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resistors & caps AND tone

2/22/2004 5:19 AM
steven oda
resistors & caps AND tone
Doesn't every resistor or cap in the signal path effect the tone? I thought that this was one of the TEN COMMANDMENTS of amp building.  
Even 'lytics, in the PS and voltage dropping resistors off the PS effect the tone. That's why choke loading anode plates is popular with some amps.  
Inquiring minds want to know.  
2/22/2004 11:54 AM
Chris ( CMW amps )

Hi Steve  
Yep : about everything between input-jack and speaker-out does effect the tone .  
Regarding the G2 resistors ( after the choke-node ) going higher then really needed ( all imho & to my ears ) :  
- loss of "punch"  
- loss of output-power  
- slight loss of clarity  
Don't forget to rebias after changing those resistors .  
2/22/2004 12:47 PM
Dan Lurie

Thanks guys... Lots of good info. I will leave the amp with the standard Fender 470ohm for now. See my next question regarding Preamp Grid Resistors.
2/22/2004 2:46 PM
Philip Morrison

I'd like to chime in to say that I've had similar experiences as these guys above. Some time back I became concerned about screen protection, primarily in EL84 & 6V6 amps, because of the higher voltages that predominate in todays "Off the shelf" power transformers. After reading some of the discussions here on ampage & reading & experimenting I've ended up increasing wattage capacity in similar fashion to the guys above. I think Chris hit it on the nose with the larger wattage "traditional" values.  
In the TUT series Kevin O'Conner advocates increasing the screen circuit resistance for the purpose of protecting against misaligned screen & control grids. Such an the increase will hedge against the possibility of hot spots, exceeding the tube's intended screen current value and early tube failure. He also claims that perceived tonal differences are "psycho-acoustic".  
IMHO his point is well taken with regard to tube protection, however, I have a/b'd two, otherwise identical, amps(EL84 voxy/match style)with the traditional 100R screen resistor values vs. O'Conner recomended screen R's of 1K1W & found there to be a real difference in tone(increased screen resistance tended to make the amp sound kind of sterile). After experimentation with other topologies & tube types I came to the same conclusion.  
Additionally, thinking that if I couldn't do much of anything about screen current because of the tonal sacrifice, I went for lower voltage PTs to try & keep the screen voltage down closer to manufacturers max ratings. I don't think this really addressed O'Conner's concerns but, coupled with the larger wattage rating screen Rs, I'm not having tube failures.  
Hope this helped  
2/23/2004 9:48 PM
Jonathan Payant
Wow, would a Marshall Super Lead benefit from dropping to say 100ohm screen resistors?
2/23/2004 10:02 PM
Chris ( CMW amps )

burnt EL34s at wall shaking volumes :D  
no offense and without kidding ;  
early 50 Watt Marshalls didn't have screen resistors but the Mullards were pretty tough . Even with NOS Mullard EL34s I wouldn't try anything below 1k/5W but that's just me .  
2/24/2004 2:07 AM
Screen resistor tests
Hi Jonathan / Chris  
When I was building my 6V6 amp and doing the screen resistor tests the general consensus on this board was that as long as the screen dissapation was within its recommended limits all should be okay - even if the screen voltage was close-to or above the plate voltage. It's easy to try it (I used a potentiometer, but you can just temporarily put a second resistor in parallel with the existing one). If anyone tries it let us know what you think.  
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