pucho812

Marshall JTM45 Problem
« Reply #20 on: July 26, 2007, 11:43:54 AM »
Um blueish glow is caused by one of 2 things.

) Most Svetlana glass power tubes show FLUORESCENCE GLOW. This is a very deep blue color. It can appear wherever the electrons from the cathode can strike a solid object. It is caused by minor impurities, such as cobalt, in the object. The fast-moving electrons strike the impurity molecules, excite them, and produce photons of light of a characteristic color. This is usually observed on the interior of the plate, on the surface of the mica spacers, or on the inside of the glass envelope. THIS GLOW IS HARMLESS. It is normal and does not indicate a tube failure. Enjoy it. Many people feel it improves the appearance of the tube while in operation.
2) Occasionally a tube will develop a small leak. When air gets into the tube, AND when the high plate voltage is applied, the air molecules can ionize. The glow of ionized air is quite different from the fluorescence glow above--ionized air is a strong purple color, almost pink. This color usually appears INSIDE the plate of the tube (though not always). It does not cling to surfaces, like fluorescence, but appears in the spaces BETWEEN elements. A tube showing this glow should be replaced right away, since the gas can cause the plate current to run away and (possibly) damage the amplifier.

So you might have a leak. AS for the rattle it could be filiment rattle.

If a tube rings like a telephone it suffers from "filament rattle." Like a microphonic tube it should be replaced. Filament rattle isn't technically the same since the tube isn't acting like a microphone. Often you'll hear people mistakenly identify filament rattle as microphonics.

AS for testing caps yeah you will get an accurate reading with the cap out of circuit as you would a resistor you could get away with only one leg out as opposed to removing the cap all together however thats usually a pain so you would have to pull it out.
I ahve measured caps in the circuit and usually get enough of a close enough reading to go o.k. not really an issue.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.


plexi

Marshall JTM45 Problem
« Reply #21 on: July 26, 2007, 12:50:42 PM »
Pull the power tubes and scope the output of the phase splitter, see if the notches disappear.  If they do, try a different set of KT-66's or maybe even 6L6 if you have some good ones....I have seen similar notches when using big bottle Chinese KT-66's, it may be inherent to the "design" of the tube.

Try some really good (NOS) tubes and see what happens.

steppenwolf

Marshall JTM45 Problem
« Reply #22 on: July 26, 2007, 01:12:25 PM »
Hi!
I pulled out the KT66 and scoped the PI caps. The notch is away and it stays clean a lot longer...

I also put a sine on the secondary of the OT and scoped the primaries. Sine goes in and a clean sine comes out, even at higher levels and at low and high frequencies... :?

pucho812

Marshall JTM45 Problem
« Reply #23 on: July 26, 2007, 01:16:20 PM »
If it's clean without the Power tubes, try putting new KT66's :thumb:
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

steppenwolf

Marshall JTM45 Problem
« Reply #24 on: July 26, 2007, 02:25:18 PM »
I just installed two pretty new 6l6 GC Svetlana tubes, the same problem!

There's still that notch, but if I disconnect the negative feedback, the notch disappears at the PI but still persists at the output...

So the problem should be somewhere behind the PI, am I right?

CJ

Marshall JTM45 Problem
« Reply #25 on: July 26, 2007, 09:23:10 PM »
different pwr tubes need different bias voltage-so watch that.
EL-34 takes about -35, 6l6 more like -50 in a fender, KT 66 will depend on when it was made.

the flickering blue is totally normal, and indigenous to all marshalls, except the solid state crap.

which KT 66 do you have-smoked glass genelex or other?

Sounds like your neg feedback is really pos feedback, try swapping the two merc output pri wires, they go to the pwr tube sockets.

12ax for the inverter, was gonna say to hot, like the pultec guy who had the wrong tubes in there, so no joy there.

transformers never distort the high end, just the bottom gets real mushy and nice.

tight butts drive me nuts, not mushy ones with cottage cheese (celulite)
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

plexi

Marshall JTM45 Problem
« Reply #26 on: July 27, 2007, 03:10:24 AM »
By removing the feedback, you removed the "notch" signal from the phase splitter, it's output will be clean.  You have a problem in the power stage that is creating the notch.

plexi

Marshall JTM45 Problem
« Reply #27 on: July 27, 2007, 03:26:49 AM »
Quote
1k you can see it, 2k even more and at 4k it is obvious....but it is only there, when the distortion begins and grows, you can't see it with a clean sine...


If you can't see it w/ a clean sine wave and it only comes in when the amp is close to full output it is crossover dist.  

At higher frequencies at full output Marshalls will start to slew distort, most guitar amp transformers are not so good...


Does the amp sound ok even though it comes on quickly on the dial?

steppenwolf

Marshall JTM45 Problem
« Reply #28 on: July 27, 2007, 04:56:23 AM »
Quote
which KT 66 do you have-smoked glass genelex or other?

I have those : http://www.tube-town.net/ttstore/product_info.php/language/en/info/p529_TT-KT66-Brown-Base---TT-XMatching.html
Should be the chinese KT66 Genalex copy.

Quote
Sounds like your neg feedback is really pos feedback, try swapping the two merc output pri wires, they go to the pwr tube sockets.
When I first puttet in the OT Paul Petronete from MM adiviced me to swap the primary wires if the amp should "squeal like a dying pig :wink: ". He told me thats because the negative feedb turns into positive. That happend to me, it really squeals but after I swapped the wires, this squeal went away...

Quote
Does the amp sound ok even though it comes on quickly on the dial?

Well, yes, it sounds OK, maybe a little mushy but ok. The main problem was the lack of headroom and that the distortion starts so early. Also, the distortion is fuzzy and not warm. (compared to my pals JTM45)...
That brought me to the thought that something isn't allright, and thats still my opinion. I thought that notch could be it and it stills looks strange on the scope as it appears not only near full tilt, but close after half power... :?

EDIT:

I just scoped the signal directly at the KT66 anodes. The Notch appears and even more evident than after the OT on both anodes. So, I can exclude the OT right?

What I noticed. Before the sine clips, it is pretty clean but it is flickering and threre are like shadows around it. Then I triggered on a a lower frequencies, around 100HZ and the hole 2k sine is moving up and down at around 100HZ. That should be hum that is canceledout later...
BUT: When I turn up the volume, I don't only see this 100HZ modulation on top of the sine, but it appears once or even twice in the middle of it getting clearer the more it distorts. There is a strange 100HZ wave in the middle of my 2k sine...and when I increase the triggerfreq. instead of this wave, I can see the notch...
 :shock:
Has anybody an idea?
I'm really confused right now...

plexi

Marshall JTM45 Problem
« Reply #29 on: July 27, 2007, 01:14:00 PM »
Sounds like you are seeing some hum (normal) as long as you don't hear it...looking at the KT66 plates will show PS ripple that does cancel in the OT.

I still think you got that sucker biased funny and you are seeing crossover notch come in as the output comes up.  Check the bias and value of your grid stopping resistors on the control grid, pin 5.

I never did like the spongy gainy JTM45 sound.  Like it was Marshalls copy of the bassman, before they got it "right".  After they changed to EL34's and dropped the primary impedance it started to sound much better (to me) more like a Marshall...

Try El34's, re-bias, and set impedance selector to 16 but connect 8 ohm load.  Measure power output and look at distortion waveform.

Or post a picture of the notch waveform.


I'm out of ideas.........


pucho812

Marshall JTM45 Problem
« Reply #30 on: July 27, 2007, 01:53:50 PM »
Bias was my first thought too :cool:

From my first post in this thread.
Quote from: "pucho812"
New Tubes? Did you re-Bias? Whenever you change power tubes, You should re-bias the tubes. What tubes are in the power section....
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

steppenwolf

Marshall JTM45 Problem
« Reply #31 on: July 27, 2007, 02:18:06 PM »
Thanks for your advice guys!

I have a bias probe with a 1OHM resistor in series and I measure the voltage drop across it...I measure around 40mv = 40mA, am I right?

How can I post a picture?

plexi

Marshall JTM45 Problem
« Reply #32 on: July 27, 2007, 04:23:13 PM »
40 mA should be close, but I like to set it by the scope, then check the plate/bias voltage, then line current draw.  But mainly adjust for best waveform at close to full output, then make sure it's not running too hot on the line current draw.  

look up the icons on the reply post page and you can add a picture.  I never did it though....

pucho812

Marshall JTM45 Problem
« Reply #33 on: July 27, 2007, 04:27:04 PM »
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

Junction

Marshall JTM45 Problem
« Reply #34 on: July 29, 2007, 08:13:49 PM »
I normally bias the JTM45, between 35 - 38mA, but 40mA should be okay.

Try measuring the resistance with an ohm meter, across the primary side of the OT and then measure from each side to the centre tap, I cant remember exactly what I measure, but it could be something like 100 ohms and then it should measure 50 ohms from each leg to the centre tap. You can do this with OT in circuit, we are looking for any fundamental problems with the OT, i.e. maybe it has some shorted turns, if you see a big imbalance between the legs and the centre tap, then there is something obviously wrong.

I've built 3 ceriatone amps, but with their transformers with great success, they sound great.

Michael

steppenwolf

Marshall JTM45 Problem
« Reply #35 on: July 30, 2007, 12:17:28 PM »
Hi!

OT measures around 160Ohms, 90Ohms from left to center, 70 Ohms from right to center.
I added a picture of the sine at the output. Just click on it to get better resolution!



Hope that somebody can tell me whats going on...

Thank you so much!
Stefan

CJ

Marshall JTM45 Problem
« Reply #36 on: July 30, 2007, 02:50:39 PM »
that dont look too bad.

another idea, did you use audio taper pots?

re bias by ear, see if you can get it to sound good with a pick in one hand, and a screw driver in the other.
lay the git on the carpet so you can play it without being grounded.

hit open strings as you turn the pot,
hopefully no explosions.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Junction

Marshall JTM45 Problem
« Reply #37 on: July 30, 2007, 08:35:30 PM »
Quote from: "steppenwolf"
OT measures around 160Ohms, 90Ohms from left to center, 70 Ohms from right to center


Something doesn't sound right here,. when you say left to centre and right to centre, are you referring to each leg of the primary to it's centre tap. The DC resistance from each primary leg to center should be about the same, maybe +/- 10% difference.

If you are measuting 900 ohms from one leg and 70 ohms from the other then there is something wrong. Also these two readings should add up to equal the total resistance between the two legs (was this 1600 ohms?)

Try pulling out the KT66 tubes and then re-measure, to make sure the tubes are not affecting your readings.

Obviously take these measurements only when the amp is switched off.

Michael

plexi

Marshall JTM45 Problem
« Reply #38 on: July 31, 2007, 01:54:35 AM »
That does look strange, some crossover notch, but also some breaking thru at the very peaks.  Maybe a layout problem, it does kinda look like positive feedback, but if you swapped the wires and it squeals then definitely not at the OT.  You could try to move some of the power tube grid wires or PS plate wires, feedback wire from speaker, even back as far as the tone stack output....keep going back and reposition wires and see if anything changes.  Or try a 12at7 and see if it gets better or worse, or load the secondary down 1/2 and see what happens to it, or disconnect the 47 pF, see it it clears up.....try and find something that affects it.

Last resort, swap in a known to be good OT and see if it changes.

VacuumVoodoo

Marshall JTM45 Problem
« Reply #39 on: July 31, 2007, 05:06:26 AM »
I'm thinking power supply...
Have you tried to monitor the B+ DC voltage while increasing the volume? How much does it sag?
Did you check the rectifier tube?
Are you getting full wave rectification i.e. ripple freqency at B+ should be 120Hz. Does it look on the scope like a nice sawtooth wave?
If your rectifier is bad it could be that you're not getting enough DC current out of power supply once the power tubes start pulling more than bias current.

Just another possibility.
Alex Niemand
_____________________________________
Life's a party but you get invited only once...
Tubewonder amps
"L


 

Related Topics

  Subject / Started by Replies Last post
6 Replies
4059 Views
Last post June 13, 2007, 05:04:17 PM
by CJ
7 Replies
6824 Views
Last post November 11, 2008, 09:02:23 AM
by API
4 Replies
1434 Views
Last post May 03, 2018, 11:59:03 AM
by nashkato
19 Replies
2795 Views
Last post November 24, 2018, 04:31:03 PM
by alexc