Need help with PT specs for Marshall Plexi/800 Build

[Mike_J]

I am in the process of starting my Marshall Plexi/800 project that was inspired by sluckey's post related to the same.  The project is essentially sluckey's idea with relays added to switch between the 800 MV and an added PPIMV for the Plexi.  It also switches between the Plexi and 800 preamps and switches in and out the .68uF on V3a.  (Note:  This would be V2a on a normal Marshall schematic)

What I would like is some input on the power transformer specifications below.  I am having Heyboer custom make the transformer for me for the project.  They are also making me their version of the Plexi era Drake output transformer and choke.

The 100 watt transformer has the following specifications:
M-50 steel
Shield
173-145-0-145-173 @ 400mA HT (approximately 406 and 485 VDC)
0 - 100V bias tap
6.3 VCT @ 8A for tube heaters and on-off LED
6.3 VCT @ 1A for relays and footswitch LEDs

I still have a few days to make changes before the PT will be made.  I have a Dumble SSS clone for my cleans.  I mainly want this amp for the Marshall overdriven sound.

  http://home.comcast.net/~seluckey/amps/misc/Marshall_Dual_50.pdf

[Mike_J]

Help I am looking for is primarily related to two things.  One (1) I want the mA rating on the HT tap to be as low as possible to maximize sag.  However, I do not want the transformer to exceed 130 degrees Fahrenheit so it will last and not damage other components.  Two (2) given my objective of having mostly an overdrive amp the maximum B+ this transformer will generate in the circuit is 485 VDC.  Can anyone make an argument as to why I would benefit from more B+?

[HotBluePlates]

... I want the mA rating on the HT tap to be as low as possible to maximize sag.  However, I do not want the transformer to exceed 130 degrees Fahrenheit so it will last and not damage other components. ...

I don't think you really want to attempt to tax the transformer's current capability for sag.


You're already going to be using a solid-state rectifier (I presume), so you won't be getting sag from that. You can make the screen resistors bigger than 1kΩ to induce sag. Something like 3.3kΩ (more or less) seems to work for many people to create the effect. Ideally, you'd experiment to find the correct value for your taste.


Sag, whether due to power transformer voltage droop, rectifier impedance/voltage drop or high screen resistance, occurs when you're pushing the amp's maximum power output. So be aware that no matter what method you use to get there, it won't happen until you've got that 100w beast turned wide open (or very close).


You could screen resistors switchable for your amp to select either 1kΩ or the higher value you select so you're not stuck with one sound.


Oh... and I'm avoiding answering the exact current draw for a 100w Marshall to induce sag because it's a very involved calculation. That, or someone would have had to choose to get the sag effect the same way (or accidentally used an undersized transformer) and have experimental results. Either way, the screen resistor approach is very much easier to figure out, use and reverse if need be.

[Mike_J]

Thanks for the response HotBluePlates.  So do you have any opinion about mA rating for the Plexi HT (for example would you say bigger is better)?  The problem I see with that is it gets you into Hiwatt territory which is where I don't want to be.  I have always liked Jimmy Page's tone over Pete Townsend's and from my little bit of research on the subject the beefed up PT in the Hiwatt has something to do with the difference in tone.   Also, could you please provide your opinion regarding M-50 versus M-6 iron as far as tone is concerned?  Seems like M-6 would keep the PT cooler and also have more head room.  I was leaning towards M-50 because I am not looking for a lot of head room.

You are right that a power transformer is an expensive place to experiment.

[HotBluePlates]

I'll have to defer to Marshall aficionado's to answer most of your questions. Hopefully Ritchie200 will chime in, as well as others.


I say that because while I once had a '73 50w Marshall (the "Model 1987" JMP Mk2), I haven't dabbled with them much. The 100w head is bigger than I could ever see myself playing.

...  I have always liked Jimmy Page's tone over Pete Townsend's and from my little bit of research on the subject the beefed up PT in the Hiwatt has something to do with the difference in tone.  ...

Remember Page used a LOT of things you might not associate with his sound at first.


But I think the Hiwatt is an entirely different amp, and its sound is more than just a power transformer. The Fane speakers are a big component of the sound.


Funny enough, at the same time I had the 50w Marshall, I had an early-70's DR504. I played those amps (and a Matchless Clubman) through a (maybe late-60's) basketweave 100w cabinet with G12H30's. Each of the amps sound similar enough when clean through that cabinet, though they each had their own quirks for controls and their own voice when cranked up.


I'd guess the speakers and output transformer, along with the overall amp circuit, will make the biggest difference in overall tone. Sorry, but I just lack the experience with enough Marshall amps to have an opinion about the different power transformers (and mainly output voltages) Marshall used in their amps over the years.

Also, could you please provide your opinion regarding M-50 versus M-6 iron as far as tone is concerned?  Seems like M-6 would keep the PT cooler and also have more head room.

Mostly Greek to me... you'd have to ask the transformer builder.


My understanding is the lower the "M-number" the lower loss in the core. The transformer designer is going to be thinking from the standpoint of "power throughput" and designing the core size based on the power needed and the core material used. So I'd bet if they used a lossier material, they'd make the stack bigger to compensate. In other words, the M-50 100w PT may just be a bigger lump of iron than a M-6 100w PT, as the manufacturer adjusts for the increased loss.


I would think that I'd want the better, lighter M-6 core... but again, I'd have to defer to those who've built and tinkered with this particular style of amp more than me.


Reiterating an earlier point, I'd think the output transformer would have vastly more impact on the final sound than the power transformer, assuming the power transformer isn't grossly under-sized (like trying to make a 100w amp with a 20w power transformer).

...  I was leaning towards M-50 because I am not looking for a lot of head room. ...

But you're gonna build a 100w amp?    You play stadiums a lot?


I saw Eric Johnson at a club in Nashville in the late 90's. Most of his amps were off the stage (or under the stage, I'm not sure which), mainly so he didn't kill people in the audience with the cranked Marshall for his lead tones.


Keep in mind Sluckey's plan has a master volume, which you'd dial down for preamp distortion. If you were gonna use the amp like my old 50w without a master volume, you'd have to crank the amp way up. Will that wind up too loud for you to use? The folks who've built Trainwreck clones (50w amps) talking about how brutally loud they are is what comes to mind for me.


Then again, I don't know your particular situation or needs.

[Mike_J]

I am making a 100 watt amp because I have two unused quads of EL34s and I can always remove the outside tubes for fifty watts of power.  If I am ever asked to play in a stadium then I can plug the tubes back in an I am all set to go.

HotBluePlates, you are right from what I have read the M-50 transformers are made thicker to make up for the losses.  It is my understanding the Drake 1203-80 used in late 1968 was thicker than the Dagnall T2562 which Marshall used in their 100 watt amps from 1968 through 1973.  The difference in the thickness is because like you said the Dagnall used better steel.  The reason I asked is because one of the big suppliers of the Drake 1203-80 clones uses the M-6 iron in their transformer but I think I will use M-50 iron because the original used the lower grade steel.  I agree the OT and speakers will make a much bigger difference in tone versus the power transformer but I get enjoyment out of learning about how things were made.

I decided to make the HT 400mA because Magnetic Components makes a Plexi PT with a HT of 290mA.  Some people have commented that it runs hot.  The company that uses the M-6 iron has their transformers wound at 450mA.  I chose 400mA because it is in the middle and has enough additional beef that it shouldn't have the heat problem. 

I am using the Drake 1203-80 PT and Drake 1203-132 combo because it supposedly sounds better with Strats which I play most of the time.

[Mike_J]

Correction: The Drake 1203-80 PT was used in late 1967.

[HotBluePlates]

It is my understanding the Drake 1203-80 used in late 1968 was thicker than the Dagnall T2562 which Marshall used in their 100 watt amps from 1968 through 1973.  The difference in the thickness is because like you said the Dagnall used better steel.  The reason I asked is because one of the big suppliers of the Drake 1203-80 clones uses the M-6 iron in their transformer but I think I will use M-50 iron because the original used the lower grade steel.

The transformer guys know what makes their stuff tick, and some of the companies do remarkable reproductions of vintage transformers, even if they use new materials. I'd tend to trust their judgement. Then again, what you're saying is not without merit.

I decided to make the HT 400mA because Magnetic Components makes a Plexi PT with a HT of 290mA.  Some people have commented that it runs hot. 

Well, it is sitting right next to a 100w space heater! 


FWIW, Heyboer seems to rate their 100w Marshall PT at ~300mA as well. Extra current draw may raise temp some, but really it mostly lowers output voltage. But using a transformer with extra capacity is fine.

I am making a 100 watt amp because I have two unused quads of EL34s and I can always remove the outside tubes for fifty watts of power.  If I am ever asked to play in a stadium then I can plug the tubes back in an I am all set to go.

This makes the whole issue of PT current moot. With only 2 EL34's, you're never gonna draw 300mA, so PT temp rise is a non-issue. So you could buy whichever is the cheaper 100w PT as you're only gonna use it to deliver 50w most of the time. If you run with 2 EL34's, voltage sag is also never gonna happen with a 100w PT, regardless of core material.


Otherwise, I'm sure you realize that you're stating conflicting desires for the transformer, right? It might run hot at ~300mA, so you want to uprate to 400mA. But then you want a lossy core material so the transformer will sag. And actually losses in the core are what amount to heating (just like power lost in a resistor is the heat it radiates into the air).


But I've been guilty of over-thinking amp stuff, and using tweak parts just because I thought they might confer an advantage. My experience has been that most of the time, such parts cost 10x more for a 10% improvement (or even no improvement). So even if you wind up finding out that worrying about certain parts of your amp wasn't worthwhile, the experience will be worth it.

[HotBluePlates]

You know, I just read your post on the "Which 2 amps" thread...


It seems to me you have plenty of build experience to know what you want. Please view my comments as "thinking out loud, but with another person" more than me suggesting you shouldn't do something.


I think maybe you were spit-balling some ideas and hopefully I've helped you think through those a bit.


HBP

[Mike_J]

HotBluePlates

You comments and a little research on my own have made me confident in the decision I made in how to have Heyboer custom build my power transformer for my Plexi/800 build.  I appreciate your help as I needed a sounding board.  Their is no specific winding information on the net concerning the Plexi era transformers and I wouldn't expect anyone who has reversed engineered them and builds them for a living to share the information.

Thanks again
Mike