BM Build Complete

[ayan]

Hi guys,

Just finished my BM build yesterday (pictures to follow later) and here are my initial impressions:

1.The BM has great cleans and a boatload of gain on tap as its TS is not nearly as lossy as in previous Dumble amps.

2. OD is good -- haven't fine tuned it yet -- with HRM controls halfway up.

3. Mega PAB (lifting the TS ground) is very harsh in OD mode, and sounds somewhat like a fuzztone [not suprised here, since I had toyed with this PAB in a Skyliner amp and hated it].

4. "Conventional" PAB [a.k.a., the Jazz/Rock in between position, i.e., disconnecting the output of the treble pot from the bass pot and disconnecting the wiper of the bass pot from the input] is not really a boost in this amp since it yields less signal than the amp without PAB when the tone controls are dimed. It makes total sense that Dumble would implement something different in these amps. According to the TS calculator, there is not too much one can do to get a boost out of the BM TS besides lifting the ground.

5. The amp responds very much like 234 when I tried it years ago. The MID switch is very subtle, the jazz/rock difference is subtle and the sound is very always springy. However, I remember liking the PAB in 234, so... maybe we're missing something (same as the LC amp, perhaps).

Overall I love the cleans, but the amp is limited when compared to my non-HRMs. The fact that the PAB on top of OD only sounds good if the OD trimmer is way low and the OD set low on the front panel as well is a bummer to me. So, tweaking time I guess... any suggestions from BM veterans?

Details: pretty much the BM schematic that's floating around on the board with Fender PT and HB multi-tap Twin OT.

Cheers,

Gil

[tomrasdf]

I had the same problem with the boost in mine. I placed a trimmer wired as a variable resistor between the PAB relay and ground. I found a sweet spot that works really well. Haven't measured it yet, it's a 1M pot at about half, so try a 470 maybe?

[v00d00blues79]

I noticed that mine has massive amounts of bass when in the rock position... I'm not sure if I would benefit from dropping OD2's Ck to 1uF from the 5uF thats in there now...


Andy

[Bob-I]

I did the pot thing on the PAB also, replaced it with a 470k, close enough and takes away that harshness.

[BobW]

Allen Amps calls this pot the RAW control, but agree, once I find the sweet spot, I tend to leave it alone.

[ayan]

Hi guys,

Just finished my BM build yesterday (pictures to follow later) and here are my initial impressions:

Put two pictures in the files section.

Gil

[odourboy]

Does anyone know if the BM PI values result in more gain than the 'standard' values?

[ayan]

Does anyone know if the BM PI values result in more gain than the 'standard' values?

The BM PI and differernt NFB resistor values (for less overall negative feedback), coupled with the less lossy tone stack definitely make this amp a gain machine. I can dial metal gain settings in my amp, and I can get AC/DC type of crunch out of the clean channel without PAB.

Gil

[odourboy]

Thanks Gil!