Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Paulycoo on August 02, 2018, 03:51:29 pm
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Hello again to all.
I've just rebuilt a 1978 Fender Princeton, a non reverb model with USA power transformer.
It sounds amazing....I'm using a uk to usa voltage transformer of good quality.
Ive got 443v on pin 3 from the OT and 20mA across the 1 ohm resisitor from pin 8 to ground so its biased at 70pc.
The datasheet says max plate voltage is 315v
Can anyone help?
I dont wanna burn these jan sylvanias up in a week!!
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Max. voltage has been exceeded on 90% of all modern designs.
Don't let it bother you. It's max. dissipation that truly matters and we've even managed to exceed that.
Tubes are mechanical instruments. Run them hard and they will fail sooner than later. It's a game we all play.
The Sylvania's are a hearty tube and a good choice.
You are well within reason at 20mA. The amp should sound great and the tubes will last as long as they will last.
If you run it cranked 7 days a week they will die far sooner than if you practice in your bedroom once a week with the volume on 3.
Personally, I run mine cranked for 3 hours, once a week and they die about once a year. When I finally hit the stage I'll be carrying spares.
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Thank you for your help much appreciated Paul
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> The datasheet says max plate voltage is 315v
That's if you do not test-measure every radio you build-- lots of room for variation.
Later sheets say 350V, if you actually control (measure) voltage on every build.
And actually-- the numbers on 6V6 were always very conservative. In the 1930s they wanted the leeway to use cheap plate-stuff. If you needed a bigger radio they would rather sell you a 6L6 (at higher price). But even in the days your vintage tubes were made, they all used the same plate-stuff and would stand pretty much the same voltages.