Bluesmaster HRM question:

[Raoul Duke]

Poking around and looking at different designs to try and increase my knowledge base and see that the Bluesmaster design seems to have some variation in pot values - which leads me to the following question:

Stock values are treble: J taper 2-35 A, mid: 50k B, and bass: 500k A. I’m assuming the layout is from an amp that was tailored to the buyer (and not necessarily a generally flexible control array) and that’s why this pot combo is the way it is - leading to the next observation:

What I’m seeing in the various build threads is the mid pot stays constant, but many favor a 250k A bass pot and 250k B audio pot.

The question is: “why” and what (logically) would be the best configuration for versatility? Is the above (oft-used) alteration a more practical way to go?

Any info is definitely appreciated! Thanks!

[erwin_ve]

The Bluesmaster tonestack is not a skyliner nor a classic tonestack.
While a skyliner might be low mid heavy, the Bluesmaster might be low bass heavy.
Some might shave off some bass pot value to get a more usually pot swipe setting.
Paying attention to the first tube voltages is imo the best solution to control the bass.
Are you sure were taking about the font panel pots? Or are you referring to the internal od tonestack?

[Raoul Duke]

I was talking about the front panel pots; I hadn’t seen any real variation on the HRM board.

I’m on a quest to better understand how the composition of the primary tone stack lays the foundation for an amp’s character and saw all the variations in the Bluesmaster; so I thought it might be a good learning platform.

Thanks Erwin, I always appreciate your guidance!

[erwin_ve]

I was talking about the front panel pots; I hadn’t seen any real variation on the HRM board.

I’m on a quest to better understand how the composition of the primary tone stack lays the foundation for an amp’s character and saw all the variations in the Bluesmaster; so I thought it might be a good learning platform.

Thanks Erwin, I always appreciate your guidance!

I haven't seen that variations in the front panel tonestack. The internal od tonestack is where the most builders lower the bass trimpot to 250k instead of the 1M.

[Raoul Duke]

Oh, ok - maybe I misunderstood what I was reading. I’ll go back through and re-read a few of these. Yours was very informative, thanks for documenting it

[Raoul Duke]

Went back and checked and I did mix up the HRM bass trimmer value change (250k replacing 1M).

I did see however that a couple of folks replaced the front panel treble control with a 250k linear pot vs the 2-35 J taper in their builds. This change leads me to wonder:

1. Why was the J taper spec’d originally?
2. What is lost if the 250k linear is subbed?

Interesting variation for someone new to the many different Dumble circuits to learn about…

[GAStan]

It's not that anything is lost. Both 250kA and 250kB adjust from 0 to 250k ohms. The terms Audio (logarithmic) vs Linear just describe how they adjust. Here's a simple plot showing the differences, it changes how the adjustment feels as you turn the control knob. A J-taper plot falls between the Linear and Audio (log) tapers.

audio-v-linear-pots_1024x1024.png

[Raoul Duke]

Thanks Glenn!
The graphic makes it much more clear. I think I understand the difference between audio and linear, but is there an advantage to the J taper or is it just a “feel” thing?

[GAStan]

To the best of my knowledge, no. But once again it's based on my limited guitar amp experience. The vast majority of my experience is with RF frequencies well above Audio where Linear taper 20 turn variable resistors are more common.

There is nothing "magical" with the different tapers, they are all, in the end, simply variable resistors.

Hopefully some of the more experienced techs will help us out.

[martin manning]

Taper is about where in the rotation you get a given percentage of resistance, and also describes the change in percentage per degree along the way, which makes a significant difference in the usability of the control. An extreme case would be a linear volume pot, which results in a high volume level right away, where an audio taper brings it up slowly. That's much better for finer adjustment at moderate volume where it is most often used, and a better fit with the way human hearing perceives loudness. A common indicator of taper is percent resistance at 50% rotation: 10% or 15% for audio, 30% for J, and 50% for linear (but that will be called B taper, or linear). J taper is useful if you want the effect to come on a little faster than audio, and is the traditional choice for the Treble control in Fender tone stacks. See snip below from '65 Twin Reverb, where pot tapers are specified. 30A is J, 10A is audio. The bias pot (not shown) on that schematic is marked 10k LIN.
Here's a classic article by RGKeen on pots: http://www.geofex.com/article_folders/p ... tscret.htm

[Raoul Duke]

Read the article and learned quite a bit, thanks Martin and everyone else who tried to shine some light for me. Very much appreciated as I try to get smarter about these things!

[Raoul Duke]

Ok, one other quick clarification needed as I research the Bluesmaster HRM circuit:

I notice in Tony’s layout that the resistor coming from the PI grids up through the NFB/presence circuit value is 9.09k, but the same resistor in the DIY board plan in the “Files” section is 24k.

Question: Being that the board plan is newer - is this an improvement discovered over time; increasing the value from 9.09k to 24k? Everything else around it looks to be the same value.

Any info is appreciated!
Thanks!

[martin manning]

Looks like I just missed updating that resistor for the BM. I’ll fix it and upload a new .pdf.

[Raoul Duke]

Oh, ok - the 9.09k is correct then? Thanks Martin!

And a another quick follow-up:

If building as a 50w, would this be another case of running the NFB to the 8 ohm tap on the impedance switch or would I stick with the 4 ohm if all the values still match the original circuit and this would apply only if the NFB 100k was a different value?

Also (hypothetically) - would I be ok to use a black panel Bassman PT with a Marshall spec OT as long as the OT primary impedance is in the right ballpark? I see these BMs (along with 183s) seem to use Marshall iron - curious about that; so I’m reading up on the differences.

Thanks again!

[Dr d]

Oh, ok - the 9.09k is correct then? Thanks Martin!

And a another quick follow-up:

If building as a 50w, would this be another case of running the NFB to the 8 ohm tap on the impedance switch or would I stick with the 4 ohm if all the values still match the original circuit and this would apply only if the NFB 100k was a different value?

Also (hypothetically) - would I be ok to use a black panel Bassman PT with a Marshall spec OT as long as the OT primary impedance is in the right ballpark? I see these BMs (along with 183s) seem to use Marshall iron - curious about that; so I’m reading up on the differences.

Thanks again!

Hi Raoul,
Using the Fender PT with a Marshall OT should be fine and stick with the 4ohm tap initially and see what you think. Remember that this was Dumbles take on a gainy 80s style JCM.