Peavey Delta Blues - Blown Fuse (2x now)

[Renegade44]

OK Guys, I'm an idiot. But, i love electronics, music, and tinkering. I have build several pedals and stuff. With that said, i am comfortable working on my amp.

My Delta Blues is old - a year 1 amp. I have had it since it was new. I love this bad boy!!! It blew the fuse. I replaced both and tubes. I just figured a power tube let go and took a fuse with it. I powered it back on and it ate another fuse. It was the same thing. Powered on the amp - it made a squealing noise - then blew the fuse.

So, question - Where do i start troubleshooting? Is it a transformer? ? ?


Thanks for the help

stephen

[mikeboone]

Schematic Attached

See attached... Lower left area of schematic shows P/S.

Also, one of the first things you will want to do is make a "light bulb" current limiting rig like the one in the attached sketch. It'll save you some time and fuses.

I would try taking all the tubes out and powering it up to see how it behaves.

If that yields no result, you'll have to being isolating the P/S components to see when you can power up without blowing a fuse. Your current limiter rig will be helpful.

[Renegade44]

Thanks,

Any idea the Voltage i should see at J27/J28 and J19/J26?

I'm sure i am overlooking it?

Thanks,
stephen

[mikeboone]

Those points appear to be the AC line input - 120VAC +/- a few V's

[Enzo]

Look at CR4, CR6 near the power tubes. Either one shorted?

Mike - and Rene - those are not the 120VAC mains. Those are the power transformer secondaries. The red wires are for the high voltage, probably about 240vAC. The other wires are about 25VAC. I seriously doubt they are the problem.

Which fuse is blowing?

You can unplug the OT primary - the blue, brown, and red wires on a Molex conector. That will eliminate the OT and power tube plate circuits from any fuses.

Les likely but anything is possible - look for a shorted rectifier diode on either supply, and the associated filter caps.

[Renegade44]

I will check those out

Thanks, I will look into those this weekend. I appreciate all the help here guys.

stephen

[mikeboone]

Thanks Enzo...

I often find myself on the opposite side of the compass while giving directions.

Sorry renegade... I didn't mean to send you on a wild goose chase.

[Renegade44]

No problem - I just appreciate all the help. I wish i had an oscilloscope to play with on it. I wish i could see the wave forms as i play.

[Renegade44]

Success

Sorry it took so long to reply. I have had the diodes but no time. Today, I had time. So, i opened it up. Rather than look for the a bad diode, i just replaced all 8. I also replaced the fuses. I had a new set of tubes i also stuck into it.

Powered it up - nothing! Frustration set in quick. So, i plugged into my Classic 30 and played for a half hour to clear my noodle. I got to thinking. What do the tubes look like? They are new. But it wouldn't be the first time i have bought a bad tube. Sure enough, bad tube. But, the tube was probably good. I think the retainer clip knocked the tip off and let the magic gas out. Threw in an old tube - BAM! it singing like 90 year-old blues man!!! Wonderful growl Sweet smooth clean tones. Aaahhhh. . . It's great to have the bad boy back!!


Thanks to the guys here, i was able to get it working. Thank you so much for y'alls help. It is very apreciated.


Blessings,
stephen


Stephen H. King
Off 61 Home Page
stephen@off61.com

[plexi50]

Power Tube Protection Diodes / Trouble Part

First there is no reason to even have CR4 & CR6 in the circuit
These diodes are overkill and actually can cause more damage in the circuit
The purpose of them is if one of your power tubes goes south and shorts out,the diode supposedly will take the brunt of the shorting tube and save the tube. Thats a fairy tale. The B+ is still on the power tube regardless of weather the diode blows or not
If a power tube shorts out then it is over for that tube long before that diode can act
If you leave that diode in the circuit and it is bad,you put in new power tubes your going to the secondary blow fuse anyway and possably kill new power tubes as well
These diodes in theory are supposed to protect. But all they really are is bad trouble waiting to happen
Simply cut these diodes off the board and get rid of them. If you want to protect your output transformer from a shorting tube, simply install a fuse link at the output transformer CT. This is real OT protection and not some mythical diode gamble

[Enzo]

Hi, welcome to the forum, we're glad you have joined.


However, I think you misunderstand those diodes. They have nothing to do with protecting the tubes. They are there to protect your transformer. If the tube shorts out, it is history already, nothing external will save it. Note that the diodes are reverse biased for B+ current to flow anyway.

Those diodes are called flyback diodes. they are there to protect your output transformer from a lack of speaker load. If you crank a signal through the amp without a speaker or load, the voltage at the secondary can kick back a high voltage spike into the primary that can easily arc the windings. And that can destroy the transformer.

Of you clip out the diodes, and the amp never has a bad speaker connection or open speaker or inadvertant disconnected speaker load, then the lack of them won;t matter, just as not wearing a seatbelt won;t matter if you never crash.

But unterminated secondaries can result in primary spikes of several times the B+, and can be easily enough to arc. THAT is what the diodes are for. The only damage they can cause is if they are shorted, they will ground off the B+ and blow a fuse. If B+ is shorted to ground, it can;t hurt a new tube, after all, there would be no voltage there. And killing the plate B+ will also take the screen supply, so no chance of the tube screen suffering.

The diodes are not protecting anything from a tube, they protect the transformer from the secondary unloaded.

[Renegade44]

Thanks - it's been my experience that a $200 part will always sacrifice it's self to protect a $.02 fuse.

Y'all have a great Thanksgiving and Wonderful Christmas.

Blessings,
stephen