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katopan
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Post by katopan »

Where are you and what shop is your preferred source? Was thinking I could look through a website for you and make a recommendation.

The MJE340 was the best choice for what is available here in Aust through one of our last surviving electronics retail store chains. It's always hard putting part numbers on schematics because different countries have easier access to different parts.
Last edited by katopan on Fri 11/11/11 4:49 pm, edited 1 time in total.
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Post by asd »

I'm in Italy, unfortunately I have no preferred shop, I just phisically go to the electronic shop since it's on my way to work, but I can easily order from anywhere in the EU : )
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Post by katopan »

Any luck finding a transistor asd? The MJE340 is overkill. What are all the npn options you have from your local store?
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Post by asd »

Any luck finding a transistor asd? The MJE340 is overkill. What are all the npn options you have from your local store?
I can go at the local shop tomorrow. What kind of transistor I should be asking for? Any npn 300V rated transistor?

BTW: is it possible to change my nickname or at least my password? I'm still using that account found on the internet, virtually anyone can log in with this account...
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Post by katopan »

It would be a pain but I don't know if you could get a list of the npn transistors they have in stock, and then we could pick the most suitable.

300V is overkill. This thing sits on the ground side of the coupling caps, so it's all signal voltage with no DC. Looking at a test waveform document I put together ages ago, the PI output / power grid input is no more than 50V peak. Worst case peak current is if you had the VVR turned all the way up and the variable zener all the way down - from the 12AX7 curves it's about 3.5mA peak, but let's allow for 5mA. We're only clipping one side so half the cycle is 5mA peak and the other side our blocking diodes prevent current flow through the transistor. Average power dissipation is going to be something like 50V x 5mA x 0.5 duty cycle = 0.125W. Geez it's looking like this could be done with a small signal transistor like a BC546.

To be honest I would go for something in a bigger case. Typical TO-92 thermal resistance junction to ambient is something like 200 degC/W so our 0.125W is going to result in a temperature rise of 25 degC above ambient. But I'd want to have this thing bulletproof. I think the minimum spec you'd want would be TO-126 case or bigger, and 80V or more rating. Anything that meets that will be able to take the 5ma current. The protection diodes cover everything else you'd be worried about.

Let us know what you find. :D
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Post by asd »

The guy at the shop consulted a big book for ten minutes and then gave me two mighty TIP41C's.
Are the good enough?
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Post by katopan »

Man, even more overkill than the MJEs! Big power transistors don't have as high a beta factor (current gain) so it might load the pot with a bit more base current. But we're talking small current here anyway so yeah, they should do the job.
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Post by asd »

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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Post by katopan »

It would loose its function if you did all that. It's the ratio of resistance across the diode and emitter to base vs the resistance on the other side of the base that sets the clipping voltage.
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Post by asd »

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.
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Post by katopan »

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.
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Post by asd »

Ok, I'm confident I can have it done for next week, let's see if it works first : )
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Post by asd »

Sorry man, I'm still here, I had some personal matters and few gigs, so I had no time to do anything on the amp...
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katopan
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Post by katopan »

Life's like that. No worries, we'll all still be here when you get a chance.
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Post by asd »

Great : ) In the meantime my best wishes for christmas and happy new year to you all!!!
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Post by Bronsboi »

I have built a lite 2b with VVR on PA and PI. Did anyone get round to building and testing the "variable ruby mod" mentioned earlier in the thread?
Just curious as I also get the fizz with the VVR turned down but not when its turned up full.
Dave
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Post by katopan »

I don't think anyone got to try it.
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Post by katopan »

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
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Post by Bronsboi »

Thanks Katoplan for taking the time writing such a thorough reply. I have read the thread and I'm going to trial the lower rated zeners. I will post my findings in a week or two.
thanks again :D
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Post by katopan »

Cool. Definitely let us know how it goes.
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