Ideas for a Fat PAB mod?

[odourboy]

MDRoberrts and I held a mini ampfest last week in which we compared his 'Sunnyland' BM HRM build to my Sky HRM with BM PI and my non-HRM 50 watter. We also threw four speakers into the mix - an EVM 12L, a 1265, a C-Rex and a RW&B.

Aside that vast quantities of alcohol were being consumed, one thing was very apparent. The full BM HRM sounded incredibly fat in OD/PAB. My HRM was only able to get into that ballpark with OD, PAB and Mid boost all pushing - and then while it was close, it wasn't quite as satisfying.

I don't really want to go to the full BM stack but I was wondering if anybody can suggest a tweak for my PAB or my OD circuit to fatten it up. I'd prefer not to mess with the mid-boost - like to keep that in reserve, but I'm open to hearing any ideas.

Thanks!

BTW - Here's a photo of the gear for your amusement:

[keithrick]

Nice collection of gear it looks like you guys had fun! Unfortunatley, I have no advice as I currently have a non-hrm. Out of curioustiy how did your non-hrm stack up? Also where did you get the plans for the 1x12 ported cabs?

[dave g]

Check out:

https://tubeamparchive.com/viewtopic.php?t=3727

[odourboy]

Nice collection of gear it looks like you guys had fun! Unfortunatley, I have no advice as I currently have a non-hrm. Out of curioustiy how did your non-hrm stack up? Also where did you get the plans for the 1x12 ported cabs?

Thanks! The non-HRM stood up fine, but it's a recent build and I'm still playing with it. It has spring clips (on loan from MDRoberts!)on all the V1 and V2 plates and cathodes so I can do quick mods - so it's really still in a state of flux. However, our attention was really focused on the two HRM amps to be honest.

The 1X12s are modified versions of the EV TL806 design. I don't have the link handy, but someone may spot this thrad and post it, or try googling TL806. Should show up.

[odourboy]

Check out:

https://tubeamparchive.com/viewtopic.php?t=3727

I actually participated in that thread but it sort of pooped up without clear resolution and it slipped my mind. But I should try out yeahyeah's PAB mod. Thanks for reminding me!

[greiswig]

If/when I ever get my fizz taken care of on my OD channel, I plan to use the unused side on my PAB relay to hook something around a .001 cap in parallel with the treble cap. (Credit to dogears for this suggestion) Try it! That brings back a lot of the body.

[mdroberts1243]

Check out:

https://tubeamparchive.com/viewtopic.php?t=3727

I actually participated in that thread but it sort of pooped up without clear resolution and it slipped my mind. But I should try out yeahyeah's PAB mod. Thanks for reminding me!

After reviewing the thread again, it seems like a lot of the phat effect you're after might come just from adjusting the PAB lift resistor... most of the BM builders seemed to have gone for a pretty low one with the fender-ish stack (I ended up at 72k). Maybe 4.7M is sufficient for the skyliner stack. You might not have to go as far as modifying the PAB switching and adding in the additional treble cap.

[odourboy]

greiswig - your suggestion is similar to what yeahyeah had presented in the thread dave g posted.

I just ran the HRM pre spice model I have to test a 500p cap as yeahyeah suggested. It looks very promising on paper. I'm going to try that tomorrow. Of course the last 'very promising' tweak i tried was ridiculously grainy and had to go, so I'm not going to get too excited yet.

[odourboy]

After reviewing the thread again, it seems like a lot of the phat effect you're after might come just from adjusting the PAB lift resistor... most of the BM builders seemed to have gone for a pretty low one with the fender-ish stack (I ended up at 72k). Maybe 4.7M is sufficient for the skyliner stack. You might not have to go as far as modifying the PAB switching and adding in the additional treble cap.

I think it would actually be easier to slip in that cap than to change out the PAB lift resistors...

None the less, I had dismissed that option because I thought lowering the lift resistors simply dimished the PAB effect and allowe the stack to influence a bit. I'm not unhappy with the boost I get from my PAB so I didn't see much value in reducing the lift. Easy enough to try out in a model though. Maybe there's a happy point where the stack influence fattens up the tone enough.

[greiswig]

After reviewing the thread again, it seems like a lot of the phat effect you're after might come just from adjusting the PAB lift resistor... most of the BM builders seemed to have gone for a pretty low one with the fender-ish stack (I ended up at 72k). Maybe 4.7M is sufficient for the skyliner stack. You might not have to go as far as modifying the PAB switching and adding in the additional treble cap.

I think it would actually be easier to slip in that cap than to change out the PAB lift resistors...

None the less, I had dismissed that option because I thought lowering the lift resistors simply dimished the PAB effect and allowe the stack to influence a bit. I'm not unhappy with the boost I get from my PAB so I didn't see much value in reducing the lift. Easy enough to try out in a model though. Maybe there's a happy point where the stack influence fattens up the tone enough.

I think we may end up wanting both...the increased cap across the treble cap AND the pad to reduce the overall gain. I haven't tried that yet, though.

[stelligan]

odourboy,

Slipping in that cap IS what I have been toying with. That eliminates having to gang the mid boost with the PAB. Seems to be getting me somewhere. This has been my all consuming quest - to moderate and fatten the PAB. Most problematic in my HRM build. I am still yet to try even smaller resistor values than 4M7 as padding resistors. It seems yeahyeah's graph concerning resistor values on the old thread also influenced the amount of lowend that can be preserved. I hope to post more findings on the old thread soon. It has been going slow due to a sore right hand I also believe that once I have the PAB tamed - I will have more luck with my Strats in the amp.

[dave g]

FWIW, I tried the 4M7 and 500p in my lastest non-HRM w/skyliner. My thoughts are that 4M7 is still WAY too much for my tastes, and 500p is still a bit too big.

I'm gonna throw a 330p in there instead and probably go down to 1M resistors, then see where that lands me.

[odourboy]

odourboy,

Slipping in that cap IS what I have been toying with. That eliminates having to gang the mid boost with the PAB. Seems to be getting me somewhere. This has been my all consuming quest - to moderate and fatten the PAB. Most problematic in my HRM build. I am still yet to try even smaller resistor values than 4M7 as padding resistors. It seems yeahyeah's graph concerning resistor values on the old thread also influenced the amount of lowend that can be preserved. I hope to post more findings on the old thread soon. It has been going slow due to a sore right hand I also believe that once I have the PAB tamed - I will have more luck with my Strats in the amp.

Hey Stelligan - I was hoping you'd wade into this thread! Sorry about your hand.

Well all I can say is I'm not completey unhappy with the boost I get with both PAB and Mid Boost, and the 500p cap gives me a touch less than the Mid boost on paper. I'm just going to have to see for myself how performs for real in my build.

I think I better 'tack solder' the cap in though. I have a feeling those resistors are going to get changed too and this is going to take a while

[bcook]

Try soldering a 1M pot between the low treble pot lug and the high bass pot lug, and see if you can dial in your PAB.
-Bob

[stelligan]

Great idea. I should clip lead one in there and measure what I like.
Thanks Bob.