AB763 Blown Fuse

[plaidzebra]

Hey all,

I was playing my Deluxe Reverb clone when I hit the low e string, let it ring, and after 2 seconds the fuse blew. I had some problems originally when I build the amp in that every time I turned the amp off of standby, it blew a fuse because I had accidentally wired some of the filter capacitors backwards. At the time the volume was set to around 8, the GZ34 was in the amp, and it was running two 8 ohm Jupiter Condenser speakers in series (I put in an upgraded output transformer from Classic Tone that puts out 4, 8, and 16 ohm loads and it was on 16 ohm). The speakers are both 8 ohm, but one is rated for 25 watts and the other is rated for 50.

I'm currently running NOS Sovtek 6V6GTs that I found in a little music shop in upstate New York, so I have no idea whether or not they were matched. I'm fairly certain that they were biased correctly, because I biased the amp for those tubes running at about 30 mA for one tube and around 27 mA for the other. Did these power tubes and/or bad biasing on my part potentially cause the failure?

In addition to these problems, the amp is muddy at high volumes and doesn't put out the same quality distortion that I've seen in videos. It's muddy, not very bright, and doesn't respond well to lower frequencies.

What can I do to fix these problems?

[shooter]

after 2 seconds the fuse blew

does it repeat?, does it blow idling?  Has it been working then just blew? 

[plaidzebra]

It's been working fine since I got it working about a week or two ago. I haven't been able to try it out again because I don't have any more fuses, fortunately I'll have ten more tomorrow thanks to Amazon. I haven't been pushing the volume up too high for the most part, and I haven't run into any problems in normal use until now. The only other times a fuse blew were when I had some capacitors backwards, and that was before I even plugged it into a speaker.

[plaidzebra]

Another possibility: I was not using slo-blo fuses; instead I was using one amp fuses I got from an auto parts store. Could it have just been a bad fuse? If that's the case, then how do I fix the poor tone at high volume?

[shooter]

not using slo-blo fuses

if you watch B+ at power-on, it will go way higher than ilde - inrush? as the caps go from basically short to fully charged, slo-blo can deal with this better than standard, but your problem happened after that.

muddy bass is one of those tweaks you learn over time - you'll come up with a *usual suspects* list to "fix" it, or avoid it.  Start with a forum search for "muddy bass".  you'll find a lot of "decrease V1/v2 cathode by-pass caps from 22uF to 1-5uF".  decrease pre-amp gain by swapping in various 12A*7's

[plaidzebra]

I did just put a new 12AX7 in the tremolo channel gain stage, so that might be part of the reason. I'll do some more testing today when I get those fuses, but I still can't figure any reason why it would just blow out of nowhere.

[sluckey]

Schematic calls for 1A SLO-BLO.

[plaidzebra]

Gotcha, didn't realize there was too much of a difference. Dad just explained it to me, so I'm glad I got the right ones from Amazon.

[plaidzebra]

More information. Amazon sent the wrong fuses, so I haven't had a chance to fire it up until tonight. I turned the amp on with the volume for the tremolo channel all the way down, but as soon as I turned it up the fuse blew. This happens regardless of what channel that I use on the amp.

I'm thinking now that it's a problem with the power section rather than the gain section of either channel, does that make sense? Hopefully I can learn a thing or two from all these mishaps :)

[sluckey]

How old is this clone? Has it ever worked properly?

[plaidzebra]

I finished it up about 3 weeks ago and until now, it's been working without any problems.

[shooter]

I'd look for something loose, cracked, touching.  Hopefully you scratched down DC readings when you finished it so you can compare now.  blown fuses means increased current, ch1 and 2 share Node D.  take your time and you should have it flushed out pretty soon.

[plaidzebra]

Well, I was checking some voltages with the amp powered up and saw that the plates on one of the output tubes were glowing cherry red. I might not know very much about amps yet but I know that is definitely not supposed to happen. i'm probably going to take a break from it for a while because I don't want to break anything else. Dad gets back from work at the end of the week, so I'll be able to look at it with him.

I'd look for something loose, cracked, touching. 

Couldn't find any of that, unfortunately. This makes me think it's a power transformer problem, which really makes me hope that I missed something.

[sluckey]

Mark the cherry red tube with some masking tape (or whatever) then pull both 6V6s and put aside. Measure voltage on pin 5 of each 6V6 socket. Should be about -35VDC on each socket and you should be able to adjust it with the bias pot. If not, examine the simple bias supply and find out why. Then set it for maximum voltage for now. Plug the 6V6s back in. Does the marked tube still glow red? If so, the tube is bad. Swap the 6v6s. Does the marked tube still glow red? If so, it's really bad.

[plaidzebra]

Well that definitely helped, thanks a bunch Sluckey. At maximum bias voltage both tube's plates were glowing, so I brought pin 5 down back to about 35 volts. The marked tube glowed in both tube positions, so I think it's safe to say the tube is bad. 

I just threw in some different power tubes and the amp works without blowing a fuse 

Seriously, thanks again for helping me out with this problem.

[sluckey]

At maximum bias voltage both tube's plates were glowing

You must mean "At minimum bias voltage both tube's plates were glowing". Right?

[plaidzebra]

Whoops, you're correct.

[plaidzebra]

One last question - I've been searching for a few hours and haven't been able to find a definitive answer. Can I put a coke bottle 5U4G into this Deluxe Reverb without any problems? I tested it and shortly afterwards one of the power tubes failed. Was that because of the different rectifier?
I'm not worried about filament current - the power transformer that I have is good for blackface and silverface Deluxe Reverbs, so I'm not worried about overworking that.

[shooter]

Can I put a coke bottle 5U4G

Each rectifier tube, or SS will give you a different B+, higher/lower B+ might mean you need to re-adjust bias.  I didn't look up the specs between the tubes.  I use this site to get datasheets, compare tubes etc.
http://tubedata.milbert.com/sheets62.html

be careful on pinouts, they might all have 8 pins, but they don't always go to the same place.

[plaidzebra]

Thanks a bunch for that page, I'll definitely look into this.

[sluckey]

I'm not worried about filament current - the power transformer that I have is good for blackface and silverface Deluxe Reverbs, so I'm not worried about overworking that.

Does that mean your PT is rated for 5V @ 3A? Look at the data sheet for your PT.

5U4 needs 3A for the filament whereas the GZ34 only needs 2A. Either tube will work with the same socket wiring. I prefer the GZ34 because it has an indirectly heated cathode which causes the warmup time to be much longer than a 5U4. This allows the B+ to come up slowly.

Why do you want to put a 5U4 in the amp?

[plaidzebra]

Why do you want to put a 5U4 in the amp?

I'm just curious about the way it sounds. I don't expect there to be much of a difference, but I've read a lot about "sag," and I wanted to hear exactly what that meant. And if I'm being honest, another reason is because I think it looks really cool    I should have mounted it where it's visible, though...

Chances are I'll see if I can hear a difference, and if I can and I like the way it sounds, I'll keep it in there. If I don't, I'll probably just keep it on my desk.

[HotBluePlates]

You likely won't notice much sag unless the amp's output section is being pushed to/beyond its maximum clean output power.

[Willabe]

You likely won't notice much sag unless the amp's output section is being pushed to/beyond its maximum clean output power.

HBP knows waaaayyyy more about this than I do but I'll add this;

but I've read a lot about "sag," and I wanted to hear exactly what that meant.

If the power tubes load is not very much (a pair of 6V6's don't draw as much current as a pair of 6L6's) compared to the current passing abilities of the rectifier tube then you won't hear much if any sag.

Sag also has to with/occurs because of the PT's B+ current abilities, well lack of. So a bigger PT with higher B+ current could out deliver the rectifier tubes current passing abilities and vise/versa.

Then there's how the power tubes are set up by the designer, generally with lower B+ dcv they use more current, with higher B+ dcv they use lower current to get their target power output.

Then with a class A amp, the power supply is pretty much all ready sagged at idol. Going to full tilt, it hasn't much left to sag.

And the OT's primary impedance/loading of the power tubes along with their B+ changes the current draw for the whole thing to work.           

It's all somewhat of a balancing act.   

[plaidzebra]

Interesting - my guess was that I wouldn't hear any sag until the amp was loud enough to annoy everyone in the house. Thanks for all this information, it definitely helps me understand more about what's going on in this amp.

[Willabe]

Interesting - my guess was that I wouldn't hear any sag until the amp was loud enough to annoy everyone in the house.

Probably. 

.....it definitely helps me understand more about what's going on in this amp.

In all amps, they all have the same things going on. For good or for bad, it depends on what you want.   

Some amp companies tried to get rid of the sag, like Highwatt, Fender Dual Showman and others. So they got rid of the tube rectifier and beefed up the PT B+ current and beefed up the B+ filter caps. Along with a bigger wattage OT, with higher rated speakers to get the compression (sag) out of the back end.