why use bluesmaster pi

[jcsifu]

I have been searching to see what the difference is in a regular OTS (non hrm) pi and the bluesmaster pi and have not come up with what this would consist of or why one would be of interested/preference. Can anyone tell me what this mod would be to my pi of my OTS? Is it just the .02 caps instead of .01 on the 2 pi halves? I would assume this would let more bass and more signal into the pi right?

[amplifiednation]

From my experience with it, the BM opens up the power amp. I'd be curious to everyone'scomments though

[jcsifu]

From my experience with it, the BM opens up the power amp. I'd be curious to everyone'scomments though

That seems like it would be desirable. So would the open-ness come from the .02 caps letting more signal in or am I missing something else that is different than the standard pi?

[martin manning]

The usual PI coupling caps are 0.1uF (not 0.01), so the BM's 0.02 are five times smaller. This will reduce the bass fed into the power amp (the rollover is ~36 Hz vs 7). The amount of bias excursion, and therefore the possibility of blocking distortion will also be reduced. The BM PI has a ~10k ohm reduction in the tail resistance, which will gve it a bit more headroom but make it a little more unbalanced (depending on the plate loads too, of course).

[jcsifu]

The usual PI coupling caps are 0.1uF (not 0.01), so the BM's 0.02 are five times smaller. This will reduce the bass fed into the power amp (the rollover is ~36 Hz vs 7). The amount of bias excursion, and therefore the possibility of blocking distortion will also be reduced. The BM PI has a ~10k ohm reduction in the tail resistance, which will gve it a bit more headroom but make it a little more unbalanced (depending on the plate loads too, of course).

Martin,
Thanks for your time.
As you point out, caps are .1, shouldn't have that 0 in there.

I don't think that raising the rollover to 36ish Hz would even be noticed audibly by the human ear as guitar (6 string) bottoms out at around 80Hz IIRC. The blocking distortion explanation makes a lot of sense as to why this pi is different and what advantage it might hold over the standard. I am curious as to what the difference will be once I swap out the .1 for .02. I am looking forward to a test of this at stage volume, but won't be able to until next weekend.

On a side note, how integral is it to add the 47pf smoothing cap? Do most people switching to a BM pi add that cap or is it negligible and only the .1 caps are switched?

[martin manning]

I'm pretty sure you'll notice a little reduction in bass. There will be a 1dB reduction an octave higher than the fo. Re the 47p that is a small cap, sometimes as much as 10x that value is used. It wil take a little fizz off the OD and help kill oscillation.

[wjdunham]

One thing to note is that all the Ultraphonix amps seem to have a BM PI - seems to contribute to that nice spongy, singing quality that the cleans on all those amps have. Use a 100K/10K GNFB off the 4ohm tap of the OT.

Bill

www.sebagosound.com

[jcsifu]

I'm pretty sure you'll notice a little reduction in bass. There will be a 1dB reduction an octave higher than the fo. Re the 47p that is a small cap, sometimes as much as 10x that value is used. It wil take a little fizz off the OD and help kill oscillation.

I won't miss the extra bass as my OTS seem to have that in spades, so that won't be any disappointment here.

I seem to remember seeing the much higher value on SF fender amps that was to help kill oscillation. I also won't miss any fizz that might be departing after the 47pf goes in. I'm starting to think I might be one of the people who is happy with the BM pi.

Thanks again for taking time to answer Martin.

[jcsifu]

One thing to note is that all the Ultraphonix amps seem to have a BM PI - seems to contribute to that nice spongy, singing quality that the cleans on all those amps have. Use a 100K/10K GNFB off the 4ohm tap of the OT.

Bill

www.sebagosound.com

Hi Bill,
That is what I am hoping for, the singing and also the almost feedback tones that I already have but would like just a little more of. I can always use guitar vol to pull back if I need, better to have a little too much on tap than too little.

Thanks for catching that I missed the GNFB difference. I see thee 100k but what 10k are you referring to?

Thanks for your time as well!

[wjdunham]

One thing to note is that all the Ultraphonix amps seem to have a BM PI - seems to contribute to that nice spongy, singing quality that the cleans on all those amps have. Use a 100K/10K GNFB off the 4ohm tap of the OT.

Bill

www.sebagosound.com

Hi Bill,
That is what I am hoping for, the singing and also the almost feedback tones that I already have but would like just a little more of. I can always use guitar vol to pull back if I need, better to have a little too much on tap than too little.

Thanks for catching that I missed the GNFB difference. I see thee 100k but what 10k are you referring to?

Thanks for your time as well!

Sorry, I was not too clear there - 100K NFB resistor. The BM PI has 470R on the cathode of the PI, with a 10K tail (normally a 24K on the ODS amps), and 4.7K to ground for the NFB divider. So the feedback ratio is 100K/4.7K. That should all be on one of the BM layouts Tony did in the files section.
.02 input coupler and .02 output couplers. The UP amps have a huge low end so the smaller cap values in the PI help keep it under control.

Bill

[martin manning]

Also note the change in the presence cap from 1.0uF to 0.1uF. That goes along with the 4k7 to ground.

[talbany]

FWIW.. I have a love hate relationship with this PI..Some people like them others (more in the traditional D-camp) might not!..The BM PI is a modified 5F6a Bassman design..Like Bill and Martin mentions, lower .02 couplers to control the Bass response and speed up bias excursion (recovery) speed..The combination of 470R/10k (tail) and amount of GNFB helps to increase gain,sensitivity and therefore clips differently,so the result (due mostly by the 10k tail and 470R) is less headroom and more harmonic content that produces more of a 3rd harmonic clipping pattern when pushed hard, which to my ears can be rather useful if you like a little breakup on the clean channel, BUT can tend to make the OD sound a bit fuzzy to my ears..SO if you are a rock guy and like that kind of breakup the BM pi may be something useful to you..I also personally think this type of PI shines (in BM) when using EL-34's (Rock Tone)..Also in and HRM style (since the OD section has less gain than the Non HRM)...My Take anyway

Side note:..The coolest thing to me about this PI (Vrs the AB763)is it does make the amp more responsive,sensitive,dynamic (more so in 100w),a bit faster and more prone to feedback..

Macdaddy kind of say's that here in his opinion of the UP modded Showman,that has this PI..IMO

He would take the original design of the amp he was working on and would improve the amp's attribute's of it

https://tubeamparchive.com/viewtopic.ph ... sc&start=0

Tony

[jcsifu]

Bill,
Thanks for clearing that up, I should have noticed that the 9k1 on the layout I was using would translate to 10k. Sometimes I miss the forest for the trees.


Martin,
Thanks for the point out, I am starting to see there are indeed more changes than I first noticed.


Tony,
This is much more involved than I had thought, thank you very much for adding tone/sound comments to help me to better understand what this mod would accomplish. I think after reading your post I will sit back and think for a little bit before I heat up the iron. While imagining the result of the changes mentioned, I believe I have an understanding of why you have a love/hate with this pi. I think my best direction as of now will be to compile a list (based on the sonic aspects you mention) of desirable and undesirable traits and go from there.

I like fast recovery, 3rd order, and wouldn't mind a little more break up while on the clean. It might be that since currently I am needing blues and classic rock tones, and I have as of yet not been able to find the right amount of clean volume and ratio volume mix to get the tones I need "on the fly", this might work well.
I don't like fizz that much though. I have a SLO that I played for a few years with the last band I was in and it was hard to enjoy some of the tones for just that reason. I might be a little "ear shy" as a result of having to play a tone that I was hired to play, but would have liked to have changed.

I have recently put a 3 way switch on V2b snubs to add 250 or 500pf, (had a lot of old ceramic 250s laying around), maybe this will help with the fizz that might be added. I am also wondering if the 47pf between the two .02s might be raised a little to help. Another thought might be the high cut rotary switch mod that Dr Ika has mentioned.

BTW, my OTS is a non HRM 50 and has 6l6. V1b LNF switch and the 3 way mentioned earlier.

[talbany]

IMO
Snubbers, Feedback loop (47pf across plates),Treble bleeders, Longer coax runs, Doesn't really solve the Fizz factor just throws a blanket over it..another thing to try would be a BM PI w/12AT7 (compromise) mo current,less gain

See I am not such a bad guy.. I gave a guy w/ Ceriatone advice!!..

Tony

[jcsifu]

IMO
Snubbers, Feedback loop (47pf across plates),Treble bleeders, Longer coax runs, Doesn't really solve the Fizz factor just throws a blanket over it..another thing to try would be a BM PI w/12AT7 (compromise) mo current,less gain

See I am not such a bad guy.. I gave a guy w/ Ceriatone advice!!..

Tony

Gotcha Tony, cut the treble or cut the pi gain and see what it sounds like. I think with all the info you guys have given me I have more than enough ideas to fiddle with.

On a side note:
I know I am involved in a private joke, and I have no problem helping out with that, but I sincerely hope I am ok asking about Ceriatone products.

I greatly appreciate your experienced advice Tony.