Re-fusing an amplifier

jamesvwaal · Post by **jamesvwaal** » Sat Apr 17, 2004 12:47 am

My Laney VC30 amp has blown a fuse and I cannot find a replacement here in the outback of Malaysia. The two fuses in the amp are labelled "T630mA" (the blown one) and "T500mA". The T = time delay, and I already tried replacing the T630mA with a "F630mA", which blew immediately upon turning the amp on.
Two people have suggested trying a F830mA fuse in place, but I am concerned about trying a fuse outside the stated specs. I prefer that the 30-cent fuse blow to protect the $50 tubes, not the other way around.

Any suggestions as to sourcing Laney spec fuses? I tried their dealer in Malaysia (Yamaha Music), but they don't sell too many amps, thus do not carry parts.

milo · Post by **milo** » Sat Apr 17, 2004 7:02 am

Hi James,
If I knew the dimensions of the fuses and how the part numbering converts to our American numbering I would offer to buy you a couple of 4-packs of each size over here and ship them to you. It sounds like those are probably the same as our Slo-blo 230 volt 3 amp and 5 amp fuses but I can't be sure. They usually cost less than $1 per fuse over here. As an alternative source for an answer to your question I would recommend registering and asking your question at the Fender Forum at www.fenderforum.com! There is a catagory there for "Amp mods, repairs, and projects" and many good US and International amp techs post there. I'm sure one of them could hook you up with a source for the fuses.

rictified · Post by **rictified** » Sat Apr 17, 2004 9:47 am

Hi James, the real question is why did the fuse blow? I always replace slow blows with regular fuses (I don't like them, there is more time for parts to fry if there is a problem) and have no problems, if the same amperage fuse blew out immediately I would suspect a problem, probably in the power supply section. The time delay is mainly for the surge that occurs when they are first turned on. Do you turn the power on before you turn the stand by on? (You are supposed to about 30 seconds earlier it prolongs tube life.) If you do, does the fuse still blow?

jamesvwaal · Post by **jamesvwaal** » Sat Apr 17, 2004 6:56 pm

Robert, I always turn on the amp for 1-2 minutes before I turn the standby on (from 0 to 1). The replacement fuse (F630mA) blew in less than one second, so yeah maybe the power supply is a problem. I haven't done anything with the amp other than move it around so maybe I need to check the power cable to ensure that the ground wire is intact.

Jeff, the fuse is 2-cm in length exactly and the same diameter as normal fuses used in cars. The Laney user manual indicates that the equivalent fuse in the 100-130 Voltage setup is 'T1.5 SA', also time delay (it shows 'T1.5A' on the amp itself). I don't know if that helps but it is all that I have to go on. The secondary HT fuse is only listed as 'T500mA (time delay)'.

Of course, I am assuming that 630mA = 0.63A and that 500mA = 0.5A. Do other amps use fuses rated in the 3-5 A range? Seems like a big difference in current allowance.

I will check on the internet to see if I can get a part conversion. I do have a friend in Alaska willing to send replacement fuses, but like me he doesn't know how to match USA fuses with the Brit equivalents.

Thanks for the input.

kranz · Post by **kranz** » Sun Apr 18, 2004 1:20 am

At the risk of having too many Rosenkranzes in here, I'll add my two cents.

Check out the online catalog for Allied electronics. If it isn't in here, it probably isn't made. They have a section for european fuses.

http://www.alliedelec.com/Catalog/Indices/IndexHandler2.asp?ID=Fuses&seq=269&IND=FP

jps · Post by **jps** » Sun Apr 18, 2004 2:47 am

"At the risk of having too many Rosenkranzes in here"

Hey, we have more Jeffs here than you can shake a stick at so why not a few Rosenkranzes!

jamesvwaal · Post by **jamesvwaal** » Sun Apr 18, 2004 3:43 am

Well in this case it was not one Rosenkranz too many, nor one James too many. I just checked out the Allied link you sent and they have the fuse type for which I search. Little 5 X 20 mm time delays (FST), and available in 0.5 and 0.63 Amp. Much obliged. You can come and Rosenkranz or James up the site anytime!

milo · Post by **milo** » Sun Apr 18, 2004 6:38 am

Hi James V.,
As Robert mentioned, the reason for the fuse blowing may need to be sorted out. If the new time delay doesn't blow I suspect you'll be okay. If the correct replacement blows, I believe that you'll want to try replacing the power tubes and/or the rectifier tube if it has one. I think these are the next stage in troubleshooting that problem, but I can't remember for sure. The techs at the Fender Forum could point you in the right direction if you continue to have a problem.

Mr. Rosenkranz,
I think you're only the second person I have run across with the same spelling that wasn't related to me. I do know that my Great-Grandparents left at least one son in Germany back in the early 1900's, but I don't know how common that spelling is over there.

philco · Post by **philco** » Sun Apr 18, 2004 10:37 am

Sometimes a tube or tranny shorts out and blows a fuse. This happened on an output tube once in my YCV40, and took out the grid resistor and series grid power supply resistor as well. Since it blew the the series resistor open, it came on after replacing the fuse, but no output. Remove the tubes and check in a tube tester. If they test good, check the power tranny for a short. If a tube is blown, look at any series components as well. Check components for discoloration from heat or any physical damage. If fuses keep blowing, you have a problem.

kranz · Post by **kranz** » Mon Apr 19, 2004 12:36 am

James V- Glad you found that fuse and I could be of help. That Allied catalog has saved me a number of times.

Milo- I've found that the Rosenkranz name is quite rare, having only run into a couple others. My father came from Schlochau, West Prussia (now a part of Poland and not far from Gdansk) and immigrated to the US in 1921 and settled in northern Wisconsin. BTW the Ellis Island records are now online on the web. I found my father's immigration records there. You might find it interesting to see if you can find your relatives in there as well.

rictified · Post by **rictified** » Mon Apr 19, 2004 11:14 am

James is that a 220 amp? A 630 ma. fuse is very low for a 110V tube amp, but is possible. Is it all tube or SS and tube? If your amp blows another fuse try taking out the power tubes and tring again. Or better yet, try taking all the tubes out and if it doesn't blow try repacing them one at a time starting with the rectifier tube, (if you have one) with a pre amp tube if you don't right in line. replace both power tubes at the same time. (shut off the amp between tries)
If your power tubes are bad also check the plate resistors, in fact all resistors that have to do with the power tubes. The resistance on a power transformers primary windings are pretty low something like under 10 ohms in SVT's (I think if I remember correctly) If it's at 0 it's bad. If it were open you wouldn't be blowing a fuse. and blowing a time delay fuse in one second indicates to me a dead short.

jamesvwaal · Post by **jamesvwaal** » Mon Apr 19, 2004 2:07 pm

Robert, it is wired for 220 V. Under the 220-V configuration, the power fuse is 630mA; the user manual shows that under the 110-V configuration, the fuse should be 1.5A.

The amp has 3 pre-amp input tubes (ECC83/7025) and 4 power amp output tubes (EL84). I have already pulled all 7 tubes out to see if there was discoloration, but all seem to be okay. Again, the amp is fairly new (played only 4 months) although it may have sat in the dealer's showroom for several years; the user manual is dated January 1995.

Yesterday, I found a supplier for these fuses on the other side of the country. I have to obtain fuses before I proceed further. If the fuse continues to blow, then I will try out your method of checking each tube individually.

Thanks again for the advice gentlemen. My 17-year-old son has a gig this Saturday and he wants his old man to back him up! Can you believe the generosity of today's youth?!

rictified · Post by **rictified** » Tue Apr 20, 2004 8:47 am

James,
Does your fuse blow before you put the standby switch to the on position? If it does that simplifies things considerably, at least you will know that you most likely don't have a shorted power tube. Did you check the diodes?

jamesvwaal · Post by **jamesvwaal** » Tue Apr 20, 2004 1:38 pm

The only replacement fuse that I tried blew as soon as I turned the power on, with the standby engaged (in the off position). But, the fuse was the wrong type (F) and, thus, I need to buy a bunch of fuses of the right type (T) before I try again. Sourcing the fuses has been hard, but I think I have found a source in Singapore.

rictified · Post by **rictified** » Wed Apr 21, 2004 3:02 pm

I wouldn't keep trying different fuses if I were you James, it sounds like there is a problem and you may just make it worse and ruin more companents by repeatedly blowing fuses like that. I never use slo blos in any of my amps, and never have any problems blowing fuses. A fuse is usually rated at double the power an amp uses it's just for the surge when you first turn on the standby switch or in a SS amp for when it first goes on.
If you are blowing fuses with the standby switch off you definitely have a problem somewhere because your amp is not supplying the high voltage to the plates of the power tubes which is where the majority of the amperage is used in a tube amp. I have had the same fast blo fuses in some of my amps for years.