Marshall DSL 50 OT, & C12 Mod Questions

[shortfuse]

I have a DSL 50 with a bad OT.  It is a friend of mines and it went to about a 1/3 power while he was playing live.  Now the amp just quacks when you hit the strings.  I know it is the OT because I have 2 DSL 50's here at the house and I tried one of my OT's in it and it worked fine. The swap was very easy, quick, and conclusive as the transformers have spade connectors on them so no soldering 4 screws and 7 wire connections at the tranny. But what would have been the correct method of testing an OT to deem it good or bad, good I know it will work but is there a way of testing to see if it is on the way out?  I ask this because it makes me wonder about one of mine as it made that quack sound twice on me while jamming with some friends about a week ago.  Only did it being pushed it has never done it here at the house at low volume.  When it did it, it did it and then kept playing fine.  1st time i thought it was one of the speakers but it sounded just like his amp does now all the time.

Also does any one have any suggestions with real trial results as far as a replacement one goes.  I have looked at Mercury (great tranny but too much money I may choose this for my own replacement), Hammond, Classic Tone, and of course the Marshall direct replacement (but gotta believe Hammond or classic-tone would be a better option.  Seems that with the classic tone i would be splicing wires, not sure I want to be splicing wires I do have some 600v heat shrink would that be acceptable to use?  Hammond looks to be spade connector as the OE.  Pros & Cons or other suggestions for the replacement OT?  I have read posts on other sites but respect the advice and suggestions here.

Has anyone done the C12 mod on one of these which from what little research I have done says it is a remove the C12 treble bleed cap from the board mod?  Seems simple and harmless to try or am i missing something?  It is supposed to smooth out the red channel.  I use the green classic crunch channel 90% of the time, clean 5% and the red with the switch out 5%.  The red channel with the switch in sounds like mush to me I know a lot of people like it but I do not.  Wondering if it may make the red channel more usable for me.
Pros & Cons?

[HotBluePlates]

...  I know it is the OT because I have 2 DSL 50's here at the house and I tried one of my OT's in it and it worked fine. ... But what would have been the correct method of testing an OT ...

Try his OT in one of your amps.

When you can just push-on spade connectors, it sure makes me think there might have been a loose connection. Unless there is other damage to the amp and/or other symptoms you haven't mentioned. Output transformers don't "go bad" they get killed by some other fault.

...  Seems that with the classic tone i would be splicing wires ...  Hammond looks to be spade connector as the OE.  ...

No output transformer from any maker is gonna come with spade connectors already on them (except possibly the Marshall replacement, and even them maybe not).

You shouldn't splice anything as a means of installation, but instead run the appropriate wires from the new OT to the correct points within the amp circuit. Since you might be dealing with quick disconnect terminals on the p.c. board, you'll likely need to crimp & solder new spade connectors onto the wires of the OT you select.

... Has anyone done the C12 mod on one of these which from what little research I have done says it is a remove the C12 treble bleed cap from the board mod?  Seems simple and harmless to try or am i missing something?  It is supposed to smooth out the red channel.  ...

You oughta include a schematic or a link to explain what you're talking about. Assuming I found the correct schematic, C12 looks like it simply rolls off highs in the 2nd gain stage. Cutting it out just makes that stage a little brighter, and probably raspier.

No way for me to know if that will be a change that's worth it to you. And Marshall made a hot mess of that schematic to where I'm not sure I follow all of where the different connectors run (for the various switching and interconnections).

[shortfuse]

HBP
Thanks for the comments. 

I did not install his tranny in my amp but did clean the connections and retested in his amp after I pulled mine back out with no change from what it was originally doing. 

This morning I looked up the classic tone one on their site and it says it comes with extra-long wires to do what you said run it to the board and splice (solder) a new female spade connector.  Looking at the pic of the Hammond it appears to have solder connector on the bottom not spade connectors as the original.  I believe I am going to go with the classic tone.

The cap thing does not sound like it is going to do what was described.  I will try to find that link and repost.

[shortfuse]

This seems to be the most informative on the mod. 
If it is a treble bleed cap how would that get rid of the highs seem like what you said it would make it brighter.  I am not to fond of the whole tone stack in this amp maybe I should just re-value the whole thing.  But how to figure out what values to use to use I'm not sure where to start.
Maybe the best thing is to just leave it be.

[HotBluePlates]

This seems to be the most informative on the mod. 

How did you hear about the mod in the first place? Maybe linking back to that info would help.

If it is a treble bleed cap ...

The picture helps, if only to call into question the function of "C12".

In the schematic I linked, C12 is in parallel with R12, a plate load resistor. In that situation, it rolls off highs of the tube stage that R12 is attached to.

But in your picture, C12 is in parallel with a 470kΩ resistor ("R19"?).

In many Marshall amps, there is an interstage voltage divider in the preamp, generally made of two 470kΩ resistors. The "upper resistor" of these is generally bypassed with a small cap, which cuts bass (or you could call that boosting highs). This helps keep highly-distorted riffs from being muddy.

*If* C12 is a bypass cap for a voltage divider like I described, then clipping it out will seem to restore/boost bass. It might make high-gain sounds a muddy mess. Your ear is the only thing that can determine if it will be a good change.

This is why we need a schematic to figure out the function of a given part... Obviously the schematic I found isn't right (none of the resistors around C12 are the 100kΩ I see in the schematic I linked).

[Ritchie200]

Shortfuse,

Having seen and experienced some spectacular OT failures - all due to very high voltage issues, I would have to agree with HBP.  OT's just don't go bad.  Throwing another tranny in there without diagnosing the original cause and you may be buying yet another.  No fuses popped on this?  Something smells here....  Check the traces around the terminals very carefully.  I would still check the "bad" tranny in one of your amps.  The DSL's are also notorious for bad terminal connectors, bad speaker jacks, and impedance selectors.  There is a mystery here!

Good luck!
Jim

[shortfuse]

I did as HBP suggested (I have learned on this forum when some of the senior guys that help suggest something it is usually for a good reason so I dont question it I do as suggested and report back) and tried his OT in my amp and it reacts the same in mine as in his.  I agree wholeheartedly that something took it out.  Now that something could be an unknown as in someone that helps him set up when he plays out may have plugged two cab's into the wrong jacks and or selected the wrong impedance as well.  Heck they may have thought it was plugged into a cab and it wasn't.  But that info we will never know.   My friend is a true professional guitarist, he played with Pat Travers for several years back in the 80's and had his own band as well.  I have no reason to doubt what he told me, he said they were in the middle of a set and it just lost power and sounded horrible so he switched over to his DSL 100 and finished out the night.  Usually when he plays the 50 he has the levels up around 7-8 and the gains aprox the same.  So it is being pushed pretty good and he sometimes plays 2 to 3 times a week.
I did order the OT today and will get into it a little deeper this weekend and post voltages but everything seems in line to me, even able to set the bias as the tubes are good and I assume power supply is good.  Just kinda squawks fizzles and pops when you hit the strings with his OT installed with mine installed it plays fine played it for about an hour.
I read where these amps have a run away bias or drifting to higher bias generated with heat while being played hard and a new bias board kit that resolves that.  Here is the article I found  http://www.drtube.com/en/modifications/jcm2000-stable-bias-mod    But if that were the case it would have taken out the tubes and not the OT.  Or the tubes 1st and then the OT so the tubes would still be bad which they are not, correct?
I am all ears if you have a suggestion as to where to start looking for a bad component.  I will get the correct board number and date code tomorrow and try to post that with the correct schematics.  This will also help with the C12 question.
Thank you both for trying to save me from buying another OT god knows I usually learn the hard way.
Steve

[sluckey]

I can't say about the Marshall OT, but the Sears Silvertone 1484 "Twin Twelve" had an odd shaped OT that was notorious for failing when pushed hard. I don't know if the winding wire was just too small or what. My OT failed and I replaced the OT in 3 or 4 others as well. Always the same symptom. Playing wide open, OT got hot, and then the volume just dropped way down and sounded crappy. The OT was always the only failed component. Sears was famous for cheap electronics in the '60s. The best cure I found was a Super Reverb!   

[Ritchie200]

Is the player/your friend Pat Thrall?   Again, check those traces!

Jim

[shortfuse]

Steve
I know a few people with these amps DLS TSL 50, 60, & 100 that have popped OT's while playing and that was all it was.  They put in a Hammond or Classic tone and been fine ever since.

Ritchie
No Jerry Riggs

[Ritchie200]

Yup, Jerry is a playah!!!

Jerry Riggs - Little Wing (rare live)

[shortfuse]

Finally received my OT was back ordered that's a story in itself.  I am getting ready to make final connections but I have a question on the Center Tap.  Looking at the OT wireing diagram (see attached PDF) I first assumed it was the brown wire in the middle but I then noticed the black dot at the top on the white wire which across from that on the secondary side the black common has a black dot next to it as well.  

The stock transformer the ct going to W5 (see dsl schematic) on the schematic was the 1st lug on the transformer at the bottom of the transformer (see attached pic) which is the blue wire, purple is W4, and white is W3.

So would it hook up
white = W3
brown = W5
red = W4

or

white = W5
Red = W4
brown = W3

Using my meter if I hold 1 lead on the brown I get 39 ohm's from the red and the white.  If I hold one lead on the white I get 40 from brown and 78 from red.  So I may have answered my own question, would that mean the brown is the CT?  Is that the correct way to check the OT for the CT which ever lead splits the difference is the CT?

[sluckey]

Using my meter if I hold 1 lead on the brown I get 39 ohm's from the red and the white.  If I hold one lead on the white I get 40 from brown and 78 from red.  So I may have answered my own question, would that mean the brown is the CT?  Is that the correct way to check the OT for the CT which ever lead splits the difference is the CT?

According to the diagram, brown is the CT. Your test with the ohm meter confirms that fact.

I then noticed the black dot at the top on the white wire which across from that on the secondary side the black common has a black dot next to it as well. 

Those dots indicate relative phase. A signal on the primary white wire would be in phase with the signal on the secondary black wire.

[shortfuse]

Thanks Steve

Well it was another aggravating day of amp repairs for me, I always seem to get these ones that have an awkward problem or create one myself. 
Got up this morning double checked the post I made last night which Sluckey confirmed Brown was the CT so I connected the wires appropriately.  Turned the bias all the way down and fired up the amp and as I started bringing up the bias when I hit 37ma it jumped to 47ma and started to climb rapidly along with making a hissing sound.  I shut it down immediately and stared wondering to myself what the hell did you do this time to create a problem.  I was very careful in everything I did took notes for all the connection plugs to from the rear board, tube board, and front board.  Took good close up gut shot pic's as I was working.  So i started disassembling down to the tube board again as this is where the majority of the soldering took place.  I Changed the the 1k/5w and 5.6 grid resistors and removed the 270/7w resistor and installed the choke.  I had also removed C12 on the front board but was pretty confident that was not the problem.  I thought I had possibly bled solder across pads that were not supposed to have continuity.  Upon dis-assembly, inspection and verifying circuit flow with the meter I could find nothing that I had done wrong.  So I re-assembled again checking and double checking all the connections again, I even popped my chassis out of my DSL 50 to double check the connection notes to make sure I did not make a typo error.  Again I found nothing wrong (with the wires attached to the board).  Started the same process again, started bringing up the bias and everything was fine this time.  So Im thinking I had to have hooked up something wrong the first time.  Took the probes off and started to play.  It played ok but didn't sound right I cant think of the correct way to describe the way it sounded but just sounded a bit odd to me.  Shut it down after playing for about 15 minutes and started cleaning off the bench and putting things away.  This is when I found the yellow wire that connects W2 on the tube board to W4 on the rear board (see pdfs) on the bench.  I took it off this time for some reason and did not leave it attached to the tube board as I had the 1st time.  So I looked at the schematics to see what it connects to and I reinstall the wire.  I also decided to hook up the bias probes again and to check and see what happens when I fire it up with this wire attached looks to me like it is for the 4 ohm tap (I had been testing with 16 ohm 412).  As soon as i flipped the stand by switch on tubes started to run away and made the hissing sound again, shut it down immediately.  Disconnected the 4 ohm wire from the board turned the stand by sw back on and it was now like it was without the yellow wire attached to W2 and W4.  Now I am really scratching my head my gut is telling me it is a bad OT.  So I decided to pull my transformer out of my amp and put it in this one.  With my OT in the amp everything works just fine.  When I went to install the transformer it had a broken Isolator for one of the mounting bolts and had 2 star washers in it place (see Pic) didn't think much of it at the time just went down to ace and got another isolator and installed.  This probably has nothing to do with it but maybe the head of the isolator broke at the factory or distributor and they put the 2 star washers on.  I know chances of receiving a brand new trans bad out of the box is like slim and none and slim just left, but I think putting a good known OT in and the amp working fine is pretty good proof something is wrong with it.
I did not try to put the classictone trans in my amp as I felt it was unnecessary and if there is something wrong with it why install it back into anything.
Comments or suggestions????

[sluckey]

This is when I found the yellow wire that connects W2 on the tube board to W4 on the rear board (see pdfs) on the bench.

That wire is in the NFB loop. Your problem indicates the NFB phase is reversed. Swap the output transformer plate leads by swapping W4 and W5 as indicated in the attached pic.

[Ed_Chambley]

This is when I found the yellow wire that connects W2 on the tube board to W4 on the rear board (see pdfs) on the bench.

That wire is in the NFB loop. Your problem indicates the NFB phase is reversed. Swap the output transformer plate leads by swapping W4 and W5 as indicated in the attached pic.

How could phase cause runaway bias?

[sluckey]

How could phase cause runaway bias?

When the amp starts squealing (squealing is a signal even if it's too high frequency to hear, right?) the output tubes begin to draw a lot of current.

[Ed_Chambley]

[quote
squealing is a signal even if it's too high frequency to hear, right?

Yup, just never left one on long enough to encounter this issue, but it is feedback, right?  If the negative voltage is present why does it not bias the tube?

How is this different from letting a guitar feedback?

[sluckey]

[quote
squealing is a signal even if it's too high frequency to hear, right?

Yup, just never left one on long enough to encounter this issue, but it is feedback, right?  If the negative voltage is present why does it not bias the tube?

How is this different from letting a guitar feedback?

Yes, it is feedback.

The negative voltage does bias the tube. That's for static (no signal) conditions. As you adjust the bias voltage to cause the tube to run hotter (conduct more current) you reach a point where the positive feedback overcomes the bias voltage such that the output tubes become an oscillator rather than an amplifier. At this point the current thru the tube begins to run away regardless of the static bias adjustment.

It's the same principle as guitar feedback, but the loop is much shorter. The short loop is from the OT secondary back to the PI, and thru the output tubes, OT primary, and finally back to the OT secondary. With the guitar feedback the loop is much bigger, from speaker to guitar pickups, to amp input, thru the entire amp to the speaker, and back to the guitar pickups again.

[shortfuse]

Steve if that is all it is that would be wonderful I will reverse the wires.  Is that why the white lead has the dot in the schematic you commented on my earlier post was that supposed to have been landed on a specific tube because I don't see anything on the schematic that would indicate so?
Will report back this evening. 

[sluckey]

Is that why the white lead has the dot in the schematic you commented on my earlier post was that supposed to have been landed on a specific tube because I don't see anything on the schematic that would indicate so?

no

Some manufacturers put the phase dots on their transformers. Some don't. The dots are handy for design work but don't specifically relate to a particular circuit. This is all you need to know about those dots... "Those dots indicate relative phase. A signal on the primary white wire would be in phase with the signal on the secondary black wire."

[shortfuse]

I believe I have read that quote on the dots here very recently LOL and understand what you are saying.
I reversed the plate wires and all is well thank you again.  So I again learned something again because I never thought of reversing the wires because I thought it would do the same thing.  Thanks for the explanation on the run away bias that makes a lot more sense now and is good information to know in the future.
The amp sounds really good with the choke especially at higher volumes.  I did do the C12 mod and like it as well I will be pulling that cap on my amp very soon. 
The only thing I can say to watch out for when installing the choke is its position in relation to the reverb tank I had to shift the tank about an inch or so.

[Attila]

More Mods

DSL 50/100 – Joey Voltage Mods

FRONT BOARD:
1.     1M resistor from wiper to ground on VR3
Put a 1M resistor from the wiper to ground of Vr3: This has a dual function. It limits the amount of drive signal possible for low frequencies below 169hz (not taking in account for any other low frequency roll off  below this point of course. full up, frequencies below this point will be down -6db, and flat. it also slightly alters the peak response of the shelving filter created by R19,C11, and Vr3.
2.     R20: removed and jumpered
jumper R20: adjusts the strength of the aforementioned shelving filter.
 
3.     C10: replaced with .0022uF/ 2.2nf / 222k @63v
Replace C10 with a .0022uf: Reduces the amount of low frequencies passed on to the next stage in conjunction with R19, Vr3, and the source impedance of the driving stage. this has the benefit of tightening the sound by somewhat tailoring the bass content within the guitars useable range, and can encourage a higher mix of second order harmonic content (but that’s for another discussion). This will also help with potential blocking.

4.     C12: cut out or replaced with  150pF/.15nf /151k @63v,
Keep C12 or remove:  Keeping C12 alters the R19,C11,Vr3 shelving filter further by moving the beginning of the upward shelf high at lower gain settings (because the top half of Vr3, and C12 are now in series with R19/C11 ignoring other circuit impedances of course) giving an edgier sound. when about 3/4 of the way up the shelving effect becomes negligible, and eventually the cap is shorted out by Vr3. Removing it obviously will darken the tone of OD1, and OD2 by removing this effect, but also does so because this now puts larger resistance in series with the grid of the next valve, whose input capacitances will create a low pass filter with this resistance, and bleed highs further.
5.     ****C8 replace with .0015uf / 1,5nf/ 152k  - only if boomy
 
 
MAIN BOARD:
****C46: replaced with same value of 22pF @ 1000v  Not installed
6.     C9: replaced with 1000pf/1nf / 102k 500v  Polyprop
C9 1000pf (.001uf)creates a single pole high pass roll off of around 3.2Khz which is around the fundamental of highest note of the guitar. there are also very few useful harmonics for guitar above this, or that can really be reproduced by most guitar speakers. it also can reduce hiss, as well as rolls off the glassy harmonics of the 220K plate load.
7.     C12: replaced with 1uF /102k @ 63v Electrolytic
C12 1Uf provides a more useable mid shelving for overdriven guitar while retaining some current induced feedback for low frequencies. also sounds good for the clean mode as well
 
8.     Replace R30 (470k)with 68k to 220k resistor
Replace R30 (470k)with 68k to 220k resistor this will Decrease the Fizziness and the OVERALL GAIN as well and also brighten the Amp. I personally Piggybacked, (using a pair of test leads), a range of resistors across the Legs of R30 from 82k to 1M and decided I liked 1M the Best. This brought the Value of R30 to 320k. I then soldered it in place across the legs of R30. This way it is easily changed if I decide on a different value without pulling the PIA Mainboard!!! This particular Mod is pretty much season to taste. One of the Techs or Myself can figure out what value you need for "Piggybacking", or you can try and figure it out yourself:
 
R1XR2 Divided By R1+R2= the Total resistance of Both Resistors combined. Use the Value of R30 (470)k for R1 and the resistor You want to try as the value for R2. (Try: 82k, 120k, 220k, 330k, 470k and 1m)
 
9.     *****BIAS RESISTORS REPLACED:
Newer rev of the board 2008 have already been sorted
R67, R69 with 220K, 1/2 watt, metal film @ 1%
R68, with 33K, 1/2 watt, metal film @ 1%
R77: original stayed in amp because it wasn't a value of 10K as stated in our schematic, but 3.9k instead. We weren't sure if it was a good idea to replace this with a 10K value — and why