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Modify the BBE BMAX input stage...

#21
Here are the resistors I have in my cart at Mouser.


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Correct specs?

Now would it behoove me to upgrade these parts since they are so cheap and I will pay 6-7 bucks shipping anyway? Would going to 1/2 watt or 1 watt resistors or use tighter tolerances make any difference? Since I will only do this mod one time I want to put the best components in there.

Also, wondering about modding this unit with better caps as well. I did this with a cheap Class T amp from Parts Express and it made an enormous difference in the sound. Very easily done too and would even be easier with this preamp since there is so much room in it.

Any one have any thoughts?
 
#23
After digging around in my parts bucket I realized I have the resistors to do this mod. I did it yesterday and put the 12AX7 back in. It does work well. Id say the tube change was more drastic of a change than the resistor swap but this mod works well. If anyone has questions let me know ill help if I can.
 
#25
spego said:
Vlad if you didn't already order the resistors.....I have extras.....PM me your address ill mail them to you.
Click to expand...

Ach! Ordered this afternoon. Thanks anyway, really appreciate it.

I may need your help with the install. Do you have a schematic or are the positions silk screened on the board? I havent looked inside at the components yet. Figure I'll get the parts sometime next week and have to locate my solder sucker.

The resistors that are removed, you just take them out right? Don't have to bridge the holes with something to complete the circuit, correct?

Thanks again!
 
#27
spego said:
The board is labeled. Its an easy one. I took those right out too....no bridge.
Click to expand...

Performed the operation last night. The hard part was figuring out how to take this thing apart while avoiding getting lit up by a cap.

Also, I refered above to the two resistors that are removed completely, C8 and C9. These are not resistors but two small capacitors. Hence the "C" designation. Duh!

Stuck a JJ 12AX7 in and it sounds FANTASTIC! Seriously, I have owned both the Alembic and a Demeter tube preamps and I think this unit sounds better than both of them!

Thanks for you help.

PS: I have the two resistors for the bmax-T so if anyone needs to do this mod, send me a PM.
 
#28
Sorry to dig up an old thread, but I have what I think might be helpful info if anyone is still considering these preamps.

I bought a new old stock (early serial) BMAX (non tube) preamp for cheap and found it the distortion issue using a bass with hot signal, even in the "active" input. I had already read this thread and decided it likely needed the mod. I also looked at the clipping on a scope and found it was clipping one rail and not the other, it was asymmetric, FWIW. Whatever, I called BBE tech support and talked to Judd, and he was extremely helpful. We discussed the changes for the non tube BMAX, he sent me the schematics, and I ordered up some matching metal film 1% resistors at Digi Key.

I changed out the 4 resistors as spego noted in his revised mod post earlier in this thread, which agreed with revision 1.3 of the official schematic:

1) R3 changes from a 46.4k to a 68.1k resistor
2) R8 changes from a 46.4k to a 21.5k resistor
3) R12 changes from a 6.81k to a 14.7k resistor
4) R14 changes from a 5.62k to a 10.0k resistor

I still noted a bit of clipping with an Ibanez SR300 with active EQ with everything up full on the axe and the preamp. Being all transistor and opamps, the clipping was not a pleasant fuzz at all, but I've decided it's adequate, as I never play in that manner anyway. BTW, using the compressor usually fixes the clipping, and the sonic maximizer circuit also affects it, so it's actually happening in an opamp stage after those circuits or the sonic maximizer IC, I didn't bother to check that, as it's actually adequate now.



Moving on, the other day I was perusing around the net and found a used BMAX-T tube version for sale at Guitar Center in the Edina store for a decent price. Of course they had to add shipping and tax, but it was still decent compared to offerings on ebay. It too was an early unit (ver 1.0 circuit board with the Jensen DI xformer!) and had not been modded. The clipping was certainly a more pleasant fuzz, but I thought the active input could use the mod, so I did the mod as C-5KO listed earlier. Again I got the schematics from Judd for the BMAX-T, but this time versions 1.0 and 1.4.

Here are the changes as C-5KO listed earlier:

1) R7 gets changed from a 46.4k resistor to a 68.1k
2) R10 changes from a 46.k to 21.5k resistor
3) C8 is removed completely
4) C9 is removed completely

I left the original 12AX7 Groove Tube in it, as having the tube gain there isn't the issue, IMO, it's just the attenuation network of those input resistors that was incorrect.

This BMAX-T sounds awesome! It's got that great tube "punch" when I get after it on the finger picking, that extra tube dimension. The non tube one sounds a bit thinner and 1 dimensional, even even with the same settings. They're both quite clean, and not too noisy if I watch the gain structure, and run the main amp lower and the preamp hot.

I'm using a Carvin DCM3800L power amp, with the inputs set to about -15dB (10 o'clock) and it really gets with it, but keeps the noise lower. I'm also using an Avatar SB126 and a dual 10 cab.

I found it's the sonic maximizer circuit that tends to make the most noise in these when it's on, but I like the effect of using it. The noise isn't all that bad, I've had much worse. In the old days, they were all noisier than this. ;)

Hope that helps someone. g
 
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#29
genesound said:
Sorry to dig up an old thread, but I have what I think might be helpful info if anyone is still considering these preamps.

I bought a new old stock (early serial) BMAX (non tube) preamp for cheap and found it the distortion issue using a bass with hot signal, even in the "active" input. I had already read this thread and decided it likely needed the mod. I also looked at the clipping on a scope and found it was clipping one rail and not the other, it was asymmetric, FWIW. Whatever, I called BBE tech support and talked to Judd, and he was extremely helpful. We discussed the changes for the non tube BMAX, he sent me the schematics, and I ordered up some matching metal film 1% resistors at Digi Key.

I changed out the 4 resistors as spego noted in his revised mod post earlier in this thread, which agreed with revision 1.3 of the official schematic:

1) R3 changes from a 46.4k to a 68.1k resistor
2) R8 changes from a 46.4k to a 21.5k resistor
3) R12 changes from a 6.81k to a 14.7k resistor
4) R14 changes from a 5.62k to a 10.0k resistor

I still noted a bit of clipping with an Ibanez SR300 with active EQ with everything up full on the axe and the preamp. Being all transistor and opamps, the clipping was not a pleasant fuzz at all, but I've decided it's adequate, as I never play in that manner anyway. BTW, using the compressor usually fixes the clipping, and the sonic maximizer circuit also affects it, so it's actually happening in an opamp stage after those circuits or the sonic maximizer IC, I didn't bother to check that, as it's actually adequate now.



Moving on, the other day I was perusing around the net and found a used BMAX-T tube version for sale at Guitar Center in the Edina store for a decent price. Of course they had to add shipping and tax, but it was still decent compared to offerings on ebay. It too was an early unit (ver 1.0 circuit board with the Jensen DI xformer!) and had not been modded. The clipping was certainly a more pleasant fuzz, but I thought the active input could use the mod, so I did the mod as C-5KO listed earlier. Again I got the schematics from Judd for the BMAX-T, but this time versions 1.0 and 1.4.

Here are the changes as C-5KO listed earlier:

1) R7 gets changed from a 46.4k resistor to a 68.1k
2) R10 changes from a 46.k to 21.5k resistor
3) C8 is removed completely
4) C9 is removed completely

I left the original 12AX7 Groove Tube in it, as having the tube gain there isn't the issue, IMO, it's just the attenuation network of those input resistors that was incorrect.

This BMAX-T sounds awesome! It's got that great tube "punch" when I get after it on the finger picking, that extra tube dimension. The non tube one sounds a bit thinner and 1 dimensional, even even with the same settings. They're both quite clean, and not too noisy if I watch the gain structure, and run the main amp lower and the preamp hot.

I'm using a Carvin DCM3800L power amp, with the inputs set to about -15dB (10 o'clock) and it really gets with it, but keeps the noise lower. I'm also using an Avatar SB126 and a dual 10 cab.

I found it's the sonic maximizer circuit that tends to make the most noise in these when it's on, but I like the effect of using it. The noise isn't all that bad, I've had much worse. In the old days, they were all noisier than this. ;)

Hope that helps someone. g
Click to expand...
 
#30
I purchased a MAX-T locally and had horrible distortion even on active input when I would crank the volume past 10:00. This mod fixed my issue 100%. The Mod procedure was not too bad. You can unscrow the front and back panels, a couple of screws on the bottom (not the transformer) and tilt the board up to un-solider the capacitors and the 2 resistors. I found the on e-Bay for $3.00.
BBE Mod.jpg
 
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