MkIIC+ inspired preamp.. DIYLC layout drafting

[owlexifry]

I was wondering if you had the Tip and Sleeve swapped. A guitar or transformer coupled device might still work, but most things won’t. Glad you got it sorted. Thanks for the sound clip BTW—very cool. Nice to see it all come together. I’m happy for you. I’m pretty sure you’ll figure out what to do on the reversed MV.

thanks so much for your help. i am in your debt.
had a real good session with it tonight after a few changes......

so tonight i addressed the MV wiring thingo. that's all sorted.

but after having a go with the TMB controls (and I know these are weird if you're not used to them), I wasn't quite sure about the wiring around the mid and bass controls in my layouts above...

especially after i had a look at a layout of a twin reverb that has the exact same tonestack (if you ignore the treble caps)

MK2C+ twin reverb


twin reverb

the layouts go in different directions, but you can see here that what i've done is different.

- above, the bass and mid pots have nothing connected to lug 3.
- my initial layouts have lug 3 connected to lug 2 on bass and mid pots.

so here's what i did:

old layout with corrections annotated:



updated layout:



and after making these changes, and playing with the TMB controls (just to confuse myself more), I'm still really not sure if the old layout was correct, or if the updated/current layout is, and im wondering if there was any difference

serious question, does it even make a difference if lug 3 + 2 are connected on the bass and mid controls?

have i got it right?

apologies for the dumb questions.

 

[andrewRneumann]

It looks pretty good, but I might be missing something. Are the controls not behaving the way you would expect? What are your symptoms?

You don’t have to jumper the extra lug when in variable resistor mode with a pot.

 

[owlexifry]

You don’t have to jumper the extra lug when in variable resistor mode with a pot.

thanks - i think this was the answer i was looking for
(i often get confused translating pots in schematics to layouts)

it would then appear that it probably made no difference (whether lug 3 is jumpered to lug 2 or not -> on the bass + mid pots)

Are the controls not behaving the way you would expect? What are your symptoms?

honestly I'm not really sure what to expect with these TMB controls.
I've never played a mesa or a mark series.
i've only ever played amps with tonestacks at the end of the preamp (eg. marshalls, peavey 5150/6505, SLO), but as we know, these mesa mark circuits have the tonestack at the front (after 1st stage), so it's hard to tell if what i'm observing is 'normal'..

the treble control is obvious.
the bass and mid controls aren't very obvious / perceptible (mild differences) but maybe that's normal when there's a ludicrous amount of gain/overdrive available (this thing is nuts, has even more drive/grunt than the SLO thing i did previously).

i guess there's a good reason why they came with a GEQ... (next thing i need to build)

 

[owlexifry]

made some changes.

the author of the DIY schematic laid out the tonestack so that the 'treble shift' control is left in the 'pulled out' position (750pF added in parallel to 250pF on the original), with a 1000pF treble cap.
initially, for this build, i thought this 1000pF cap would be the ideal setting and I wouldn't need or want the switching option.

then i started playing with tonestack calculators...

graphs below have the same settings (T-8, M-7, B-1):


1000pF treble cap




250pF treble cap


then i considered whether it would be possible/sensible to retroactively install another switch...


managed to squeeze it in.





before



after


not the prettiest outcome
but i guess it feels better to have the treble shift option added in. makes quite a difference.

hopefully should look a bit tidier when i get some more black dymo tape and redo the labels (only got blue for now)

here's a 'final'/updated layout for anyone interested:

 

[owlexifry]

few updates:

- redid the treble shift circuit with a new switch, different caps, and shielded cable this time

- new labels


built the 5 band EQ:
https://www.tdpri.com/threads/mesa-boogie-inspired-5-band-gyrator-eq-vero-strip.1089433/

and here’s a demo:
(note: bad chip in EQ at the moment)

 

[owlexifry]

the 5-band EQ is now running on a good TC1044SCPA chip instead of the bad/noisy/fake ‘ICL7660S’ that was previously there.

so here’s another short demo, but this time through a power amp (4xEL34) + real 4x12, recorded on iphone sitting roughly 30-45cm from the cab (instead of into DAW + cab sims)

 

[Ocirnelooc]

Very cool project. I've been looking at it, but in your turret picture, it represents a 47uF orange drop, but according to the schematic the value should be 47nF. I assume this is a bug in your layout? I couldn't find anywhere in this thread that this has been changed.
I also find the value of the 547pF curious. At least I can't find any silver mica with this value. In the original this is a 47pF parallel with a 680K resistor. Here it is a 547pF parallel with an 87K resistor.
I see that the whole value selection of resistors and capacitors between V2 and V3 has been changed from the original MKIIC+ schematic.
Bancika had also done this in his first version. But in his 2nd version, where he could switch between Lead and Rhythm, he kept the original values again.
Do you have any idea what this could affect? And what is the reason for the change in values?

 

[owlexifry]

Very cool project. I've been looking at it, but in your turret picture, it represents a 47uF orange drop, but according to the schematic the value should be 47nF. I assume this is a bug in your layout?

cheers!
correct- thanks for finding a typo.
i definitely used 47nf.

the value of the 547pF curious. At least I can't find any silver mica with this value. In the original this is a 47pF parallel with a 680K resistor. Here it is a 547pF parallel with an 87K resistor.
I see that the whole value selection of resistors and capacitors between V2 and V3 has been changed from the original MKIIC+ schematic.

looking at the mkiic+ schematic, on the output of the ‘lead circuit’ there is a resistor and cap pair, to ground (100K+500pF), then when this lead circuit mixes back in, it meets another resistor+cap pair to ground (680K+47pf).
these pairs are in parallel, and exist on each side of where lead switching cuts off (between v2, v3, as you say)
since this DIY design has no lead channel switching, these pairs can be ‘combined’.
the two resistors (680K, 100K) in parallel equate to a single value of 87K. so i guess 82k was chosen as the closest value.
547pf doesn’t exist, so i used 500pf+47pf in parallel.
(caps in parallel are additive).

Bancika had also done this in his first version. But in his 2nd version, where he could switch between Lead and Rhythm, he kept the original values again.
Do you have any idea what this could affect? And what is the reason for the change in values?

his second version includes switching between lead and rhythm modes, which means switching out that ‘lead circuit’, which means those two resistor+cap pairs that i mentioned earlier, cannot be ‘combined’. they need to be seperate as per the original.
in this version, where there is no switching, these pairs can be combined to a single equivalent value, which is why you see the 82K and 547pf values.

it’s a good question, i queried this as well… eventually figured it out after staring at schematics long enough haha

 

[Ocirnelooc]

Sorry for the late response. But your explanation is very enlightening and now I understand how you arrive at those values.
I wonder why you didn't use Orange drops everywhere, but a combination of Orange drops and Mallory?!
What is the reason for this?

 

[owlexifry]

Sorry for the late response. But your explanation is very enlightening and now I understand how you arrive at those values.
I wonder why you didn't use Orange drops everywhere, but a combination of Orange drops and Mallory?!
What is the reason for this?

no worries, happy to clarify.

not sure, it's been a while.... it looks like i've used orange drops for all coupling cap positions, and then mallory, silver mica, or ceramic for everything else (cathode bypass, ground shunt, treble/'bright' caps)

or maybe it came down to pricing and availability on mouser? i don't remember exactly, but it really doesn't matter though.

 

[Ocirnelooc]

Okay that's good to hear. Thank you for your reply.
Maybe you can also help me with the following question.
I want to build the preamp in a 19" 1U size case.
As a result, the height of a (toroidal core) transformer may not be more than 44mm.
I can't find any of 300V secondary voltage.

I did find one of 36mm high. However, this 250-0-250V has secondary windings.

3N1904 (XN1904) voedingstrafo voor buizenversterkers 71VA

Specificaties : Vermogen : 100VA Primaire wikkeling : 2x115V / 230V Secundaire wikkelingen : 290V - 250mA 6.3V - 3.8A 40V - 100mA Diamete

www.toroidal-transformer.com

500V is too much for this circuit. Any idea if this is easy to lower without unwinding the transformer?

Another has 275V secondary winding. But this voltage is too low.

2N1586 voedingstrafo voor buizenversterkers 50VA

Specificaties : Vermogen : 44VA Primaire wikkeling : 2x115V / 230V Secundaire wikkelingen : 200V - 100mA 8V - 1.5A 8V - 1.5A Diameter : 8

www.toroidal-transformer.com

Do you have any suggestions?

 

[owlexifry]

Okay that's good to hear. Thank you for your reply.
Maybe you can also help me with the following question.
I want to build the preamp in a 19" 1U size case.
As a result, the height of a (toroidal core) transformer may not be more than 44mm.
I can't find any of 300V secondary voltage.

I did find one of 36mm high. However, this 250-0-250V has secondary windings.

3N1904 (XN1904) voedingstrafo voor buizenversterkers 71VA

Specificaties : Vermogen : 100VA Primaire wikkeling : 2x115V / 230V Secundaire wikkelingen : 290V - 250mA 6.3V - 3.8A 40V - 100mA Diamete

www.toroidal-transformer.com

500V is too much for this circuit. Any idea if this is easy to lower without unwinding the transformer?

Another has 275V secondary winding. But this voltage is too low.

2N1586 voedingstrafo voor buizenversterkers 50VA

Specificaties : Vermogen : 44VA Primaire wikkeling : 2x115V / 230V Secundaire wikkelingen : 200V - 100mA 8V - 1.5A 8V - 1.5A Diameter : 8

www.toroidal-transformer.com

Do you have any suggestions?

because you're limited to the capacity of a 1RU chassis, finding a suitable toroidal PT will be difficult.

i would suggest this one, but its 2.5" tall (too high)

AS-2T300 - 200VA 300V Tube Transformer

Toroidal transformers and power supplies

www.antekinc.com

the one you have pointed out appears to be the closest fit:

2N1586 voedingstrafo voor buizenversterkers 50VA

Specificaties : Vermogen : 44VA Primaire wikkeling : 2x115V / 230V Secundaire wikkelingen : 200V - 100mA 8V - 1.5A 8V - 1.5A Diameter : 8

www.toroidal-transformer.com

with a primary voltage supply 230V AC, the secondary 275V AC rectified to DC will end up around 387V DC, a few volts shy of the target (~400V DC).

I know there's plenty of folks out there that have built MK2C+ preamps like this in a compact 1RU chassis, but I would have no idea where they're sourcing toroidal PTs. it's possible they've been made to order to desired specs.

 

[owlexifry]

- layout updated (.pdf attached) - thanks to @Ocirnelooc for combing through and finding the error


Another update.

to be honest, I wasn't real happy with how it was sounding with the 5 band EQ circuit. Without the EQ I could hear this thing had massive potential, but for my purposes, that MB 5BEQ is absolutely essential.
Where it was really letting me down was when it was playing at big volume with a drummer [through FX return, 100W EL34 amp] and it was quite honestly unpleasant and shrill.

How to make it sound better? Well, i thought i'd see what i could replace the x2 TL074 ICs with..

- some googling lead me to the OPA4134.
- great opamp for audio, used in high end gear etc.
- so i thought yep this is the one.
- went looking,unfortunately the DIP version is out of production, SMD version is about $20 or so (and SMD->DIP adaptor boards go for even sillier prices).. wasn't keen to spend heaps to try out something that I may not like..
- kinda gave up a bit


..and then one day i saw these on ebay (yes, horrendous for fakes) but i thought, eh i'll give it a gamble, and ordered x2 of them.



they arrived a little while ago, installed them earlier this week.

well, they weren't dead ....1st test - passed an audio signal, yep, that's good.

then set it up with:
guitar -> (overdrive) -> preamp -> 5-band EQ -> FX return (peavey windsor 4xEL34 100W) -> mesa 4x12 cab

great success.

- noticeable improvement over TL074 (both running at +12V)
- actually pleasant to listen to.
- worked well for big volume (with a drummer).
- ?better dynamics/feel
- turns out these were used ICs (legs were tinned), geez they might actually be genuine burr-brown ICs..

here's a demo

 

[Ocirnelooc]

Okay, I'm going to try the amplifier with a 250V transformer instead of 300V.
That's the only one I can find that fits decently in a 1unit high 19" housing. And that's a requirement for me.
But if I understand correctly, that high voltage is not so necessary. In the original this is needed for the B+ for the power amplifier, but you obviously don't have that here.
Most plate voltages will be set between 200V and 250V.
So I started drawing a loadline for the first stage. This is the red line.
I just went with the original schematic.

Rtot = 150K+1K5
Rtot = 151K5

392V/151K5 = 2.6mA

2V/1K5=1.33mA
1.5V/1K5=1mA

Then I can draw the loadline.
The intersection point is approximately -1.7V.

Then I did the above with a voltage of 352.5V (the DC voltage at 250V transformer).
Then you get the purple loadline.
This shows that there is almost no difference in the amplification.
My idea is that the difference in sound is nil and not audible.

I also want to omit the 10K resistor in the power supply, as it has no added value in my view.

Anyone have any suggestions or comments on this?
I like to hear it.

 

[Ocirnelooc]

And one more question, is it true that you didn't include the 22nF capacitor in your design?
I don't see it coming back in your layout.

 

[owlexifry]

Okay, I'm going to try the amplifier with a 250V transformer instead of 300V.
That's the only one I can find that fits decently in a 1unit high 19" housing. And that's a requirement for me.

why not use the PT with 275V AC secondaries that you identified previously?

2N1586 voedingstrafo voor buizenversterkers 50VA

Specificaties : Vermogen : 44VA Primaire wikkeling : 2x115V / 230V Secundaire wikkelingen : 200V - 100mA 8V - 1.5A 8V - 1.5A Diameter : 8

www.toroidal-transformer.com

This one should get your B+1 voltage to 387V DC

But if I understand correctly, that high voltage is not so necessary. In the original this is needed for the B+ for the power amplifier, but you obviously don't have that here.

in my experience the preamp B+ supply voltages to the plates absolutely does affect the resulting tone.

i had this experience with an SLO DIY build where the B+ supply was too high and resulted in an overly harsh / bright tone. so I lowered the B+ with the different (higher) dropping resistor values to hit the target B+ and this achieved a better, 'more accurate' tone to my ears.

similarly with this MK2C+, if the supply B+ voltage is too low, the tone will be darker, smoother, warmer.

if you want it to sound as close as possible to the real deal, you'll want to hit those target B+ voltages [as seen on pg.1, 'C' - 397V, 'D' = 392V]

I didn't hit them spot on, but i got as close as i reasonably could.

I also want to omit the 10K resistor in the power supply, as it has no added value in my view.

im not really an expert, but I'm pretty sure that 10K resistor is necessary to decouple the 47uF filter cap from the next B+ node filter cap (22uF), otherwise the 47uF and 22uF caps are in parallel.

so you probably do need one there. maybe you can get away with something small like 470R? if not, 1K.


I lowered that 10K resistor down to 2K to increase B+.

And one more question, is it true that you didn't include the 22nF capacitor in your design?
I don't see it coming back in your layout.
View attachment 1027048

good question.
this 22nF cap does appear here in the schematic i followed, and i did include it in my original draft layouts seen on pg.1.

but ultimately I decided it was redundant to have the 47nF coupling cap, followed by another 22nF coupling cap, especially with this 'lead channel always on' design where there isn't any switching.
so i dropped that 22nF cap completely.

maybe it does matter? i don't really know.
I guess I could always insert one in and see what difference it makes

 

[Ocirnelooc]

The proposed 275V transformer will not fit due to this size. The transformer is 43mm high. And a 1U 19" housing is 44mm.
But I found another transformer with a secondary voltage of 300V. And it does fit. I wanted to post the link to this transformer, but with the link in the message I couldn't post the message..

With your hammond transformer, there you have a CT of the heater voltage. You use this to elevated the heater voltage in combination with the voltage divider 220K/47K.
But if you don't have CT on your 12.6V you can also use the voltage divider according to the first power supply picture and connect it to pin 9. In this picture it is only described to V3 pin 9. But that must be on all tubes and not just V3?

Because of the 300V transformer I will keep the power supply the same as you suggested.
So you converted 10K to 2K and the 2 resistors that follow then become 3K3 instead of 1K.
Capacitors are also 22uF instead of 10uF?!

Concerning the 22nF, I will include it in my design. Let's see how that plays out.
Thanks for answering my questions.

 

[owlexifry]

With your hammond transformer, there you have a CT of the heater voltage. You use this to elevated the heater voltage in combination with the voltage divider 220K/47K.
But if you don't have CT on your 12.6V you can also use the voltage divider according to the first power supply picture and connect it to pin 9. In this picture it is only described to V3 pin 9. But that must be on all tubes and not just V3?

im not exactly sure how the original author of the schematic designed his heater circuit, but to me it looks like he used the 'series' method with 12.6V. i think pin 9 is grounded with that arrangement?

I'm only familiar with the usual 'parallel' @ 6.3V method, and wired it up exactly as per the pdf layout above.

The heater 'elevation function' works as the 220K/47K voltage divider creates a ~70V DC 'ground' reference point for the AC heater circuit. (roughly 70V DC, if given a 400V DC input).

if your chosen PT doesn't have a centre tap for the heater secondaries (6.3V AC / 12.6V AC), you can create an artificial one, with x2 100R (or 220R) resistors, like this:

pic taken from great resource here:

The Valve Wizard

another example (artificial centre tap taken from power tube, but same principle)

some more good info here:

Amp Mods

Generic Guitar Tube Amplifier Modifications

robrobinette.com

Because of the 300V transformer I will keep the power supply the same as you suggested.
So you converted 10K to 2K and the 2 resistors that follow then become 3K3 instead of 1K.
Capacitors are also 22uF instead of 10uF?!

yep, first dropping resistor, 10K, was dropped to *2.2K (just checked the layout)

and yes, the following B+ dropping resistors were changed to 3.3K (from 1K), as per a suggestion made my by a wiser poster, earlier on in this thread.
im not going to claim that i can explain why 3.3K is a better choice over 1K, but apparently it's for better filtering.

To have separate nodes, there must be dropping resistors. I would suggest no less than 3.3k for excellent filtering.

- yes, B+2,3,4 filter caps changed from 10uF to 22uF.
(but that was my arbitrary choice, 10uF would probably be fine)

 

[Ocirnelooc]

Okay, thank you for your detailed explanation. Thanks for the links. I'll definitely check that one out.
Then I'm going to take a good look at how I'm going to wire my heater voltage.
Below is a first draft of how my version will be.

 

[Ocirnelooc]

im not exactly sure how the original author of the schematic designed his heater circuit, but to me it looks like he used the 'series' method with 12.6V. i think pin 9 is grounded with that arrangement?

I'm only familiar with the usual 'parallel' @ 6.3V method, and wired it up exactly as per the pdf layout above.

The heater 'elevation function' works as the 220K/47K voltage divider creates a ~70V DC 'ground' reference point for the AC heater circuit. (roughly 70V DC, if given a 400V DC input).

if your chosen PT doesn't have a centre tap for the heater secondaries (6.3V AC / 12.6V AC), you can create an artificial one, with x2 100R (or 220R) resistors, like this:

View attachment 1028770

Yes, exactly. In his 2 channel version he has tackled it a little more extensively. In this he says the same as what you say above. Just don't understand what he means by his channel version with only pin9 of V3.
But I can give it a try as described here.
Note that he has lowered the voltage a bit from 70V to 48V.