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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sun Apr 27, 2014 10:10 am
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Roadie
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I always stick with the schematic values for both resistors and caps with the exception of voltage rating. I replaced a bias cap in the Princeton 25uf/50v stock with a 50uf/100. The amp sounded a little stiff. So, I switched to 25uf/100v. The amp sounds fuller-more bass response. Grid resistors see very low voltage. The screen resistors are the ones that blow if a power tube fails.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sun Apr 27, 2014 10:19 am
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Krankster wrote:



I replaced a bias cap in the Princeton 25uf/50v stock with a 50uf/100. The amp sounded a little stiff. So, I switched to 25uf/100v. The amp sounds fuller-more bass response. Grid resistors see very low voltage. .



Sure of that ? This is no sense to me.

Sure you do something more than that .


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sun Apr 27, 2014 6:57 pm
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Krankster wrote:
I always stick with the schematic values for both resistors and caps with the exception of voltage rating. I replaced a bias cap in the Princeton 25uf/50v stock with a 50uf/100. The amp sounded a little stiff. So, I switched to 25uf/100v. The amp sounds fuller-more bass response. Grid resistors see very low voltage. The screen resistors are the ones that blow if a power tube fails.



Check the voltage with the 50/100 and with the 25/100 cap. My bet is the resultant negative voltage is different. And thus, the idle bias point is different between the two caps used in the bias supply.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Mon Apr 28, 2014 1:20 pm
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Roadie
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Could be, plus a little less filtration. I'm not changing the 27K bias resistor though. I notice the difference when the amp is cranked. With my old ES-335, I can dime the volume and control everything from the guitar. It has a tweed like distortion. The Jupiter caps have broken in nicely.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Tue May 06, 2014 11:17 am
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yomammaaho wrote:
But other recommendations have been using higher than 1 watt for voltages over 400.


Carbon comps can handle high voltage much better than metal oxide. A 3 Watt metal oxide is usually used in place of a 1 Watt carbon comp.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Tue May 06, 2014 4:56 pm
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Krankster wrote:
Screen resistors are 470ohm 1W. Grid is 1500ohm 1/2W. I use carbon comps, but most use metal oxide. You don't want to exceed the wattage rating on the resistors. If a power tube blows, a high wattage resistor will no blow allowing the voltage spike to hit the transformers. I rebuit an early 70's twin that had carbon film and metal oxide resitors. Replacing them with carbon comps completely changed the tone for the better.
Image


This maybe true for the grid stopper (which should be CC or good metal oxide, as they are in the signal path). HOWEVER for the screen resistors, increased wattage rating is a good thing. You do not want those resistors to pop, as the screen helps to control the output tube from instabilities. You want that screen grid working all the time. If you lose the screen grid voltage (as when that screen resistor pops) --- you run a bigger risk of a runaway tube.

I like wire-wound 5-watt screen resistors. They are more stable to heat and sudden current transients, compared to CC or even metal oxide resistors. Some folk increase the value of the screen resistor to 1k-ohm (versus the nominal 470-ohm). This extra resistance drops the screen voltage another 10VDC compared to the anode voltage. The added voltage difference helps control secondary emissions. At a slight lost to the output dissipation.

Anyhow, here is a photo of a Super Reverb fitted with a "sand-box" 5 watt wire-wound 1k-ohm resistor. Taped pot is a feedback mod which was later removed.


http://en.wikipedia.org/wiki/Tetrode



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