Weird 5150 block problem.

This forum is for talking about all kinds of Peavey guitar amplifiers.
Post Reply
darthpoodle
Member
Posts: 12
Joined: Thu Oct 07, 2010 4:34 pm

Weird 5150 block problem.

Post by darthpoodle » Sat Jun 26, 2021 10:40 am

I picked up a used block letter for cheap. Worked fine when I got it but was a bit noisy. It sat for a while until I revisited it. Worked just like I remembered it. Took a break and came back and turned it on and all I got was a whisper quiet signal. Preamp gain worked on both channels as well as channel switching and resonance and presence dials, but post volume knobs would neither get louder or quieter. All leds are on.

I changed out the tubes with a known good set and still no change. I opened her up and saw why she was so cheap. A lot of mods, looks like most resistors were changed out for metal film, bias mod, choke mod, a 47pf across the .047uf PI caps, probably more but that is what I could see.

All tubes glow and the fuses check good. Plate voltages on power tubes is high at 550 volts and the amp thumps when placing a meter on them. PI thumps when metering it as well. Can’t really get to the preamp voltages because of all the ribbon cable in the way. A lot of ribbon cable. Looked for cold solder joints and touched up some that might be a problem including the ribbon cable pins. Cables tested for continuity and didn’t find any problems.

Effect send and preamp send have no signal and plugging guitar into effects return has no signal.

Might have to go to the shop but closest shop is 100 miles away. Thought I would ask before I took a long drive.

darthpoodle
Member
Posts: 12
Joined: Thu Oct 07, 2010 4:34 pm

Re: Weird 5150 block problem.

Post by darthpoodle » Sat Jun 26, 2021 5:34 pm

Playing with this more with my limited knowledge but the voltages seem all weird especially on the heaters reading 26 volts. Will check with another meter but…. Rectifier diodes all check good.

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: Weird 5150 block problem.

Post by Enzo » Sun Jun 27, 2021 12:28 am

What matters to a tube heater is the voltage ACROSS it - from one end to the other, NOT the voltage to ground

If the tube heaters are all glowing, then don't worry, it is OK. At least the heaters are OK.

darthpoodle
Member
Posts: 12
Joined: Thu Oct 07, 2010 4:34 pm

Re: Weird 5150 block problem.

Post by darthpoodle » Sun Jun 27, 2021 12:46 am

Thank you! I have limited knowledge on tube amps other than the basics. Any idea on what it could be? I was going down the filter cap road, but my limited experience was they started to sound bad not completely fail. But, the amp is 30 years old.

K-B
Member
Posts: 13
Joined: Wed Jan 08, 2020 7:20 am

Re: Weird 5150 block problem.

Post by K-B » Sun Jun 27, 2021 6:50 am

Try patching a cord from send to return. Try sending a signal into the return and see if you get sound.

darthpoodle
Member
Posts: 12
Joined: Thu Oct 07, 2010 4:34 pm

Re: Weird 5150 block problem.

Post by darthpoodle » Sun Jun 27, 2021 9:58 am

I should have mentioned I tried that and no joy.

K-B
Member
Posts: 13
Joined: Wed Jan 08, 2020 7:20 am

Re: Weird 5150 block problem.

Post by K-B » Sun Jun 27, 2021 10:36 am

So at this point it's down to doing some voltage checks and resistance checks. You could pull a tube and check the voltages on the tube sockets to make sure you are getting HV but be careful there is over 500 on pins 3 and 4. If you don't get 450vdc to 500vdc on pin 4 it's likely you have an open screen resistor.

Pin 1=0 vdc
Pin 3= 455 to 505 vdc
Pin 4= 450 to 500vdc
Pin 5= -55 vdc (negative)

darthpoodle
Member
Posts: 12
Joined: Thu Oct 07, 2010 4:34 pm

Re: Weird 5150 block problem.

Post by darthpoodle » Sun Jun 27, 2021 11:04 am

Pin 1 = p
Pin 3 = 560
Pin 4 = 556
Pin 5 = -042

darthpoodle
Member
Posts: 12
Joined: Thu Oct 07, 2010 4:34 pm

Re: Weird 5150 block problem.

Post by darthpoodle » Sun Jun 27, 2021 11:04 am

Pin 1 = 0. Typo

K-B
Member
Posts: 13
Joined: Wed Jan 08, 2020 7:20 am

Re: Weird 5150 block problem.

Post by K-B » Mon Jun 28, 2021 5:35 am

So voltages are good so it's likely a connection or tube / socket problem. This is where an O'scope comes in as you inject a signal to the input and trace it with the scope.

Post Reply