about to start on a fender 5c1 "clone" pentode into 6v6 or 6l6. these i believe are grid bias how will it go with pedals, should i make it cathode bias instead 🙄
anyone have any experiences to share
anyone have any experiences to share
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This style of bias is called grid leak bias. There will be a small negative voltage on the grid due to accumulation of electrons from the space charge on the grid. The resistor "leaks" them off at a controlled rate to establish a negative bias voltage. Every tube will be different, and they will all degrade over time due to ion contamination (gas). This type of bias is not normally used any longer.
I started out building 5C1 derived amps in the mid 60's. I was a kid then and didn't really understand what I was doing, but that didn't stop me. I had made a few amps using parts pulled from old TV's and radios. I usually used a 6SJ7 for the input tube because they were very common, but the output tube was anything from a 6V6 to a TV sweep tube.
I made a fuzz tone box using germanium transistors that resembled the Vox Tone Bender. Plugging the fuzz box into the champ clone resulted in an ugly mess of noise with the sound actually cutting completely off at times.
I took the amp over to a local ham radio guy who had given me the hand drawn 5C1 schematic originally. He poked around in the amp with a voltmeter, and noticed that the problem was nearly cured when the voltmeter probe was on the grid of the 6SJ7. He suggested lowering the resistor value on the input tube grid. This helped but the amp didn't sound as good.
A few weeks later he gave me a new circuit with a resistor in the cathode and the amp worked much better.
I don't think either one of us really knew what was happening then, but looking back I realize that we had a big case of blocking distortion.
Driving the grid of the 6SJ7 with a couple of volts of signal results in a large negative voltage on the grid since the grid and cathode form a diode that rectifies the signal. This charges the coupling cap leaving that voltage to discharge slowly through that 5 meg resistor. The time constant can be long depending on the cap value.
Every pentode input guitar amp I have built since the 60's has been cathode biased.
I started out building 5C1 derived amps in the mid 60's. I was a kid then and didn't really understand what I was doing, but that didn't stop me. I had made a few amps using parts pulled from old TV's and radios. I usually used a 6SJ7 for the input tube because they were very common, but the output tube was anything from a 6V6 to a TV sweep tube.
I made a fuzz tone box using germanium transistors that resembled the Vox Tone Bender. Plugging the fuzz box into the champ clone resulted in an ugly mess of noise with the sound actually cutting completely off at times.
I took the amp over to a local ham radio guy who had given me the hand drawn 5C1 schematic originally. He poked around in the amp with a voltmeter, and noticed that the problem was nearly cured when the voltmeter probe was on the grid of the 6SJ7. He suggested lowering the resistor value on the input tube grid. This helped but the amp didn't sound as good.
A few weeks later he gave me a new circuit with a resistor in the cathode and the amp worked much better.
I don't think either one of us really knew what was happening then, but looking back I realize that we had a big case of blocking distortion.
Driving the grid of the 6SJ7 with a couple of volts of signal results in a large negative voltage on the grid since the grid and cathode form a diode that rectifies the signal. This charges the coupling cap leaving that voltage to discharge slowly through that 5 meg resistor. The time constant can be long depending on the cap value.
Every pentode input guitar amp I have built since the 60's has been cathode biased.
The problem perhaps arises from a little error in those items.
Control grids in a tube, has two current components:
1) a positive one mainly generated from grid emission,
2) a negative one, by catching electrons (Or the lasts impacting in the former).
Both of them varies over time, temperature, ageing, plate/screen currents, voltages in the electrodes, and many other variables. So, excepting tubes in which plate's current is very low and limited by a large plate lead resistor in non linear amplifiers (Class C for example), usualy it is undesirably bias a tube only with grid leakage.
Control grids in a tube, has two current components:
1) a positive one mainly generated from grid emission,
2) a negative one, by catching electrons (Or the lasts impacting in the former).
Both of them varies over time, temperature, ageing, plate/screen currents, voltages in the electrodes, and many other variables. So, excepting tubes in which plate's current is very low and limited by a large plate lead resistor in non linear amplifiers (Class C for example), usualy it is undesirably bias a tube only with grid leakage.
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Thanks guys SO cathode it is. that's sorta what I thought. I remembered hearing these amps were not too pedal friendly, now I know why
and thank you George for the turbo champ schematic from another post, I built one of those with a Kt88 apparently it's too loud lol.
and thank you George for the turbo champ schematic from another post, I built one of those with a Kt88 apparently it's too loud lol.
One of mine was loud enough to make the neighbor angry while my daughter's drum set didn't.....and his own kid played in a death metal band.
The Turbo Champ I used had a 6V6 in it. It seems that the kids are allowed to make a racket, but the husband hears "turn it down."
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