Variac questions

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Skeeter
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Variac questions

Post by Skeeter » Mon Jul 17, 2017 5:59 pm

I know that some of you guys have been running plexis on variacs for a long time, so this seems to be the perfect place to ask these questions...

I have a Mojave Scorpion that is designed for European wall outlets (230vac/50hz) and I want to try it with a Variac. Eddie used to run his plexi at 90V which is only about 75% of the initial wall voltage so by that logic I'd need to run my amp at about 170v. How safe is that voltage for a plexi style amp? Some people say that it vastly shortens the tube life and sometimes even damages electronic components...

Also, would I need a voltage stabilizer before the variac? I only intend to use it at home, so I won't be gigging with this variac setup...

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Re: Variac questions

Post by johnnybgoood » Tue Jul 18, 2017 11:48 pm

Regarding the RCA tests on shortness of tube life due to starved filaments. I have the same tubes in my amps since 2010 and have played for hundreds of hours, possibly thousands, using a variac. No issues to this point to report and no need to replace tubes.

With a European Variac you can go down to 140 volts.

Twistynecks tests at 60volts (North American)
1971 Marshall Super Lead dimed at 60 volts
https://www.youtube.com/watch?v=m0be2IqIQ1Y

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Re: Variac questions

Post by Skeeter » Wed Jul 19, 2017 12:25 am

Have you had to rebias your plexi due to these lower voltages in order to keep it stable for such a long period of time or did you leave the bias at the 120v optimum?

I've seen quite a few vids of people using a variac, but I'm never sure if they did the rebiasing or not. Most forum posts I've read about this topic claimed that rebiasing is mandatory for voltage drops im excess of 5%.

With regards to the Mojave Scorpion: I've read that some users have used a variac with their Mojave Peacemaker, but I'm not sure how similar the topologies of these two amps are. The Scorpion uses 4 preamp tubes, though the extra tube has mainly noise reduction purposes, and also an independent filament transformer along with the power dampening circuit. Is it ok to use the variac with the Scorpion, too?

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Re: Variac questions

Post by jape88 » Wed Jul 19, 2017 6:23 am

I've been of the belief that lowering the voltages puts less stress on the valves and transformer? I'm no expert btw. I took me a bit to find a variac that was 6amp or above, (which were the requirements I'd read for a 100 watter) that didn't break the bank to buy! I'm still to try it myself so I can't help with the biasing Q.

If it's of interest staalhoofd, who used to be here did a great demo with euro voltage, plus this Czech guy shows you step settings on the fly (no idea what he says though!).

Hope it helps.

https://www.youtube.com/watch?v=KGeJsMdBrzw

https://www.youtube.com/watch?v=F2NB5bbg6bY

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Re: Variac questions

Post by johnnybgoood » Wed Jul 19, 2017 7:19 am

With regards to rebiasing, I keep my amp biased to 70% plate dissipation. Earlier I used to max out the bias, but in my case when I reduce the voltage down to 70 volts (roughly 140 v Europe) the tubes are so starved at that point, I don't notice an audible difference.

Mohave Scorpion already has 3-50 Watt power scaling according to their website.

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Re: Variac questions

Post by Skeeter » Wed Jul 19, 2017 8:26 am

johnnybgoood wrote:With regards to rebiasing, I keep my amp biased to 70% plate dissipation. Earlier I used to max out the bias, but in my case when I reduce the voltage down to 70 volts (roughly 140 v Europe) the tubes are so starved at that point, I don't notice an audible difference.

Mohave Scorpion already has 3-50 Watt power scaling according to their website.
Not sure if I'm reading this correctly: Have you biased the amp for the 70v optimum or have you left it at the 120v optimum?

If I wanted to use the same setup as you, would I need to rebias my amp, which is currently biased for 230v, so that it is optimized for 140v or would I leave the bias the way it is and just lower the voltage via variac?

The Mojave Scorpion doesn't use power scaling the way John Suhrs amps are using them (I think it was developed by Kevin O'Connor). The Scorpion has power dampening which is a whole different thing altogether. I don't know what it is exactly, but John Suhr said that it's way different than the power scaling he's using.

But the question remains: Can I use the variac with the Mojave Scorpion?

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Re: Variac questions

Post by johnnybgoood » Wed Jul 19, 2017 11:25 am

The amp does not need to be rebiased. My amp is set for 70% plate dissipation (optimum) for standard wall voltage. I use the variac from this starting point.

I looked at a low-resolution picture of the chassis layout for a Scorpion amp. It is a Plexi clone.

The variac is safe to use if the power dampening circuit is similar to this.

http://www.trinityamps.com/ForumGallery ... pening.jpg
Image

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Re: Variac questions

Post by Skeeter » Wed Jul 19, 2017 3:04 pm

Oh, ok. And you had no problems with cathode stripping over all those years running the amp at such low voltages and without rebiasing? No problems with internal electronic components?

Since I'm fairly new to this amp board and you're somewhat of a veteran, do you know many other guys who have run their plexis this way (no rebiasing, low voltages) without any problems?

Yes, I think that's the power dampening circuit. I checked the trinity amp forum where this schematic was uploaded and they were specifically talking about Mojave power damping.

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Re: Variac questions

Post by jnew » Sat Jul 29, 2017 4:01 am

Lower plate voltages is only a small part of the full variac effect. Colder bias and reduced heater voltages also play into how the amps feels and behaves. 8)
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Re: Variac questions

Post by jnew » Sat Jul 29, 2017 12:15 pm

:rockon:
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Re: Variac questions

Post by jnew » Sat Jul 29, 2017 12:26 pm

:rockon: 8)
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Re: Variac questions

Post by jnew » Sat Jul 29, 2017 1:37 pm

8)
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Re: Variac questions

Post by jnew » Sat Jul 29, 2017 2:28 pm

:rock:
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Re: Variac questions

Post by jnew » Sat Jul 29, 2017 2:51 pm

:rocker:
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Re: Variac questions

Post by jnew » Sat Jul 29, 2017 7:33 pm

:thumbsup:
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