Thread: Grid resistor Q's Traynor Bass mate

I'm trying to make a Traynor Bass Mate a little more guitar friendly. In doing so, I wanted to simply bypass the cathode resistors. Each time I try, it sounds overloaded. I'm tring to bring up the mids/highs a bit.

The cathode resistors were both 5.6K. I reduced one to 1.5K. I can't lower the other without the overload type sputtering sound.

So, I thought if I lowered the gain by lowering the grid resistor from 1M to 470K, that may leave me enough room to boost the signal back up by bypassing the cathode resistors.

I installed a 470K for the grid. I tried a .68uf cap over the 1.5K and 5.6K resistor. I'm still getting the shorting or sputtering sound.

When I remove the cap, the amp works fine. I realize I can alter other parts to get the results. But, after looking at a Marshall type schematic, I can't figure out why I cannot bypass the cathode resistors. 5.6K resistor seems a really high value. I was thinking a .68uf cap would be small enough to work. Compared to a Marshall 1987 preamp, why won't the Traynor allow the bypass caps?

If I can't do it, I at least want to know why!

http://www.lynx.bc.ca/~jc/720606_YBA2_AB.gif

Originally Posted by JDW3

I'm trying to make a Traynor Bass Mate a little more guitar friendly. In doing so, I wanted to simply bypass the cathode resistors. Each time I try, it sounds overloaded. I'm tring to bring up the mids/highs a bit.

The cathode resistors were both 5.6K. I reduced one to 1.5K. I can't lower the other without the overload type sputtering sound.

So, I thought if I lowered the gain by lowering the grid resistor from 1M to 470K, that may leave me enough room to boost the signal back up by bypassing the cathode resistors.

I installed a 470K for the grid. I tried a .68uf cap over the 1.5K and 5.6K resistor. I'm still getting the shorting or sputtering sound.

When I remove the cap, the amp works fine. I realize I can alter other parts to get the results. But, after looking at a Marshall type schematic, I can't figure out why I cannot bypass the cathode resistors. 5.6K resistor seems a really high value. I was thinking a .68uf cap would be small enough to work. Compared to a Marshall 1987 preamp, why won't the Traynor allow the bypass caps?

If I can't do it, I at least want to know why!

When you lower the value of the cathode resistors you dramatically increase the gain of the stage! When you use a bypass cap you are also increasing the gain. The usual circuit has some negative feedback to it that is higher at the lower, more bassy frequencies. Using a bypass cap is like putting in a very low value resistor but only for the AC signal. So the gain at those lower signals goes way up but the dc current stays the same.

Every amp is designed to have a reasonable amount of gain in each stage that gets multiplied as you add more stages. What you are doing is jacking the gain of an individual stage into orbit! Stages running super high amounts of gain tend to be unstable. The itty bitty bits of capacitance between wires or parts running close to each other actually can act like capacitors and feedback some of the output signal from a stage into its own input. If the signal happens to be in phase then the stage oscillates, just like putting a microphone in front of a speaker.

The grid bypass capacitor doesn't influence the gain like you think, unless you make it REALLY lower than 1meg! This isn't the way to fix your problem.

I suggest you put that 1K5 value back to the stock 5K6. Put the 1 meg back as well. With everything stock THEN try adding bypass caps! The bypass caps act like lower value resistors to the AC signal but not to the DC power flowing through the tube. It's important to remember that there are TWO different circuits in a tube stage! The DC voltages set bias levels and such. The AC signal runs through these resistors but also through coupling and bypass capacitors. The value of each capacitor is like an AC only resistance, which is the definition of REACTANCE.

As I said, every amp is designed to have a certain amount of gain per stage. When you play with bypass caps, do them one at a time. By putting caps on both stages you simply had too much gain for things to be stable!