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DIY 100W Bassman...again

8/1/1999 2:20 AM
mosrite man
DIY 100W Bassman...again
Great to have the amp BBS back again! I'm slowly ironing the kinks out of this project (5F6 Bassman), and I seem to be in the home stretch. (I hope) {:} What is happening is this annoying distortion that occurs when I increase the volume past a very low critical point. I scoped the preamp circuit and found that It first occurs in the cicuit on the grid of the first preamp tube, where the 2 68K resistors are connected. This seems to be nutz because it occurs before the volume control. Anyone out there experience this or have any ideas? Thanks in advance!
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8/2/1999 2:48 AM
mosrite man

I think I may have found the problem. The plate voltage on my first preamp tube is about 220V when it should be 150V. It seems that this excess could hike the gain of the first preamp stage, causing distortion. I am using the 100K plate resistors as called for in the schematic (5F6). Can the fact that I'm using a different transformer (from an old organ/theater amp) be causing this excess voltage? If so, could the plate voltage be lowered by simply using higher value plate resistors, or would this cause problems elsewhere? All other voltages are within 10% or so of what's on the schematic.Any thoughts would be appreciated!
8/2/1999 5:54 PM

Actually having a higher B+ will give more headroom thereby delaying the onset of distortion. Check the voltage on both sides of the plate resistor. Hopefully they aren't the smae. If they are, check the cathode circuit for a ground problem.  
Just for the hell of it check the ground on the cathode followers load resistor. I had a bassman with an open resistor there one time that caused the nasty distortion thing.  
8/3/1999 1:41 AM
mosrite man

There is a goodly drop across the plate resistor, it just doesn't drop to the value predicted in the schem. I'm getting frustrated, since everything else seems to be working. Thanks for the suggestions... I'll try these and post the results.
8/3/1999 2:55 AM
Steve A.

mosrite man:  
    The voltages you mention probably aren't causing the distortion (see my other post about removing the 25uF Ck on V2A at least temporarily).  
    If you do want to lower the B+ voltages going to the preamp tubes you can experiment with different valued resistors in the power supply. The schematic shows 4k7 resistor after the choke followed by a 10k. Maybe you can post the B+ voltages you have at the PI, the dc-coupled pair and the preamp tube, along with the voltages right at the tube pins so that we can see how they compare to the +/- 20% voltages printed on the schematic. (B+ is listed as 385vdc to the PI and 325vdc to the other preamp tubes, with the plate voltages being 325v(PI) 180v(V2) and 150v(V1).) IMHO 150v is a bit low for the 12AX7 tube you are presumably using for V1 (I'd aim for maybe 180v to 190v). A lot of the modern high-gain amps will run the plates around 140v to 150v just to get a lot of distortion but I don't think it cleans up very well when you turn the volume down... (Since you are building a tweed bassman clone I'd assume that you don't want it to sound like the super distortion channel of a Mesa Boogie Mark IV...)  
    If your preamp voltages are all a bit high you might try replacing the 4k7 resistor with a 10k/2w resistor and then measure everything all over again. If the V1 and V2 voltages are still a bit high then you might want to try a 15k or 18k resistor instead of the 10k one.  
    Good luck!  
Steve Ahola
8/3/1999 2:03 PM
mosrite man

Thanks! I'll post these as soon as I get them written down. Things seem to be smoothing out a bit. I re-routed some things, but the problem seems to rest with the bright channel. If I increase the volume on it, even with no input signal, EXTREME motorboating occurs. Doesn't happen with the normal channel, and the two channels seem to interact too much, like they are acting as successive gain stages. I'll keep you posted. Many thanks!
8/4/1999 3:50 PM
mosrite man

The voltages I measure on the tube pins are as follows:  
pin# 1 2 3 4 5 6 7 8 9  
Preamp 12ax7 247 0 1.9 6.2 6.2 239 0 1.9 2.1  
12ax7 142 0 1.8 6.5 6.5 354 143 147 2.3  
12ax7 271 20 29 6.5 6.5 255 20 29 2.2  
6L6 NC 2.1 420 420 -52 6.2 0  
same for the remaining three 6L6's.  
Any insight or suggestions would be greatly appreciated. Thanks in advance, I can't express how much help you all have been! I can imagine how my rantings my be annoying!

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