Help improve pp-18 tone
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Is it ok if I leave the leak resistors on board and use only the wiper and ground connection of the pot? Basically I'd remove the 56k resistor and connect the diode at the top of the schematics and the ground accross the leak resistor as the ruby zener+diode would... and could it be possible to use just a single potentiometer so the bases share the same pot wiper? I mean if I swap the 150k resistor with the pot so each transistor base is connected to its 150k resistor and then share the wiper pot to ground? Or am I dreaming? : )
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And the two bases connected together is a big no no I guess?
I'm absolutely not whining, instead I'm greatly thankful that you draw that schematic fot me, I'm just asking because a triple gang pot is hard to find at best (if not impossible), and if I'm going to build the circuit like it is, it's a separate control for the buzz fix on the backpanel of the amp, that I have to deal to the same amount as the VVR control on the front. Not so convenient.
I'm going to build it in anycase to prove the concept, but I don't know if it will stay unless a reliable 3 gang pot source is found or the circuit can be done with a single ganged pot.
What do you think?
PS. I'm already contacting some custom pot dealer to find out if they require a minimum order or not.
I'm absolutely not whining, instead I'm greatly thankful that you draw that schematic fot me, I'm just asking because a triple gang pot is hard to find at best (if not impossible), and if I'm going to build the circuit like it is, it's a separate control for the buzz fix on the backpanel of the amp, that I have to deal to the same amount as the VVR control on the front. Not so convenient.
I'm going to build it in anycase to prove the concept, but I don't know if it will stay unless a reliable 3 gang pot source is found or the circuit can be done with a single ganged pot.
What do you think?
PS. I'm already contacting some custom pot dealer to find out if they require a minimum order or not.
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Custom pot.... wow. I'd want to test that it works first before shelling out for a custom pot. As I said, I haven't tested this circuit myself yet but it should do what it's supposed to.
All the ways I looked at it I couldn't work out how to do it with one pot, and especially a 1M pot which would allow you to use a dual gang for the VVR and variable zener. It always came down to anything shared between the two sides provided a conduction path between the two power grid inputs and stuffed everything up. But I might be missing something. I'll have another look over it. When I drew it up I kept thinking "how can I simplify this" and nothing came of it.
All the ways I looked at it I couldn't work out how to do it with one pot, and especially a 1M pot which would allow you to use a dual gang for the VVR and variable zener. It always came down to anything shared between the two sides provided a conduction path between the two power grid inputs and stuffed everything up. But I might be missing something. I'll have another look over it. When I drew it up I kept thinking "how can I simplify this" and nothing came of it.
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Someone on The Amp Garage was having this problem where the Ruby mod fixed the fizz problem at high VVR settings but became ineffective as the VVR was turned down. After trying out a number of different suggestions I was going to post the variable zener circuit which I'd put up in this thread, saying it was very much unproven. Before getting that far he tried zeners with much smaller threshold voltages and seems to have found a very simple answer.
He used 3V 5W zeners in the standard Ruby mod and says that it now doesn't fizz across the whole VVR range. Using zeners of too low a value risks clipping the negative side of the power grid signal before the power valve is cut off. This would introduce unwanted distortion. Paul Ruby's document suggests picking zeners a couple of volts higher than the cathode bias voltage. He aimed to make the clipping around symmetrical with the grid clipping on the positive side. But in theory there's no reason the zeners can't be lower than that as long as they don't clip before the power valve reaches cutoff.
I've never tested the cutoff voltage of EL84s in 18W amp conditions. From the data sheet curves it might be something like -20V. Bias is typically 10.5 - 12.5V, but that cools off under signal even before output clipping. So the 3V isn't that unreasonable. Anyway, something for people to try if the Ruby mod is the only fix to their fizz problems and they want VVR as well.
Here's the thread for those with a login at TAG:
http://ampgarage.com/forum/viewtopic.php?t=18157
He used 3V 5W zeners in the standard Ruby mod and says that it now doesn't fizz across the whole VVR range. Using zeners of too low a value risks clipping the negative side of the power grid signal before the power valve is cut off. This would introduce unwanted distortion. Paul Ruby's document suggests picking zeners a couple of volts higher than the cathode bias voltage. He aimed to make the clipping around symmetrical with the grid clipping on the positive side. But in theory there's no reason the zeners can't be lower than that as long as they don't clip before the power valve reaches cutoff.
I've never tested the cutoff voltage of EL84s in 18W amp conditions. From the data sheet curves it might be something like -20V. Bias is typically 10.5 - 12.5V, but that cools off under signal even before output clipping. So the 3V isn't that unreasonable. Anyway, something for people to try if the Ruby mod is the only fix to their fizz problems and they want VVR as well.
Here's the thread for those with a login at TAG:
http://ampgarage.com/forum/viewtopic.php?t=18157
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Now that's really cool information!He used 3V 5W zeners in the standard Ruby mod and says that it now doesn't fizz across the whole VVR range. Using zeners of too low a value risks clipping the negative side of the power grid signal before the power valve is cut off. This would introduce unwanted distortion. Paul Ruby's document suggests picking zeners a couple of volts higher than the cathode bias voltage. He aimed to make the clipping around symmetrical with the grid clipping on the positive side. But in theory there's no reason the zeners can't be lower than that as long as they don't clip before the power valve reaches cutoff.
I've never tested the cutoff voltage of EL84s in 18W amp conditions. From the data sheet curves it might be something like -20V. Bias is typically 10.5 - 12.5V, but that cools off under signal even before output clipping. So the 3V isn't that unreasonable. Anyway, something for people to try if the Ruby mod is the only fix to their fizz problems and they want VVR as well.
Here's the thread for those with a login at TAG:
http://ampgarage.com/forum/viewtopic.php?t=18157
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I actually tried this out using my rotary zmulti value zener fix. The lower zener value does work over the whole usable range of the VVR pot. (From about 10 o clock)
However, when the VVR is maxed, you do lose a little of the swirl at the end of the note decay. But, not anything you would notice in the mix with a band behind you. I'll try dig up the lowest zener value I used, but from my experience so far, it works.
(Of course, since I've already drilled a hole for the rotary, it stays!)
Thomas
However, when the VVR is maxed, you do lose a little of the swirl at the end of the note decay. But, not anything you would notice in the mix with a band behind you. I'll try dig up the lowest zener value I used, but from my experience so far, it works.
(Of course, since I've already drilled a hole for the rotary, it stays!)
Thomas
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