note modulation issue in ultralinear marshall

[marshall]

Hello all, had a thread on my amp build issue , on note fizz issue . After reading alot of info on  web about my issue   , I found the correct wording for my issue .   amp is 100 watt marshall major clone ultralinear build . 12ax7/12ax7/12au7/   2-kt88’s.   600 volts b+ . biased 28mv///  I read online about a guy who posted issue same as mine , that solved  his issue by varying the filter caps values.   was told by another tech prior to reading this  to try the same  thing .  in the pwr supply there are 2 jj 390uf  450 volt can caps in series   .   also noticed that there are no  pwr resistors   on the caps .   just wondering if  I need higher or lower filtering  in that area . othe caps are 1-50uf 500v//2-100uf 500v /// lmkif anyone has some input on this fix.. i changed out resistors , tried a fiiz cap (47pf)   didnt help . had ppimv removed and gain mod (cascading) removed aswell snd ran it stock .

[pdf64]

Without a schematic of your amp, the validity of any response is questionable 
Has the amp always exhibited this issue?

UL / DL power amps are generally far better supplied by a pi filtered HT eg http://schems.com/schematicheaven.net/manu/sunn/sunn_model_t.pdf
Without that, the power tube matching will need to be very good (ie not just at idle), and the reservoir cap ESR very low.

[Ed_Chambley]

Is your fizz on the string attack or decay?


Sure you need to filter the pi, but if your fizz is on the attack side and you do not hear it as it trails off, probably not caps.


For 2 kt88, I can see no reason for series 390uf, probably fine and not a problem. What UL  transformer are you using. What percent are the screens tapped?


Getting voltages up at 600 plus with new production tubes is a risk. I have built a few push  pull kt88 amps, UL designs. The previous posted Sunn schematic is proven. That is what I used to base my builds. I did change the PI to a 12at7 and balanced the signal gain.

[Ed_Chambley]

What I meant is the  Sunn was proven trouble free while the Major has fly back issues by design.

[pdf64]

Just to be clear, I meant 'pi' as the math symbol, which represents a CLC filter, as per the Model T HT arrangement 

[Ritchie200]

Marshall,
Please don't take this wrong - several people, including myself, have asked you for a schematic on the last thread and this one as well.  Nobody has time to sit down and go through your pics to check values and many connections are just not visible.  There are folks here who would like to give you a hand, but you have to "help me, help you!"  Any advice we can give is a shot in the dark without a schematic that references EXACTLY what you have.  Here is just one reason why...  You mention over and over that it is a clone, but some values you are giving are not close to a Major.  Please, if you were not provided a schematic by the builder, draw one out, or use the original 1967 (that has been fixed) as a guide and fill in the blanks.  Check and double check the values and connections. This will also help you with your understanding of the amp.

Ed,
I don't think you can get an off the shelf tranny with other than 40% taps.  I don't think that is the problem here.  The flyback issues that you mention were inherent with the Major are the same with EVERY UL amp.  The problem with the Major was the crappy OP transformers were not up to the task of absorbing these transient spikes during overdrive conditions.

Jim

[pdf64]

Sorry I'm late to this, what's the flyback issue with these?
I can see that flyback may be worse if the OT was scrimped on and had excessive leakage paths.
And that the high HT would increase the potential for damage from flyback.
But not how UL would make things any worse?
Rather the opposite, ie the lower output impedance (UL compared to pentode) may act to reduce and damp down flyback.

[marshall]

no schematic , was built by moratto ampshop  . prob used a 200w version as a guide .   pwr tranny is a hammond 278cx    800v ct.530ma///6.3v ct@6A..I’m using 2 input wall voltages via 3-position toggle  to get 110v/125v  to get b+  600v/540v...the o/t is a special wound merren audio tranny  to  handle the high b+... / 4 k prim imp . 43% tap.  I had explained to chris  what i would be using   [kt88’s with high plate voltage ].      as for the   fizzy distortion  im getting  it on pic attack only  , the amp is dead quiet   even on   max volume , no issues other than the pic attack  fizz  . the fizz  is not on the decay . tried different  value coupling  caps . I have  checked and rechecked solder connections ,  metered all resistor values  , ppimv and cascading was removed and put to stock . I have merren audios  pwr transformer here that i didnt use    because it only   had 460/500/540 v taps ....I am questioning the filter cap values     there a 2-seies  jj caps 390uf  with no balancing resistors , 2-100uf   with balancing resistors , and a 50uf in the pre amp.... is it possible that the 2-390uf  caps in series is high .  there is a stiffness to the  overall sound  of the amp but  to be honest never actually heard this amp without  the fizz note distortion .     would high filtering    and no balancing resistors on the caps warrant this fizz issue ?

[marshall]

Is your fizz on the string attack or decay?




yes its on pic attack , at most  volume levels , amp is dead quiet  otherwise , the kt88's handle the 600v   n/p    no red plating    i tried high bias as high as  50mv and still ok  , my normal bias is setting  around 25/28mv.  phase inverter tube   is a 12au7 , an at7 seems to make the amp  to gainey , with some funky  noises  .    I'm using the 8 ohm spk tap for the neg feedback   , tried the tap  on 16/4 ohms as well  ,    tried   coupling  cap values [.1/.05/.02]... stayed with the astron .05   .......merren audio has built the ul output tranny for other people as well       with no issues , so it is  unlikely its the o/t   .    have an email    sent to him                                                                                                                       

[sluckey]

I am questioning the filter cap values     there a 2-seies  jj caps 390uf  with no ballanging resistors

The Major model 1967 uses two 375µF caps in series and they don't require balancing resistors because the HT center tap is connected to the junction of the two caps. This arrangement forces equal voltage across each cap. Those 390 caps are fine.

[marshall]

im using 100 watt pwr transformer      2-kt88s

[marshall]

just got this schematic with new updates ,     as per micheal r/t . not  sure if  moratto used this on my build . no longer dealing with him because of this build . 

[pdf64]

Fizz often indicates instability, especially at higher frequencies.
The amp's design looks poor, as the tone controls are within the NFB loop, hence will introduce variable degrees of phase shift within the loop depending on the particular settings.
The shared, unbypassed cathode resistor at the driver stage may present opportunity for a local positive feedback path to manifest when the plate currents are not well balanced, eg during overdrive, poor balanced sections.

I suggest to fully bypass the driver stage cathode (eg 100uF), and to add grid stoppers (eg 10k), mounted at the socket terminal, to every stage where they're not currently present, and then re-appraise the issue.

[Ed_Chambley]

I am questioning the filter cap values     there a 2-seies  jj caps 390uf  with no ballanging resistors

The Major model 1967 uses two 375µF caps in series and they don't require balancing resistors because the HT center tap is connected to the junction of the two caps. This arrangement forces equal voltage across each cap. Those 390 caps are fine.

sure if it is wired that way. if it is a can.

[marshall]

dropped it off the church audio , getting the transformers  scoped , and checking  the circut l design build .    updates to follow in a few days

[Ritchie200]

One thing you might try is another NOS 12AU7.  Or I should yell NOS!!!!  New 12AU7's suck in my amp, it has to be NOS.  That tube is used to swing the big current on the original Major's 4 KT88's and it gets worked hard if you are pushing them.  Not sure how hard it is working in your amp but it wouldn't hurt as that is an easy switch.   You said the AT7 was gainy with weird noises, but did it still have the attack fizz?  I've read where people supposedly plop a 12AX7 in that position and claim it turns the amp into a blazing Ritchie Blackmore special - they are lying.  It just thins it out at high volume.  You need the 12AU7 to handle the current. 

If that doesn't work, you need to put one hand in your pocket and give us voltages.  There used to be a link somewhere to a voltage chart that you can use and fill in, maybe someone can provide it.  I'm sure it's right under my nose.....

[shooter]

New 12AU7's suck in my amp

I use AU's in most builds, the 1st 10+hrs I get upset and think, this time I'm sending them back, but as they get older least the last 4 came into their own

[Ritchie200]

In regards to the flyback conditions in UL circuits (In particular the Major) question:  When a cascaded gain stage, booster, or fuzz is used to cause distortion on the preamp side at HIGH volume, it causes the 600V of B+ to overshoot to 1800V.  Why?  When the square wave hits the output tranny it starts switching like a flyback transformer in an old TV.  The UL design does not play well with distortion combined with the high voltages.  These voltages smacking the crappy Dagnall OP tranny and poof the tranny is smoke.  Or you get arcing between pins 2 and 3 of the KT88's.  Or inside the tube itself. Higher quality tranny's can do a better job of absorbing these transient spikes.  There was an old guy in New York that custom wound repair replacements back in the 70's for the Major by the name of Otto.  These are prized as being absolutely bulletproof in the Major and what was in all of Ritchie's amps.

Jim

[Ritchie200]

New 12AU7's suck in my amp

I use AU's in most builds, the 1st 10+hrs I get upset and think, this time I'm sending them back, but as they get older least the last 4 came into their own

I have a load of old ones and I happened to put a newer Russian one in a few years ago and didn't think anything of it.  Worked fine until I cranked it to do some recording and poof...nothing.  Put in another one and it suddenly sounded horrid and tested bad.  Mentioned it to Dawk as I was praying it wasn't the OP tranny.  He basically rapped my knuckles with his knowledge ruler and said "I told you how many times to use NOS in the PI/driver? That tube gets worked too hard in that amp for the new crap they are making these days. First two positions, use anything that sounds good."  I think I mumbled something about being hard of hearing to try and maintain my dignity (didn't work).

Jim

[pdf64]

...distortion on the preamp side at HIGH volume, it causes the 600V of B+ to overshoot to 1800V.  Why?  When the square wave hits the output tranny it starts switching like a flyback transformer in an old TV...

Have you tried a x100 probe on the power tube plate of a sqarewaving power amp, driving an inductive load?
I get more than that with my AC30
Hence the spikes with a 600V HT and a poorly interleaved OT would likely be fearsome

...The UL design does not play well with distortion combined with the high voltages...

I don't get why that should be the case?
I see the root cause of the issue as being that the OT was low spec with poor parasitic characteristics, and the UL arrangement would act to alleviate the problems that caused.
Such that if the UL taps were isolated and the amp configured as a regular pentode, the flyback from pushing a full power square wave into the inductive load presented by a speaker cab would be even worse, and the amp yet more prone to failure.
Note that a 4k primary looks to be a little under the value provided in the tube info for fixed bias UL at 560V HT http://www.mif.pg.gda.pl/homepages/frank/sheets/084/k/KT88.pdf

[DummyLoad]

New 12AU7's suck in my amp

I use AU's in most builds, the 1st 10+hrs I get upset and think, this time I'm sending them back, but as they get older least the last 4 came into their own

I have a load of old ones and I happened to put a newer Russian one in a few years ago and didn't think anything of it.  Worked fine until I cranked it to do some recording and poof...nothing.  Put in another one and it suddenly sounded horrid and tested bad.  Mentioned it to Dawk as I was praying it wasn't the OP tranny.  He basically rapped my knuckles with his knowledge ruler and said "I told you how many times to use NOS in the PI/driver? That tube gets worked too hard in that amp for the new crap they are making these days. First two positions, use anything that sounds good."  I think I mumbled something about being hard of hearing to try and maintain my dignity (didn't work).

Jim

seems like a 12BH7 or 6CG7 would work much better there. also, you could try the JJ ECC99 - bet it works well...  ;-)


--pete

[Ed_Chambley]

New 12AU7's suck in my amp

I use AU's in most builds, the 1st 10+hrs I get upset and think, this time I'm sending them back, but as they get older least the last 4 came into their own

I have a load of old ones and I happened to put a newer Russian one in a few years ago and didn't think anything of it.  Worked fine until I cranked it to do some recording and poof...nothing.  Put in another one and it suddenly sounded horrid and tested bad.  Mentioned it to Dawk as I was praying it wasn't the OP tranny.  He basically rapped my knuckles with his knowledge ruler and said "I told you how many times to use NOS in the PI/driver? That tube gets worked too hard in that amp for the new crap they are making these days. First two positions, use anything that sounds good."  I think I mumbled something about being hard of hearing to try and maintain my dignity (didn't work).

Jim

Clear top RCA! 12au7. Not all UL have fly back sir. In the Major, when you hit the front hard the whole circuit gains up. True, 1800 volts, but if you have seen one catch fire, you gotta consider the amperage contribution.


I have never seen a UL HIFI amp if designed and built correctly get toasted. I had Dynaco amps for years and you know Sabbath was wide open.


I hate it when you are wrong. Don't you?


Burning down the house that was made in Japan and killed my woman from Tokyo.

[Ritchie200]

"I have never seen a UL HIFI amp if designed and built correctly get toasted. I had Dynaco amps for years and you know Sabbath was wide open."

I believe that is exactly what I said.  However, "Mel Tillis does Gospel" is not the equivalent of Tony Iomi.  That's a different Sabbath.  You are Marie trying desperately to be Donnie.  Get rid of that goofy shaped guitar if you want to be taken seriously here Clem.

Jim - I'm ALWAYS right

[Ed_Chambley]

"I have never seen a UL HIFI amp if designed and built correctly get toasted. I had Dynaco amps for years and you know Sabbath was wide open."

I believe that is exactly what I said.  However, "Mel Tillis does Gospel" is not the equivalent of Tony Iomi.  That's a different Sabbath.  You are Marie trying desperately to be Donnie.  Get rid of that goofy shaped guitar if you want to be taken seriously here Clem.

Jim - I'm ALWAYS right

your mind is getting slow with your aging metal mentality. I expected an intelligent response and Mel Tillis is the best you can do. Elvis sung gospel, lots of it. I did a rock version of a or Gospel standard last night. Tempting to insult me with inference to God certainly limits one's humility. I really do not care the coca cola cowboy, but he did have more success in music than you or I have.


So, I guess you may have insulted yourself.


Now that is a reply of correctly functional thought process

[marshall]

pre tubes are brimar in v1/brimarv2 / jan   b/n nos  12au7  in P/I .changed them out with   phillips minwatt in v1/v2/  and a brimar 12au7 in P/I... still did exact same thing .  the amp is dead quiet   other than this note fizz .   Hoping to hear back from church audio this week  on his bench test  of transformers  and the circuit build/layout for issues .   

[Ritchie200]

Marshall

Does the fizz only start to happen at a certain volume?  All the time?

Jim

[marshall]

all the time  at all volumes . emailed  chris merren  about the issue to  see if its possible its the    UL O/T   .   He mentioned that the pre  tubes  could possibly  biased  to cold   causing the fizz .