50 watt jmp

[tvjr]

Can anyone tell me roughly what the MA numbers should be reading on my 6550 tubes. Somewhere in the 40's? When i got the amp about a year ago , it wasn't working properly. Couldn't get hardly any sound,but when i did it sounded like a farting noise through the speakers.I had a local tech fix it.He replaced some caps ,new 6550s and a rebais. worked fine after that. Yeaterday i got a celestion vintage thirty speaker ,hooked it up,sounded ok for a few minites ,then the sound started to fade, then come up again,then down again. then it just sounded like paper tearing,crappie.I put the old speaker back in ,but no change ,still sounds bad.I checked the speaker ohmages and all is correct. Any ideas. thanks in advance TVJR.

[tvjr]

jmp 50 watt 76

gentleman, My original problem was with very low volume,still is. I figured out how to use a multimeter,kind of ,and have been poking around a bit. Thank god i havn't fried myself yet. Power tubes are 6550's pin 3 reads 390 and pin 5 reads -40 this reading applies to both tubes..Pins 2 and 9 read 2.9. preamp tubes pins 4,5 and 9 read 2.9. pin 1 reading right to left are 316,115,135. The power and preamp tube should be reading higher,right. I have confidance that someone out there can help. Chuck where are you? thanks in advance for your help . tvjr

[Jazz P Bass]

pre tubes read right to left pin 1 316,115,135. .

This statement has me confused.
Are you only reading pin#1 of the three 12AX7's, right to left?
Ideally in a static test you would read the Vdc of the preamp tubes at pin#1 & pin#3, pin#6 & pin#8.
That reading will tell whether or not the circuit is conducting or not.
Pins 4, 5 & 9 of the 12AX7 are the heaters. It is best to read those across pin 4/5 & 9. Vac. (4 & 5 are connected).
The output tube static voltages look good.

[Chuck H]

Nice work. That's not all the info but it's a place to start. I'm guessing by the info so far that your problem is V1 (first preamp tube) possibly just V1A (first triode, first preamp tube), the associated circuitry or that tube socket.

The low reading on the filaments is because your measuring pin to ground with your DMM set for DC. You should be reading from either end of the filament wind with the DMM set for AC. Set your DMM for AC, put the red probe on pin 4/5 and the black probe on pin 9 of any preamp tube. You should read close to 6.5 volts.

Have you tried plugging into the low sensitivity input? Does it work correctly?

Have you tried replacing V1 with a known good tube? If that doesn't help>
On V1 pin 3 goes to a 2.7k resistor (with a .68 cap across it). Pin 8 goes to a 10k resistor. These two resistors are joined at their other ends on the board and from there a wire should go to ground. Look for faults in this circuit. Loose connections, broken board traces, test between the junction of those two resistors and the chassis, it should read as a dead short, grounded.

[tvjr]

jmp 50 watt 76

Sorry for the confusion.The pre amp tubes, pins # 4,5,9 all have the same readings of 2.9.maybe i'm doing this wrong,my multimeter negitive lead is grounded to the chassis and the pos is touching the pin. Correct? On pin #1 reads 316 on the first tube closest to the power tube, then 115 on the next and 135 on the last. Sorry.tvjr

[Chuck H]

Each preamp tube actually has two triodes inside. So when you read pin 1 your reading the HV for only one of the triodes. JPB's post for how to take readings is correct. Pin's 1 and 6 of each preamp tube has high voltage. The readings on pins 3 and 8 are also important. See my post above, we were posting at the same time. We may not need any more information if my hunch is correct.

[tvjr]

Chuck, thanks for straighting me out.You were correct about the ddm.Set to ac, checked across 4,5 to pin 9 and presto 6.3. thats as far as i got ,wiil try the rest today. thanks much,tvjr

[Chuck H]

OK, hows this?

[tvjr]

Thanks Chuck . You must have been reading my mind,i was having trouble understanding what you asked. Really ,you ..must have known, because it was taking so long for me to respond. Thanks again, tvjr

[tvjr]

jmp 50 watt 76

Chuck ,thanks for the photo,you know what they say, a pictures worth a thousand words. checked allthe letters with the meterand all read 000 except for x to x that read 1. From x to x is just a wire,bad solder joint???. And to note on my board there's a small can capasitor in between where you marked a and b. I don't know if it means anything ,just thought i'd ket you know. Thanks tvjr

[Chuck H]

all read 000 except for x to x that read 1.

1. what? 1k, 1ohm, 1megohm ?

Anyway, it should read as 0 as the others I suppose. Your last voltage readings indicated that V1A was not conducting current. If the X to X connection is bad for any reason it would cause this condition, and your problem. It's probably a cold solder joint. If you have a soldering iron and some flux core solder you should "re flow" both ends of the wire. You will need to lift the board to do this properly. You won't need to lif it much. Just enough to expose the joint. You can prop it up with a stick of some kind and (here's a trick I used when I had two of those amps) gently bend the tip of your iron so it is curved. These two things should allow you to re flow the board joint without having to remove other wires. Don't use too much solder. Just enough new solder to melt the joint quickly and add a tiny bit of flux from the new solder.

And to note on my board there's a small can capasitor in between where you marked a and b. I don't know if it means anything

I remember from an earlier post we discussed the "Master lead" model. If this is a master vol. model then that cap is part of a modification someone did. If this is a four input non master model the cap is supposed to be there.


P.S. That's an old amp. I don't know what capacitors the last service guy replaced but I do know that nine times out of ten when a "tech" does a "cap job" they don't replace the bias supply caps. These should be replaced if they haven't already been. If your going to lift the board anyway you may as well take care of this. It's all of five bucks for the two caps and they're redily available at any Fry's or Radio Shack. If you decide to do the work yourself I can post another image.

[tvjr]

jmp 50 watt 76

Chuck, I'm sorry ,but i made an error ,all the connections were o.k. In regards to the amp it's a50watt jmp two input master vol and yes it's been teched.Replaced most of the caps on the board, and if you mean the larger caps next to the bais pot,yes they have been replaced. At some point someone did the el 34 conversion, but it has since been changed back to 6550's. Pin # 3 on the power tubes read 390 is this a low # for these tubes? Thanks, Richy aka tvjr

[Chuck H]

No, That's where the voltage on both of mine was. Give or take five volts. Mine sounded much better with el34's than 6550's.

So, that cap you indicate as being on the board is a modification. Either your amp has been converted to two seperate input triodes like a four input non master model (instead of cascade gain stages as that came) or it's part of a gain boosting circuit to give the amp more preamp overdrive.

Have you tried a known good tube in the V1 socket yet???

[tvjr]

I switched all three around and had little or no effect. With the master and the preamp maxed out it's no louder the a person speaking in a normal voice,but it sounds like a sheet of looseleaf paper laying across the strings.Thanks chuck,i'll talk with you again tomorrow, got to get up early.tvjr

[Chuck H]

Well I would expect something different to happen changing the tubes around, but it would be subtle at best because a bad tube anywhere in the system still means no sound. Since we already have line of sight on that V1A triode don't switch 'em around any more. Get a good tube and plug it in. It'll cost you ten to fifteen bucks and you'll use the tube at some point anyway.

Sleep tight.