Help improve pp-18 tone

asd · Post by **asd** » Tue 11/15/11 9:04 am

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

katopan · Post by **katopan** » Tue 11/15/11 4:26 pm

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.

asd · Post by **asd** » Wed 11/16/11 9:41 pm

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? : )

katopan · Post by **katopan** » Wed 11/16/11 11:30 pm

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.

asd · Post by **asd** » Thu 11/17/11 10:48 am

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.

katopan · Post by **katopan** » Thu 11/17/11 4:05 pm

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.

asd · Post by **asd** » Thu 11/17/11 9:50 pm

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

asd · Post by **asd** » Tue 12/20/11 10:35 am

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...

katopan · Post by **katopan** » Tue 12/20/11 2:59 pm

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

asd · Post by **asd** » Wed 12/21/11 7:38 am

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

Bronsboi · Post by **Bronsboi** » Tue 05/15/12 2:42 pm

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

katopan · Post by **katopan** » Tue 05/15/12 5:12 pm

I don't think anyone got to try it.

katopan · Post by **katopan** » Sun 05/27/12 5:31 am

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

Bronsboi · Post by **Bronsboi** » Mon 05/28/12 3:29 pm

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

katopan · Post by **katopan** » Mon 05/28/12 4:00 pm

Cool. Definitely let us know how it goes.

Post by **zaphod_phil** » Sun 06/03/12 8:47 pm

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

Now that's really cool information!

TomOlsen1 · Post by **TomOlsen1** » Mon 07/02/12 5:42 am

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

katopan · Post by **katopan** » Mon 07/02/12 4:50 pm

Good to have more confirmation that this works. Thank for letting us know Thomas.

Bronsboi · Post by **Bronsboi** » Mon 07/02/12 5:02 pm

Yeah, Thanks Thomas.
I've been meaning to test this for ages. I'll definitely try this out this week and post my findings.
Dave

Gary · Post by **Gary** » Tue 07/03/12 5:10 am

Thomas

are you saying you prefer a bit of fizz on the decay and that it is eradicated at minimum voltage?

How are you defining usable range of the vvr pot? Mine is usable down to tv-like volume. What does your 10 o'clock equate to?

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Help improve pp-18 tone