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Post subject: Grid Stopper and Screen Resistors in 69' Princeton (AA964)
Posted: Fri Dec 06, 2013 10:17 pm
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Roadie
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I picked up a 1969 Princeton amp a while back for cheap. The amp was hacked. While rebuilding it, I noticed it had 1500 ohm grid stopper and 470 ohm screen resistors on the 6V6's. They are old carbon comps and look like they could be original, so I left them alone. Did Fender use these resistors on some Princetons?


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Dec 07, 2013 3:08 am
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I don't see them on many Princeton Schematic .

About the 470 ohms , there no need , these 6V6 's grid are "protected" by the 1K resistor on the B+ .
This 1K act like 500 ohms for each tube .

Look original ! Are you sure of that ? This mod can be there since long time. Old carbon type are available .


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Dec 07, 2013 4:46 am
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Yeah Stratele, no grid stoppers on the 6V6GT signal grids. This maybe a good mod, to help reduce instabilities --- like parasitic oscillations. With a slight effect on the tone.

Right, about the 1k-ohm resistor in the power rail. This drops the B+ about 10VDC. So, the screen grid run around 10VDC below the anodes.

Are these resistors on the socket tangs, similar to how Fender soldered the grid stopper and screen grid resistors on 6L6GC amps? If they seem well mounted and soldered --- let them be. If they are hanging off the sockets with a lot of lead --- remove or replace them.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Dec 07, 2013 10:19 am
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Roadie
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They're on the tangs like 6L6 amps and the solder joints looks solid. They may not be stock, but they look like they've been in there a while. I did some research and found that Fender put these on some Princeton and PR's that had oscillation problems. The amp is not as loud as other Princeton's I've played. Would removing these resistors give me more power?


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Dec 07, 2013 12:23 pm
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Krankster wrote:
. Would removing these resistors give me more power?


IMO , no , 1500 ohms is not high enough to change volume. I'm not sure .

You can try just put a jumper on it . No need to remove them.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Dec 07, 2013 12:39 pm
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So the resistors are soldered across the socket tangs. From screen grid connection (pin #4) to an empty, non-connected (NC) tang (pin #6), like in the photo? And another resistor from pin #1 to Pin #5 --- connection for the grid stoppers.

The added 470-ohm screen resistor will lead to a greater voltage drop on the screen versus the anode. About a 15 total VDC drop, in your amp's case. Greater voltage difference between the screen and the anode = bigger drop in output power.

You can try clipping the 470-ohm resistors. But, BE SURE that the line from the PSU tap point now goes directly to pin #4. As you can see from the photo, the screen resistor is across pin#6 (NC) and pin #4. Removing the screen resistor means you must connect the wires from the tagboard, directly to pin #4.

The 1.5k-ohm grid stopper alters the tone very slightly. But, like Stratele stated, does not change the total output of the 6V6GT.



6V6GT pinout:

http://tdsl.duncanamps.com/show.php?des=6V6GT




Image


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Dec 07, 2013 1:52 pm
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Krankster wrote:
They are old carbon comps and look like they could be original, so I left them alone. Did Fender use these resistors on some Princetons?


Some photos of your amps may help us .


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Thu Apr 24, 2014 8:08 am
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Roadie
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The plate voltage is 402. Instead of removing the screen resitor, et al., I changed the bias resistor from 27K to 26K giving me 24ma on the tubes. BTW, the schematic calls for a 25uf/50v bias cap. Bullpucky, they come with 50uf/70v. Good ol' Fender schematics.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Thu Apr 24, 2014 8:53 am
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Krankster wrote:
The plate voltage is 402. Instead of removing the screen resitor, et al., I changed the bias resistor from 27K to 26K giving me 24ma on the tubes. BTW, the schematic calls for a 25uf/50v bias cap. Bullpucky, they come with 50uf/70v. Good ol' Fender schematics.


Usually a 1k-ohm change in this resistor (27K to 26K) doesn't affect the idle bias that much. I normally go in increments of 3-5K-ohms. But, if it worked no problems.

Capacitance value on the bias supply is not that critical. From 50-160 mfd is fine. Voltage value shouldn't drop below 50VDC. 100VDC is better. Just don't go hog-wild and put in 400 mfd worth of filtering. As you run the risk of the bias supply voltage lagging behind the main PSU filtering. Esp on cold starts.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Thu Apr 24, 2014 11:01 am
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Roadie
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I have a Sprague 25uf/50v in there now. It was the only cap I had when I rebuilt the amp. It works fine, but I'll drop a 50/100 in when I place another parts order. The Jupiter (yellow) tone caps sound great-worth every penny. I'll post a pic. I'm proud of my lead dress work. It's dead quiet.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Fri Apr 25, 2014 8:26 pm
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Roadie
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Here are a couple pics of the Princeton rebuild. I'm proud of the lead dress work. Notice the screen resistors.

Image

Image


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Apr 26, 2014 4:43 am
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While we're talking screen grid and grid stopper resistors here, I'm recapping a 69 twin and having to replace the scorched carbon comps on the 6L6 sockets.

Already have the 2 watt metal oxide 420 ohm resistors just in, but gonna replace the stopper 1.5k resistors while I'm in there.
Anyone chime in if 1 watt grid stopper 1.5 k metal oxide resistors are high enough rated?


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Apr 26, 2014 7:04 am
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Roadie
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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


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Apr 26, 2014 7:20 am
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Interesting. Was just about to post I found my answer (2 watt) grid stoppers, but to protect the tranny I'll sacrifice a tube any day.
Maybe I'll order 1 watt and put them on the new micalex octal sockets I also decided to replace here.


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Post subject: Re: Grid Stopper and Screen Resistors in 69' Princeton (AA96
Posted: Sat Apr 26, 2014 7:26 am
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But other recommendations have been using higher than 1 watt for voltages over 400.


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