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Can't escape crap distortion

5/27/2000 9:41 PM
Chris Harden
Can't escape crap distortion
Many of you may have allready read posts relating to my amp experiments, and I'm grateful for the help I have recieved so far.  
Here's my latest question for you to ponder. My preamp currently has 3 gain stages feeding into a single 6V6. A basic description follows.  
Stage 1: 1/2 12AX7 no bypass capacitor, 10nF coupling to stage 2. 260V B+  
Stage 2: 1/2 12AT7 1uF cathode bypass, 22nF coupling to passive Bandaxall tone control. 280V B+  
Stage 3: 1/2 12AT7 no bypass, 280V B+  
Unfortunately it isn't sounding too good. The distortion is very rough, especially on lower notes, probably what you describe as farting due to blocking distortion. I've tried reducing the size of the coupling capacitors and increasing the grid stoppers but it doesn't help.  
Would the fact that my power supply may be slightly under-rated, make a large difference to the tone. There is plently of smoothing in the main filter bank, and there is power supply decoupling between, the power valve and stage 1 and stages 2,3.  
Any ideas please. If you require more info to work with then I can supply it.  
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5/27/2000 11:16 PM

Have you tried moving the tone stack before the second stage? Let the second stage recover any tone lost in the TS and let the 3rd stage work it into overdrive?  
Never used a 12AT7 in the signal path myself. Haven't heard the best reports of it in that position.  
If you could throw up a schem, I'm sure there lots of folks here who could help.  
5/28/2000 10:12 AM

Hi Chris!  
You could try to replace the first 12ax7 by a 12at7,a bit less gain helped a lot in my bassman.  
Best regards.  
5/28/2000 11:16 AM
Steve Ahola

    As BW suggested you might try moving the tone stack so that it is after the first stage. After doing that you might try using two 500k trim pots wired as variable resistors after the 2nd stage coupling cap. The first pot would supply a series resistance after the coupling cap to adjust the H.F. content. The second pot would be wired to ground as a grid load and as you increase the resistance it will raise the signal level and the L.F. content. Depending on how hot of a signal you have at that point the higher settings of the second pot will control the amount of overdrive and distortion. If you can't find a "sweet spot" by adjusting those two controls, then try adding in a grid stopper (like 100k for starters). Once you have determined what settings for the trim pots work best, you can replace one or both of them with fixed resistors, or add a front panel pot if you want to be able to make adjustments on the fly.  
    FWIW in my HRT I found that a 130k series resistor and a 120k resistor to ground worked best for me. I then replaced the 130k series resistor with a 56k fixed resistor in series with a 100k linear pot wired as a variable resistor. With that arrangement I can set the series resistance between 56k and 156k. BTW 56k seems to work really well with the Clean settings...  
    Before stumbling across the use of these trim pots to fine-tune the circuit I was using typical Marshall-like RC networks such as a 470k resistor bypassed with a 560pF cap- which gave the OD mode a "one trick pony" type of midrangy voicing; by using just resistors the sound seems to be more versatile... although I did just come up with a "Plus" version based on the Electroplex Rocket 50 that uses a p-p pot to toggle over to a 470k/.0015uF RC network there:  
--Good luck!  
Steve Ahola
5/28/2000 1:16 PM
Nate S.

What's the B+ for the 6V6? I made an amp that had seemingly endless crappy distortion problems. It had 250VDC on the plate. I finally upped the B+ to 365VDC and biased it hot (of course this is required for SE), and that was it. Fantastic distortion when cranked, nice clean when quiet. It seems to me that 6V6's like higher B+ and hot bias, especially after a similar problem wth my Priceton.
5/28/2000 1:53 PM
Carl Z

First thing that comes to mind is that this amp probably doesn't have enough preamp gain.  
Unbypassed ax7 is good for what...30dB. The second stage is probably lower than that. The Baxandall stack is going to eat close to 50dB and your recovery stage isn't coming close to recovering the signal. You may not even be hitting the power tube with two volts.  
I'm going to guess that you're way outside of linear operation of the tubes. I don't think you've got enough gain here to go into blocking distortion. You may even have some sort of parasitic oscillation. That can lead to really harsh, grainy distortion too.  
Give us some plate loads, bias voltages and a little more detail on the topology.  
Carl Z
5/28/2000 11:11 PM
Stephen Conner

Hi Chris,  
Like Carl said, there doesn't seem to be enough gain in your amp to make the preamp fart out. So it must be the power stage. Try reducing the grid coupling cap and cathode bypass cap on the 6V6. You could try increasing the screen voltage too and rebiasing (give you more headroom before you run into grid current)  
P.S. I think you are right to use the 12AT7, they sound great when overdriven. Folk recommend putting them in Boogies to make them sound good ;)  
Steve C.

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