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Re: Project is done!!

5/30/1998 5:19 PM
Re: Project is done!!
Hi Steve... it works... but the biggest problem i've got is the processor being overdriven by the preamp... i've only got the "Marshall" pre-amp built so far, (i.e. no BF yet...)... the processors I've tried so far are a Korg G3, an older Korg A5, and a el-cheapo DOD TR3M... actually, the DOD works the best so far, but I'm sure that's because i'm over-driving the hell outta the others...  
the reverb... i KNEW there was something i was forgetting to change in the schematic.... it's wrong as drawn... basically what i ended up doing was using the circuit as found in the Ampegs, et. al.  
only instead of a 6CG7 i used a 12AX7... it DOES work, but it's a bit brittle sounding so i need to tweak out the response of the reverb circuits... thanx for the offer of doing the drawing for me, but i've got it... the one i put up on my ftp site was just a quickie i did to just have something there...  
I did notice a few people looked, anybody else have any comments?  
5/31/1998 3:50 AM
Steve A.

    For the heck of it have you tried a 12AT7 for the reverb driver tube? You mentioned the 12AX7 being a bit brittle sounding; maybe the lower gain of the 12AT7 would help...  
Steve Ahola
5/31/1998 11:49 AM

not yet, no... mostly cuz i aint got one.. although that would probably help... the 12AX7 is only rated at 1W plate dissipation while the 12AT7 is rated at 2.5W... the way i've got it biased right now, it's running right at 1W so i'm pushing the hell out of a sovtek 12AX7... i've got an order infor some NOS tubes and when it comes i'm sure that'll solve a few of the problems i've got... you wanna peek at the new schematic? same address....  
5/31/1998 3:09 PM
Steve, again

    Can you post the B+ voltages you are using? I'm working on a similiar project with a SR AA763 preamp for the Normal channel and a modded 5F6A preamp for the "Abnormal" channel, but the B+ voltages from my over-modded 65 Pro are way too high for the Bassman design...  
Steve Ahola  
P.S. The Abnormal channel is based (bassed?) on the Pignose G40V schematic, which you might want to check out..." target="_blank">">  
Note: C10 is a .022uF cap, not a .047uF cap
5/31/1998 4:06 PM

Steve: using the reference id's on my schematic:  
B+0 = 420V  
B+1 = 418V  
B+2 = 400V  
B+3 = 350V  
B+4 = 300V  
these are approximations as I havent got EVERYTHING wired up yet... both poweramps are done as well as the "Marshall" preamp and reverb. I can't imagine the BF preamp drawing them down too much though... also that big-assed 272JX power transformer is regulating very well... i've measured the output at about 32 watts per channel (with little distortion yet), using a sinusoidal input and B+ only dropped about 5 volts...  
5/31/1998 11:48 PM
Steve, again

Maybe you can give me some advice on lowering the B+'s for the 65 Pro project I'm working on:  
B+0=477V (to choke)  
B+1=475V (to 1K resistor)  
B+2=456V (to 4K7 resistor)  
B+3=416V (to 10K resistor)  
Can I keep adding 10K resistors and filter caps to get B+"X" down to 325V (5F6A specs) or even lower (280-300V) for the 5F6A-style preamp? Could I use a 22K resistor to get a bigger voltage drop?  
Steve Ahola  
P.S. What is the effect of running a BF preamp at the 300V B+4 on your schematic? The AA763 SR spells out a 410V B+ for the preamp tubes, which I believe would give you a lot of clean headroom. Am I correct in assuming that a lowered B+ of 300V would distort easier (which may be the effect you are trying to achieve)? With the project I'm working on, I'm trying to get the Normal channel to be as close to a SR as possible and have the other channel duplicate a modded 5F6A design (hence the wide range of preamp B+'s I'm trying to get).
6/1/1998 9:03 PM
Can I keep adding 10K resistors and filter caps to get B+"X" down to 325V (5F6A specs) or even lower (280-300V) for the 5F6A-style preamp? Could I use a 22K resistor to get a bigger voltage drop?  
yes... to get it where you need it, all you have to do is some simple Ohm's law...

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