6L6 PP home built amp:
I wanted to put a switch to choose lower plate voltage by choosing between two dropping resistors. I did this with a cathode biased amp w/o problems. Thinking about it for a fixed bias amp makes me realize the bias will change if I change the plate voltage.
The reason for this mod is to 'mimic' the brown sound of Eddie Van Halen w/o dropping the heater voltages too.
Any ideas? The only thing I can come up with would be a DPDT with the other side changing the resistors in the bias network to be close to what I want wrt current. Of course there still is a bias pot to adjust for optimum performance, but my understanding of the fixed bias network is quite limited. I can read, copy and measure, but I don't know enough to plan it myself.
Thanks,
Daniel
I wanted to put a switch to choose lower plate voltage by choosing between two dropping resistors. I did this with a cathode biased amp w/o problems. Thinking about it for a fixed bias amp makes me realize the bias will change if I change the plate voltage.
The reason for this mod is to 'mimic' the brown sound of Eddie Van Halen w/o dropping the heater voltages too.
Any ideas? The only thing I can come up with would be a DPDT with the other side changing the resistors in the bias network to be close to what I want wrt current. Of course there still is a bias pot to adjust for optimum performance, but my understanding of the fixed bias network is quite limited. I can read, copy and measure, but I don't know enough to plan it myself.
Thanks,
Daniel
I would suggest just trying it and see what happens, monitor both the current through the valve, and the sound it produces. Basically you're trying to produce even greater distortion, so I don't see as trying to maintain accurate bias really matters.
IF you can't get a sound you like, try making the bias adjustable as well, and alter both the dropping resistor values and the bias, see if you can find an optimum point here as well.
If it's better with adjustable bias, then you may need to do it with a DPDT switch.
IF you can't get a sound you like, try making the bias adjustable as well, and alter both the dropping resistor values and the bias, see if you can find an optimum point here as well.
If it's better with adjustable bias, then you may need to do it with a DPDT switch.
Thanks Nigel,
That's where my understanding breaks down in this part of amps. Through calculations I would be running lower current if I left the bias at 47mA, which is pretty close to what I should have with 445v on the plates. If I switch to around 300v and using the 70% rule as my upper limit that would require 70mA.
About the only thing I really understand is that the current is not too much for the plates. I'm not sure how the current effects the distortion though, and yes that is what I'm after, especially for a smaller gig or practice.
But I will try it and listen to the sound like you suggest, so far that's been my real test equipment anyway.
That's where my understanding breaks down in this part of amps. Through calculations I would be running lower current if I left the bias at 47mA, which is pretty close to what I should have with 445v on the plates. If I switch to around 300v and using the 70% rule as my upper limit that would require 70mA.
About the only thing I really understand is that the current is not too much for the plates. I'm not sure how the current effects the distortion though, and yes that is what I'm after, especially for a smaller gig or practice.
But I will try it and listen to the sound like you suggest, so far that's been my real test equipment anyway.
I forgot to mention, after what I read about bias, there is so much conflicting advice as to what is perfect or what is standard, it's nice to hear from someone with more knowledge than myself that I have somewhat of a window to fall within,
Thanks again...
Thanks again...
First off, the 70% rule, is just a rule of thumb a lot of guitar guys like to tubes. A LOUD gain monster like the PV 5150 can come out of the box at 25-30%
Look at amps with a high/low power switch. Not the ones that turn off half the power tubes. Let me see if I can recall a specific Fender I have in my mind. They drop the B+ and at the same time change the bias voltage.
Here, "The Twin." Not the one I was thinking, but it serves as an example.
http://www.webphix.com/schematic heaven/www.schematicheaven.com/fenderamps/the_twin.pdf
Look at amps with a high/low power switch. Not the ones that turn off half the power tubes. Let me see if I can recall a specific Fender I have in my mind. They drop the B+ and at the same time change the bias voltage.
Here, "The Twin." Not the one I was thinking, but it serves as an example.
http://www.webphix.com/schematic heaven/www.schematicheaven.com/fenderamps/the_twin.pdf
Thanks Enzo, I see how they put the 6.8K as a voltage divider switched in and out to drop the negative voltage, yes?
With that PV 5150 statement, does that mean lower current produces more distortion and vice versa? I mean to a point, knowing you can't go too low or high.
When I get this put together I will experiment with different bias settings in both power modes now that I know it is okay to do so. Which leads me to ask:
Why don't manufacturer's use a pot that will allow one to change the bias but keep the range within 'safe' operating mode? I think I will try that, say adjust from 20% to 70% of the plate dissipation. That is quite a range and if it's not going to hurt anything, why not make it available to use for sonic changes?
With that PV 5150 statement, does that mean lower current produces more distortion and vice versa? I mean to a point, knowing you can't go too low or high.
When I get this put together I will experiment with different bias settings in both power modes now that I know it is okay to do so. Which leads me to ask:
Why don't manufacturer's use a pot that will allow one to change the bias but keep the range within 'safe' operating mode? I think I will try that, say adjust from 20% to 70% of the plate dissipation. That is quite a range and if it's not going to hurt anything, why not make it available to use for sonic changes?
Asking that last question just made me realize how it all works.
The bias voltage sets the current flow (with its negative repulsion) from the cathode to the plate. Like a river, if it is low, will cause the incoming alternating current-guitar-to reach it's amplitude peaks faster as the current does not 'drag' it as much, thus going into square wave distortion. More current will 'stretch' out the signal producing larger amplitude peaks thereby making it more difficult to go into the square wave form.
Do I understand this correctly?
The bias voltage sets the current flow (with its negative repulsion) from the cathode to the plate. Like a river, if it is low, will cause the incoming alternating current-guitar-to reach it's amplitude peaks faster as the current does not 'drag' it as much, thus going into square wave distortion. More current will 'stretch' out the signal producing larger amplitude peaks thereby making it more difficult to go into the square wave form.
Do I understand this correctly?
For the same reason you can't sell baby toys with little bits that can be swallowed - you know they're gonna do it. If you tried to return a system to a hifi shop becuse the speakers blew up when you turned the volume knob all the way up, they'd laugh at you. A guitar store will just replace the amp.
When I was younger, I got glorious tones by raising the plate voltage. For some reason though, I couldn't interest anyone in an amp that needed to be retubed every two hours.
Oh many guys have remarked that the best tone they ever got from some amp was when the tubes were cherry red, and the great tone came in the couple minutes the tubes lasted.
PV for many years made their tube amps not adjustable, and their reasoning was stated - if it were adjustable, it made it far too easy for someone to misadjust the amp. And in fact on some later designs they did as you suggested - they added an adjustment that had limited range, so someone could twiddle it, but probably not get in trouble.
The thing is in a design like the 5150, the tone is in the preamp, so they did not design it for ease of breaking up the power tubes. I only brought it up to point out that "70%" is not a necessary condition. Set "cold" like they are stock, the power amp is strong and clean.
PV has some interesting white papers in their little technical library, you might find what Mr Peavey has to say about bias of interest. And read also what Mesa says about biasing. "Bias" has become a mantra for tube amp enthusiasts in the guitar world, and they make it sound like some ultra-critical setting. It is not. SOmeone may prefer the thing on the edge of melt-down, but there is a wide range of perfectly valid bias idle current settings.
PV for many years made their tube amps not adjustable, and their reasoning was stated - if it were adjustable, it made it far too easy for someone to misadjust the amp. And in fact on some later designs they did as you suggested - they added an adjustment that had limited range, so someone could twiddle it, but probably not get in trouble.
The thing is in a design like the 5150, the tone is in the preamp, so they did not design it for ease of breaking up the power tubes. I only brought it up to point out that "70%" is not a necessary condition. Set "cold" like they are stock, the power amp is strong and clean.
PV has some interesting white papers in their little technical library, you might find what Mr Peavey has to say about bias of interest. And read also what Mesa says about biasing. "Bias" has become a mantra for tube amp enthusiasts in the guitar world, and they make it sound like some ultra-critical setting. It is not. SOmeone may prefer the thing on the edge of melt-down, but there is a wide range of perfectly valid bias idle current settings.
What you need to remember with power tubes is the way they operate in class A and class AB. the colder (more negative the bias) the closer to class B the amp is pushed, as a result more crossover distortion can occur which can be tamed by the use of negative feedback. biasing colder also provides slightly more headroom.
Amplifier classes
this should explain classes.
Amplifier classes
this should explain classes.
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