Crate Palomino noises

[Randall]

Got a Crate Palomino V32 in for the usual dirty pots and jacks. Once I got that in order I find a sputtering, muffled noises in the clean channel that sounds like distant thunder. It stops when a plug is inserted into the Line In plug, so I trace it back to V2. Shorting grid 2 has no effect, but shorting grid 7 silences it. OK, but why just the clean channel? This amp uses 5 vactrols for channel switching. The first 3 determine which channel passes signal, but pin 2 is after the point where they mix, and grounding it has no effect. The other two determine whether the master level is activated or not, and that is downstream of grid pin 7.

I suspect the coupling cap C36 between the two triodes of V2, but this board is going to be a bit of a pain to get under, so I was hoping for some insight before I dig into it, hoping to have to do it only once. Heck, I may even replace a couple of those resistors while I have it out just to be safe.

PS, changing the tube made no change.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

I think you're on the right track. Suspects would be C36, R40, R43, R44. Is there any DC voltage on that second grid (pin 7)? It could be a bad cap either way, but if there's substantial DC there, it's almost certainly bad.

[Randall]

No voltage on Grid 7. And what's up with these odd value 1% resistors? 274K, 221K?

[The Dude]

Those resistors aren't anything I'd worry about. Just use the closest standard value. I'd challenge someone to prove to me they could hear the difference.

[Leo_Gnardo]

Those resistors aren't anything I'd worry about. Just use the closest standard value. I'd challenge someone to prove to me they could hear the difference.

'Zackly. Someone at Crate must have had a hankering for precision. Or maybe Crate got a super deal on a bunch of 1% resistors. No need for that, 274K use 270K, 221K use 220K, like that. Like Enzo says, these are guitar amps, no need for NASA specifications. "Close enough for rock 'n roll."

[The Dude]

...... maybe Crate got a super deal on a bunch of 1% resistors......

^^^^^^I vote that. It doesn't make sense to spec such a peculiar/specific resistor for a tube circuit, otherwise. We all know how consistent tube gain and biasing is.

[Randall]

I replaced C36, R40, 43, and 44. No change. Also, all volumes are turned off. Still sounds like a dull murmur with popping. Switching to the dirt channel stops it. Both channels work as they should otherwise.

[The Dude]

Maybe a socket pin or solder issue?

[nosaj]

Maybe a socket pin or solder issue?

Wiggle them tubes, Dude.

nosaj

[Randall]

I already did that as part of the initial cleaning and inspection.

[Chuck H]

IMHE with these amps the pots and jacks are very prone to board connection failures. The pots actually act as support for the board via their solder joints (poor design) and they are under considerable stress because the current in those amps is high at idle and the cathode resistor is right under the pots. Some players have reported barely being able to handle the knobs because of how hot they get. I moved the cathode resistor off the board and mounted an aluminum cased model (of a slightly higher value) on the chassis away from the electronics. Then I replaced all the pots (several were questionable). I mounted the pots upside down and ran flying leads to the board and retrofitted a different mount to stabilize the board. IIRC I had to do away with the tube retainers because they simply put too much stress on the board when used because the retainer base is on the chassis and the sockets are board mounted. So using the retainers collectively applies considerable spring load and bends the board. This would have been another stress on the pot lugs and their board connections. I think what you're looking for is failing connections at these stress points.

A very good sounding design (well, except for the reverb ) that is built to fail in my opinion.

EDIT: P.S. I also had to put new and different knobs on the amp because the original pots have a weird, skinny knurled spline that the original knobs are specifically made to fit.

[Randall]

I'm not sure re-engineering the amp is going to be what is going to happen here. But, I will pull the board again and look for solder connections. I still don't understand the metrics of why grounding pin 7 silences the disturbance, and why it is only on the clean channel.

[Randall]

Well, this is frustrating. I hit all the pots, socket pins, and almost everything else around the sockets in the preamp section. No change. I did however come to realize that the noise is also in the dirt channel to some extent, but I wasn't hearing it because that channel has a master volume control downstream of V2 that was turned full down, whereas the clean channel has not.

But, just focusing on the clean channel for now, grounding V2 pin 2 does not stop the noise, but grounding pin 7 does. What could it be?

Could a bypassed cathode be a factor here? Pin 8 say?

[Chuck H]

Check the cathode circuit pads and contacts for J2b pin8. If they're ok I might try replacing C38

[Randall]

Found it. My eye kept going to the fact that I could see disturbances, albiet slight, at several different places that corresponded with the noise. So I took another look at the power supply, thinking maybe I had a bad electrolytic. The screen supply was nice and strong, but the 275v node was a little low at 259v, the 260 node was at 198v, and the 250 node was 195v. I figured C4 might be the culprit if it was leaking. So I replaced C4, C7 and C8, and all three had some brown spludge under the glue.

Now all is well. It's not a real quiet amp as it now sits, but that darned noide is gone. I wonder if the two 100/450v caps could stand to be replaced as well. That will have to be another future episode I think, because the customer wants/needs the amp for the weekend.

As always, thanks for all the help guys!