Bluesmaster HRM question:

[Raoul Duke]

Understood, thanks Doc.

Also looking at the dropping string and bleeders on the PS board - should those be the correct values based on the differences in the overall circuit (I.e. x-formers, main board values etc)? If I were to choose this as my next project I’d start with Martin’s PS board - which my understanding is will work with any circuit in the plans. Guess I’m just getting tripped up by the small(?) differences.

Also I notice the layout doesn’t have voltages - which I would assume may be different given the Marshall influence on this circuit?

[martin manning]

Unless someone has some voltage information to reveal, I'd start with the dropping resistors as shown on the layout. IIRC TF-130 PT resulted in voltages on the high side, which would go with the 3k4 first dropping resistor. The "universal" PS board layout will work fine, Axial or radial cap version is your choice, but you might as well do something different this time ;^)

[Raoul Duke]

Thanks Martin, I completely agree with your suggestion about something different! Forces me to learn a different approach.
Would you suggest the 470k bleeders as well (vs 270k)?

Haven’t fully decided yet, but I’m leaning toward this being next. I was also considering the Tweedle Dee and TW Liverpool; but the BM/HRM becomes more interesting to me as I dig deeper into it.

Deciding on the iron seems like it might be tricky - but I’m in no hurry - so I’ll keep researching and trying to find more information.

[martin manning]

270k 1W on the 220u's just like the all-axial version, and 470k 1/2W on the 22u radial filters.

[Raoul Duke]

Trying to understand the “mega boost”/PAB in this amp. I understand that there’s some variation based on the individual amp - so the first question is:

1. Is PAB the same as “mega boost”? If not - what is the difference? Is it something unique not in the layout or schematic?

For the FET: layout looks like it uses two 8k2 for the source and voltage (?) while the schematic looks like 5k1 and 8k2.

2. Would Martin’s 18v bias process still work with this circuit?

Last - I see a lot of folks replace the bass trimmer on the HRM with a 250k (vs 1M). I think the reason (from what I’ve read) is that the 1M useful range of adjustment is in the final 25-30% of the trimmer’s travel. Is there any second or third order consequences to making this change or does this affect any of the other HRM board values?

Apologies if these are basic questions - couldn’t find definitive answers using Search. There’s a fair bit of BM/HRM info here, but not as consistent as say a 124 or 102. I figure that means either there’s not a lot of hard and fast documentation on the originals or the design is pretty flexible - thereby making each build unique?

Any info is appreciated!

[erwin_ve]

Trying to understand the “mega boost”/PAB in this amp. I understand that there’s some variation based on the individual amp - so the first question is:

1. Is PAB the same as “mega boost”? If not - what is the difference? Is it something unique not in the layout or schematic?

For the FET: layout looks like it uses two 8k2 for the source and voltage (?) while the schematic looks like 5k1 and 8k2.

2. Would Martin’s 18v bias process still work with this circuit?

Last - I see a lot of folks replace the bass trimmer on the HRM with a 250k (vs 1M). I think the reason (from what I’ve read) is that the 1M useful range of adjustment is in the final 25-30% of the trimmer’s travel. Is there any second or third order consequences to making this change or does this affect any of the other HRM board values?

Apologies if these are basic questions - couldn’t find definitive answers using Search. There’s a fair bit of BM/HRM info here, but not as consistent as say a 124 or 102. I figure that means either there’s not a lot of hard and fast documentation on the originals or the design is pretty flexible - thereby making each build unique?

Any info is appreciated!

1. A tonestack basically sent signal to ground. A bluesmaster tonestack is sending more signal to ground then let's say a Skyliner tonestack, both with " normal" playing settings.
When engaging the PAB there isn't sent signal to ground. So a Bluesmaster tonestack vs bluesmaster pab has a greater difference in signal than a skyliner ts hence the Mega boost.
Short: it's the difference in signal level between ts and pab.

2. Yes

Last: 250k bass hrm has no consequences as long as the setting would be less than 250k.

[Raoul Duke]

Thanks Erwin!

So most BM/HRM circuits’ optimal bass adjustment setting is under 250k then based on what I’m reading. Is that fairly consistent with your experience and impressions of others’ builds?

Thanks again!

[erwin_ve]

Thanks Erwin!

So most BM/HRM circuits’ optimal bass adjustment setting is under 250k then based on what I’m reading. Is that fairly consistent with your experience and impressions of others’ builds?

Thanks again!

Yes correct, and 250k makes life easier for finding a bass sweet spot.

[martin manning]

Trying to understand the “mega boost”/PAB in this amp. I understand that there’s some variation based on the individual amp - so the first question is:
1. Is PAB the same as “mega boost”? If not - what is the difference? Is it something unique not in the layout or schematic?

BM stack has a single ground point, so lifting that (as the BM PAB does) lifts the entire stack and makes the response essentially flat with almost no signal loss, AKA "mega-boost." Some people keep some resistance to ground to limit the boost, which retains limited function of the controls. The Skyliner PAB breaks the mid-pot/bass pot connection and the bass pot wiper-CW short, which results in a bass roll-off when PAB is engaged.

For the FET: layout looks like it uses two 8k2 for the source and voltage (?) while the schematic looks like 5k1 and 8k2.
2. Would Martin’s 18v bias process still work with this circuit?

With 8k2 lower divider resistor the FET Vdd will be lower than 18V, more like 10V, so if you stay with that use a similar DC voltage on the drain resistor (opposite the drain terminal) to bench bias it.

[Raoul Duke]

Ahhh; PAB understood (now), thanks Martin.

Regarding the FET: so I could start with the plans values and work from there then because the “mega” isn’t so much dependent on the actual values - but rather the end result (however you get there)?

I find the whole FET process an interesting exercise - from the bench to the final playing test. Call me weird…

Looking at the HRM and I think I understand the signal path. If I have it right - the top trimmer is mids (20k), bass is center (1M but I’ll likely go with Erwin’s suggestion for 250k), and the bottom (250k) is treble. Any preferences on the small (470p) cap? Looks like ceramic disc; but wondering if anyone has tried mica, film etc?

Thanks all!

[martin manning]

Regarding the FET: so I could start with the plans values and work from there then because the “mega” isn’t so much dependent on the actual values - but rather the end result (however you get there)?

I don't understand what you're saying there. You could use the 8k2 in the divider (with the 150k) but you will still have to find the source resistor value that biases it correctly (the other 8k2 in the layout drawing).

Looking at the HRM and I think I understand the signal path. If I have it right - the top trimmer is mids (20k), bass is center (1M but I’ll likely go with Erwin’s suggestion for 250k), and the bottom (250k) is treble. Any preferences on the small (470p) cap? Looks like ceramic disc; but wondering if anyone has tried mica, film etc?

Correct. It's nothing more than a slightly modified Marshall tone stack. I'd go with NP0/C0G ceramic for the 470p.

[Raoul Duke]

What I meant was that one could start with any value for R1 and Rs (within reason) and as long as they can get to half of Vdd and ~ 9v, they will achieve the desired effect.

I get stuck on the different values from layout to layout thinking R1 and Rs are part of the bigger “recipe” when in fact they are a means to an end (I think?).

Do the BMs typically have the clean boost or the FET distortion? Judging by the values in the layout - I’m guessing distortion?

[martin manning]

What I meant was that one could start with any value for R1 and Rs (within reason) and as long as they can get to half of Vdd and ~ 9v, they will achieve the desired effect.
I get stuck on the different values from layout to layout thinking R1 and Rs are part of the bigger “recipe” when in fact they are a means to an end (I think?).

No, smaller R1 means lower Vdd, and less headroom/more THD for the same input signal.

Do the BMs typically have the clean boost or the FET distortion? Judging by the values in the layout - I’m guessing distortion?

The 150k/8k2 divider says low Vdd, can't say for sure where the bias is for the particular FET.

[Raoul Duke]

Understood. Went back and re-read the bench bias process and see that now.

If I decide to build this one - not sure which FET I’d go with. On one hand - if the cleans are as good as everyone says - more should be better, right? On the other hand - the layout has the 8k2; must be for a reason?

Decisions, lol…

[Raoul Duke]

Bit of a sidetrack here, but I have a quick question regarding VRs:

Are the LM7812CT and L7812CV functionally interchangeable?

I’ve looked at both data sheets and based on my limited understanding - it would appear they are; but figured I’d check in case there’s something I misinterpreted or some actual practical experience from those more knowledgeable.

Thanks all!