Crate VC50 blowing B+ fuse

[Dave Rice]

Hey all, been away from amps/Ampage for a while, hope all is well with all y'all. A friend of mine brought me a Crate Vintage Club 50 that crapped out on him mid song. He recently replaced the 10 year old extremely microphonic EL84s with a quartet of Ruby Tubes. I opened it up and it's blown the 1/4 amp fuse on the B+ Here's the schem for the VC30 (all I could find, looks pretty close):

http://www.schematicheaven.com/newam...2_poweramp.pdf

With no signal it's only drawing .13 amp on the B+. Are the Ruby Tubes drawing more than the old Sovteks at some point? The old 84s have some interesting scorch marks on the glass, could running these things at the edge of thier envelope for too long have whacked other component values (haven't really gone there yet...)? He tells me it pooped out while he was playing so that seems to eliminate a spike upon kicking in the B+. Haven't got a 1/4 amp fuse here, gotta pick those up tomorrow, but it seems fine with the meter in there. I dunno, I'm scratching my head. Any ideas? Again, hi to all, hope to hear back.

Dave

[Ray Ivers]

Dave,

After you replace the fuse - I'd suggest getting some slo-blo .25A's to use when the amp's fixed, and some .5A fast-blows for repair/testing - I'd be very suspicious of the output tubes, as one or more may have shorted. When you have the amp up and running, I'd then check TP17 for DC voltage - the stock schematic calls for 9.7VDC, resulting in about 104% plate dissipation from the EL84's; I wonder if they ran hot?

Again, just from my perspective... I would replace R43 with a value that results in a less hellish bias point (68 to 82 ohms, 15W), install a fan, or preferably both. Also, any time you have a fuse blow it's a good idea to check around for collateral damage, as you alluded to in your post.

Of course, the VC 50 watter may have a different circuit - if it's getting 50 watts from four EL84's, it almost certainly does - making all the above meaningless.

Ray

[Dave Rice]

Thanks. Ray. Checked things against the schem tonight and it seems to check out, though they've got a 47ohm cathode resistor instead of the 60 on the schematic, but it reads bang on 9.7v. Huh. It seems to follow the VC30 schem pretty closely, although it's hitting the p/i plates with 219 instead of 170. Guess that'd give it something of a boost. Haven't traced everything out, as pc board amps give me the rickets, but I've had it running for about four hours now with no blown fuse (fast-acting .250 at that, damn you Radio Shack!!!), though I've got no dummy load here and haven't been able to crank it up (I need a shop, damn you, Bay Area!!!). so I still feel a little hinky about just popping in a slo blo and giving it back to him, even though he wants to gig with it tomorrow. I dunno. Ever see one of these pop a b+ fuse just as a fluke? The 84s are, of course, toasty warm, but no glowing plates or anything scorched on the board. I'm inclined to let him know it's possible it'll blow again and make sure he has a spare amp and take it from there. Maybe replace that 47ohm with something sane later. A fan is an excellent suggestion, by the way, as it was like working in front of a space heater. Sheesh. Thanks for listening and let me know if you have any other input.

Dave

[SeanF]

I've had a VC5310 since 1995. The difference between the 30-watt and the 50-watt models is that the 50-watt models are fixed-bias nonadjustable. If you look on the board between the center power tube sockets, you should see a big blank rectangle marked "R43". That's where the cathode resistor would go, if there was one. I'm in the process of converting mine to an 18-watt by removing two tubes, changing to cathode bias and replacing the OT with a Heyboer, which is the only reason I discovered this. It's not hard to do, the board's laid out for it, you just have to remove two jumpers, add a jumper, and install the cathode resistor/cap.

The 5-watt 47-ohm resistor you're seeing is one of two used in the +10/-10V DC supply.

mine only blew the .25A fuse when one of the power tubes failed, iirc, it hasn't happened in about 10 years. Lots of the solder joints on the board are marginal, though, you might wanna do a close inspection and hit every one that looks borderline, especially on the tube sockets and pots.

[R.G.]

Read the "Tube Amp Debugging Page" at GEO for some ideas on why fuses pop.

There are about a zillion reasons why something would pop a fuse when it got good and hot. Intermittent bias loss (or short on cathode bias cap) though about sixteen different ways, intermittent decoupling cap, or choke or DC wiring, or intermittent short in a tube or... or... or...

[Dave Rice]

Thanks guys. I convinced him to just go with his spare while I sort out his Crate, and I'm glad I did. Thanks for that tip about R43, as I could've spent some fruitless time going after that one. Yes, that empty spot's right where you said it should be. Duh. That also explains why the one I was looking at (held down with a huge blob of number-obscuring glue) is only a 5W. Funny that it reads the same as R43 should. Looks like I've got some work/research ahead of me. Has anyone seen the schematic for the VC50 out there?

Thanks again,
Dave

[SeanF]

I've never seen the 50-watt schematics on the web, but as far as I know, the bias style is the only difference from the 30-watt models. I guess you could always call Loud Technologies and see if they'll send you one. (800) 258-6883.

[Dave Rice]

Well, I think I found the problem. Removed the 84s and in one of them the getter was merrily rattling around inside. Let's hear it for quality control. Seems like a prime candidate for an intermittent short to me. Glad I didn't go nuts and just start replacing stuff, although I still like the fan idea. Thanks to all for the input.

Dave