JTM45 issue...

[SleepLess]

Hi.
You thought I'd let you be for a couple of months huh? Well, I'm back for yet another issue and this one makes me totally crazy!
I have just finished a JTM45 build. Smooth build. With this stuff:
Marstran JTM45 103 OT
Marstran JTM45 1202-55 PT
Marstran 3H choke

On to testing time:

No tubes in, just AC cord in: 229V at the wall and as much at the Power hot terminal (I'm in EU). Perfect!

GZ34 IN Power ON: Perfect.

GZ34 IN 3 12AX7 IN: Perfect readings 3.23VAC on pins 4/5 and 9. Forgot to check DC negative voltage at pins 5 of the output tubes...
ALL tubes in: Impossible to bias the amp cuz I had almost no readings and tturning the bias pot wasn't changing anything...

I swapped a few wires (OT red and black) and flipped it on: blew the 500mA internal fuse...

That's when I discovered that I had soldered the yellow wires (+ side of the 32uF cap to pin 6 of V5 then pin 6 of V4 then board) to pin 5 instead of pin 6... 

I soldered where they should be and now I get perfect readings with the GZ34 and 12AX7 but the KT66 are strange on pins 8 and 5. The DC negative voltage is perfect when no KT66s are installed (-52VDC) but once in V4 gets -42 and V5 -11...WTF! And the strangest thing is that pin 8, which is supposed to give a 0 reading gives me 457VDC in V5... I should just be reading mV there to be able to bias the amp as per Ohs law (1ohm resistor attached to pins 8 and 1 then ground...) That's when I turned the amp OFF because V5 was getting red like lava and told me "turn me off or I'll blow up in your face!" I lightly touched the tube about one minute after turning it off and my finger still hurts, I'm red burnt...

Here's a few pics... I hope it's not blown tubes because I don't have any replacements at hand... Thanks a lot for your help!

[tubeswell]

the KT66 are strange on pins 8 and 5. The DC negative voltage is perfect when no KT66s are installed (-52VDC) but once in V4 gets -42 and V5 -11...... I hope it's not blown tubes because I don't have any replacements at hand... Thanks a lot for your help!

Well if you swap the output tubes around, do the readings change sockets?

[plexi50]

Isnt your pin 8 on the octal sockets soldered to ground? Or do you have a 1r resistor on pin 8 going to ground. It sounds like you blew a power tube. Never seen high voltage on pin 8 cathode of a power tube like that. Shorted power tube. I would not swap the power tubes around to check the tubes for any difference if i found 457VDC on the cathode of any power tube. Make sure your OT &  PI wiring is correct. Got to have a tube tester or someone that can test the tube for you

[sluckey]

Forgot to check DC negative voltage at pins 5 of the output tubes...

That was a costly mistake. For a new build fixed bias amp I would never plug an output tube in the socket until I was certain there is proper bias voltage on the grid. If you had checked you would have seen there was approx 450VDC on that pin 5.

That's when I discovered that I had soldered the yellow wires (+ side of the 32uF cap to pin 6 of V5 then pin 6 of V4 then board) to pin 5 instead of pin 6..

IOW, you had approx 450VDC connected to pin 5, control grid. I expect that tube pooped and probably took out the 1Ω resistor on it's cathode too.

I'm pretty sure you will need some more big tubes. Maybe play with some cheap EL34s 'til you get it right.

[SleepLess]

Damn... What a fool... Sluckey you were right the 1 ohm resistor of V5 blew, I couldn't get any readings when I measured it. I have replaced it with a new one that give sme a correct reading (1,4 ohms instead of 1 ohm, but the one on V4 reads 1,4 ohms as weel)... The tube looks OK though. So I tried a matched pair of known-to-work EL34s... Loud noise. Turned it OFF as I didn't want to burn these as well... Took my measurements again, all seem to be OK: Everything in DC except heaters and pins 2/8 of V6. NO power tubes installed but the amp fully on with the GZ34 and the three 12AX7s...

V1:                       V2:                      V3                   V4                 V5                   V6
1     237                1     193               1     263           1     0             1     0                  
2                                                                           2     3.24         2     3.24         2     480
3     1.8                 3     1.28              3     48.1          3     480          3     480          
4     3.24               4     3.24              4     3.24          4     478           4     478          4     354
5     3.24               5     3.24              5     3.24          5     -52.7         5     -52.7      
6     237                6     348               6     256           6     478           6     478          6     354
7                                                                           7     3.24          7     3.24
8     1.8                 8     193               8     48.1          8     0              8     0             8     480
9     3.24               9      3.24             9     3.24

Then I installed a pair of EL34s and here's waht I have on them, Power ON but STANDBY OFF:

V4:             V5:
1    0          1      0
2    3.15      2    3.15
3    0          3    0
4    0          4    0
5    -51       5    -51
6    0          6    0
7    3.16      7    3.16
8    0          8     0


I'm lost... I absolutely can't understand how an amp that has good readings on all tube pins doesn't work...  
The only thing that puzzles me is the OT primary: I have red on V4 and black on V5 and here is the diagram:



THANKS!

[stratele52]

I'm lost... I absolutely can't understand how an amp that has good readings on all tube pins doesn't work... 
The only thing that puzzles me is the OT primary: I have red on V4 and black on V5 and here is the diagram:

 -SleepLess

______________________________________________________________________________

Voltage reading is only one parameter to trouble shooting. Take your time to re-check your wiring . Every body can make some mystake when building and don't forget ......... Sleepmore .     ; )

[plexi50]

Your plate wiring looks good on Marstran 103 OT. Go back over the power supply feed capacitors and resistor nodes. Starting from the bridge recto and standby switch all the way to the last preamp stage. Sounds like there is HV wire that is possably grounded in the power supply chain

I make plenty of mistakes myself. Finding them is fun. (sorta)

[sluckey]

So I tried a matched pair of known-to-work EL34s... Loud noise.

Could be NFB loop related. Disconnect the loop per the attached pic. If noise dissapears, swap the OT primary plate leads and reconnect NFB loop resistor.

If not NFB related, describe the noise better.

[SleepLess]

Sluckey I exactly did what you said.
NFB resistor disconnected: no noise.
NFB reconnected and red/black wires of the OT reversed: no noise!
The amp is WORKING and sounds good (Jeez, I didn't think it would sound so well with EL34s...!)

I biased it at 22mA for V4 and 29mA for V5. I'm full up on the bias pot... Which means that it will be too low for KT66s which will require about 40mA... The biasing range goes from 2mA to 29mA... What can I do to increase the values (20mA to 50mA would be good...) ?

Sluckey once again, you're a genius!!!!!

Just one last thing: do you think I can throw away my KT66s or can I try them in? Is there any risk for the amp if one or both of them is fried? They look good and unharmed, but now that the amp sounds good I don't want to ruin my working amp...

Thanks to everyone! You guys rock!!! 

[sluckey]

I biased it at 22mA for V4 and 29mA for V5. I'm full up on the bias pot... Which means that it will be too low for KT66s which will require about 40mA... The biasing range goes from 2mA to 29mA... What can I do to increase the values (20mA to 50mA would be good...) ?

First, tell us how much the negative bias on pin 5 swings when you turn the bias pot from end to end.

Just one last thing: do you think I can throw away my KT66s or can I try them in? Is there any risk for the amp if one or both of them is fried? They look good and unharmed, but now that the amp sounds good I don't want to ruin my working amp...

Didn't you say it was blowing fuses even after you fixed the wiring error? One of those KTs has been severely stressed and is probably shot. The other 'may' be OK. It would be best to put them on a tube tester. But, you can put them in the amp and watch, smell, and listen carefully. Keep one hand on the power switch. If that tube is shot, it will probably take out another 1Ω resistor. Worse case, it may take out your OT also. I know, it's a gamble. Callahan once said, "Do you feel lucky, punk?"   

BTW, get some 1Ω 1watt 1% resistors from Doug and replace those 1.4Ω cathode resistors. They are throwing off your bias current readings unless you are taking that into consideration.

You did learn a valuable lesson, right? NEW BUILD... FIXED BIAS... CHECK THE BIAS VOLTAGE ON THE TUBE SOCKET BEFORE PLUGGING IN ANY TUBES.

[SleepLess]

I did learn a lesson for sure... a $90 lesson if my tubes are indeed useless...

So pin 5 variation is -51VDC to -41DC from one end to the other... I have 453VDC B+ at pins 3 of both tubes...

I actually remember that the fuse blew BEFORE I found out about my wiring mistake... Don't know if I should try the KT66s... All I remember about one of them is that it got red as hell and that the 1 ohm resistor above it blew...

Thanks!

[plexi50]

Dont try the KT66 tubes. Dont take chances with your OT unless it has a fused CT. If you blew a cathode resistor and the tube had 457VDC on the cathode then you can pretty much be guaranteed that tube is shorted and dead

[stratele52]

Often, when I power "ON" a new amp I built , I use current limiter, you know the 100 watts/ 120 volts AC  bulb in series with wall outlet and amp ?  This protect your amp.

Or I use variac with monitoring B+ and bias . Starting with low voltage.

[SleepLess]

Dont try the KT66 tubes. Dont take chances with your OT unless it has a fused CT. If you blew a cathode resistor and the tube had 457VDC on the cathode then you can pretty much be guaranteed that tube is shorted and dead

OK I won't. I guess I'm headed for another pair of Gold Lions then... 
Meanwhile if you know how to increase my bias range...
Thanks to everyone, I'll post a pic of the fully finished build!

[sluckey]

You need more cathode current, so follow the instructions in the pic. For the 15K, try 22k, then 27K, then 33K. One of those will probably get you there. Or, for the 56K, try 47K, then 33K, then 27K. One of those should get you there also. You probably won't need to change both resistors.

EDIT... I just realized you have a 68K resistor where there is usually a 56K, so change that to a 56K first to check the range.

Be sure to check the neg. voltage swing on pin 5 after each resistor change with the amp on standby or the GZ34 pulled to kill B+. Apply B+ after verifying neg. bias voltage.

[SleepLess]

OK... I have soldered a 56K instead of the 68K and now I'm good... Bias is set at 41mA for V5 and 34mA for V4 with a 445V B+. I am 3/4 up on the bias pot. I could maybe have a 47K instead but at least it is safer the way it is... I have also ordered a new matched pair of KT66s to fully finish the amp and install it in its cab (Bluesbreaker 2X12!)...

Just one thing that doesn't have any link to the JTM45 thing: I built a 5F4 three months ago and everything was going perfectly well. But now when I plug a guitar cord in either one of the input jacks a pop is heard. The same pop occurs when I switch on each pedal I have. I have checked the exact same rig on a twin 57 and the pedals are quiet when I switch them on or off. Have you got any idea as to what could cause that? Bad tube? Something else?

Thanks again to everyone!!!!

[sluckey]

I'm thinking maybe a bad connection on an input jack or the jack itself, or a resistor on the jack.

BTW, I like that layout for the JTM45. Where did you get it?

[SleepLess]

I'm thinking maybe a bad connection on an input jack or the jack itself, or a resistor on the jack.

Damn... It worked well since last weekend... Resistor gone bad...? 

BTW, I like that layout for the JTM45. Where did you get it?

Steve it is the stock MetroAmp layout... You can get it off the Metro website Wiki. They have all their layouts there. The only thing I changed is the grounding layout. I used Larry star grounding instead of the original Marshall grounding. I think it is quieter.

[Willabe]

I really like your work. Solder joints and wire dress look _realy_ good to me.

Why are you painting(?) the solder joints red? I know Dr. Z does this and I figured he did it to know if someone was messing around in an amp he sold, voids warranty?     


                    Brad     

[SleepLess]

I really like your work. Solder joints and wire dress look _realy_ good to me.
Why are you painting(?) the solder joints red? I know Dr. Z does this and I figured he did it to know if someone was messing around in an amp he sold, voids warranty?     
                    Brad     

Thanks a lot!!! It's the frst time I paint them red and I chose to do so because:
1. Marshall did it this way...
2. It looks darn good...
3. I've heard it might keep moisture away so the solder remains forever shiny underneath...

And you're right I think it might serve as warranty proof as well... Unless someone knows exactly what I use...

[plexi50]

I was going to say red nail polish.  But i think it is a red magic marker on the solder joints.

[SleepLess]

I was going to say red nail polish.  But i think it is a red magic marker on the solder joints.

Spot on! 

[SleepLess]

Hi!
I was writing about a popping sound issue on my 5F4 a little earlier on. The amp emits a noise each time I plug a cord in the input jacks or each time I switch ON/OFF any pedal I bring in... If these two pictures help, please advise on what to do and check...
Thanks!

[sluckey]

Do those jacks have insulating bushings to isolate the jack ground from chassis?

[SleepLess]

Do those jacks have insulating bushings to isolate the jack ground from chassis?

Mmm... I don't remember if they came with insulating bushings... But I suppose if they didn't it means that they need them... The mystery is that the amp worked dead quiet for more than a month before that popping issue came along... If it's just the bushings, wouldn't it have popped right from the start?

[sluckey]

I wasn't thinking the bushings would solve the problem. I was thinking they may be the problem. I say that because you did not hard wire the low jacks' ground lugs to chassis. If you don't have bushings then the jack wiring looks OK. Take a look. You'll see the insulating bushing under the jack nut.

Do you get a pop when plugging into any of the four jacks?

[SleepLess]

Do you get a pop when plugging into any of the four jacks?

Yes I do. The lower inputs are quieter though, but I assume that is normal considering they don't have the 1M resistors...

[SleepLess]

I have checked on the jacks... They do not have any insulating bushings...

[stratele52]

Do you get a pop when plugging into any of the four jacks?

Yes I do. The lower inputs are quieter though, but I assume that is normal considering they don't have the 1M resistors...

Those kind of metal jacks are never insulated from the chassis . Each of  1 meg is connected at two jack . This is good.

[sluckey]

Those kind of metal jacks are never insulated from the chassis .

Unless you put bushings on them yourself. Switchcraft makes a bushing set just for this purpose. You can buy them here at Doug's store. I have a couple builds using them.

[SleepLess]

My 5F4 never had those bushings and worked flawlessly for a month... What the heck is going on...? 

[sluckey]

Nothing is going on. I just asked a question. You don't have any bushings so that cannot be a factor. I asked because your pic looks like there 'may' be a bushing on a jack and that jack does not have a wire connected from ground lug to chassis.

Maybe reflow your solder connections on the jacks, the 68K resistors on the board, and the wires going to V1 pins 2 and 7.

[stratele52]

My 5F4 never had those bushings and worked flawlessly for a month... What the heck is going on...? 

If you use insulating bushing but you ground ( like you should ) the jack with some wire to chassis  ground, insulating bushing is useless.

Or your chassis is not at ground potential , ground is only on the board ?