamp pot: 22.7K
vol pot: 348K

If I back off slightly (set at one or two) the resistance becomes higher

preamp pot: 366K (this is the highest I can get)
vol pot: 460K (this is the highest I can get)

Do I need new pots and could these be contributing to my HISS problem?

Thanks, Mark