Peavey 5150 ll SGR Replacement Help

[gainfreak12]

I had a tube fail and fry a screen grid resistor in my 5150 ll. I've already ordered the new sgr's 470ohm/5watt and want to replace them all. I'm pretty good with electronics, soldering etc. but I'm not sure of the location of the resistors. I'm a complete noob at schematics but I really want to do this myself so I can learn along the way. I'm aware of the potential danger present without draining the juice. I know I can do it with some assistance. I also want to add a choke as well. Anyone care to guide me in the right direction? Thanks a ton in advance.

 

[TheWinterSnow]

First you want to drill out the rivets that hold the power tube board in the chassis. Once that is done, they SGR's will be white square ceramic resistors labeled 100R5W, cut them out, desolder the leads and solder in the new resistors. After that is all said on done, install the board back in with new rivets (you will need rivets and a rivet gun). I hope that you ordered metal film resistors instead of wirewound as its the wirewound that causes the failure in the first place and adds inductance to the tube which is by all means not a good thing. When I replaced mine, I used 2 2.2K 2W metal film resistor in parallel for each tube and has helped out the tone, the stability of the tubes, and they run much cooler and happier.

With the choke, you will just buy the choke drill the location in the chassis for it, and you are using it to replace the skinny long blue 410 ohms resistor that is elevated on the power board, much easier than replacing the SGR's.

As for dangerous voltages, most modern amps have bleed off resistors, the 5150/6505 series amps have these. All you really need to do to be safe is use a multimeter and check the taps, where the power transformer connects to the board via molex connectors. There will be 4 diodes at each tap, connect the ground lead from your multimeter to the chassis and use the positive lead and check each lead of the diode. If they all read 0v then the leak resistors did their job and you will not have to worry about electrical shock.

 

[sprack]

Parallel SGR's.

Just went back through mine. It started getting volume surges and popping. Resistors and tubes are good. Found 3 bad caps with my shiny new ESR meter though. :-D

 

[gainfreak12]

First you want to drill out the rivets that hold the power tube board in the chassis. Once that is done, they SGR's will be white square ceramic resistors labeled 100R5W, cut them out, desolder the leads and solder in the new resistors. After that is all said on done, install the board back in with new rivets (you will need rivets and a rivet gun). I hope that you ordered metal film resistors instead of wirewound as its the wirewound that causes the failure in the first place and adds inductance to the tube which is by all means not a good thing. When I replaced mine, I used 2 2.2K 2W metal film resistor in parallel for each tube and has helped out the tone, the stability of the tubes, and they run much cooler and happier.

With the choke, you will just buy the choke drill the location in the chassis for it, and you are using it to replace the skinny long blue 410 ohms resistor that is elevated on the power board, much easier than replacing the SGR's.

As for dangerous voltages, most modern amps have bleed off resistors, the 5150/6505 series amps have these. All you really need to do to be safe is use a multimeter and check the taps, where the power transformer connects to the board via molex connectors. There will be 4 diodes at each tap, connect the ground lead from your multimeter to the chassis and use the positive lead and check each lead of the diode. If they all read 0v then the leak resistors did their job and you will not have to worry about electrical shock.

Thanks man. Very cool of you to lend a hand. I found the SGR's today but the board wasn't riveted in. The screws that mount the spring metal things that hold your tubes in place actually attached the board to the chassis as well. I was glad I didn't have to drill out rivets. lol.
I did order metal film resistors (from Weber). I had read somewhere to avoid wirewound. I haven't ordered a choke yet. Any recomendations?
Thanks again. This is a HUGE help to me.

 

[sprack]

I got a 10H multichoke from Mercury Magnetics. Weber sell smaller ones too. Somewhere in the range of 6H is about right for it.

 

[Red Phoenix]

Hey,
last year i wanted to take out the power tube board also because there is still a failure but i didn't found out how to take it off. I had noticed that the tube sockets are mounted with rivets. Is the only way to take the board off from the chassis with drilling out the rivets?
I don't understand this sentence "The screws that mount the spring metal things that hold your tubes in place actually attached the board to the chassis as well. I was glad I didn't have to drill out rivets." Could somebody explain the first part?

 

[sprack]

It looks like he had a board that someone else remounted with screws. The stock configuration is with rivets. Unfortunately you do have to drill them to remove the board. If all you need to do though is replace the screen grid resistors though Enzo at the Peavey forum swears its easier to desolder them on the mounted board, let them drop and solder the replacements to the other side.

 

[Red Phoenix]

Ah thanks, then i have to drill..too bad. My problem is that i don't know what exactly is damaged and i can't look at the downside of the board. I use a little mirror but i see nothing. I only know the error pattern and from this and looking at schematics i guess it is something on the power board. Or maybe the output transformer is half broken. Cause one tube pair keeps the amp silent. the other one not..but all tube heating works.

 

[sprack]

So one pair, the inner or the outer power tubes does not work, but if you pull one of those it work? Which pair?

 

[gainfreak12]

Hey,
last year i wanted to take out the power tube board also because there is still a failure but i didn't found out how to take it off. I had noticed that the tube sockets are mounted with rivets. Is the only way to take the board off from the chassis with drilling out the rivets?
I don't understand this sentence "The screws that mount the spring metal things that hold your tubes in place actually attached the board to the chassis as well. I was glad I didn't have to drill out rivets." Could somebody explain the first part?

I'm referring to the metal rings at the base of each power tube. I'm not sure what they're called but they hold your tubes in place. You have to push down on each side of them to remove that tube. Anyway, the screws that attach those to the chassis also hold the tube sockets and board in place. I'll have take some pictures, I know I suck at explaining this. lol.

 

[Red Phoenix]

When i put only the inner tube pair in, the amp is totally silent. with the outer pair everything works fine. Tube heating works on all pairs. But the inner pair doesn't react on playing. No "electron transfer" between cathode and anode. So i guessed the control grid was defective respectively some resistors in the cable. But when i measure it it seems all to be normal. But When i measure these square ceramic resistors which go to the screen grid..i can only measure the outer ones. The two ceramics from the inner pair are "dead". But measuring electornical components while beeing in a pcb isn't that good because they get affected by the hole circuit. So maybe it's not that expressive what i measure. But i hope it is the same fault like the one from the threadstarter. That would hopefully be an easy/cheap repair as long as i can find new resistors. You wrote that metal film res can also be used. But why did the circuit designer used ceramic ones? there have to be a reason why they used them instead of metal film or carbon or other more common resistors. 5W loadability exists also for metal film etc.. and in schematic the ceramic ones have 100ohm and you used 2,2k parallel ..so that is 1,1k in result. So you changed the value. why?

My Metalrings are attached with rivets. And you have screws instead of rivets? Like sprack explained? Or do you mean some other screws?

 

[sprack]

At the time ceramics would have the best heat dissipation for the cost. MO are now a better choice in power section and considerable cheaper than they were 18yrs ago. I think you're misreading the resistor values, I used 2 200 Ohm 5W Xicon's in parallel. Here's the link.

 

[Red Phoenix]

Ah ok thanks. MO i can get everywhere And now i see that you have posted the picture with the changed resistors and the one who wrote about 2,2k res was "TheWinterSnow" in post #2. I mixed that up, sorry. Then my only problem will be the mechanical part. how to get the pcb out of the chassis without damaging and getting it back....best without rivets again..i'll think about it

 

[TheWinterSnow]

Proper SGR values for 6L6s and EL34s rang from 470 to 1K. 100 is way too low, that is why they blow up so easily. SGR are merely current limiters, as you want the anode to pick up the current not the screen. The screen just needs a voltage, too much current from having a low value SGR will cause tubes to have less life and will cause the resistors to die usually when the tubes go. Higher values in a push-pull topology that most 50-120 watt 6L6/EL34 etc amps have will not effect the tone, but ensure that the pentode itself is operating more to datasheet spec.

 

[sprack]

I drilled the rivets with the chassis upside down and a shopvac running to catch all shavings. Once that's done it comes out very easily. Took me the longest to find a hardware store with the right size screws.

Wintersnow's tune his SGR's to his taste. There's some room to play on those values as it will change the plate dissipation and consequently the compression of the signal. If you like the stock tone for now and just want it reliable just get some big MO resistors in place and put it back together.

I'm up to 4 bad caps and 5 more that are marginal (higher end of ESR values).

 

[gainfreak12]

When i put only the inner tube pair in, the amp is totally silent. with the outer pair everything works fine. Tube heating works on all pairs. But the inner pair doesn't react on playing. No "electron transfer" between cathode and anode. So i guessed the control grid was defective respectively some resistors in the cable. But when i measure it it seems all to be normal. But When i measure these square ceramic resistors which go to the screen grid..i can only measure the outer ones. The two ceramics from the inner pair are "dead". But measuring electornical components while beeing in a pcb isn't that good because they get affected by the hole circuit. So maybe it's not that expressive what i measure. But i hope it is the same fault like the one from the threadstarter. That would hopefully be an easy/cheap repair as long as i can find new resistors. You wrote that metal film res can also be used. But why did the circuit designer used ceramic ones? there have to be a reason why they used them instead of metal film or carbon or other more common resistors. 5W loadability exists also for metal film etc.. and in schematic the ceramic ones have 100ohm and you used 2,2k parallel ..so that is 1,1k in result. So you changed the value. why?

My Metalrings are attached with rivets. And you have screws instead of rivets? Like sprack explained? Or do you mean some other screws?

This is exactly right. My tube sockets are mounted with screws instead of rivets.

 

[Red Phoenix]

Ah that's good to know TheWinterSnow, thanks for this info when higher values really not affect the tone i will take them.

@sprack
do you still know which screw size it was? And which driller size i have to use?

Is there something other i have to mind while doing this? Except to vacuum the metal cutting (not sure if that is really the right translation oO)?

@gainfreak
lucky you