Failed bias circuit

[jim]

This is from a Peavey Classic 50. The fuses are open. The secondary still supplies 42V.  The diodes appear to be conducting both ways.  1/4 watt R61 and R62 are burned. The fault occurred within seconds after the power tubes were replaced.   Why are the resistors in parallel--why instead not use 10 ohm 1/2 watt resistor which I have?

[sluckey]

If the diodes are indeed conducting both ways you should replace them. 1/4 watt for those resistors seems low to me. Are you sure?

[jim]

Looks to be. (like a long grain rice)    scant docs don't help.  frustrated.

[labb]

Our Buddies over in Mississippi still support the Classic 50. Times I have spoken with them they have been really helpful.


per the parts list  R61 and R62 are 1/4 Watt-5%-CF-T&R. Not sure what CF an T&R stand for.


Check the 4558 op amp  U1

[shooter]

The secondary still supplies 42V

Is that measured where the schematic shows -27?

I fixed 2 classic 50's, 10ish years ago, both flamed out, the 2nd one I just bought a new "mother board" from peavey.  They did send me a great schematic and good tips

[PRR]

Lift one end of C41. Do the diodes still go both ways?

If that is not it, you have a SHORT on the 27V feed. Find it before you make the fuse company rich.

[tubeswell]

Bad filter cap is my humble guess.

[labb]

I'll  bet a six pack of Diet Dr. Peppers on U1.

[guitardude57]

Our Buddies over in Mississippi still support the Classic 50. Times I have spoken with them they have been really helpful.


per the parts list  R61 and R62 are 1/4 Watt-5%-CF-T&R. Not sure what CF an T&R stand for.


Check the 4558 op amp  U1

CF = Carbon Film resistor...

[PRR]

"T&R" is "tape and reel". How the factory needs the parts by the thousands: resistors on long tape and tape on a reel.

For repair: any half-watt (or larger) you can make a good connection to. Since you just need one or two, you don't need a reel of tape. Carbon Film is the price; metal film is fine but the factory didn't want to spend that much.

Whatever: that resistor(s) didn't smoke by itself. There is a short.

[guitardude57]

Yup Tape and reel.  Those are for pick and place machines, that auto insert parts.
They do that with through hole, and SM (surface mount) parts too.  Parts come on a reel, or a magazine...

[jim]

Lift one end of C41. Do the diodes still go both ways?

If that is not it, you have a SHORT on the 27V feed. Find it before you make the fuse company rich.

and the resistor company.....so the diodes now test OK ....the -27V is not raw bias (this was confusing me)--it goes to dual op amp (4558) reverb driver which is cracked in half.  And guess what! Its socketed!
The failure mode is interesting.  The customer told me the amp failed quickly after he changed the power tubes.  When I opened the cab I found a mounting screw missing from the power tube daugherboard.
It was where my needle nose pliers are in the photo.   He pushed the power tube home--the board flexed upward and pushed a connector block against the IC cracking it in half.  You can see the crack.


Happy.   Jim

[jim]

Here is a better pic.

[bnwitt]

Looks like Labb wins a case of Diet Dr. Pepper

[labb]

       By the way,,be really careful when connecting the ribbon cables..Make sure you get them seated good...

[bnwitt]

I looked at some photos of that amp's innards on the web and man what a rat's nest of jumper wires between boards.  I would imagine it is prone to cracked solder joints if it is ROHS compliant.

[jim]

I traced the line all the the way to the IC,   Thanks Labb--- I should have just looked there first.  Jim

[shooter]

what a rat's nest of jumper wires

Yup!!, and someone in a hurry can easily plug them in backward, IIRC even miss a pin.  Once I fixed my Son's the 2nd time I told him SELL it, or don't call me to fix it

[labb]

First tube amp I bought for my Son was a Peavey Classic 50-410. That would have been some 16 years ago. Rescued it from a Pawn Shop for $100. They said it would blow the fuse ever time they turned it on. Bought a hand full of fuses from Radio Shack. Plugged the thing in and let it play for 4 hours. Never missed a lick. To this day I don't know what the pawn shops problem was...Kid gigged with it all through high school and college. Then loaned it out to a friend that was on the road. Got it back and then loaned it to a club for a club amp. Call me one day and said "Dad, the Peavey died." I told him to bring it home and let me look at it. Had the same problem you had. Fixed for less than $10.00 not counting tubes. Gave it back to him and I don't know where it is now. He gave me some story about it...That was one tough SOB of an amp. It took more abuse that any amp should have to go through. With the help of this board I built the Son a 5F6-A Bassman and he quit the Peavey for ever more. Then he gave up the Guitar for some reason or other. Shame, he was quiet good at it. Kinda hurt me. But that the way things go.

[bnwitt]

Well since ROHS went into affect in 2006 I assume that old classic 50 wasn't lead free solder.

[labb]

Not lead free. It was the real deal. I never bothered to date it. It had to be at least 5 years old when we got it, from looking at it. That would make it about "96. The cab was straight across not like the new style you see today. Lot of folks don't care for Peavey's but that amp was a heck of an amp for a working musician. I would have retired it to my basement if the kid hadn't let it get away. It was the only good deal I ever got from a pawn shop.