5150 gurus...I need help please

[Lord Toneking]

So in an earlier thread I mentioned that I bought a 5150 head for $350 knowing that it sounded like crap but had old tubes and even one mismatched power tube

Well, I took it home where I have a buddy's 6505 he let me borrow for the week (which made me gas for a 5150/6505 in the first place! ). Long story short, I took the tubes out of the 6505 and put them in the 5150 and it fired up and sounded good....for about 30-40 seconds. While it was still sounding good on the lead channel I switched to the ryhthm channel and thats when it just popped!....I watched as the green light by the standby switch went off and the sound went away, the red power light stayed on but I turned it off real fast

The next thing I did was start pulling the tubes. As I touched them (facing the back of the amp) the two right ones were just warm like they should be. The two on the left were sizzling HOT!...I mean HOT. I was sure I blew his tubes which really pissed me off (I put them back in his amp and played for awhile and thank God they're fine)

Anyways, I pulled the chassis out and there is 4 fuses and one of them fried. I checked out the board and everything looks good...no burn marks or anything fried. What my concern is is the left side tubes that were so hot

any ideas??

 

[mrkmas]

Did you check the bias? I am unfamiliar with 5150's as I have never owned one, But a high bias will cook a tube and make it super hot. That could explain why one side of tubes was fine since you can bias both sides seperately. Not sure if it would blow a fuse though.

 

[chumbucket]

Screen grid resistors?

 

[O'Rourke]

The screen grid resistors on those are on the underside of the power tube board if it's set up like the II's. Sounds like the left two redplated if they got that hot that quickly, does either amp have a bias mod and did you check the bias when swapping the power tubes?

 

[glpg80]

The screen grid resistors on those are on the underside of the power tube board if it's set up like the II's. Sounds like the left two redplated if they got that hot that quickly, does either amp have a bias mod and did you check the bias when swapping the power tubes?

+1

strike of bad luck to be honest if you popped the screen grid resistors having just purchased a 5150 - i know people who have owned them for years and played them on 6.5 volume day after day, year after year and never saw one let go.

5150's ride the screens close to the plate voltage, inrush surge current or a tube that is faulty can cause the screen grid resistors to go. at this point the power board needs to be pulled and checked, and the bias circuit needs to be checked for functionality as well. two tubes blazing hot and two that are mildly warm/hot sounds like someone might have attempted to do a bias modification and fucked up badly - all tubes should be within the same heat range.

 

[OBGYNkanobi]

Yikes, this fix will be a pain. I had that happen to a 5150 I got dirt cheap off of Craigslist and you do have to pull the power board to get to the screen resistors. What makes it such a hassle is that the power board...at least mine was...is held in place with pop revits instead of screws, and it was a real headache to get them out without damaging anything.

The 5150's had a fixed bias, and it's actually fairly low; however, the combo amps were biased a bit hotter. JerryP used to have an instruction thread with a bias mod for the 5150 somewhere, but he pulled it because some guys were performing the bias mod on their 5150s, changing a few caps and then selling their amps as "FJA" mods. I did it to my 5150 and it really helps bring it to life; it's warmer and punchier. IMHO, this is why some people prefer the combo to the heads. While you're in there, drop in a 3H 100ma Plexi choke. It's a cheap fix that will help tighten up the sound. I bought the 10H Merc choke and it was super stiff, tight and sterile; horrible sounding! The Marstran choke, bias mod and a few tweaks made that amp sing.

 

[primerib]

yup. sounds like the screen grid resistors. not that big of a deal. the amp is probably 15 or 20 yrs old. but IIRC, pulling the board is a bit of a pain in those

 

[Lord Toneking]

but I put good tubes in it...Rubys bought from Laura...and it brought it to life! But when I switched to the ryhthm channel it popped one of the fuses on the board

Just cant figure why the two tubes on the left were BLAZING hot

btw, the board if I remember correctly was held in by phillips screws. It all looked very 'stock' inside the guts. Quite a few cobwebs which concerned me

 

[JerryP]

Screen grid resistors are the result of a tube problem, not the cause. An open resistor will just cause that tube to not work, won't hurt the tube or make it red plate.
Screen grid resistors do fasil on the 5150 all the time, very common.
Most times a tube red plates it's a bad tube. If it's not a tube you have a problem with the bias circuit on the amp. Not very common.
Almost everytime I see that 2A fuse on the board blow it's a power tube problem.
Jerry

 

[Lord Toneking]

Screen grid resistors are the result of a tube problem, not the cause. An open resistor will just cause that tube to not work, won't hurt the tube or make it red plate.
Screen grid resistors do fasil on the 5150 all the time, very common.
Most times a tube red plates it's a bad tube. If it's not a tube you have a problem with the bias circuit on the amp. Not very common.
Almost everytime I see that 2A fuse on the board blow it's a power tube problem.
Jerry

Man I was hoping you'd pop in here Jerry

I just remembered that the dude told me he replaced the plug on the cord. Well, I took the plug apart and he put the hot (black) on the wrong side. Would that cause an issue?

 

[JerryP]

Nope, that didn't cause the fuse to blow.

 

[Lord Toneking]

Nope, that didn't cause the fuse to blow.

How do I test the bias circuit?

and can I use just 2 6L6's and set my ohms to half? I only say this because I only have a pair of 6L6's that are good (besiides the ones in my buddys 6505 that I dont want to fry )

thanks

 

[Lord Toneking]

The screen grid resistors on those are on the underside of the power tube board if it's set up like the II's. Sounds like the left two redplated if they got that hot that quickly, does either amp have a bias mod and did you check the bias when swapping the power tubes?

+1

strike of bad luck to be honest if you popped the screen grid resistors having just purchased a 5150 - i know people who have owned them for years and played them on 6.5 volume day after day, year after year and never saw one let go.

5150's ride the screens close to the plate voltage, inrush surge current or a tube that is faulty can cause the screen grid resistors to go. at this point the power board needs to be pulled and checked, and the bias circuit needs to be checked for functionality as well. two tubes blazing hot and two that are mildly warm/hot sounds like someone might have attempted to do a bias modification and fucked up badly - all tubes should be within the same heat range.

Got the main board and the power section board out. Everything checks out ok as far as the values with a meter. One of the ceramic screen resistors is a little brown but still reads 98 ohms. I went ahead and bought a two pack and also the blue 100mfd 25v capacitor which I had no way of checking

Im not feeling good about this. Is there anything else to look for? I cant understand whty just the two left side pwer tubes are red plateing

oh, this is all 100% stock 5150...no mods. Do you know where the bias circuit is?...is there a bias circuit?

Sorry, I dont know jack crap about this

 

[glpg80]

is there a bias circuit?

if you're asking these questions you have no business poking around in there. How do you expect to troubleshoot a circuit that you do not even know exists, let alone how it works?

Put the 5150 back together, take the parts to a technician, and tell them the symptoms. what you are doing is called shotgunning the circuit - throwing parts at it blindly without knowing the cause to a solution. Its expensive and it is making the ordeal worse than it really is.

you cant troubleshoot an amplifier over the internet, especially when you do not know its history or have any means to put it on a bench or have the skills to safely work on it. There is just no way. Things that are that cheap are that cheap for a reason - you got a good deal but do not continue to take shortcuts - pay someone that knows what they are doing OR send it to peavey directly.

 

[Lord Toneking]

is there a bias circuit?

if you're asking these questions you have no business poking around in there. How do you expect to troubleshoot a circuit that you do not even know exists, let alone how it works?

Put the 5150 back together, take the parts to a technician, and tell them the symptoms. what you are doing is called shotgunning the circuit - throwing parts at it blindly without knowing the cause to a solution. Its expensive and it is making the ordeal worse than it really is.

you cant troubleshoot an amplifier over the internet, especially when you do not know its history or have any means to put it on a bench or have the skills to safely work on it. There is just no way. Things that are that cheap are that cheap for a reason - you got a good deal but do not continue to take shortcuts - pay someone that knows what they are doing OR send it to peavey directly.

I appreciate what youre sayin and understand. Though I claim I dont know crap I do know how to use a multimeter...does that count?

I spent about 45mins on the phone with a tech from Peavey. Hell of a nice guy. Anyways after gong over the schematic and having me check everything he could think of he claimed the only reason those two tubes would red plate was because the ribbon cable from the main board to the power board was not getting voltage to pin 8. I checked continuity on ALL the ribbon cables and cleaned EVERY connection there is and guess what?....still red plating

I called a tech today and am dropping it off later. I just don't really have the extra cash to throw at an amp right now

 

[glpg80]

I appologize if i sounded too forward, working a meter correctly does have its benefits

peavey techs are nice guys, i have dealt with them on a few occasions tracking down intermittent problems or getting second opinions - thats why i recommended sending it to them or at least calling tech support.

IMHO i would have just packed it up and sent it to them. normally the cost to send it to them + labor which is $50 an hour is all you're in for. rarely do they charge more than that.

keep us updated and when you get it back, post your oh shit this sounds great post in the other 5150 love topic

 

[Lord Toneking]

I appologize if i sounded too forward, working a meter correctly does have its benefits

peavey techs are nice guys, i have dealt with them on a few occasions tracking down intermittent problems or getting second opinions - thats why i recommended sending it to them or at least calling tech support.

IMHO i would have just packed it up and sent it to them. normally the cost to send it to them + labor which is $50 an hour is all you're in for. rarely do they charge more than that.

keep us updated and when you get it back, post your oh shit this sounds great post in the other 5150 love topic

no worries, you're 100% correct on what you said. Fact is I am just a paycheck to paycheck guy and my wife is totally pissed at me for spending $350 on an amp...an amp I really dont need. Now I just gave her the news that it's jacked up and that I need to have it repaired so needless to say I am in the dog house But man was I hoping it was just gonna be a simple screen grid resistor or a bad connection

I guess for $350 if I can get by with a couple hundred extra bucks in it it'll be well worth it in the end

...I hope

 

[glpg80]

I appologize if i sounded too forward, working a meter correctly does have its benefits

peavey techs are nice guys, i have dealt with them on a few occasions tracking down intermittent problems or getting second opinions - thats why i recommended sending it to them or at least calling tech support.

IMHO i would have just packed it up and sent it to them. normally the cost to send it to them + labor which is $50 an hour is all you're in for. rarely do they charge more than that.

keep us updated and when you get it back, post your oh shit this sounds great post in the other 5150 love topic

no worries, you're 100% correct on what you said. Fact is I am just a paycheck to paycheck guy and my wife is totally pissed at me for spending $350 on an amp...an amp I really dont need. Now I just gave her the news that it's jacked up and that I need to have it repaired so needless to say I am in the dog house But man was I hoping it was just gonna be a simple screen grid resistor or a bad connection

I guess for $350 if I can get by with a couple hundred extra bucks in it it'll be well worth it in the end

...I hope

in the dog house?

all of that time spent sleeping on the couch for the next few months will be worth it in the end when you get the chance to crank it

its quite an experience 5150's are work horses, its a wise investment. tell your wife what they normally go for

 

[JerryP]

Did you try different power tubes?
Jerry

 

[Lord Toneking]

Did you try different power tubes?
Jerry

I only tried the =C='s that were in the 6505 that sounded awesome in that amp...and still do.

As soon as kick it off standby they started red plating so I would turned it off before it would damage the tubes. Put the tubes back in the 6505 and it roars and sounds great. I dont have any other 6L6's laying around

The Peavey tech was VERY helpful and spent at least 45 mins on the phone having me check things with my meter while he looked at the schematic. They were closing and he couldnt help me but gave me his direct extension so I could call him monday.

According to him the ONLY way those two tubes could redplate was because there was no voltage from the main board to pin 8 (he called it pin 8 but actually there is only 7 pins and one blank spot). I had the power board out and had access to everything. He also told me to quit asking questions on forums He seemed to know this thing inside and out but yet what he told me didnt work. I cleaned and checked continuity on EVERY connection in there

I was/am really hoping it's something simple. This dang thing is depressing me

You know Jerry, one thing I noticed that I wasnt able to tell the Peavey tech because they already closed was that V1 and V3 didnt seem to want to glow equally compared to the other pre tubes. They would take a few seconds longer to get going...even when I swapped them around