blowing fuses in BF vibrolux reverb

[peachhead25]

Let me start by giving a background on the amp.Its a 66 BF Vibrolux Reverb in beautiful condition that my father is the original owner of.The amp sat for about 20 years in the closet, seeing maybe 30 mins of use 0nce every 2 years. Anyway,about 2 years ago I decided to completely rebuild the entire amp for him,as I repair hammond organs and leslies and dabble in tube amp repair.I called Dan Torres and he sent me a complete rebuild. Well I changed everything except trannies and the cosmetics.I kept all the parts in baggies in case he ever sells it. Well the amp never quite sounded as warm and tube like as I thought it should but I put it off as break in,new tubes,new speakers and so on. The amp saw weekly use and stayed on for 2+ hours a week since the rebuild,now the fun stuff. Dad calls and its blowin fuses!!! Ok so I check everything out looks good maybe bad recti,nope tried 3 different ones 2 of which are new and they all glow red and arc after 10 sec POP,fuse blows.it only blows when the recto is in with or without the other tubes. I checked the filter caps and bias caps and all seem to be good they hold a charge and have no shorts.I have examined the amp under a microscope and have found no shorts,burned parts,etc.I went over solder joints everywhere everything looks good I notice that after I flip the standby switch that the pilot lamp dims and stays dim till the recto glows and fuse blows.The fuse does not blow without the recti so I've been told that that means the trannies are prolly good? I am at a loss and where we live there are hardly any good trustworthy techs around. I appreciate any help I can get
Thanks,
Joshua Roques

p.s. srry for the long first post!!!!

[ziess]

Have you checked AC voltages on all the PT secondaries with no tubes installed?
What kind of rectifier tubes did you try? Current production, NOS, original tubes?

Tommy.

[peachhead25]

I tried to new 5ar4 chinese(valve art) I tried a known good old sylvania gz34 all had the same results. I dont have the schems anymore so I dont know where and what voltages to check for on the power tubes.Also I thought I wouldnt get any B+ since the recti has to be out to keep the fuse from poppin?

[peachhead25]

I just read your reply again Tommy No I didn't check the secondaries.Where do I hook my meter for that and how much voltage should I read?

[ziess]

You're correct in saying that you won't get any B+ with the rectifier removed. What I was asking was for you to check for AC voltage coming out of the power transformer with all tubes (including the rectifier) removed: mesuare for AC voltage on pins 4 and 6 (you should see about 315VAC) and pins 2 and 8 (should see 5VAC here).

Schematics are available here:

http://lib.store.yahoo.net/lib/thetu...everbaa964.gif
and here:
http://lib.store.yahoo.net/lib/thetu...everbab568.gif

Depending on your revision.

Remember that you've rewired the amp so only you will know if the amp differs from the schematic in any way.
Above all, stay safe. If you're not sure, take it to a tech.

Best,
Tommy.

[peachhead25]

Ok Please god dont be bad news.With all tubes removed and amp on and standby on i get nothing on 4 and 6 on the power tubes these pins are connected by a resistor.PINS 2 and 8 read 3 volts and pins 2 and 7 (heater wires) read 6.3.

[ziess]

Ignoring your readings for just a second...
Sorry, I should have been more specific there, I meant mesuare them at the rectifier socket (the point of contact for the PT secondaries into the circuit).

Best,
Tommy.

[peachhead25]

ok I got 710 AND 5 VOLTS

[ziess]

Yup, i'd reflow all the connections for the heaters on all the tubes.
I'd also reflow all the connections in the powersupply and bias section just to be sure.

Those readings are good by the way so the PT sounds fine.

Tommy.

[ziess]

Just thinking about it a bit more, check there are no strands of wire between pins on the power tubes.
What voltage are the first two filter caps rated at?

Tommy.

[peachhead25]

I checked the heater wiring for shorts and thought i found a short and fixed it but same problem. bias supply seems good no burned resistors or bad solder joints and caps hold voltage and show resistance so they arent open. Whats wrong with Torres other than price was outrageous,luckily it was a donation from a pool of friends that payed for the kit and my work.

[ziess]

Problems with electrolytic caps can often (in my experience anyway) only show up when they have a high voltage across them. Do they all look ok?

Tommy.

[peachhead25]

so far it only works with no tubes.Recti tube alone last maybe 10 secs before glowin red and poppin fuse.I dont let it get to that point, I can tell by the pilot light still dimming to like half brightness that whatever it is shorting out its still doing it.Also the caps where changed at the same time with sprague atoms.All components look new no burns browning bulges drippings etc....also no components feel like they are getting hot. I just examined tube contacts again and no bad joints, wire strands or anything fishy.

[ziess]

Is it blowing the fuse with the rectifier installed and the standby set to 'Standby'?

Out of interest, when you rebuilt the amp did you reuse the original circuit board and just replace the parts or was it a new drop in?

Tommy.

[peachhead25]

Im going to start tearing it down tommorrow night. I have a feeling its a cloth wire touching chassis somewhere.

[peachhead25]

it was a kit with all the parts and i reused the fiber board, wires and i got lazy and did not change the preamp tube sockets.

[peachhead25]

it blows the fuse when switched out of standby amp on mode .With just the rectifier tube installed it blows the fuse after 10 sec of standby off, it starts to glow red on the plates and then the fuse pops.

[peachhead25]

how do i check the choke and the Ot? Im going to start tracing everthing tommorrow but if theres a quick meter test I can check right quick for the choke or ot let me know

[peachhead25]

ok Im off of work and back to troubleshootin this thing I will keep you guys up to date.Thanks for the awesome help so far guys!

[peachhead25]

heres something I just noticed, The recti tube has to warm up to cause the problem.Let me explain, If i flip the standby switch right after the power switch the pilot light doesnt dim and fuse is cool.if I wait till it warms up like 10 secs and flip it the light dims and 10 secs later the problem starts.Also I changed the standby switch to a new carling and that was not it,I had a guy tell me it could be arcing.Im going thru the schem and this where I start getting lost, I've never been this stuck before on an amp.

[ziess]

It sure sounds like a short on your B+ rail... With no power applied to the circuit and the standby switch set to 'ON', check for continuity between pin 8 of the rectifier and ground.

Keep going, you'll figure it out!

Tommy.

[peachhead25]

ok how do I test the choke? If I disconnect one of the black leads from the choke no more dimming and fuse popping

[peachhead25]

ok no continuity with power unplugged and standby in any position between 8 and ground

[peachhead25]

ok I pulled what I believe to be the center tap on the OT(red wire) and same thing.I pull one leg of the choke and no more pops I get 151 ohm from that choke leg to ground and continuity from the other leg of choke to ground.It reads 151 ohms between both legs of the choke.

[JAM]

151 ohms sounds about right for the choke itself, but the side that reads continuity to ground has a fault. Is it the side that connects to the OT CT or the screens? If its the screen side, check the filter cap at that point.

[peachhead25]

no its reading continuity to ground on one side of the choke disconnected from the circuit completely.

[peachhead25]

It seems as though the electrolytics are fine I checked em again.It just seems as though the choke is the problem.I pulled the choke out and one leg is open to ground(side of choke) the other reads 150 ohms.That does not seem right to me.

[peachhead25]

do my readings on the choke sound right? One leg of the choke is reading continuity to side of choke.

[JAM]

do my readings on the choke sound right? One leg of the choke is reading continuity to side of choke.

No, that's not right. Looks like you've got a bad choke.

[peachhead25]

No offense Jam, but could I get a second opion on the choke.Also if everyone thinks its the choke, who do you guys recommend I should get one from?

[peachhead25]

oh and is there something I should look for as to why it died after 41 years.Basically why would a choke die?

[JAM]

No offense Jam, but could I get a second opion on the choke.Also if everyone thinks its the choke, who do you guys recommend I should get one from?

No offense taken. Here's a link to where I got mine.

[peachhead25]

It was the choke. Thanks so much guys!!! Now I need help figuring out hooking up a new hammond 273bx xformer to a 1972 peavey vintage 2x12 that blew the xformer years ago. I am unsure of the correct connections for the high power leads.I have line and filament connected but im stuck there.

[Plugger]

It was the choke. Thanks so much guys!!! Now I need help figuring out hooking up a new hammond 273bx xformer to a 1972 peavey vintage 2x12 that blew the xformer years ago. I am unsure of the correct connections for the high power leads.I have line and filament connected but im stuck there.

I think you need to start a new thread.

Congratulations on the call on the choke, Fezz and JAM! Always fun to watch you guys at work...

-Mark