183 build keeps blowing fuses.....

[noworrybeefcurry]

Ok......so i finally finished wiring up my 183 build. I hit the power switch for the first time and all seemed fine, I did a voltage check on the filaments and bias-all was fine. I hit the standby and nothing blew up...but there was no sound. I realized that i was a jackass and i didnt wire the screen pins to the B+. I wired that up, then turned the amp on....again, no sound. There was nothing coming through the speakers, but there was what sounded like an oscillation coming from the output transformer. I was trying to trouble shoot and i eventually turned my master volume all the way up.....probably not a good idea in hindsight....the fuse blew again. I then changed my output tubes and the fuse blew instantly...again and again.

Does anyone have any ideas?

[ToneMerc]

Does anyone have any ideas?

Well pics always help but here goes;it possibly appears that you two separate problems here and what started out as a no sound manifested itself into a short of some type as well or you could have had two separate problems from the jump. With limited info at hand I'm going to assume that it is wired correctly.
I would address the short first by dividing and conquering the B+ power supply rail. I would cut it in half by disconnecting B3 thru B5 and for grins I would disconnect the filament supply at V7 as well.

TM

[amplifiednation]

TM knows better than I, but I was thinking some additional voltage readings with the output tubes removed would be useful. Then you may be able to isolate the problem.

Pictures would be good even if it IS wired correctly!!

[ToneMerc]

Pictures would be good even if it IS wired correctly!!

Yes, because my amp troubleshooting crystal ball is still cloudy. I think it's those cheap far east flux capacitors that I grabbed from eBay

TM

[talbany]

First

Pull all the tubes and see if it blows with the screen supply hooked up
If not recheck all your voltages including bias range.

fuse blows instantly,re-check your PS wiring especially the screen supply . If the PS looks ok there is a good chance your rectifier has shorted out. Bridges fail more readily because all it takes is for one of the 4 segments to short . If you don't have a bridge, then usually there is a string of 3 or 4 diodes, so you get a few "lives" before your rectifier totally shorts out. Either way, I would check that first and then go from there.

If the fuse doesn't blow and all the voltages look good, install the output tubes.(set -DC volts to cut off) Bring the amp up and watch the bias carefully on each tube.

If it blows when you install the tubes chances are it's either wired wrong or you have a bad tube

If the amp comes up and the tubes idle out and everything looks OK install the PI tube and see if you have any positive feedback or if the oscillation returns.( keep an eye on your bias)

If not turn the master up about half and take one side of your DMM and the other grounded set to DC volts and touch the input to the PI cap and you should get a loud thump.
It really is a good idea to do all this with a light bulb tester wired to the AC.
It's a start!!..Get back with us at any point

Tony

[Structo]

Be sure to follow Tony's instructions, he knows his stuff.

But I wanted to throw this out there because a lot of newbies don't understand the fusing on these amps.

You must use a Slo Blo Fuse on the primary of the power transformer.
When the amp is first powered up there is an inrush of current while the main filter caps charge.
As they charge the current demand goes down to what it will be when the amp is functioning.

If your amp is a 100w build then you should be using a 5 amp fuse (if you are on 120vac house current).

[noworrybeefcurry]

Sorry guys...I should mention that i did add a thing or two of my own to this build. One of those things was adding a fuse between the power transformers HV secondary center tap and ground, currently that fuse is a 500ma fast blow-this is the fuse that keeps going, sorry for not clarifying.

Also, when i first turned the amp on with out the screen supply connected I was able to get several voltage readings. The Plate voltage and bias on all preamp tubes was fine, slightly high but nothing dangerous at all. The plate voltage on the output tubes was about 500v...i was a little hesitant when i saw that, but i didnt think it was a problem.

Once the fuses started blowing i checked for shorts along the B+ string and all seemed fine, i did not check the rectifier though.

I blew through atleast 6 or 7 fuses last night so i am currently out. After class and work today ill pick up a bunch more and start troubleshooting again and try to get some pictures up.

[ToneMerc]

Sorry guys...I should mention that i did add a thing or two of my own to this build. One of those things was adding a fuse between the power transformers HV secondary center tap and ground, currently that fuse is a 500ma fast blow-this is the fuse that keeps going, sorry for not clarifying.

I would change the HT center to 500mA slo blow and the mains to 3A slo blow as well.

TM

[noworrybeefcurry]

Ok guys, heres a quick update...I pulled the tubes and powered the amp up, all voltages seemed to be fine at that point. I put the output tubes in and powered it up once more....but the fuse blew instantly. I even changed the output tubes and the same thing happened. I figure it has to be the tubes drawing to much current, so i put in a pair of 6v6's just to see what happened. I powered it up and the fuse did not blow! I began to turn the volume up and then that oscillation from my OT came back..and i mean the sound is physically coming from the output transformer! So i turned it off, but one i went to turn it on again the fuse blew instantly. I guess im back at square one.......im really stumped here??

[noworrybeefcurry]

the amp now blows fuses instantly with no tubes of any kind installed.....

[brentm]

How about some pictures!??? I'm sure this could be figured out quickly by folks here if a set of detailed pictures were posted.

[Structo]

Are you using EL34 tubes?

And if so do you have pins 1 & 8 jumpered on the sockets?

[noworrybeefcurry]

I have tried both 6l6's and EL34's and i have pins 1 and 8 tied together and then grounded.

Please excuse the messy lead dress in the pictures : / The little PC board is a power supply for the effects loop in there, it runs off of the screen B+. Ive been trouble shooting with the little PC board disconnected though.

[John_P_WI]

Where are the rectifier diodes? Under the board? I am assuming the bias diode is in the heat shrink next to the power resistor? Is the band in the right direction?

Are you using a bridge and grounding a center tap or other that could be causing a problem?

I'm also having a hard time understanding the ball of connected wires - orange - etc at the upper left corner of the power transformer.

Regardless, I would disconnect the B+ Tap and heaters and see if the fuse still blows, then add the heaters back and then the B+.

Your filter cap leads are not shorting on the caps are they?

BTW, in a normal working amp it is common for the OT to sing....

Good luck,

John

[alvarezh]

Not to say this will fix your problem but, here it goes:

Your red wire going to V1b is your plate wire to pin 6, and you have another red wire point to pin 1 (plate) of V2a, correct? If so, you could easily be getting oscillation at high frequencies which you can't hear because those two wires are running almost parallel and too close together. You could be easily be running your power tubes with excess current. Usually they red plate, but in your case, I will guess you are being protected beforehand by the fuse.

Again, that may not be the culprit, but I would separate your plate wires in a hurry. As a matter of fact, I strongly suggest you tidy up your lead dress as per the layout. Long leads are not truly recommended specially for grids. Also, notice that there are certain connections that should be done with a properly grounded coaxial wire.

All the best.