Weird rattle in Crate V-18

[Steve A.]

I've been tracking down and fixing most of the rattles in my new Crate V-18 amp but there is this one last one that has me baffled a bit.

It sounds like it might be the EL84 output tubes rattling internally. I switched between a pair of 1999 and 2007 Sovtek tubes with no change. I just put in a pair of JJ's which seemed to have improved things a lot.

What is weird is that I get the rattle/buzz on notes between low F# and B, but only if the master volume is higher than 6. Even if the gain is set very low. If I turn the gain up higher the rattle/buzz is not there if I leave the MV at 6 or lower.

The power amp section of this amp is a bit strange. What would be bias splitters are 470k resistors rather than the more common values of 220k or 100k. But this is a cathode biased amp so I guess that are just considered to be the grid load. The grid stoppers are 10k, which seems a bit high to me. I usually use something between 1k5 and 5k7 on EL84's. Anything higher than that seems to make the amp sound a bit dull to my ears.

Any thoughts here would be appreciated. Prayers, too!

Steve Ahola

[Gtr_tech]

Prob'ly output tube rattle. My VC3112 will rattle those 84s loose internally and cause them to start adding thier own contribution to the signal. Thats just what happens when they're placed right on top of a speaker.

Try using an external spkr cab placed far enough away that it can't rattle the tubes. No more strange noises?

[Steve A.]

Prob'ly output tube rattle. My VC3112 will rattle those 84s loose internally and cause them to start adding thier own contribution to the signal. Thats just what happens when they're placed right on top of a speaker.

Try using an external spkr cab placed far enough away that it can't rattle the tubes. No more strange noises?

I guess I should have mentioned that I did try using an external speaker cabinet and when I still heard that noise I started to think it was something else besides the speaker rattling various components. And like I said, setting the master above 6 would produce the noise even with the volume set very low.

Here are the plate and cathode voltages:

V1a 183 / 1.54
V1b not used (I bypassed stage for Clean Channel mod)
V2a 242 / 3.75
V2b 192 / 1.60
V3 plates 208 / 195 [LTP]
V3 cathodes 45.2 / 45.2
V3 grids 26.5 / 29.6

V4 & 5 plates 324
V4 & 5 screens 321
V4 G3 10.6

Steve

[Gtr_tech]

Check the pcb over real close for cracked solder then. Especially the tube socket pins.