MARSHALL MINOR 2026

[TIMBO]

Hi guys, The local electronics store (Jaycar) were having a clearout of old stock.
The power transformers were used in SS amplifiers and are now obsolete.
The MM2026 35v-0-35v 175VA sold for $4 and the MM2027 47v-0-47v sold for $8
So I cleaned out their remaining stock for $40
I have used the MM2026 here
https://el34world.com/Forum/index.php?topic=16656.msg165574#msg165574
Thanks to Tubenit and Dummyload for producing a great schematic that fits the PT perfectly.



Another recycled chassis with the right valve lineup.............

[tubenit]

I will follow this build with great interest!  Thanks for sharing the project.  As far as I know, no one has built this one yet?

I have a favor to ask, if convenient for you ………………………

When it's all built and tweaked,  could you please do a recording of the amp?  I'd love to hear how the thing sounds?

You continue to build some remarkably cool designs/amps.  Bravo!

With respect, Tubenit

[Ritchie200]

TIMBO!

Yes, other than the original (pirated from Tubenit and DL) version from another site..... that was voiced to sound like EVH?  Still don't get that....  Anyway, I think this is the first build?

I am interested as well!  That was about a 6 month thread with a lot of great input.  Glad someone took the reins to this wonderful amp.  Crank it and spank those KT88's!

Jim

[silverfox]

So what is the output of the voltage doublers** ?silverfox.

[TIMBO]

Should hit around the 380v

[shooter]

what is the output

was wondering the same, but did find in Tim's link 400vdc.  I was guessing 360 -400, but can't wrap my brain on the current, my SE 88 is eating 104mA at idle, ~380vdc, so I was guessing 2 PP deep AB would be ~~ the same 

[DummyLoad]

ideal is 395V. (70V rms * 1.414 * 4 = 395V) add the diode drops - about another 4V +/-  so call it 390V ideal, give or take...there are other losses.

sim hints at between 365V to 350V loaded with 100mA to 150mA respectively.

IMO the 94Vrms $8 part makes better numbers for KT88.

+1 on the sound clip(s) when dialed in. 

--pete

[TIMBO]

Thanks guys, Heater supply and quadrupler PCB in place.


I'm getting 412v unloaded, wall mains are setting at about average.

[TIMBO]

Hi guys, A couple of progress pics.




Just waiting on a couple of components and we're good to go.

[tubenit]

Great job!  The point to point wiring is pretty cool.  Really looking forward to hearing a review of the amp and hopefully sound clips!

THANKS for sharing this project!  Will continue to follow this with great interest.

with respect, Tubenit

[TIMBO]

Hi guys, No problems on start up.
Haven't had a chance to crank just yet but does sound very marshally.

Very happy with the voltages from the MM2026 and think some 6L6s would be a treat.
This datasheet

Shows some similar voltages and proposed power output.
But when I use the bias calculator https://robrobinette.com/Tube_Bias_Calculator.htm
The KT88s are not running at max plate dissipation, which I thought that cathode bias was all about.

I need some educating. :yep

[shooter]

I need some educating.

cathode/self biasing is a frustrating balance between Rk, B+, and tube specs.  lower Rk, current will increase, but that will drag plate volts down some, so then increase plate volts.
A smaller Rk will affect headroom, usually lowering it, think of Vk as "headroom", the lower it is the sooner you start bending the PA tubes

[DummyLoad]

The KT88s are not running at max plate dissipation, which I thought that cathode bias was all about.

not running in class AB1 - should be 60-70% of max. you're maybe thinking of class A P-P or class A single ended. for class A P-P you'd be running with a lower B+. 

--pete

[PRR]

In self-bias, to get the most WATTS out of your tubes, you do want to run near 90%-99% idle dissipation.

BUT the KT88 is a BIG tube. And UL mode suggests Hi-Fi. Hi-Fi guys can't change their own tubes. And a 100W fix-bias amp is a BEAST. More than most hi-fi guys needed back in the day. So the suggestion is for long trouble-free life and 30W out at a safe 68% Pdiss, or 50W out at 90% Pdiss; designer's choice.

[TIMBO]

Thanks guys.
As per https://robrobinette.com/Tube_Bias_Calculator.htm
406ohm (II 820/820ohm)resistor=62.9%
363ohm (II 1000/560ohm)resistor=69.5%

[DummyLoad]

Thanks guys.
As per https://robrobinette.com/Tube_Bias_Calculator.htm
406ohm (II 820/820ohm)resistor=62.9%
363ohm (II 1000/560ohm)resistor=69.5%

maybe try in II: 1k|1K|820R = 310R - should get around 75%.


--pete

[TIMBO]

Thanks pete, Any particular reason for pushing the extra 5%

[DummyLoad]

maybe might sound better? others have stated KT88s in UL mode sound better pushed hard.i were i to experiment with that ckt., i'd up the B+ and push them 80-85% as a compromise.


--pete

[TIMBO]

Thanks mate.
At this stage it is a bit of trial and error.
If the there was a relay to switch out V1b I think the amp would be a bit bland and the power amp would need to be pushed to get the grind.
There is a much better voicing by blending the two triodes.
So the relay is on hold for the moment.
BUT, a clean sound is a little hard to achieve.
Removed the 250uf and this cleans it up a bit, but just seams to loose a bit of the MOJO.

OH YEAH.........

I forgot to mention......

LOUD VERY LOUD

[shooter]

a clean sound is a little hard to achieve.

did you try an AU in V1 to knock out some gain
I can't see where you can get more B+, but that would help get you more clean swing

[TIMBO]

Hi guys, Finally got around to making a cab for this one....



Just got to work on a faceplate.

[TIMBO]

A couple of extra pics..

[DummyLoad]

very nice! how does it sound?


--pete

[Ritchie200]

a clean sound is a little hard to achieve.

did you try an AU in V1 to knock out some gain
I can't see where you can get more B+, but that would help get you more clean swing

What you have here is the equivalent of an EL34 running at 200v B+.  I know, I know, that's what the data sheets recommend.  However, why would you (not you, the data sheet folks!) use such an expensive tube for that wattage when others would be cheaper all around for the same performance?  I never understood what I would consider starved performance for that tube - like putting an old Rochester one barrel carburetor on a big block Ford.  I would imagine anything you do on the preamp side will just sound thin? The secret sauce of the Major is that balance of the preamp grind just pushing the KT88's into distortion.  You are just shy of 60 watts on tubes that would be happy pushing 100.  If you like the current grind on the KT88's that's great, but if you want that clean chimey headroom the Major is known for, you may have to find a few more volts.  Of course it will not be any quieter.

That cabinet is TOO cool!!!!    Cant wait to see the faceplate!  Wow!

Oh and you did NOT say the nasty "L" word...  If you put lifeless 6L6's in this wonderful amp I will never talk to Silvergun ever again!!!!!

Jim

[Ritchie200]

PS, not sure how much heat you will have but the Major was open in the back and had a vent on top as well.  Like SG makes fun of, yes it can heat a small room...  Maybe just keep an eye on the temps in there?

Jim

[Ritchie200]

Woah, wait just a minute.  Voltage is WAY down on the preamp side. Compared to the original Major we have:

Minor
        1       6
V1   147   201
V2   235   173
V3   216   206

Major
        1       6
V1  190    235
V2  210    300
V3  310    310

Are your sure the distortion is coming from the KT88's?

Jim

[TIMBO]

Hey Jim, Thanks for that info.
I don't have a enough sound proofing in my shed to max it out.
I'm waiting for the local guys to arrange a gear day so I can let it loose.

What am I going get by increasing the preamp voltages.

[Ritchie200]

With the lower B+ throughout, I'm not really sure how all this is interacting.  However, you would typically get more headroom.  You mentioned that it is very Marshally.  The Major was never "Marshally", at ANY setting.  It is loud and clear with a low end that would disembowel.  Your preamp voltages are more typical of the 50 and 100 watt models - models with a very distinct sound.  I am painting with a broad brush with my assumptions here.  I am not even sure of the voicing differences between the two.  Maybe some of the more experienced Marshall guys will jump in here.  I know Tubenit did a TON of work on that schematic, maybe he can weigh in on the differences.  If reproduced faithfully, you should not start hearing any grind until about 7-8 and very little at that.

Jim

ps, sorry I would do some research but I am buried in work and just do not have the time.

[TIMBO]

All good thanks mate.
I'll get the cab finished and I'll work on a clip with the circuit as is.

[uki]

Wow Timbo always come up with the cool stuff!!

I'm very curios about the power supply! Why everyone uses big irons instead of that? How the circuit increase the voltage from 35v to 380v ? Impressive !!

[TIMBO]

Thanks Uki, The PT has a 35v-0-35v secondary, so dropping the CT for 70v. This is put through a QUADROUPLER.

When I get a chance I'll revisit this project and do a bit more tweaking.

[uki]

Quadroupler?! That is two doublers yes?

I'm working on similar amp, check it out: KT88 PP amp

[TIMBO]

Hi guys, Got some time on my hands to FINISH a few builds.
Thought it would be a good idea to make the "one wire mod" a bit more usable by adding a foot switch.


A simple PSU tapped off the heater supply to power the relay.
Makes much easier to get cleans and OD.
Face plate is on order and this one will be done.

[TIMBO]

Guys, I was not totally happy with the preamp voltages as the PT does not deliver the same HT as the original Marshall would.
So a little tweak of the dropping resistors has helped.
Unfortunately the wall voltage has dropped a bit which is not good for the power amp.


Any advantage in making a fixed bias?

[shooter]

don't see fixed bias helping, the post MV i believe will need caps.  you might get a small volume bump, but don't believe enough to matter.
have any KT66's laying around, possibly 6V6 to experiment with?  might have to tweak speakers n bias though

[TIMBO]

Thanks shooter, I'll be taking the amp to a mates place today for a demo.
I'll get it crank'n and see what the volume is like.

[TIMBO]

Hi guys, I can confirm that the drop in volts has done no harm.
Sounds great and still plenty loud, maybe 30w.
Stunt guitarist gave it the nod, so sound clip soon.

[TIMBO]

Hi guys, Just wanting to address another little problem.
The valves heat when the power switch is on.
I would generally wait 10-15 sec before throwing the standby switch.
A hum is heard for for about 5 sec then it will subside and disappear.
I would think it's the KT88s powering up.
I have put a 100k bleed resistor across the standby switch to energize the filter caps.
This didn't help.

[shooter]

if you want to get some readings;
put your meter on Tap A to ground.;
with standby OPEN measure VDC, then measure VAC
close standby and re-measure

[sluckey]

I don't think it's of any concern. Very possible that one tube heats up a bit faster than the other. During this time only one tube is conducting and there will be no common mode hum cancelling going on. When the other tube catches up the hum is cancelled. Just some theory but it sounds good to me.   

[TIMBO]

Hi guys, The minor is back on the bench for some more tweaking and a faceplate.
As per Jim's voltages on reply#25, my preamp is running a little low.
It seems different parts of where you live here have different wall voltage, atm the moment it's around 245v which is good as the preamp voltage are close to Jim's.
But the phase inverter is about 100v lower.....
Would changing the plate resistors to as low as 22k be of help...

[tubenit]

Remember with a 12AU7, the plate voltages will be lower.  Have you tried a 12AT7 or 12AX7 in that position and if so, what were those voltages?  And how did it sound with a 12AT7?

And yes, I think you can try a 22k in the plate resistors. You may need to change the cathode resistor also?

With respect, Tubenit

[TIMBO]

Hi T, Thanks for the input.
The 12AU7 does sound the best with the higher voltage, approx. 260v on the plates.

[TIMBO]

Hi guys, Got one of the local guys trying to calculate the operating point on a loadline chart.... it's a little over our heads...
So tweaking some resistors to 47k plate and 3.3k cathode, I now get 265v on the plates and 14.4v on the cathode.
I can only guess the operating point could be close to the Major?????

[shooter]

no clue what the Major is
my crayons came up with 14.4vdc / 3.3k ohms ~= 4mA so each 1/2 "sees" ~ 2mA at idle

[TIMBO]

Hey shooter, I guess what I trying to do is to mimic the Marshall Major 12AU7 operating point but at a lower voltage.
If this can be done with adjusting the plate and cathode resistors.
  

[shooter]

We're all old now, just tell the young'ns, "that's what a Marshall sounded like in the way back days" 
the worlds just making History up as we go, so..................... 

[TIMBO]

Hi guys, Tweaked a few things and i have lined up a mate to put it to the test.
I would love to be able to have more tech savvy to be able to fine tune these builds.
So I'll leave it up to the EAR to make a call

[shooter]

I'll leave it up to the EAR to make a call

That's the "final", a Tech can get close, but it's the guitarist that gets it across the finish line

[TIMBO]

Some pics..