Comments on Skyliner to BM conversion...

[Tonegeek]

I just finished a BM conversion and I am really excited about the direction this takes my tone, especially on the clean channel. I have long had issues with my clean channel to the point I just gave up on it and spent all my energy on getting a great OD sound. Well I started thinking the BM circuit might be the ticket, not because of the tone stack values, but because of the bias points set by the plate and cathode values. I always wanted more 'hair' on the clean channel and I knew going to 100k plates would help out there. After reading some great threads here comparing the 100k to 220k amps, I was afraid that going to 100k plates for the clean channel might mess up my OD channel. The BM circuit looked like a good compromise so I dove in. I was thinking I would just keep my tone cap values as these were the same as in my '64 Bassman which I dearly love the cleans on (except I have a .01 mid cap in the clone instead of the .047). so, I swapped out the plate and cathode values and gave it a go. It did sound better, but still not right. THen I pushed on with the other changes, putting in the .02 caps and 47k slope. Now we are getting somewhere, but it was all too dark, which is what my original problem was, among others. At least it was breaking up a bit now in a good way. Lastly, I put back the 100k slope which I had originally in the Skyline stack (I know, it should have been 150k, but that's another story!) So now every thing is BM spec (as far as I know, piecing together a couple of different schematics). I already had a BM PI. The one thing different is my treble cap is 235p instead of 326. This is in the ball park with a lot of other amps using the same circuit.

Now this amp sounds great! Its got more sparkle than it had and the crossover points seem to be in a better position (at least for a Strat). I finally have a bit of hair on my cleans. The OD channel is reacting differently too, but in a positive way. My transistion to OD is smoother than it ever was. It seems fuller and more articulate. I used to play mostly on OD with PAB and mid boost on. This setting is now a bit more Marshally sounding (a little more bite to it) than it was. I imagine it would cut through a mix slightly better than it did before. I shall probably have to tweak my HRM settings a bit for this change, but not much.

I must say I did not change any of my relay switching. PAB still works like it did, breaking the treble CCW leg from the bass CW leg. I did not implement the mega boost switching but plan to give it a try sometime. I shall continue to experiment with this arrangment trying different treble caps, switching, etc, but overall this change has been very worthwile.

[Structo]

Cool!
I love it when things come together in a good way.

[Tonegeek]

Cool!
I love it when things come together in a good way.

Sometimes I jump the gun on my initial assessment of changes to my amp and then later realize maybe that change wasn't so good after all. So I got up this morning and while my wife was out walking, I put the amp through its paces at club volume. Its even better than I initially thought! The BM stack (really '59 Bassman, Marshall, etc.) is a keeper and I am not looking back! Normally my OD channel also has PAB and mid boost on. The OD alone just never did it for me. Today, I was getting very nice crunchy lead tones with just OD alone. Great for doing articulate blues licks. Cranking the drive and/or master up, it just got better and better. One thing I will need to do is tame the PAB a bit as it is almost too much of a jump in level. (I think it always was a big jump, but I never noticed since I never did OD alone). Mid boost still works like it always did. I have a feeling I will be implementing the relay to lift the stack to make this a complete BM conversion but I want to play with it a while longer like it is. Can't wait to get off work and mess with this some more. I havent been this excited since I built a BN 18 watt Lite IIb a few months back. Its good to be back on the Dumble trail again.

[Structo]

Yes the PAB can be temperamental.
It really depends where you have your tone stack set to begin with.
If you have your pots at less than 12 o'clock then the boost will be big.

If the pots are up, not so big.

I'm going to try something else for the boost.

I going to put a trimmer pot on the ground lug of the mid pot paralleled by a .001-.005uf cap.
I will try lifting to that and dial it in with the trimmer.
I don't like the tone of the PAB, too full, so this would leave the bass and treble still there and might be the answer.

[Tonegeek]

I going to put a trimmer pot on the ground lug of the mid pot paralleled by a .001-.005uf cap.
I will try lifting to that and dial it in with the trimmer.
I don't like the tone of the PAB, too full, so this would leave the bass and treble still there and might be the answer.

I think that will have the opposite effect. Looks like it would shunt some high mids to ground. Maybe thats what you want, or I am missing something.

I am dealing with a similar issue as I convert my relays from Skyline functions to BM. Lifting the whole BM stack does seem louder than the Skyline PAB and Mid Boost combined. One thing I am experimenting with is the mid boost caps (the one that works with the treble cap). I am using 250p for unboosted mids and 500p with mid boost engaged. I lift the stack to 100k for a fat lead sound instead of it being completely open. I like lifting to 24k as well (on top of what the mid pot is set to) and with the mid boost engaged it gives some good upper mids to cut through a mix. (Its quite Marshally with no lift, buts gets tamer as you encrease the resistance to ground from the mid pot ground point). Unboosted mids are better at home or for getting a nice mid scooped sound with the 250p cap. I also use a 100k slope, instead of the 47k which was just too muddy for me. Even with that, my bass control is still set around 10 or 11 oclock. I use 2 speakers so wimpy bass has never been an issue.