Can I do this? Attn:amp experts like Glassman

[SlyFoxx]

As you can see the large resistor on the left is extra crispy. This is a Mesa Lonestar Special. (quad of el84) This resistor went critical over a year ago as I was cranked about max in 5 watt mode. A trip to the amp doctor means I'll be out two bills by the time I ship it and pay for repairs. I can solder decent enough to work on my guitars and make cables. Can I fix this? Or should I just bite the bullet. If yes, what am I looking at replacement wise. The color code is hard to read and I would think crispy's neighbor to the right is the same. The silver and black are obvious but I don't find cream or beige on the color chart. Plus I can't be sure the writing on the board refers to the resistor it's near or something on the other side of the board.

[DankStar]

Re: Can I do this? Attn:amp experts like Glassman

isn't that the value right on the board? 1-watt, 470 ohm (I'm not saying fix it, just sayin')

[jeremy]

Re: Can I do this? Attn:amp experts like Glassman

470 would be yellow purple brown

i mean it looks like white black white but ???

[Koreth]

Re: Can I do this? Attn:amp experts like Glassman

The way the writing is lined up with those two pairs of solder pads, I'd hazard a guess that it marks what's supposed to occupy those pads in another version of the amp, or a part occupying the other side of the board (can't tell if that's extra solder on those pads or part leads poking through).

Have you sought out a schematic to look up what that value should be? Since the other resistor in that picture looks like it has (or had, before the crispiness) the same markings, have you tried measuring the other resistor?

[SlyFoxx]

Re: Can I do this? Attn:amp experts like Glassman

Actually measure the resistor...now THERE'S a thought!

EDIT...

It read 37.5 but then again the burnt one read 37.5. Fired it up and the main fuse just poofed in 1 second. I think this one's over my head. Oh well...

[tone4days]

Re: Can I do this? Attn:amp experts like Glassman

almost impossible to measure a resistor in circuit accurately because of everything else that is in the circuit with it (essenitally in parallel with it)

[glassman]

Re: Can I do this? Attn:amp experts like Glassman

The Boogie resistor values are always printed directly under the resistor...you gotta remove it to see what it is. The one thing I can tell you is that I've never seen an EL84 equipped Boogie use a 100 ohm grid resistor...or a carbon comp for that matter; someone has modified this amp at some point. A lot of other EL84 amps will use a 100 ohm screen resistor but those amps are designed around using those. Your Boogie isn't.

The result of using the 100 ohm in there would be premature tube death. This is most likely what cracked the resistor and probably what is causing the fuse to blow. Pull all the power tubes and retest it...if the fuse holds, get a new quad of tubes (get the Boogie branded ones to be safe) and replace those resistors with the value printed on the board underneath them. You don't have to get at them from underneath; just heat the pad and gently pull up with a pair of needlenose pliers. Bend and trim the replacements, heat the pad til it drops in a bit and retouch the solder with fresh.

[ratherdashing]

Re: Can I do this? Attn:amp experts like Glassman

It's pretty hard to tell the colour from your photo, but I think that's a 1W 100 ohm 10% carbon comp resistor. I'm certain about the 1W part and the 10% part, but the colours of the bands look like they got changed when the resistor died. I'm sort of assuming those whitish bands are supposed to be brown, which would translate to 100 ohm.

If you do manage to remove the resistor the marking underneath should indicate the value. The marking you see beside it appears to be for a component that wasn't installed.

De-soldering a component from a PCB is not quite the same as de-soldering a pot or jack in a guitar. For one thing, you have to be much quicker and more careful so as not to cook the PCB. You need a hot (at least 40W) iron with a pencil tip. You also need de-soldering braid and/or a solder pump.

All that said, Mesa has a five year transferable warranty. Are you no longer covered by it?


EDIT: just read Glassman's post. Agree on the carbon comp being weird.

[SlyFoxx]

Re: Can I do this? Attn:amp experts like Glassman

almost impossible to measure a resistor in circuit accurately because of everything else that is in the circuit with it (essenitally in parallel with it)

I was thinking along those lines when I put the meter to work.


@ Gman...thanks. The amp was modified by a certified boogie tech soon after I got it. There was a known issue with this model that was causing the rectifier tube to meet an untimely end. What he did to fix it I couldn't say. And from what you said maybe he's not the guy to trust again. Humm.

I do have an older quad of el84's around someplace and all were boogie branded. They're worn but should still work at least for a while.


EDIT: pulled the 84's and the fuse still fails in an instant. Guess it's time to get Meas on the phone.

[glassman]

Re: Can I do this? Attn:amp experts like Glassman

Sounds like you have a plan. This really is something you can handle yourself...if the fuse holds without the power tubes installed that is.

I gotta run but if you post a full gut shot, I'll most likely be able to tell you what your tech did to spare the rect tube (unless he hid it) when I get back.

[SlyFoxx]

Re: Can I do this? Attn:amp experts like Glassman

OK..I did a little net surfing and ran across this thread...

http://www.el34world.com/Forum/index.php?topic=10762.0

So did he pull those but install the 100's there for some reason. Or are the values just marked to the side in the case instead of underneath?

[glassman]

Re: Can I do this? Attn:amp experts like Glassman

OK..I did a little net surfing and ran across this thread...

http://www.el34world.com/Forum/index.php?topic=10762.0

So did he pull those but install the 100's there for some reason. Or are the values just marked to the side in the case instead of underneath?

The "stock" setup would have been two 470 ohm/1 watt resistors in parallel on each input to the rectifier; effectively making a 235 ohm/2 watt resistor on each side. Rather than clipping out one on each side, which would have increased the resistance to 470 ohms and 1 watt of dissipation (no big deal), he lowered the resistance to 100 ohms...that's going in the wrong direction.

I mistakenly assumed those were screen resistors; chances are that your rectifier tube is toast.

I HATE using series resistance in front of a rectifier tube to dumb down current in a case like this; it gives an "artificial" sag to the circuit. The problem is that a 5Y3 is the wrong type of rectifier tube to use in this circuit. If you want to never worry about replacing a rectifier tube again, stick a 5V4 in (NOS only) and call it a day. You could keep the 100 ohm resistors in there or better yet, replace them with jumper wires (no resistance). The 5V4 will still give you the rectifier "feel" without worries about failure and no artificial sag. It will give you another 10 or 15 volts at the plates which isn't enough to worry about bias and probably give you more punch.

Option B is still to put the 470 ohm/1 watt resistors in where the 100 ohms are and run a 5Y3.

[SlyFoxx]

Re: Can I do this? Attn:amp experts like Glassman

Looks like jumpers and a new tube is the ticket. I looked and NOS 5V4 are available. I assume the extra letters on the end of the model number are like the a c and r you see on 12ax7.

[USAPatriot]

Re: Can I do this? Attn:amp experts like Glassman

Did anyone notice that 2 resistors are missing? R101 and R103. Whether the other two are replacements for different originals or completely new addons is a question. I think you need a schematic or a photo of another amp to find out. -Rod-

[glassman]

Re: Can I do this? Attn:amp experts like Glassman

Looks like jumpers and a new tube is the ticket. I looked and NOS 5V4 are available. I assume the extra letters on the end of the model number are like the a c and r you see on 12ax7.

Yes. The GA, GT, GTA, GB, ect. are all more "build characteristics" than a function thing. They indicate construction...like "G" means glass as opposed to a metal can type. A or B (or C) usually indicate permissible voltage which in the case of a 5V4 in a guitar amp is never a problem. I have a 5V4 in one of my scopes rectifying 1400 volts.

Did anyone notice that 2 resistors are missing? R101 and R103. Whether the other two are replacements for different originals or completely new addons is a question. I think you need a schematic or a photo of another amp to find out. -Rod-

What you are seeing is a popular mod to these amps to reduce the inrush current the rectifier sees. A 5Y3 can't handle much and this amp goes way beyond its capabilities. There should be two 470 ohm resistors on each side; the mod removes one on each side to reduce the inrush current. Someone replaced the remaining 470's with 100 ohm resistors which is a move in the opposite direction.

If it were on my bench, I would replace the 100 ohm resistors with 25volt/5watt zener diodes and use a GZ34/5AR4 so that a new production tube could be used. The 5V4 is a great option though because it will behave much like the 5Y3 as far as voltage drop and pull down but will be able to handle all the current the amp demands...in other words, it will sound and react more like the stock amp did.